Daytime Sprinkler Irrigation Effects on Net

Crop Ecology & Physiology

Daytime Sprinkler Irrigation Effects on Net Photosynthesis of Maize and Alfalfa Yenny F. Urrego-Pereira, Antonio Martínez-Cob, Victoria Fernández, and José Cavero* ABSTRACT

During sprinkler irrigation some water is lost due to drift and evaporation. After irrigation, plant-intercepted water is lost due to evaporation. The water loss causes microclimatic changes, which may involve positive or negative plant physiological responses. We studied the changes in net photosynthesis of maize (Zea mays L.) and alfalfa (Medicago sativa L.) associated with irrigation with a solid-set sprinkler system. For each species, measurements were made simultaneously in two plots, one being irrigated and the other not being irrigated. Two automatic canopy chambers connected to two CO2 infrared gas analyzers were used. Sprinkler irrigation decreased air temperature (1.5°C on maize, 1.7°C on alfalfa), air vapor pressure deficit (VPD) (0.44 kPa for both crops) and canopy temperature (5.1°C on maize, 5.9°C on alfalfa). Sprinkler irrigation decreased maize net photosynthesis on 80% of the days and the mean reduction was 19%. Sprinkler irrigation increased alfalfa net photosynthesis on 36% of days, decreased it on 14% of days, and had no effect on half of the days. The decrease of maize net photosynthesis during sprinkler irrigation was linked to the high leaf wettability (water contact angles from 60–80°) and the decrease in temperature below the optimum range for photosynthesis. The higher hydrophobicity of alfalfa leaves (water contact angles >120°) and the wide range of optimum temperature for alfalfa photosynthesis may be the reasons why photosynthesis remained unaffected by sprinkler irrigation. The results suggest that daytime sprinkler irrigation with solid-set should be avoided for maize but can be used for alfalfa.

D

uring a sprinkler irrigation event, some water is lost due to wind drift and evaporation that occurs as water travels from the sprinkler nozzles to the crop, and to evaporation that occurs for water intercepted by stems and leaves after the irrigation event (Tolk et al., 1995). The water that is evaporated produces microclimatic changes: decreases the temperature and VPD of the air (Robinson, 1970; Thompson et al., 1993; Tolk et al., 1995; Cavero et al., 2009). These changes are greater during daytime (Cavero et al., 2009). The microclimatic changes during daytime sprinkler irrigation cause physiological changes in the crops, which could affect their growth and yield. A relevant physiological change during sprinkler irrigation is the reduction in crop transpiration (McNaughton, 1981; Tolk et al., 1995; MartínezCob et al., 2008), which is considered positive because it represents a reduction of wind drift and evaporation losses (WDEL) (Martínez-Cob et al., 2008). Another major physiological change during sprinkler irrigation is the decrease in crop canopy temperature (Steiner et al., 1983; Tolk et al., 1995; Saadia et al., 1996; Cavero et al., 2009), which could have a positive effect on photosynthesis when leaf temperatures are too high (Mahan et al., 1995; Wanjura and Upchurch,

Y.F. Urrego-Pereira, A. Martínez-Cob, J. Cavero, Dep. Soil and Water, Estación Experimental de Aula Dei (CSIC), Avda. Montañana 1005, 50059 Zaragoza, Spain; V. Fernández, Forest Genetics and Ecophysiology Research Group, School of Forest Engineering, Technical University of Madrid, 28040 Madrid, Spain. Received 8 Mar. 2013. *Corresponding author ([email protected]). Published in Agron. J. 105:1515–1528 (2013) doi:10.2134/agronj2013.0119 Copyright © 2013 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

2000); for instance, during the afternoon of summer months in semiarid climates. However, if leaf temperature decreases below an optimum value, photosynthesis could also decrease (Mahan et al., 1995). Hirasawa and Hsiao (1999) found a decrease in stomatal conductance and photosynthesis of maize during the afternoon in a well irrigated maize crop in California (high evapotranspiration). This decline in photosynthesis during the afternoon of summertime days has been observed in other studies (Huck et al., 1983; Puech-Suanzes et al., 1989; Bunce, 1990a, 1990b; Pettigrew et al., 1990). Hirasawa and Hsiao (1999) indicated that the decrease in photosynthesis occurred because the plants could not transpire at the rate imposed by the atmospheric conditions, which caused a decrease in leaf water potential (LWP). Hsiao (1990) proposed that sprinkler irrigation could reduce the water stress experienced by the plant due to microclimatic changes during sprinkling. Howell et al. (1971) found that intermittent mist irrigation increased the LWP of southern pea [Vigna unguiculata (L.) Walp.] during irrigation and led to a higher yield than the non-mistirrigated treatment. Cavero et al. (2009) found that sprinkler irrigation increased the LWP of maize from –1.2 and –1.4 MPa to –0.54 MPa. Given that photosynthesis of crops increases as LWP increases (Boyer, 1970a, 1970b; Beadle et al., 1973), sprinkler irrigation could result in increased photosynthesis. However, Cavero et al. (2008) reported a 10% reduction in maize grain yield with daytime solid-set sprinkler irrigation compared with nighttime irrigation. Abbreviations: CU, Christiansen coefficient of uniformity; ETo, reference evapotranspiration; ETc, crop evapotranspiration; FC, field capacity; GMT, Greenwich Mean Time; IRGA, infrared gas analyzer; LWP, leaf water potential; PAR, photosynthetically active radiation; VPD, vapor pressure deficit; WDEL, wind drift and evaporation losses; WP, wilting point.

A g ro n o my J o u r n a l • Vo l u m e 10 5 , I s s u e 6 • 2 013

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Table 1. Soil characteristics of the experimental fields. Depth m Exp. 1 0.0–0.3 0.3–0.6 0.6–0.9 0.9–1.2 Exp. 2 0.0–0.3 0.3–0.6 0.6–0.9 0.9–1.2

pH

C N CaCO3 Sand Silt Clay —————————————————– % —————————————————–

FC† WP‡ ———– m3 m–3————

8.4 8.4 8.4 8.6

0.86 0.72 0.44 0.38

0.110 0.102 0.088 0.075

30.9 31.6 30.7 30.3

26.5 24.0 17.4 19.1

45.4 46.9 50.0 50.3

28.1 29.1 32.6 30.6

0.351 0.351 0.344 0.329

0.197 0.217 0.196 0.171

8.1 8.2 8.3 8.4

0.82 0.52 0.38 0.30

0.070 0.045 0.036 0.028

36.0 39.4 38.2 38.8

51.0 54.4 56.4 55.8

35.5 33.8 32.8 34.4

13.4 11.8 10.8 9.7

0.269 0.250 0.243 0.243

0.096 0.083 0.071 0.065

† FC, field capacity (–0.033 MPa). ‡ WP, wilting point (–1.5 MPa).

Several studies have found that water from rainfall, fog, dew, or artificial misting can affect the photosynthesis of plants (Smith and McClean, 1989; Brewer and Smith, 1994, 1995; Ishibashi and Terashima, 1995; Hanba et al., 2004). It has been shown that the effect of leaf wetting on photosynthesis depends on leaf wettability. A material is defined as wettable when contact angles of drops deposited on to its surface are below 90° and non-wettable when angles are above 90°. The solid–liquid interactions between drops of water deposited on to plant surfaces will depend both on the physical structure (i.e., micro- and nanoroughness) and chemical composition of every particular surface (Brewer et al., 1991; Khayet and Fernández, 2012). Photosynthesis increased or was not affected by wetting when leaves were non-wettable (Smith and McClean, 1989; Hanba et al., 2004), but seemed to decrease when leaves were wettable (Smith and McClean, 1989; Brewer and Smith, 1994, 1995; Ishibashi and Terashima, 1995; Hanba et al., 2004). This different behavior is related to the negative effect on photosynthesis (lower CO2 uptake) of the water deposited on to the surface of wettable leaves that may limit gas exchange, in contrast to the positive effect for photosynthesis of the lower VPD of the air when leaves are non-wettable (Smith and McClean, 1989; Brewer et al., 1991; Brewer and Smith, 1995, 1997). The relationship between leaf wettability and photosynthesis has been studied mainly in natural ecosystems and in relation to non-agricultural species (Smith and McClean, 1989; Brewer and Smith, 1995, 1997). Studies with agricultural plant species have been performed only under controlled conditions (Brewer and Smith, 1994; Ishibashi and Terashima, 1995; Hanba et al., 2004). Liu and Kang (2006a) reported the work of Yang et al. (2000), who found increases in wheat (Triticum aestivum L.) photosynthesis in sprinkler irrigated areas. Maize and alfalfa are among the main sprinkler irrigated field crops worldwide, but there is no information available about the effect of daytime sprinkler irrigation on their photosynthesis. The objective of this study was to determine whether the net photosynthesis of maize and alfalfa is affected by sprinkler irrigation in a solid-set system during daytime irrigation events and to determine what mechanisms might affect it.

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MATERIALS AND METHODS Experimental Site Field experiments were conducted in Zaragoza, Spain (41°43´ N, 0°48´ W, 225 m asl). Maize was grown in a 2.34-ha field during 2009 and 2010 (Exp. 1) and alfalfa was grown in a 2.0-ha field during 2009, 2010, and 2011 (Exp. 2). The distance between the two experiments was about 1 km. Both of them were located in the middle of an irrigated 8000-ha area where the main crops are maize, alfalfa, and wheat. The minimum distance of the experimental plots to the edge of the irrigated area was 2.5 km. The soil was clay loam in Exp. 1 and sandy loam in Exp. 2. Both soils are classified as Typic Xerofluvent (Table 1). The climate in the area is Mediterranean semiarid. In the last 10 yr (2003–2012) annual daily mean air temperature was 14.1°C, annual total precipitation was 298 mm, and annual reference evapotranspiration (ETo) was 1243 mm. Experimental Layout A solid set sprinkler irrigation system was installed in both experimental fields (Fig. 1 and 2). The sprinkler spacing was square, 18 by 18 m in Exp. 1, and 15 by 15 m in Exp. 2. The impact sprinkler and nozzles were manufactured in brass (RC-130, Riegos Costa, Lérida, Spain). The sprinkler has a vertical throw angle of 25°, the nozzle diameters were 4.4 mm (main) and 2.4 mm (auxiliary), and the nozzle height was 2.5 m (Exp. 1) and 2.2 m (Exp. 2) above the soil surface. The nozzle operating pressure was kept around 0.3 MPa with a hydraulic pressure control valve. Sprinkler application rates were 5 mm h–1 (Exp. 1) and 7.5 mm h–1 (Exp. 2). The irrigation volume was measured with an electromagnetic flow meter (Promag 50, Endress+Hauser, Reinach, Switzerland) which has a measurement error of ±0.5%. The meteorological conditions during the irrigation events were measured with an automated weather station (thereafter named as the weather grass station) located at the experimental farm, at 1 km from Exp. 1 and 50 m from Exp. 2. The weather grass station is located over grass following the reference conditions defined by Allen et al. (1998). The field of Exp. 1 had been grown with maize the previous year and the crop residues incorporated to the soil with conventional tillage. Maize cultivar Pioneer PR34N43 (comparative relative maturity 110) was planted on 21 Apr. 2009 and 20 Apr. 2010 (Exp. 1), in rows 0.75 m apart at a Agronomy Journal • Volume 105, Issue 6 • 2013

Fig. 1. General layout of Exp. 1. Twelve irrigation sectors irrigated independently by four sprinklers each. The irrigation sectors with even numbers were irrigated during daytime in 2009 and during nighttime in 2010. The irrigation sectors with odd numbers were irrigated during nighttime in 2009 and during daytime in 2010. Irrigation was characterized (water losses and irrigation uniformity) in sectors 5 and 6. The arrow indicates where is the north (N).

planting density of 87,000 seeds ha–1. Fertilization consisted of 50 kg ha–1 N, 100 kg ha–1 P2O5 and 100 kg ha–1 K 2O applied preplant, and 200 kg ha–1 N applied with the irrigation water split between the V6 and V12 growth stages (Ritchie et al., 1996). Weeds were controlled by applying acetochlor (2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6methylphenyl)acetamide) and terbuthylazine (6-chloro-N(1,1-dimethylethyl)-N9-ethyl-1,3,5-triazine-2,4-diamine) preemergence. Chlorpyrifos (O,O-diethyl O-3,5,6-trichloro2-pirydyl phosphorothioate) was applied at V14 through the irrigation system for insect control. Alfalfa cultivar Aragon was planted after conventional tillage at 35 kg ha–1 on March 2008 (Exp. 2). Fertilization consisted of 120 kg ha–1 P2O5 and 150 kg ha–1 K 2O applied each year in March. Six cuttings were made every experimental year, starting on April and with an approximated interval of 1 mo. No herbicides were necessary in this crop but two to three insecticide applications were made each year to control pests. The field of Exp. 1 had 12 irrigation sectors (main) irrigated by four sprinklers each (Fig. 1). The borders of the field were irrigated independently of the main irrigation sectors. In this experiment, maize was irrigated at nighttime until the crop was well established (V6–V8 growth stage). Thereafter, two irrigation time treatments were established (daytime and nighttime), so six irrigation sectors were irrigated at daytime and the other six at nighttime. Thus, at each daytime irrigation event, microclimatic and physiological measurements (see later in the text) made in one of the six daytime irrigated sectors could be compared with those made in one of the six nighttime irrigated sectors (which was not irrigated at that time). The irrigation sectors irrigated during daytime in 2009 were irrigated during nighttime in 2010 (Fig. 1). Daytime irrigation generally started at 1000 GMT while nighttime irrigation started at 2200 GMT of the previous day. The weekly irrigation amount was generally applied in two

Fig. 2. General layout of Exp. 2. Two irrigation sectors (A and B) not irrigated at the same time. The area within each irrigation sector where irrigation was characterized (water losses and irrigation uniformity) is shown (IE). The arrow indicates where is the north (N).

irrigation events that lasted 4 to 6 h. The area surrounded by the four sprinklers of each irrigation sector was considered for the measurements (Fig. 1). The area outside was disregarded because it received water from two different irrigation sectors. The soil matric potential was measured with a granular matrix sensor (Watermark, Irrometer Co., Riverside, CA) in the two irrigation sectors (one daytime irrigated and the other nighttime irrigated) where net photosynthesis was measured. Eight sensors were installed in each of these two irrigation sectors at the same positions within the square defined by the four sprinklers: four within the plant rows (two at 0.2-m depth and two at 0.6-m depth) and four at the center of the interrow space (two at 0.2-m depth and two at 0.6-m depth). The eight sensors in each irrigation sector were connected to a datalogger (Microisis, Sistemes Electrónics Progres, Bellpuig, Spain), which recorded the measurements each hour. A soil matric potential threshold of –0.080 MPa, which corresponded to a 30% soil water depletion, was used to indicate probable water stress in the experiment (Urrego-Pereira et al., 2013a). Complete details of the experiment can be found in UrregoPereira et al. (2013a). The field of Exp. 2 was divided in two irrigation sectors 1.0 ha each (Fig. 2). In this experiment, alfalfa was irrigated twice or thrice per week at each irrigation sector A and B (Fig. 2). The duration of the irrigation events was generally limited to 3 h. The applied irrigation water depth was the same at each irrigation sector, but irrigations were not simultaneous. Generally, at the beginning and end of the week the irrigation sector A was irrigated during daytime periods and the irrigation sector B was irrigated during nighttime the following day. By the middle of the week, the irrigation sector A was irrigated during nighttime and the irrigation sector B during daytime the same day. Thus, at each daytime irrigation event, measurements made in the irrigation sector being irrigated could be compared with those made in the other nighttime irrigation sector, which was not irrigated at that time. Daytime irrigations started generally at 1000 GMT, while nighttime irrigations started at 100 GMT.

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Crop water requirements were computed following the FAO approach (Allen et al., 1998). Reference ETo was computed with the FAO Penman–Monteith method from meteorological data obtained from the weather grass station. Crop coefficients (Kc) were calculated in the case of maize as a function of thermal time developed in the same location (Martínez-Cob, 2008) or in the case of alfalfa from tabulated values (Allen et al., 1998) locally adjusted by García-Vera and Martínez-Cob (2004). Daily maize and alfalfa evapotranspiration (ETc) was calculated from the corresponding daily values of ETo and Kc. Next, the crop irrigation requirements were calculated weekly as the difference between ETc and the effective precipitation, which was estimated as 75% of precipitation (Dastane, 1978). The irrigation amount applied to the crop was equal to the crop irrigation requirements plus the measured water losses. To evaluate the water losses of the irrigation events, a grid of 25 catch cans separated at 3.6 (Exp. 1) or 3.0 m (Exp. 2) was installed within the square area delimited by four sprinklers in two irrigation sectors (one daytime irrigated and the other nighttime irrigated) of Exp. 1 and in the irrigation sectors A and B of Exp. 2. The catch cans were made of plastic, had a diameter of 0.18 m and were located just above the crop canopy. After each irrigation event, the water amount collected in each catch can was measured. WDEL was calculated as:

WDEL = 100

I fm − I cc I fm

where Ifm, applied irrigation amount calculated from irrigation water measured with the flow meter; Icc, mean irrigation amount measured at the 25 catch cans. The coefficient of uniformity of the irrigation (CU) was computed from the water amount collected on the catch cans using the methodology described by Christiansen (1942). Microclimatic and Canopy Temperature Changes Due to Irrigation In Exp. 1 an automatic weather station was installed in the center of one daytime irrigated sector and a second station in one nighttime irrigated sector (Fig. 1). In Exp. 2 an automatic weather station was installed in the center of each of the two irrigation sectors (Fig. 2). These four weather stations were used for continuously recording the air temperature and VPD during the crop season so comparisons could be made between the sector that was irrigated and the sector that was not irrigated in each of the experiments during the irrigation events. One temperature and relative humidity probe (HMP45C, Vaisala, Helsinki, Finland) was installed at each weather station at 1.0 m above the crop canopy of maize and at 2.0 m above the soil surface of the alfalfa experiment. Each probe was installed inside a shield URS1 (Campbell Scientific, Logan, UT) which protected it from irrigation water and solar radiation. The accuracy of the probes was ±0.3°C for temperature and ±3% for relative humidity. The air temperature and relative humidity were measured every 10 s and the 5-min mean values were computed and recorded in a datalogger (CR10X, Campbell Scientific Inc, Logan, UT). The VPD was calculated from the air temperature and relative humidity data (Allen et al., 1998). 1518

An infrared thermometer (Apogee Instruments Inc., Roseville, CA) was installed in each of the automatic weather stations to measure the crop canopy temperature. The infrared thermometer was located at 1.0 m above the crop canopy with an angle of 45° and was oriented towards the north. The model IRR-P, with an accuracy of ±0.1°C, was used in Exp. 1, and the model IRTS-P, with an accuracy of ±0.3°C, was used in Exp. 2. Canopy temperature measurements were taken at 10 s intervals and average values every 5 min were recorded in the dataloggers. Due to a technical problem canopy temperature data from 2010 were lost. Leaf Wettability and Surface Topography Leaf wettability was determined by measuring advancing contact angles of drops of double-distilled water at 25°C using a Drop Shape Analysis System (DSA100, Krüss, Hamburg, Germany). Contact angles were measured on intact adaxial and abaxial maize and alfalfa leaf surfaces (10 replications). Two microliter water drops were deposited on to the surfaces with a manual dosing system holding a 3-mL syringe (0.5 mm diam. needle). Side view images of the drops were captured at a rate of six frames s–1. Contact angles were automatically calculated by fitting the captured drop shape to the one calculated from the Young–Laplace equation (Khayet and Fernández, 2012). On 9 Aug. 2010 four maize leaves (ear leaf) were taken from plants of the irrigated and not irrigated sectors at different times (before the irrigation started, during the irrigation at 2, 3, and 4 h after the irrigation started, and at 1 and 3 h after the irrigation finished). Water contact angles were measured on five portions of each leaf. On 12 July 2010 eight plant samples were taken from the irrigated and not irrigated sectors of alfalfa just after the irrigation finished and the water contact angles were measured in five leaves of each plant. The surface topography of adaxial and abaxial gold sputtered, intact maize and alfalfa leaf surfaces was examined with a scanning electron microscope (SEM) (S-3400 N, Hitachi, Tokyo, Japan) (Khayet and Fernández, 2012). Net Photosynthesis At each measurement day, net photosynthesis was measured simultaneously in one sector being irrigated and another one not irrigated at the same time. Data were collected from 1 h before the irrigation started until 2 h after the irrigation finished, so data were collected for 6 to 9 h. For this task all the equipment (chamber, pump, gas analyzer, flowmeter, thermocouples) was replicated, so two sets of equipment were used (one in the irrigated sector and another one in the not irrigated sector). Two automated transient-state closed-system canopy chambers for CO2 exchange determination were used. The chamber (Tecno El, Formello, Italy) is similar to that described by Steduto et al. (2002). It had a main module that is a rectangular box with five transparent polycarbonate walls (1.5 mm thick), held together by a narrow aluminum angular frame. The chamber was open in the bottom, had a ground surface area of 0.75 m2 (1.0 by 0.75 m), and had a height of 0.5 m. It has a metal base that is inserted 5 cm into the soil. For the maize crop, other modules with the same ground surface area but with a height of 1.0 m and open in the base and in the Agronomy Journal • Volume 105, Issue 6 • 2013

Table 2. Characteristics of the irrigation events and mean meteorological conditions during the irrigation events measured at the weather grass station. The mean photosynthetically active radiation (PAR) reaching the crop canopy during each irrigation event is also shown. Experiment and irrigation date Exp. 1: Maize 9 July 2009 16 July 2009 23 July 2009 27 July 2009 30 July 2009 24 June 2010 28 June 2010 20 July 2010 2 Aug. 2010 9 Aug. 2010 Exp. 2: Alfalfa 29 Sept. 2009 1 Oct. 2009 5 Oct. 2009 8 Oct. 2009 12 July 2010 16 June 2011 21 June 2011 23 June 2011 18 July 2011 20 July 2011 4 Aug. 2011 19 Sep. 2011 21 Sep. 2011 28 Sep. 2011

Irrigation duration hh:mm

Irrigation applied mm

4:00 5:15 5:00 5:00 5:30 5:30 4:30 5:30 4:00 4:00

21 27 25 26 28 27 22 28 20 20

18.8 22.1 24.2 25.6 21.9 10.9 9.5 8.5 21.6 11.2

3:00 3:00 3:00 3:00 3:00 3:00 3:00 3:00 3:00 3:00 2:45 3:00 3:00 2:30

22 22 22 22 22 22 24 23 22 23 20 22 22 18

0.8 22.4 5.7 10.5 9.6 18.6 7.3 15.6 15.4 30.3 11.5 35.3 10.4 13.5

Air temp. °C

Air VPD§ kPa

Wind speed m s–1

PAR µmol m–2 s–1

87 86 72 68 82 90 84 83 67 86

25.4 31.6 29.7 31.1 28.9 29.7 26.9 30.7 27.0 30.8

2.0 2.5 2.5 2.9 2.4 3.1 1.8 1.9 1.5 2.3

2.3 2.6 3.3 4.1 3.3 1.2 2.3 2.2 3.4 2.9

1810 1694 1726 1406 1651 1818 1554 1544 1624 1310

85 82 81 78 84 87 85 89 87 77 88 76 86 84

22.8 25.3 24.7 24.9 32.0 27.9 30.3 23.8 22.0 22.5 29.0 20.3 23.9 25.7

1.3 1.6 1.2 1.6 2.9 2.2 2.4 1.5 1.5 1.5 2.2 1.3 1.3 1.7

1.2 3.4 1.7 2.0 1.3 3.2 0.9 3.2 2.6 5.2 1.3 7.0 1.1 2.6

1257 1163 1243 1278 1520 1737 2069 1929 2055 1985 1959 1584 1629 1410

WDEL† CU‡ –––––––––– % ––––––––––

† WDEL, wind drift and evaporation losses. ‡ CU, coefficient of uniformity of Christiansen. § VPD, vapor pressure deficit.

top were added as the crop grew. Thus, the canopy chamber at the maximum height of maize was composed by two modules of 1.0 m height each and on top the main module of 0.5 m height. In the case of the alfalfa crop only the main module of 0.5 m height was used. In the case of maize, the chamber was located centered in a plant row, so it covered six plants in a 1.0 m length portion of the plant row. Each module has four fans (Ebmpapst, Mulfingen, Germany) mounted in the corners which provide a total flux of 1.4 m3 min–1. The chamber top-cover has a hinge on one side, is usually open but can be moved to close the chamber to measure the CO2 exchange. The top cover was maintained opened at an angle of 75°, so some interception of the irrigation water occurred. The chamber was in place only during the measurement days in order not to interfere with the crop. Two thermocouples (Campbell Sci., TCBR-3, Shepshed, UK) not shielded were installed to measure the air temperature at a 0.5 s interval at each chamber at half of its height, one was inside the chamber while the other was close but outside of the chamber. The photosynthetically active radiation (PAR) incoming above the crop canopy was measured at an interval of 60 s (Delta-T, model BF3, Wynster, UK). A miniature diaphragm pump (model 15D1150, GAST, Benton Harbor, MI) was used to continuously extract the air from inside of the chamber (from two positions in maize: 1.5

and 2.3 m above the soil surface; from one position in alfalfa: 0.3 m above the soil surface) and to conduct it to an infrared gas analyzer (IRGA, model LI-7000, Li-Cor Inc., Lincoln, NE). A flowmeter (Dwyer, model VFB-66-SSV-BFP, Michigan City, IN) was used to get a constant flow of 5 L min–1 through the IRGA system. After the air has passed through the IRGA it was recirculated to the chamber. The IRGA was set to measure the CO2 concentration of the air every 0.5 s. The chamber top-cover was kept open, except for a 50 s period every 15 min. During the time that the chamber was closed the four fans were stirring the air in each chamber module. The net photosynthesis was calculated as the CO2 flux during the period that the top-cover was closed and was determined with the concentration regression method (Reicosky et al., 1990). We used a lag time of 10 s and a calculation window of 20 s (40 values at a 0.5 s interval). Net photosynthesis of maize was measured on 5 d on 2009 and 7 d on 2010. Measurements were done after maize tasseling except in the two dates of June 2010. Net photosynthesis of alfalfa was measured on 4 d in 2009, 5 d in 2010, and 12 d days in 2011. Maize and Alfalfa Yield In Exp. 1, each of the 12 irrigation sectors (18 by 18m) was harvested on 6 Oct. 2009 and 4 Oct. 2010 with a combine and the grain weighed with a 1-kg precision scale. A subsample of


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grain was collected from each irrigation sector to measure the grain moisture, a measurement used to adjust the grain yield to a standard 140 g kg–1 moisture content. In Exp. 2, six samples of the alfalfa aboveground biomass (0.75 m2) were taken at each sector at each of the six alfalfa cuttings made per year. The alfalfa samples were dried at 60°C. Data Analysis For each measurement day the net photosynthesis data set was divided into four periods: (i) before the irrigation (before), (ii) during the irrigation (during), (iii) 1 h after the irrigation (1 h after), and (iv) 2 h after the irrigation (2 h after). For each measurement day and period, the 15-min interval data of net photosynthesis of the two treatments (irrigated and not irrigated) were compared with a paired t test at a level of significance of P ≤ 0.05. The measurement days when the net photosynthesis before the irrigation was different between the irrigated and not irrigated treatments were rejected. Other variables measured at the same time in the irrigated and not irrigated plots (air temperature and VPD, canopy temperature, air temperature inside and outside the automated canopy chamber) were analyzed also with a paired t test. Leaf wettability in the irrigated and not irrigated plots was compared with a t test. The daily soil matric potential in the daytime and nighttime maize irrigated plots was compared with a paired t test. The effect of irrigation time on maize yield was analyzed with ANOVA. The Statgraphics 5.0 software was used to analyze the data. RESULTS Characteristics of the Irrigation Events The duration of irrigation events ranged from 4.0 to 5.5 h for maize and from 2.5 to 3 h for alfalfa (Table 2). Taking into account the sprinkler application rates for both experiments, the average irrigation depths applied at each irrigation event were similar for both species (maize: 24 mm, alfalfa: 22 mm). For irrigation events of maize, the mean air temperature and VPD ranged from 25 to 32°C and from 1.50 to 3.10 kPa, respectively. For irrigation events of alfalfa, the mean air temperature and VPD ranged from 20 to 32°C and from 1.20 to 2.90 kPa, respectively. The wind speed ranged from 1.2 to 4.1 m s–1 during the irrigation events of maize and from 0.9 to 7.0 m s–1 for those of alfalfa. The WDEL during the irrigation events of maize ranged from 8 to 26% of applied water depth, while this range was wider for irrigation events of alfalfa (1–35%) (Table 2). In general, greater WDEL occurred with higher wind speed. The greatest WDEL of maize (26%) and alfalfa (35%) were measured for wind speeds of 4.1 and 7.0 m s–1, respectively. The CU values for the irrigation events of maize ranged from 67 to 90% and for the irrigation events of alfalfa from 76 to 89%. The lower CU values were found in irrigation events with high wind speed. The PAR ranged from 1310 to 1818 µmol m–2 s–1 during the maize irrigation events, and from 1163 to 2069 µmol m–2 s–1 during the alfalfa irrigation events. The soil matric potential in Exp. 1 was similar in the daytime and nighttime sprinkler irrigated maize plots and was always higher than the water stress threshold for maize at this site (Fig. 3). 1520

Fig. 3. Daily average values of soil matric potential for the different irrigation time treatments in Exp. 1 (maize). Each value is the average from eight probes: four probes installed within the plant row (two at 0.2-m depth and another two at 0.6-m depth), and another four probes installed halfway between two plant rows (two at 0.2-m depth and another two at 0.6-m depth). The dashed line indicates the soil matric potential that can cause water stress in maize for the soil at this experiment location. The stars indicate the days when maize net photosynthesis was measured.

Microclimatic and Canopy Temperature Changes Due to Irrigation Reductions of air temperature and VPD and canopy temperature during irrigation were observed as soon as sprinkler irrigation started (Fig. 4 and 5). There was a significant reduction of the air temperature measured above the crop canopy on the irrigated treatment around 1.5°C (maize) and 1.7°C (alfalfa) (Table 3). For both crops these significant reductions lasted for 1 h after irrigation with an average reduction of 0.6°C. Two hours after the irrigation finished the air temperature values of the irrigated treatment matched the air temperature values in the not irrigated treatment. Likewise, air temperature measured with thermocouples inside the canopy chamber was significantly lower at the irrigated treatment as compared to the not irrigated treatment (on average 3.8°C for maize and 3.5°C for alfalfa). This lower air temperature determined in the irrigated treatment lasted for 1 h after the irrigation finished and accounted for 1.9°C (maize) and 1.3°C (alfalfa). Similar differences between the irrigated and not irrigated treatments were observed for the air temperature measured with thermocouples outside the canopy chamber. The air temperature reductions due to irrigation recorded with thermocouples were greater than those reductions recorded with the Vaisala probes. This result could Agronomy Journal • Volume 105, Issue 6 • 2013

Fig. 4. Air temperature and vapor pressure deficit (VPD), maize canopy temperature and net photosynthesis of maize at 15-min interval from 1 h before until 2 h after the end of the irrigation event on 30 July 2009 for the irrigated and not irrigated treatments (y axis at the left side). The photosynthetically active radiation (PAR) above the crop canopy is shown in the y axis at the right side.

Fig. 5. Air temperature and vapor pressure deficit (VPD), alfalfa canopy temperature and net photosynthesis of alfalfa at 15-min intervals from 1 h before irrigation until 2 h after the end of the irrigation event on 8 Oct. 2009 for the irrigated and not irrigated treatments (y axis at the left side). The photosynthetically active radiation (PAR) above the crop canopy is shown in the y axis at the right side.

Table 3. Average air temperature and vapor pressure deficit (VPD) (measured above the crop canopy) and canopy temperature of maize and alfalfa in the irrigated (Irrig) and not irrigated (Not irrig) plots during and after the irrigation events when photosynthesis was measured (maize, 2009 and 2010; alfalfa, 2009, 2010, and 2011). Average air temperature measured with thermocouples inside and outside the canopy chambers during the 50 s that the canopy chamber was closed is also provided. Variable and crop Air temperature (°C) Maize Alfalfa VPD (kPa) Maize Alfalfa Canopy temperature (°C) Maize Alfalfa Temperature inside chamber (°C) Maize Alfalfa Temperature outside chamber (°C) Maize Alfalfa

N†

During irrigation Irrig Not irrig

10 14

27.6b‡ 23.6b

10 14

1.75b 1.30b

29.1a 25.3a 2.19a 1.74a

1 h after irrigation Irrig Not irrig 30.5b 26.8b 2.78b 2.05b

31.1a 27.4a 2.91a 2.22a

2 h after irrigation Irrig Not irrig 30.6b 27.4b 2.90a 2.29a

30.7a 27.6a 2.90a 2.35a

5 13

24.4b 21.1b

29.5a 27.0a

28.0b 27.2b

29.9a 28.2a

29.0a 28.0a

28.8a 27.1a

10 14

25.9b 23.6b

29.7a 27.1a

27.8b 26.7b

29.7a 28.0a

28.2a 27.0a

28.7a 27.3a

10 14

26.3b 22.4b

28.7a 26.3a

28.2b 26.8b

28.9a 27.4a

28.0a 26.7a

27.9a 26.9a

† Number of irrigation events. ‡ For each variable, crop and period of measurement, the values followed by different letters are significantly different according to a paired t test at the 0.05 probability level.


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Fig. 6. Advancing contact angles of a distilled water drop with the adaxial leaf surface of (A) maize and (B) alfalfa. Surface topography (x200) of the adaxial leaf surface of (C) maize and (D) alfalfa. Surface topography (x200) of the abaxial leaf surface of (E) maize and (F) alfalfa.

be related to the fact that thermocouples provided readings inside the crop canopy while Vaisala probes provided readings above the crop canopy. Cavero et al. (2009) found that the microclimatic changes due to sprinkler irrigation are greater as the measurement height over the soil surface decreases. Sprinkler irrigation significantly reduced the air VPD during irrigation by 0.44 kPa for both crops (Table 3). During the first hour after the irrigation finished the significant VPD reductions were 0.13 kPa (maize) and 0.17 kPa (alfalfa). For both crops, the VPD recorded at the irrigated treatment matched the recorded values at the not irrigated treatment at the second hour after irrigation finished as in the case of reductions observed for air temperature. Sprinkler irrigation decreased maize canopy temperature by 5.1°C during the irrigation event (Table 3). During the first hour after the irrigation finished this decrease although lower (1.9°C in average), was significant. The alfalfa canopy temperature was also significantly reduced by 5.9°C during sprinkler irrigation and 1 h after irrigation this significant decrease was 1.0°C.

differences in water contact angles were determined in leaves collected from the irrigated and not irrigated treatments in any period of measurement (Fig. 7). For this species, lower water contact angle values were found in the abaxial as compared with the adaxial leaf surface. These water contact angles were lower than 90° indicating that the leaves of maize are wettable. Similarly, no water contact angle differences were found between the irrigated and not irrigated treatments over the adaxial and abaxial leaf surfaces of alfalfa (Fig. 8). However, the contact angles of water drops with alfalfa leaf surfaces were higher (121° for the adaxial surface and 125° for the abaxial surface), which indicates that such surfaces are more hydrophobic than maize leaves. Examination of adaxial and abaxial leaf surfaces by SEM provided evidence that the leaves of both species are amphistomatous (Fig. 6C, 6D, 6E, 6F). The maize leaf surface (Fig. 6C, 6E) was found to be more flat than the alfalfa leaf surface, which had a more rough topography associated with the shape of epidermal cells (Fig. 6D, 6F).

Leaf Wettability and Surface Topography

During the 50-s period that the canopy chamber was closed, there was a slight increase of the air temperature inside the canopy chamber as compared to the air temperature measured outside the canopy chamber. This increase occurred in both the

An example of the advancing contact angles of a distilled water drop with the adaxial maize and alfalfa leaf surfaces is provided in Fig. 6A and 6B. For maize, no significant 1522

Net Photosynthesis


Fig. 8. Contact angle of water droplets (θ) with the adaxial and abaxial alfalfa leaf surfaces of the irrigated and not irrigated treatments on 12 July 2010. Values are the means of eight plants (five leaves from each plant were measured) and error bars are the standard errors. For each type of leaf surface no differences were found between the irrigated and not irrigated treatments after a t test at P ≤ 0.05.

Fig. 7. Contact angle of water droplets (θ) with the adaxial and abaxial maize leaf surfaces of the irrigated and not irrigated treatments on 9 Aug. 2010. Values are the means of four leaves from different plants and error bars are the standard errors. For each time and type of leaf surface no differences were found between the irrigated and not irrigated treatments after a t test at P ≤ 0.05.

irrigated and not irrigated treatments and in average was below 1 and 1.2°C for maize and alfalfa, respectively (Table 3). The time evolution of the net photosynthesis of maize recorded at 15-min intervals on 30 July 2009 at the irrigated and not irrigated treatments is shown in Fig. 4. The net photosynthesis of maize was similar for both treatments before irrigation. However, 15 min after the irrigation started (the

first measurement made) the net photosynthesis of maize at the irrigated treatment decreased compared to the not irrigated treatment. This lower photosynthesis measured in the irrigated treatment remained until the end of irrigation. As soon as irrigation finished, the net photosynthesis of irrigated maize plants rose to the levels of not irrigated plants. Two of the measurement days for net photosynthesis of maize in 2010 were rejected from the analysis because there were significant differences between the two treatments before irrigation started. Thus, 10 irrigation events were available to study the effect of sprinkler irrigation on maize net photosynthesis (Table 4). Sprinkler irrigation significantly reduced the maize net photosynthesis on 8 of the 10 irrigation events during the irrigation period. In the other two irrigation events, there were no differences between the two treatments. The reduction of net photosynthesis of maize at the irrigated treatment ranged from 10 (16 July 2009) to 41% (27 July 2009). Considering all the irrigation events, a mean 19% reduction of maize net photosynthesis was found in the irrigated treatment compared to the not irrigated treatment during the irrigation event.

Table 4. Net photosynthesis of maize in the irrigated (Irrig) and not irrigated (Not irrig) plots before, during, and after the irrigation events.

Irrigation date 9 July 2009 16 July 2009 23 July 2009 27 July 2009 30 July 2009 24 June 2010 28 June 2010 20 July 2010 2 Aug. 2010 9 Aug. 2010 Mean

Crop height m 2.40 2.40 2.40 2.40 2.40 1.05 1.40 2.45 2.45 2.45

Net photosynthesis Before irrigation During irrigation 1 h after irrigation 2 h after irrigation Irrig Not irrig Irrig Not irrig Irrig Not irrig Irrig Not irrig —————————————————————— µmol m–2 s–1 ————————————————————— 47.8†a 56.5a 58.0b 70.3a 48.5a 69.3a 60.5b 68.8a 82.3a 76.2a 76.4b 85.0a 49.5a 61.3a 20.8a 15.0a 30.4a 35.8a 51.1b 68.8a 61.1a 66.8a – – 73.9a 76.2a 39.0b 66.4a 21.7b 35.9a 9.8a 19.4a 53.5a 55.6a 52.5b 66.7a 39.5a 42.6a 16.6a 17.2a 40.0a 44.5a 39.7b 47.5a 38.5a 34.7a 23.5a 20.8a 55.7a 53.9a 62.6a 63.6a 61.3a 59.0a 47.9a 52.9a 61.2a 66.9a 65.4b 82.5a 39.1a 42.0a 13.1a 22.8a 46.1a 51.8a 55.8b 81.8a 50.5b 72.0a 32.7b 51.3a 44.9a 46.8a 53.2a 52.6a 38.0a 14.5a – – 53.6 56.4 55.4 68.5 44.8 49.8 28.1 33.5

† For each irrigation date and period the values followed by different letters are significantly different according to a paired t test at the 0.05 probability level.


1523

Fig. 9. Relationship between the net photosynthesis of maize and alfalfa and the temperature inside of canopy chamber during sprinkler irrigation at the irrigated and not irrigated treatments along the irrigation seasons in 2009 and 2010 (maize) and 2009, 2010, and 2011 (alfalfa). Each point is the average value of an irrigation date. The dotted line indicates the temperature threshold when photosynthesis of maize decreases as temperature decreases (Duncan and Hesketh, 1968; Tollenaar, 1989; Labate et al., 1991; Wolfe, 1991; Kim et al., 2007). PAR is the photosynthetically active radiation, and V8 is the eight leaves growth stage (Ritchie et al., 1996).

During the first hour after the irrigation finished the net photosynthesis of maize at the irrigated treatment was significantly lower as compared to the not irrigated treatment in 2 d with a reduction of 30 to 39%. In the other 8 d there were not differences between the two treatments (Table 4). Considering all the irrigation events, during the first hour after the irrigation finished the maize net photosynthesis was 10% lower in the irrigated treatment compared to the not irrigated treatment. Finally, during the second hour after irrigation of maize finished the net photosynthesis of the irrigated treatment was lower as compared to the not irrigated treatment in 2 d with a reduction of 12 to 36%. In the other 6 d, there were not differences between the two treatments. Considering all the irrigation events, during the second hour after the irrigation finished the maize net photosynthesis was 16% lower in the irrigated treatment compared to the not irrigated treatment. The lower net photosynthesis of maize during the irrigation event at the irrigated treatment was related with the lower air temperatures inside the canopy chamber reached at this treatment compared to the not irrigated treatment (Fig. 9). Thus, the air temperature at the irrigated treatment was mostly below the optimum range of 27 to 35°C (Duncan and Hesketh, 1968; Tollenaar, 1989; Wanjura and Upchurch, 2000; CraftsBrandner and Salvucci, 2002; Kim et al., 2007) and net photosynthesis decreased as air temperature within the canopy chamber decreased below 27°C. Low values of net photosynthesis of maize at the not irrigated treatment were found with incomplete canopy cover (24 June 2010, maize height of 1.05 m) and when the PAR was low (9 Aug. 2010, 1310 µmol m–2 s–1). In the latter date similar values of net photosynthesis were measured at air temperatures of 27°C (irrigated treatment) and 30°C (not irrigated treatment). Figure 10 shows that during sprinkler irrigation the decrease of maize net photosynthesis at the irrigated treatment compared to the not irrigated treatment was less as the PAR was lower. The low PAR on 9 Aug. 2010 may explain 1524

why no significant net photosynthesis differences were found between the irrigated and not irrigated maize treatments in this irrigation event. The time evolution of the net photosynthesis of alfalfa recorded at each 15-min interval on 8 Oct. 2009 at the irrigated and not irrigated treatments is shown in Fig. 5. Before irrigation started the net photosynthesis of alfalfa was similar for both treatments. Around 1 h after the onset of the irrigation, the net photosynthesis of alfalfa was slightly higher than that of not irrigated plants. Once the irrigation finished the net photosynthesis of alfalfa at the irrigated treatment began to be similar to that of the not irrigated treatment and in the second hour after the irrigation finished the net photosynthesis of alfalfa was similar in the two treatments. Four of the measurement days for net photosynthesis of alfalfa in 2010 and three in 2011 were discarded from the

Fig. 10. Relationship of the difference between net photo synthesis of maize measured during irrigation in the irrigated and not irrigated treatments and the photosynthetically active radiation (PAR) along the irrigation seasons in 2009 and 2010. Each value corresponds to a 15-min interval measurement.


analysis because there were significant differences between the two treatments before onset of irrigation. Thus, 14 irrigation events were available to assess the effect of sprinkler irrigation on alfalfa net photosynthesis (Table 5). During the irrigation period, sprinkler irrigation did not affect the alfalfa net photosynthesis in half of the irrigation events, significantly increased the net photosynthesis in 5 of the 14 monitored irrigation events, and significantly decreased the alfalfa net photosynthesis in 2 of the 14 monitored irrigation events. The increase of net photosynthesis of alfalfa at the irrigated treatment ranged from 10% (21 June 2011) to 30% (16 June 2011), while the decrease of net photosynthesis of alfalfa at the irrigated treatment ranged from 15% (23 June 2011) to 30% (28 Sept. 2011). Considering all the irrigation events, similar net photosynthesis was found for alfalfa in the irrigated and not irrigated treatments during the irrigation event. During the first hour after the irrigation of alfalfa finished, the net photosynthesis at the irrigated treatment was higher as compared to the not irrigated treatment on 4 d with an increase of 14 to 57%. In the other 10 d, there were not differences between the two treatments. Considering all the irrigation events, during the first hour after the irrigation of alfalfa finished the net photosynthesis was 12% higher in the irrigated treatment as compared to the not irrigated treatment. Finally, during the second hour after stopping the irrigation of alfalfa, the net photosynthesis at the irrigated treatment was higher as compared to the not irrigated treatment in 3 d, with an increase of 25 to 55%. In the other 11 d, there were not differences between the two treatments. Considering all the irrigation events, during the second hour after irrigation of alfalfa finished the net photosynthesis was 13% higher in the irrigated treatment compared to the not irrigated treatment. In the case of the alfalfa crop, although the temperature inside the canopy chamber decreased due to sprinkler irrigation, no relationship was found between the net photosynthesis and the air temperature inside the canopy chamber within the range of temperature measured (20–32°C) (Fig. 9).

Maize and Alfalfa Yield In the case of the Exp. 1 the yield of nighttime irrigated maize ranged from 15.0 Mg ha–1 (2009) to 16.5 Mg ha–1 (2010). Daytime irrigation significantly (P < 0.05) decreased the maize yield by 9% compared with nighttime irrigation in 2010. There was a 5% yield decrease for daytime irrigation compared with nighttime irrigation in 2009 that was significant at P = 0.10. Complete details about the effect of irrigation time on the growth of maize can be found in UrregoPereira et al. (2013a). In Exp. 2, the alfalfa forage yield was 24.2 Mg ha–1 in 2009, 20.5 Mg ha–1 in 2010 and 22.9 Mg ha–1 in 2011. Yield differences of alfalfa forage between the two sectors ranged from 2 to 6%. DISCUSSION The microclimatic changes (reduction of air temperature and VPD) during and after sprinkler irrigation recorded in this work were similar to previous reports (Thompson et al., 1993; Tolk et al., 1995; Saadia et al., 1996; Cavero et al., 2009), and induced plant physiological responses such as the decrease in canopy temperature (Steiner et al., 1983; Tolk et al., 1995; Saadia et al., 1996; Liu and Kang, 2006a,b; Cavero et al., 2009). The decrease in canopy temperature of maize found in our study was similar to that found by Tolk et al. (1995) using a lateral-move sprinkler irrigation system and by Cavero et al. (2009) using a solid-set sprinkler irrigation system. The slightly lower mean decrease of canopy temperature of maize compared to alfalfa due to sprinkler irrigation was probably due to a lower water application rate for maize (5 mm h–1) as compared with alfalfa (7.5 mm h–1). A higher application rate can have a higher cooling effect on crops (Cavero et al., 2009). It has been reported that sprinkler irrigation increases leaf water potential of maize (Cavero et al., 2009). This plant physiological change should increase maize photosynthesis according to the work of Boyer (1970a,1970b) and Beadle et al. (1973). Thus, increased leaf water potential due to mist irrigation resulted in a yield increase for southern peas (Howell

Table 5. Net photosynthesis of alfalfa in the irrigated (Irrig) and not irrigated (Not irrig) plots before, during and after the irrigation events.

Irrigation date 29 Sept. 2009 1 Oct. 2009 5 Oct. 2009 8 Oct. 2009 12 July 2010 16 June 2011 21 June 2011 23 June 2011 18 July 2011 20 July 2011 4 Aug. 2011 19 Sept. 2011 21 Sept. 2011 28 Sept. 2011 Mean

Crop height m 0.20 0.25 0.35 0.42 0.50 0.35 0.40 0.45 0.30 0.35 0.50 0.20 0.25 0.40

Net photosynthesis Before irrigation During irrigation 1 h after irrigation 2 h after irrigation Irrig Not irrig Irrig Not irrig Irrig Not irrig Irrig Not irrig ————————————————————— µmol m–2 s–1 ————————————————————— 17.9†a 19.8a 20.5a 19.9a 19.3a 20.5a 12.4a 13.7a 25.2a 22.1a 25.8a 20.8b 23.2a 14.8b 17.5a 11.3b 26.5a 27.7a 29.3a 29.3a 24.1a 21.1b 23.7a 18.9b 26.1a 26.5a 30.3a 27.2b 21.2a 18.8a 14.9a 11.4a 34.8a 36.0a 29.9a 24.0b 30.0a 23.3a 26.2a 20.3b 26.9a 26.4a 27.8a 21.4b 22.9a 16.9b 17.1a 13.7a 30.5a 28.7a 31.3a 28.4b 29.5a 28.6a 3.6a 4.8a 35.1a 37.8a 33.6b 39.4a 35.5a 35.9a 33.0a 34.1a 26.1a 26.4a 30.4a 27.0a 29.7a 26.7a 26.0a 22.0a 31.7a 26.9a 34.2a 30.0a 31.0a 27.9a 27.9a 24.4a 25.0a 26.7a 21.8a 25.6a 22.8a 22.6a 22.1a 19.6a 20.1a 20.6a 22.3a 21.8a 23.2a 20.8a 21.8a 18.5a 23.6a 22.9a 22.3a 23.1a 26.1a 19.7b 20.4a 19.0a 28.0a 23.5a 21.4b 30.8a 23.3a 25.4a 23.4a 25.1a 27.0 26.6 27.2 26.3 25.8 23.1 20.7 18.3

† For each irrigation date and period the values followed by different letters are significantly different according to a paired t test at the 0.05 probability level.


1525

Fig. 11. Leaves of maize (wettable) and of alfalfa (nonwettable) during sprinkler irrigation.

et al., 1971). Liu and Kang (2006 a, 2006b) reported a higher yield of sprinkler irrigated winter wheat as compared to surface irrigated. Our field experiments showed that sprinkler irrigation clearly decreased the net photosynthesis of maize by an average of 19% during the irrigation event and slightly decreased the net photosynthesis of maize by an average of 13% during the 2 h following the irrigation event. However, sprinkler irrigation did not affect the net photosynthesis of alfalfa during the irrigation event and slightly increased the net photosynthesis of alfalfa by an average of 12% during the 2 h following the irrigation event. Soil matric potential data indicate that the decreased net photosynthesis of maize during daytime sprinkler irrigation could not be attributed to water stress. There is one plant morphological trait and one plant physiological trait that can explain the different response of net photosynthesis to sprinkler irrigation of maize and alfalfa. The plant morphological trait that may explain the different response of net photosynthesis to sprinkler irrigation of maize and alfalfa is leaf wettability (Smith and McClean, 1989; Brewer and Smith, 1994, 1995; Ishibashi and Terashima, 1995; Hanba et al., 2004). Measurement of advancing contact angles of water drops showed that maize leaf surfaces are wettable (water contact angles < 90°), while the alfalfa leaf surfaces are more hydrophobic and non-wettable (water contact angles >120°). It has been reported that the wettability of maize leaves changes with phenology with the first five to six leaves being non-wettable while older leaves are wettable (Bianchi et al., 1985; Beattie and Marcell, 2002), as observed in our work. Bradley et al. (2003) reported that some Medicago spp. have non-wettable leaves. In a growth chamber study, Hanba et al. (2004) reported that moistening the leaves of a wettable species (bean, Phaseolus vulgaris L.) decreased photosynthesis, while moistening the leaves of a non-wettable species (pea, Pisum sativum L.) slightly increased photosynthesis. For the maize crop (wettable species), the sprinkler irrigation water drops covered a large portion of the leaf surface (Fig. 6A, 11) and represented a barrier to CO2 diffusion into the mesophyll, which led to reduced photosynthesis. Hanba et al. (2004) found that the reduction of net photosynthesis on the wettable species occurred shortly after moistening the leaves during 10 min and that the photosynthesis increased rapidly as the layer of water evaporated. However, they found that the reduction of net photosynthesis was irreversible when the leaves have been exposed to moistening for a long period of time (72 h) due to a 16% decrease in stomatal conductance and a 55% reduction in the amount of Rubisco (ribulose-1,5-biphosphate carboxylase). In our work, we found that the net photosynthesis of maize 1526

was reduced by sprinkler irrigation shortly after the irrigation started but given the short duration of irrigation (4–6 h) the net photosynthesis of maize was only reduced in 2 out of 10 d along the first and second hour after irrigation finished. Therefore, during sprinkler irrigation of maize the CO2 diffusion and fixation processes were limited by the irrigation water drops covering the leaf surface, and photosynthesis was not totally recovered along the 2 h following the irrigation. In the case of the alfalfa crop (non-wettable species) no water barrier to CO2 uptake was formed (Fig. 11), therefore the CO2 diffusion and fixation processes were not affected and the sprinkler irrigation had a slight positive effect for photosynthesis probably because of the lower VPD (Smith and McClean, 1989; Brewer et al., 1991; Brewer and Smith, 1995, 1997). The potential uptake of water by the foliage of the wettable bean leaf was also considered by Hanba et al. (2004) in association with the reduced photosynthesis of moist leaf surfaces. Water may be taken up by the foliage via stomata, the cuticle, cuticular cracks, and modified epidermal cells such as trichomes (Fernández and Eichert, 2009). The area of contact between water drops deposited on to maize leaf surfaces is much higher than on the more hydrophobic alfalfa leaves. Hence, a larger leaf area will be covered by water drops in the more wettable maize leaf surface, which may facilitate the process of foliar water uptake. However, the actual mechanisms of foliar penetration by water are currently not fully understood, and will be affected by the chemistry and physical structure of every particular leaf surface (Khayet and Fernández, 2012). Hence, more detailed water uptake experiments with maize leaves will be required to clarify whether the photosynthesis reduction recorded for irrigated maize may be also linked to the potential absorption of irrigation water by the foliage. The plant physiological trait that may explain the different response of net photosynthesis to sprinkler irrigation of maize and alfalfa is the response of net photosynthesis to temperature of each species. Lower net photosynthesis of maize during sprinkler irrigation was related with lower temperatures reached at the irrigated treatment while the net photosynthesis of alfalfa was not affected by the lower temperatures at the irrigated treatment. This could be due to the fact that the C4 plant species (maize) have a higher temperature optimum for photosynthesis than C3 species (alfalfa) (Berry and Björkman, 1980). For maize, the optimum temperature for photosynthesis has been established around 27 to 35°C (Duncan and Hesketh, 1968; Tollenaar, 1989; Wanjura and Upchurch, 2000; Crafts-Brandner and Salvucci, 2002; Kim et al., 2007) and a reduction in photosynthesis has been found when temperature decreases below 27°C (Duncan and Hesketh, 1968; Tollenaar, 1989; Labate et al., 1991; Wolfe, 1991; Kim et al., 2007). For alfalfa, a wide range of temperature optimum for photosynthesis has been established (20–30°C) (Chatterton and Carlson, 1981; Brown and Radcliffe, 1986; Al-Hamdani and Todd 1990; Ziska and Bunce, 1994). Therefore, for maize the air temperature measured at the irrigated treatment (between 23 and 28°C) was below the optimum range for 80% of the irrigation events probably reducing net photosynthesis. However, in the case of alfalfa, the air temperature at the Agronomy Journal • Volume 105, Issue 6 • 2013

irrigated treatment (between 20 and 28°C) was within the optimum range for most irrigation events, and consequently did not affect net photosynthesis. Results from our work and previous at the same location (Cavero et al., 2008) have found that daytime sprinkler irrigation of maize decreased the grain yield by 5 to 10% compared to nighttime sprinkler irrigation. This yield decrease was mainly caused by the lower irrigation uniformity of daytime irrigation due to the higher daytime wind speed (Urrego-Pereira et al., 2013a). However, the lower net photosynthesis of maize during sprinkler irrigation found in our study indicates that part of the maize yield decrease with daytime sprinkler irrigation could be due to the reduced net photosynthesis. Most sprinkler solid-set systems at our site (18 by 18 m spaced, 5 mm h–1) must be run for 12 h wk–1 to apply enough water for maize during the main growing period. Considering a 19% reduction of net photosynthesis during the irrigation event this will result in an average 3% decrease of maize yield if sprinkler irrigation is performed during daytime. Although this is a slight maize yield decrease, considering that alfalfa photosynthesis has a slight positive response to sprinkler irrigation, when daytime sprinkler irrigation is necessary due to constraints of the irrigation system, alfalfa should be the crop daytime irrigated. In hotter environments than in our site, the decrease of temperature due to sprinkler irrigation could be beneficial for maize if the reduced temperature remains within the optimal range for photosynthesis. However, the decreased CO2 absorption due to the leaf surface deposited water barrier could also decrease maize photosynthesis. Under sprinkler moving systems the time that the plants are moistened is generally lower compared to solid-set systems although physiological changes can have a similar duration (Urrego-Pereira et al., 2013b). Further studies are needed to clarify the impact of sprinkler irrigation on the photosynthesis of field crops under different environments and sprinkler irrigation systems.

alfalfa. The decrease of air VPD due to sprinkler irrigation induced a slight increase of alfalfa net photosynthesis. Daytime sprinkler irrigation with solid set systems under similar climatic conditions to the study site should be avoided for maize due to decreased net photosynthesis and yield. Daytime sprinkler irrigation can be used for alfalfa because it did not decrease net photosynthesis but the consequences of irrigation time on yield should be studied.

CONCLUSIONS Sprinkler irrigation decreased the net photosynthesis of maize on 80% of days during the irrigation event and the mean reduction was 19%. However, sprinkler irrigation increased the net photosynthesis of alfalfa during the irrigation event on 36% of days, decreased it on 14% of days, and did not affect it on half of the days, so on average the net photosynthesis of alfalfa was not affected. In the 2 h after the sprinkler irrigation finished, the net photosynthesis of maize was reduced on 20% of days but the net photosynthesis of alfalfa was increased on 21% of days. The reduction of net photosynthesis of maize during sprinkler irrigation was related both with the high wettability of maize leaves, which reduced the exchange of CO2 , and with the reduction of canopy and air temperature, which resulted in temperatures below the optimum for photosynthesis of maize. The low wettability of the leaves of alfalfa prevented sprinkler irrigation water from interfering with the CO2 uptake and therefore did not decrease net photosynthesis. Moreover, sprinkler irrigation did not decrease air and canopy temperature below the optimum range for photosynthesis of

Boyer, J.S. 1970a. Leaf enlargement and metabolic rates in corn, soybean and sunflower at various leaf water potentials. Plant Physiol. 46:233–235. doi:10.1104/pp.46.2.233

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