Effects of regulated deficit irrigation on grain yield and ...

Effects of regulated deficit irrigation on grain yield and water use efficiency of spring wheat in an arid environment Bu-Chong Zhang1,2, Feng-Min Li1,5, Gao-Bao Huang2, Yantai Gan3, Pu-Hai Liu4, and Zi-Yong Cheng4 1State Key Laboratory of Arid Agroecology, Lanzhou University, Lanzhou, Gansu 730000, PR 2Gansu Agricultural University, Lanzhou, Gansu 730070, PR China; 3Agriculture and Agri-Food Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2; and 4Department of Water

China; Canada,

Can. J. Plant Sci. Downloaded from pubs.aic.ca by Lanzhou University on 12/04/11 For personal use only.

Resources Engineering, Gansu Agricultural University, Lanzhou, Gansu 730070, PR China. Received 29 October 2004, accepted 8 June 2005. Zhang, B.-C., Li, F.-M., Huang, G.-B., Gan, Y., Liu, P.-H. and Cheng, Z.-Y. 2005. Effects of regulated deficit irrigation on grain yield and water use efficiency of spring wheat in an arid environment. Can. J. Plant Sci. 85: 829–837. Grain yield and water use efficiency (WUE) of spring wheat (Triticum aestivum L.) in arid environments can be improved by applying irrigation selectively to allow soil water deficits to develop at non-critical stages of crop development. Field experiments were conducted on a loam soil in Zhangye district, northwest China in 2003 and 2004 to determine the grain yield, yield components, and water use characteristics of spring wheat in response to regulated deficit irrigation (RDI) schemes. Wheat grown under the RDI schemes produced 17% (in 2004) and 29% (in 2003) higher grain yield than wheat grown under water deficit-free control (5.6 t ha–1 in 2003 and 6.2 t ha–1 in 2004). Among six RDI schemes studied, wheat having a high water deficit at the jointing stage, but free from water deficit from booting to grain-filling produced highest grain yield in both 2003 (7.95 t ha–1) and 2004 (7.26 t ha–1). Compared with the control, wheat plants grown under the RDI schemes received 59 mm (or 15%) less water via irrigation, but they either extracted 41 mm more (or 74%) water from the soil profile (in 2003) or lowered (16%) evapotranspiration (ET) (in 2004). Grain yield increased as ET increased from 415 to 460 mm, and declined beyond 460 mm. The WUE values varied from 0.0116 to 0.0168 t ha–1 mm–1, and wheat grown under the RDI had 26% greater WUE compared with the control. Grain yield and WUE of spring wheat can be greatly improved by regulated deficit irrigation with reduced amounts of water. This practice is particularly valuable in arid regions where wheat production relies heavily on irrigation. Key words: Triticum aestivum, grain yield, evapotranspiration, water use efficiency, regulated deficit irrigation Zhang, B.-C., Li, F.-M., Huang, G.-B., Gan, Y., Liu, P.-H. et Cheng, Z.-Y. 2005. Incidence de l’irrigation déficitaire contrôlée sur l’efficacité d’utilisation de l’eau et le rendement grainier du blé de printemps cultivé en milieu aride. Can. J. Plant Sci. 85: 829–837. On peut améliorer le rendement grainier et l’efficacité d’utilisation de l’eau (EUE) du blé de printemps (Triticum aestivum L.) cultivé en milieu aride en irrigant celui-ci de manière sélective pour que le sol manque d’eau aux stades de développement non cruciaux de la plante. En 2003 et 2004, les auteurs ont entrepris des essais sur un loam du district de Zhangye, dans le nord-est de la Chine, en vue d’établir le rendement grainier, les composantes du rendement et les paramètres d’utilisation de l’eau du blé de printemps dans le cadre de divers programmes d’irrigation déficitaire contrôlée (IDC). Le blé cultivé de cette manière a donné un rendement grainier de 17 % (2004) et de 29 % (2003) supérieur à celui du blé cultivé sans déficit hydrique (5,6 t par hectare en 2003 et 6,2 t par hectare en 2004). Sur les six programmes d’IDC examinés, le blé soumis à un fort déficit hydrique au stade de la montaison mais sans déficit du gonflement au remplissage du grain est celui qui a donné le meilleur rendement en 2003 (7,95 t par hectare) et en 2004 (7,26 t par hectare). Comparativement aux témoins, les plants cultivés sous IDC avaient reçu 59 mm (15 %) moins d’eau par irrigation, mais en avaient extrait 41 mm (74 %) de plus du sol (en 2003) ou avaient réduit (de 16 %) leur évapotranspiration (ET) (en 2004). Le rendement grainier s’accroît quand l’ET passe de 415 à 460 mm puis diminue passé cette marque. Les valeurs EUE varient de 0,0116 à 0,0168 t par hectare et par millimètre, et l’EUE du blé cultivé sous IDC dépasse celle des plants témoins de 26 %. On pourrait améliorer considérablement le rendement grainier et l’EUE du blé de printemps en régulant le déficit d’irrigation avec un plus faible volume d’eau. Cette pratique s’avère particulièrement utile dans les régions arides où la culture du blé dépend énormément de l’irrigation. Mots clés: Triticum aestivum, rendement grainier, évapotranspiration, efficacité de l’utilisation de l’eau, irrigation déficitaire contrôlée

There is increasing competition for water resources between economic development and agricultural activity worldwide (McVicar et al. 2002). In China, there is already strong competition for water resources between fast-growing industri-

alization and the increasing food demands of the growing population. Water-efficient agriculture plays a vital role in meeting the demands of food supply without sacrificing industrialization.

5To

Abbreviations: ET, evapotranspiration; RDI, regulated deficit irrigation; SM, soil moisture; TDR, time-domain reflectometry; WUE, water use efficiency

whom correspondence should be addressed. 829


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CANADIAN JOURNAL OF PLANT SCIENCE

In the arid regions of northwest China, annual precipitation is less than 140 mm while annual evaporation (>2000 mm) is nearly 15 times that amount. The production of spring wheat, the main food crop, is only possible with irrigation (Wang et al. 2001; Deng et al. 2002). Timing and the amounts of irrigation are two key factors affecting grain yield and water use efficiency (WUE) of crops in arid environments (Arora and Gajri 1998; Ogola et al. 2002). Timely irrigations increase the grain yield of spring wheat significantly (Zhang et al. 1998a; Deng et al. 2002), whereas restricted irrigations can improve WUE substantially (Zhang et al. 2004a, b). The timely and controlled irrigations also minimize evapotranspiration and potential drainage losses (Kang et al. 2002b). Supplementary irrigation during critical stages of crop development has been used to save water while improving grain yields in wheat grown in subhumid regions of northwest China where annual precipitation varies from 350 to 450 mm (Deng et al. 2002; Kang et al. 2002a, b). Kang et al. (2002b) reported that spring wheat receiving about 20% less irrigation water during early vegetative stages produced a grain yield equal to or greater than the wheat that was irrigated to maintain soil at field capacity. Cheng (1996) and Jiao et al. (1999) estimated that by using supplementary irrigation management, the grain yield of winter wheat in the semiarid regions of northwest China could be increased by 20 to 30% from the current levels to potential yields of 7 to 9 t ha–1. Regulated deficit irrigation (RDI), as described and used by Rawson and Turner (1983) and Fabeiro et al. (2002), can further improve WUE. The RDI maintains crop plants under water deficit stress during some of the growth stages by controlling the amounts of irrigation. The adverse growing conditions allow the plants to acclimatize to better withstand further water deficits during later developmental stages (Fabeiro et al. 2002). The RDI increases root growth and water uptake during water deficit periods (Asseng et al. 1998), improve nutrient uptake (Plant et al. 1998), and facilitate the redistribution of photosynthates between vegetative tissues and reproductive organs (Cai et al. 2002). However, the period of water deficits must coincide with the least-sensitive growth stages of the plants (Arora and Gajri 1998). The previous studies were conducted in environments where the evaporative demands are relatively low (500 to 1400 mm) during a growing season. Lower demands for evaporation allow crop plants to perform normally under mild drought or moderate water deficit conditions. Little is known about the effect of the RDI on performance of spring wheat in arid environments where evaporation demands are high. The objective of our study was to test the hypothesis that the use of the RDI would enhance the grain yield of spring wheat in arid environments of northwest China where evaporation (>2000 mm) is nearly 15 times precipitation. We further aimed at determining the effects of RDI at critical growth stages on grain yield, yield components, and WUE of spring wheat. MATERIALS AND METHODS Site The field experiments were conducted in Zhangye county of Gansu Province in northwest China (99°23′E longitude,

41°13′N latitude) during 2003 and 2004. The average annual precipitation in this region is 139 mm, and the mean annual evaporation is 2048 mm. The growing season (March–July) precipitation was only 45.4 mm in 2003 and 36.7 mm in 2004; similar to the long-term average (Fig. 1). Mean temperatures during the growing season were 22.0°C in 2003 and 22.7°C in 2004; both similar to the long-term averages (Table 1). The frost-free period was 165 d. The soil at the experimental site was a silt loam with an average soil organic matter of 12.5 g kg–1, bulk density of 1.39 g cm–3, and pH (water phase) of 8.5 in the 1-m depth (Table 2). The soil from the surface to 0.6-m depth contained 0.88 g N kg–1, 0.88 g P kg–1, and 13.97 g K kg–1. Porosity and field capacity are also given in Table 2. Land Preparation and Crop Management Experimental plots were 14 × 3.5 m in size, and were separated by 1-m-wide buffer zones between plots. To prevent lateral flow of soil water, each plot was physically separated using plastic films (0.005 mm thick) prior to planting. A trench about 0.4 m wide was made (by hand) to a depth around 1.4 to 1.5 m between plots, and plastic film was placed along one wall of the trench, which was backfilled holding the plastic film in place. At planting, the plots received 135 kg N ha–1 and 33 kg P ha–1 applied as ammonium fertilizer, along with 0.3 kg m–2 dry weight of swine manure (nutrients in the manure were not determined). The spring wheat cultivar Ningchun No.18 was planted at a density of 250 seeds m–2 on Mar. 14 in both years. Plant rows were spaced 20 cm part oriented east-west. About 2 wk after emergence, the plants were thinned to approximately 180 plants m–2. Plant development stages were recorded for each year (Table 3). Weeds were removed by hand during both growing seasons, and diseases were controlled using registered fungicides. Experimental design The experiment was a randomized, complete block design with three replicates. The crop was subjected to the low (L), medium (M), and high (H) water deficits in comparison with free (F) from water deficit control. The deficit treatments were applied at each of the three phases: Phase I, during seedling emergence and jointing; Phase II, during booting and shooting; and Phase III, during grain filling and pre-physiological maturity. The following six treatments were established with water deficit levels being varied during the three phases: (1)FFF, free from water deficit in all three phases; this treatment provided a water non-limited control. (2)LLL, low water deficit in all three phases. (3)MFM, medium water deficit in phases I and III, but free from water deficit in phase II. (4)MFH, medium water deficit in phase I, free from water deficit in phase II, but high water deficit in phase III. (5)HFF, high water deficit in phase I, free from water deficit in phases II and III; (6)HFM, high water deficit in phase I, free from water deficit in phase II, and medium water deficit in phase III. Medium to high water deficit in phase II were not included in the present study because previous studies have shown

ZHANG ET AL. — EFFECTS OF REGULATED DEFICIT IRRIGATION ON GRAIN YIELD AND WUE

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20


Precipitation (mm)

2003 2004 Long- term

15

10

5

0 March

April

May

June

July

Month Fig. 1. Growing season precipitation and the monthly distribution at the experimental site of Zhangye, northwest China, in 2003 and 2004, compared with those of long-term (1970–2000) averages.

Table 1. Growing season (March–July) mean air temperatures and solar radiation in Zhangye, northwest China

Month

2003

2004

Long-termz

Total solar radiation (KJ cm–2)

March April May June July

9.9 18.0 23.8 28.3 30.2

10.7 18.6 24.3 29.2 30.8

10.5 18.2 24.1 28.7 30.7

12.8 14.3 16.6 16.7 15.9

Temperature (°C)

z1970–2000.

that high water deficit during these stages causes significant yield losses in wheat (Deng et al. 2002; Kang et al. 2002b). For the duration of the experiment, the soil moisture (SM) of each plot was monitored and recorded on a daily basis using a time-domain reflectometry (TDR) system (Soil Moisture Equipment Corp., CA), according to Kang et al. (2002a, b). Prior to planting, four moisture probes were each placed at the 0.2-, 0.4-, 0.6-, and 0.8-m soil depths in each plot by pushing the probes directly into the soil profile. The SM measured by these probes was calibrated by the TDR to produce a daily mean SM for each plot by averaging the readings from the four probes. When soil water content dropped to the lower limits of the designed range of the treatment, irrigation water was applied to the plot. The lower limits of soil water content were 45, 50, 60, and 70% of the field capacity for the H, M, L, and F water deficit levels, respectively. For example, when soil water content of the FFF plots dropped to 70% of the field capacity at the jointing stage, an irrigation of 75 mm water was applied to the treatment. After the irrigation, the soil water content was again recorded for each treatment (Table 4). The

amounts of irrigation water applied during each of the developmental stages were also recorded. The date on which irrigation was applied at a given developmental stage varied from treatment to treatment, but within the vicinity of 2 to 4 d. The growing seasons were short (≤118 d) and only five irrigations were used in 2003 and four in 2004. Data Collection and Calculations Volumetric soil water content was determined for each plot every 7 to 10 d during the growing season. Two 30-mmdiameter core soil samplers were used to take soil samples up to a depth of 1 m, and the soil core was separated into five segments corresponding to depths of 0–0.2, 0.2–0.4, 0.4–0.6, 0.6–0.8, and 0.8–1.0. These segments were oven dried and water contents of each depth were determined. These measurements were used to determine the amounts of water extracted during a period considered. At the end of the growing season (Jul. 13 in 2003, Jul. 11 in 2004), all plants were hand harvested for grain yield and aboveground biomass. Prior to harvest, 20 individual plants were sampled from each plot, and were used for measurements of kernels spike–1, grain weight spike–1, fertile spikelets spike–1, weight kernel–1, and spike length. The WUE was calculated by dividing grain yield by the cumulative seasonal ET. The ET was estimated using the water balance equation, the same equation used by Zhang et al. (1998a, b) and Kang et al. (2002b): ET = ∆W + I + P – Dr where ET is evapotranspiration, ∆W is the change in soil water extracted between two soil moisture measurements

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Table 2. Some selected physical and chemical properties of the soil at the experimental site, in Zhangye, northwest China Soil depth (cm) 0–20 20–40 40–60 60–100

Organic matter (g kg–1)

PH

14.4 12.0 11.1 10.7

Bulk density (g cm–3)

8.42 8.52 8.56 8.50

1.31 1.43 1.43 1.45

Table 3. Phenological data and plant developmental stages for spring wheat grown in Zhangye, northwest China, 2003–2004


Days after planting Plant developmental stage

2003

2004

Emergence Jointing Booting Shooting Grain filling Physiological maturity Maturity

11 46 69 79 95 111 118

9 43 65 76 93 108 116

(mm), I is the irrigation water (mm), P is the precipitation (mm), and Dr is the drainage below the bottom of the root zone (mm). The soil water contents at the 0.8- to 1.0-m depth were consistently low and there were no differences among treatments (data not shown). Therefore, drainage water below the 1.0-m soil depth was considered negligible, and thus Dr was excluded in this calculation. Statistical Analysis Data were analyzed using analysis of variance with the Statview statistical package (BrainPower Inc., Calabasas, CA). Data from the 2 yr are presented separately where year × treatment interaction was significant at P < 0.05. Linear and non-linear regression models were used to determine the relationships between yield and yield components, between yield and ET, and between ET and WUE. RESULTS Grain Yield and Yield Components Mean grain yields of spring wheat were 7.1 t ha–1 in 2003 and 6.5 t ha–1 in 2004 (Table 5). The greater (9.5%) productivity in 2003 than in 2004 was attributable to cooler conditions early in the growing season (Table 1) that promoted seedling growth and also due to 70-mm more irrigation water applied in 2003 (Table 4). The RDI significantly affected grain yields of spring wheat in both 2003 and 2004 (Table 5). On average, wheat grown under the RDI schemes (i.e., the treatments HFF, MFM, MFH, and HFM) produced 17% (in 2004) to 29% (in 2003) more grain compared with the water deficit free (FFF) control (5.6 t ha–1 in 2003 and 6.2 t ha–1 in 2004). These four RDI treatments had either a high (HFF, HFM) or a medium (MFM, MFH) water deficit at the jointing stage. Wheat grown under the HFF conditions (i.e., high water deficit at jointing and no water deficit during the rest of the growth periods) produced highest grain yields in both 2003 (7.9 t ha–1) and 2004 (7.3 t ha–1), which

Porosity (%) 50.72 46.76 46.68 46.76

Soil texture Light loam Medium loam Medium loam Medium loam

Field capacity (V. %) 29.25 31.19 32.87 34.66

was 42% (in 2003) and 17% (in 2004) greater than the control. Wheat subject to medium water deficits at jointing, grain filling, and pre-physiological maturity produced a grain yield similar to that of the control. Wheat grown under low water deficit conditions during the entire growth periods (LLL) performed similarly to the well-watered control, and produced 17% lower grain yield than when the crop was grown under medium stress at jointing and during reproductive growth (MFM). Aboveground plant biomass was between 15.6 and 20.4 t ha–1 in the 2 yr, and was a linear relationship with grain yield (Fig. 2). The mean biomass was 6.3% greater in 2003 (18.8 t ha–1) compared with 2004 (17.7 t ha–1), mainly due to more irrigation water applied in 2003 (Table 4). The treatment effects on biomass followed a similar trend as the effect on grain yield (Table 5). The RDI increased total biomass by 2 to 16% in 2003 and 6 to 31% in 2004 compared with the control. The plants grown under the HFF conditions produced the highest (20.3 t ha–1) biomass in both years, followed by the plants grown under MFM (19.4 t ha–1) and MFH (18.2 t ha–1) conditions. The increased grain yields of wheat grown under the RDI schemes were partly due to improved spike characteristics and yield components (Table 5). In both years, the plants grown under the RDI schemes increased spike length, kernels spike–1, and kernel weight compared with the control, while bearing similar numbers of spikelets spike–1. Lowered grain yields for wheat grown under LLL and FFF conditions, compared with the RDI treatments, were mainly due to reduced kernels spike–1 and spike length. The relationship between yield components and grain yield of spring wheat grown under the RDI was described using the following function: Y = 233.15 – 9.52WK + 0.0775WK2 + 0.1455WK × FSS – 0.2354FSS2 (R2 = 0.623, P < 0.05) where Y is grain yield (t ha–1), WK is weight (mg) kernel–1, and FSS is fertile spikelets per spike. This equation suggests that the grain yield of spring wheat grown in arid environments can be improved through increasing weight per kernel and fertile spikelets per spike. This function was developed using the combination of the 2 yr of data, since their responses to water deficit irrigation were similar between the 2 yr. Evapotranspiration Growing season precipitation was scarce at the experimental site (Fig. 1); thus, the value of ET was largely reflective of the


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Table 4. The amounts of irrigation water (mm) applied at the various developmental stages of spring wheat grown under different soil water deficit regimes in Zhangye, northwest China, in 2003 and 2004 2003 Treatment z


FFF LLL MFM MFH HFF HFM

Jointing 75 (93)y 75 (88) 60 (78) 60 (78) 50 (65) 50 (65)

Booting 105 (97) 90 (93) 90 (98) 75 (93) 75 (93) 75 (93)

Shooting 75 (93) 75 (88) 60 (88) 60 (88) 75 (93) 75 (93)

Grain filling

Pre-physiological maturity

2004 Jointing

Booting

Shooting

Irrigation water in millimeter (% of the field capacityy) 90 (98) 50 (85) 105 (98) 105 (93) 75 (88) 50 (80) 105 (93) 90 (88) 75 (83) 60 (78) 90 (88) 90 (93) 60 (68) 50 (65) 90 (88) 75 (93) 90 (98) 60 (88) 75 (68) 75 (93) 75 (83) 60 (78) 75 (68) 75 (93)

90 (98) 90 (93) 75 (93) 75 (93) 90 (98) 90 (98)

Grain filling 90 (93) 75 (88) 75 (83) 60 (68) 90 (98) 75 (83)

zThe treatment MFH means the soil water deficit level was controlled at a “Medium” level during jointing stage, “Free” from water deficit during booting and shooting, and at a “High” deficit level during grain-filling and pre-physiological maturity stages. The treatment LLL means Low water deficits at each of the three phases [i.e., (1) jointing, (2) booting and shooting, and (3) grain-filling and pre-physiological maturity]. The similar meanings are applicable to all the other treatments. yThe numbers in parentheses are soil moisture in % of the field capacity after the irrigation.

Table 5. Grain yield, aboveground plant biomass at maturity, and yield components of spring wheat grown under different water deficit regimes in Zhangye, northwest China, in 2003 and 2004 Yield component Treatment 2003 FFF LLL MFM MFHz HFF HFM 2004 FFF LLLz MFM MFH HFF HFM

Grain yield (t ha–1)

Biomass (t ha–1)

Spike length (cm)

Fertile spikelets (# spike–1)

Kernels (# spike–1)

Grain weight (g spike–1)

Kernel weight (mg kernel–1)

Plant height (cm)

5.58d 6.27c 7.71b 7.61bc 7.95a 7.61bc

17.5c 17.8c 19.6ab 19.1ab 20.3a 18.7b

7.6c 7.6c 8.5a 8.6a 8.2b 8.3ab

13.3ab 13.1ab 14.0a 14.0a 14.2a 14.1a

34.3b 32.2c 39.7a 38.0ab 40.1a 38.6ab

1.73c 1.73c 1.93b 1.83bc 2.18a 2.01ab

47.8c 50.1b 52.7a 52.0a 52.4a 51.0b

78.5ab 79.1ab 81.0a 79.8ab 79.8ab 78.7ab

6.20bc 5.89c 7.06ab 6.44b 7.26a 6.17bc

15.6e 16.5d 18.7b 17.7c 20.4a 17.4c

8.7c 8.9b 9.1a 8.9b 9.2a 9.1a

13.8a 14.0a 14.0a 14.1a 14.6a 14.2a

40.2b 40.1b 41.2b 40.2b 43.8a 40.8b

1.98a 2.00a 2.09a 2.07a 2.22a 2.01a

48.2ab 47.3ab 47.7ab 49.4a 48.6ab 47.5ab

74.3b 74.3b 80.6a 81.4a 81.0a 79.2ab

zThe treatment MFH means the soil water deficit level was controlled at a “Medium” level during jointing stage, “Free” from water deficit during booting and shooting, and at a “High” deficit level during grain-filling and pre-physiological maturity stages. The treatment LLL means Low water deficits at each of the three phases [i.e., (1) jointing, (2) booting and shooting, and (3) grain-filling and pre-physiological maturity]. The similar meanings are applicable to all the other treatments.

water applied through irrigation and water extracted from the soil profile during the period considered. Mean extracted soil water was higher in 2003 across all treatments except for FFF than those of 2004 (Table 6). In 2003, wheat plants grown under the RDI conditions (averaged over the treatments LLL, MFM, MFH, HFF, and HFM) received 55 mm (or 14%) less irrigation water but they extracted 41 mm (or 74%) more water from the soil profile compared with the control (Table 6). In 2004, wheat plants grown under RDI received 63 mm (or 16%) less irrigation water, but they extracted an equivalent amount of water from the soil compared with the control. The differences in response to the deficit treatments between the 2 yr were probably due to the magnitude of total amounts of water applied as well the amount of irrigation at the jointing stage. The total amount of irrigation water applied during the entire growing season of 2004 was lower (70 mm) than that of 2003 (Table 6). The limited irrigation in 2004 resulted in overall low water extraction from the soil profile. However, the plants grown in the RDI plots received 33% less

irrigation water at the jointing stage in 2003 compared with the same stage of 2004 (Table 4). The water stress conditions at the jointing stage may have promoted root systems, allowing the plants in the RDI plots to extract more water from various soil layers later in the growing season. In 2004, the low total irrigation water, coupled with high air temperatures (Table 1), may have limited the magnitude of root expansion for the plants grown under the RDI conditions. Regression of grain yield on ET revealed a quadratic relationship (Fig. 3). As ET increased from 415 to 460 mm, the grain yield increased, peaked and flatted off at around ET = 460 mm, and then declined as the ET increased further. The ET of 460 mm served as a critical value, which was about 91% of the measured maximum ET. At the critical ET, the maximum grain yield was about 7.9 t ha–1, near the measured maximum grain yield. This relationship between grain yield and ET partly explained the reason that the spring wheat grown under the HFF conditions produced the highest grain yields in both years (Table 5).


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Fig. 2. Relationships between grain yield (Y) and water use efficiency (WUE) and aboveground biomass (AB) for spring wheat grown under the regulated deficit irrigation regimes, in Zhangye, northwest China. Table 6. Amounts of irrigation water (I), extracted soil water (∆W), evapotranspiration (ET), and water use efficiency (WUE) for spring wheat grown under different water deficit regimes in Zhangye, northwest China, in 2003 and 2004 2003 Irrigation water (I)y

Extracted soil water (∆W)x (mm)

395 365 345 305 350 335

55.5 69.8 110 122 79.2 101

Treatmentz FFF LLL MFM MFH HFF HFM

2004 (ET)w

WUE (t ha–1 mm–1)

496 480 500 472 475 481

0.0126c 0.0116d 0.0154b 0.0161ab 0.0167a 0.0158b

Irrigation water (I)y

Extracted soil water (∆W)x (mm)

390 360 330 300 330 315

76.2 57.0 67.4 81.3 67.0 68.4

(ET)w

WUE (t ha–1 mm–1)

503 454 434 418 434 420

0.0117d 0.0137c 0.0163ab 0.0154b 0.0168a 0.0147bc

zThe treatment MFH means the soil water deficit level was controlled at a “Medium” level during jointing stage, “Free” from water deficit during booting and shooting, and at a “High” deficit level during grain-filling and pre-physiological maturity stages. The treatment LLL means Low water deficits at each of the three phases [i.e., (1) jointing, (2) booting and shooting, and (3) grain-filling and pre-physiological maturity]. The similar meanings are applicable to all the other treatments. yTotal amounts of water applied to the plots through irrigations. xTotal amounts of water extracted from soil profile during the period of seedling emergence to plant maturity. wSum of irrigation water + extracted soil water + growing season precipitation (45.4 mm in 2003 and 36.7 mm in 2004).

Across the two growing seasons, the grain yield was a function of ET as follows: Yield (t ha–1) = –0.00102ET2 + 0.9393ET – 207.5. This equation suggests that spring wheat grown under the RDI conditions in arid environments can increase grain yield with the ET values greater than 371 mm. Water Use Efficiency and its Relation to Yield and ET The values of WUE varied from 0.0117 to 0.0168 t ha–1 mm–1 in this study (Table 6). There were significant differences in WUE among treatments, with the control having lowest WUE (0.0117 t ha–1 mm–1) and the HFF treatment

greatest (0.0168 t ha–1 mm–1). Wheat grown under the MFM, MFH, HFF, and HFM schemes had 20% greater WUE in 2003 and 31% greater WUE in 2004 compared with the control. The improved WUE for the wheat grown under the RDI conditions was attributable to both reduced ET (Table 6) and increased grain yields (Table 5). Using regression analyses, the relationship between WUE and ET was described as: WUE = –1.705e06ET2 + 0.001545ET – 0.333 (R2 = 0.66)

(1)

As ET increased from 415 to 455, the WUE values increased, peaked as ET reaching 455 mm, and then declined as the ET further increased (Fig. 3). From Eq. 1, it



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Fig. 3. Relationships between seasonal evapotranspiration (ET) and water use efficiency (WUE) and grain yield (Y) for spring wheat grown under the regulated deficit irrigation regimes, in Zhangye, northwest China.

was estimated that the maximum WUE was 0.0192 t ha–1 mm–1 when ET = 455 mm (which was nearly 90% of the measured maximum ET). Similarly, there existed a quadratic relationship between WUE and grain yield, and the relationship was described as: WUE = –10.9e06Y2 + 1.62e05Y – 0.04374 (R2 = 0.86)

(2)

As grain yields (Y) increased, the WUE values linearly increased, and then leveled out as the yields moving from 7.4 to 7.9 t ha–1 (Fig. 2). From Eq. 2, it was estimated that the maximum WUE value was obtained when the grain yield was near 95% of the measured maximum grain yield. This relationship suggests that the maximum WUE generally coincides with the maximum grain yield for spring wheat grown in the arid environments of northwest China. Our study also suggests that further increases in grain yield may still be achievable using intensive management practices (such as more detailed, regulated irrigation schemes), but the magnitude of further increases in yield will be marginal and will result in a decreased WUE. DISCUSSION Regulated deficit irrigation schemes increased the grain yield of spring wheat by 17 (in 2004) to 29% (in 2003), and improved WUE by 20 (in 2003) to 31% (in 2004) compared with the control, which was maintained free from water deficit during critical stages of plant development. Averaged over the 2 yr, the spring wheat grown with the RDI used 58 mm (or 15%) less irrigation water compared with the control. Mechanisms responsible for the increased grain yield and WUE for spring wheat with the RDI were not well understood, but our data showed that it could be partly related to soil water extraction and ET. The wheat

plants grown under the RDI extracted more water from soil profile in 2003 and had a lower ET in 2004 compared with the control. In addition, the RDI may help improve rooting systems of the crop, particularly when a moderate water stress was imposed at the jointing stage. In previous studies conducted in more subhumid environments, Zhang et al. (1998a, b) found that wheat plants grown under the RDI developed larger root mass during the early vegetative growth period, and extracted more water from the soil starting from the booting stage. Li et al. (2001) found that wheat plants grown under mild deficit conditions at the jointing stage reduced leaf water potentials and increased sensitivity of plants to non-hydraulic root signals that promote root growth. These authors also found that drying topsoil at the jointing stage increased the proportion of root biomass and root length particularly in the middle soil layers, responsible for great soil water extraction during the later part of the vegetative stages. Supplemental irrigation with a small quantity of water positively affects the morphological and physiological characteristics of the crop plants (Zhang et al. 1998a; Zhang et al. 1999) and yield components (Ehdaie 1995). Wheat plants re-watered during the later part of the reproductive stages after receiving water deficit treatments during the vegetative stages increased photosynthetic rate and stomatal conductance compared with water-deficit-free control. Waterdeficit treatments during the early growth stages intensify the energy metabolization of the plants by improving leaf water-holding capacity (Asseng et al. 1998), increasing percolation regulation ability of the root (Plant et al. 1998), and facilitating the remobilization of photosynthetic materials to the grain (Cai et al. 2002). The values of ET in our study ranged from 418 and 503 mm in the two growing seasons, which were similar to that reported by Zhang et al. (1998a), but higher than those


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reported by others (Li et al. 1999; Wang et al. 2001; Kang et al. 2002b). This was expected because our experiment was in an environment where the annual evaporation is >2000 mm with annual precipitation