Plant and Soil (2006) 279:229–242 DOI 10.1007/s11104-005-1302-z
Springer 2006
Relationships between climatic variables and sap flow, stem water potential and maximum daily trunk shrinkage in lemon trees M.F. Ortun˜o1, Y. Garcı´ a-Orellana2, W. Conejero1, M.C. Ruiz-Sa´nchez1,3, O. Mounzer1, J.J. Alarco´n1,3 & A. Torrecillas1,3,4 1
Dpto. Riego y Salinidad, Centro de Edafologı´a y Biologı´a Aplicada del Segura (CSIC), E-30100 Espinardo, (Murcia), Spain. 2Dpto. Ingenierı´a Agrı´cola, Universidad Centro Occidental Lisandro Alvarado (UCLA), Barquisimeto, Venezuela. 3Unidad Asociada al CSIC de Horticultura Sostenible en Zonas A´ridas (UPCTCEBAS), E-30203 Cartagena, (Murcia), Spain. 4Corresponding author* Received 24 May 2005. Accepted in revised form 21 July 2005
Key words: irrigation scheduling, lemon, plant–water relations, sap flow, trunk diameter fluctuations
Abstract The feasibility of obtaining sap flow (SF), maximum daily trunk shrinkage (MDS) and midday stem water potential (Ystem) baselines or reference values for use in irrigation scheduling was studied in adult Fino lemon trees (Citrus limon (L.) Burm. fil.) grafted on sour orange (C. aurantium L.) rootstocks. Plants were irrigated daily above their water requirements in order to obtain non-limiting soil water conditions. The results indicated that baselines for plant-based water status indicators (MDS, SF and Ystem) can be obtained, even though there was a certain scattering of the data points representing the relations between the plant-based measurements and the environmental variables (reference evapotranspiration, solar radiation, vapour pressure deficit and temperature). SF was more closely associated with changes in the studied evaporative demand variables than were MDS and Ystem. SF and Ystem were more closely correlated with changes in reference evapotranspiration (ETo) (r2 = 0.93 and 0.79, respectively), while MDS behaviour was best correlated with mean daily air temperature (Tm) (r2 = 0.76). Increases in the evaporative demand induced more negative Ystem values and, as a consequence, SF increased, which, in turn, was translated into an increase in MDS. This confirmed that SF and MDS were very good predictors of the plant water status during the observation period and their continuous recording offers the promising possibility of their use in automatic irrigation scheduling in lemon trees. Abbreviations: Ystem – midday stem water potential; ETc – crop evapotranspiration; ETo – reference evapotranspiration; MDS – maximum daily trunk shrinkage; Rs – solar radiation; SF – sap flow; TDF – trunk diameter fluctuations; Tm – mean daily air temperature; Tmd – midday air temperature; VPDm – daily mean vapour pressure deficit; VPDmd – midday vapour pressure deficit Introduction In recent years the use of plant-based water status indicators have become very popular to study plant–water relations and for planning irrigation programs. The measurement of plant water sta* FAX No: +34-968-396213. E-mail:
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tus may be useful for irrigation scheduling because of its dynamic nature, which is directly related with climatic and soil conditions, as well as with crop productivity (Goldhamer et al., 2003; Remorini and Massai, 2003). The most widely used approach for evaluating tree water status has been to determine leaf water potential (Ame´glio et al., 1999; Hsiao, 1990). However, Chone´ et al. (2001), Naor (2000) and Shackel
230 et al. (1997) showed that midday stem water potential (Ystem) is a significant and more reliable plant water status indicator for scheduling the irrigation of woody crops. However, the main disadvantage of Ystem is the relatively cumbersome measurement procedure, the necessity of frequent trips to the field and a significant input of labour. The robustness of the sensors used to measure sap flow (SF) and trunk diameter fluctuations have renewed interest in using these parameters as plant water status indicators (Cohen et al., 2001; Ferna´ndez et al., 2001; Goldhamer and Fereres, 2001; Moreno et al., 1996). These techniques permit continuous and automated registers of the plant water status, and an immediate, consistent and reliable response to water deficit (Goldhamer et al., 1999; Ortun˜o et al., 2004a, b), with a reduction in the labour needed for the measurement procedure. Goldhamer and Fereres (2004) obtained promising results using only maximum daily trunk shrinkage (MDS) for scheduling almond trees irrigation. To correctly interpret the values of each plant-based water status indicator and their use for irrigation scheduling, it is necessary to develop baselines or reference relationships, which are obtained with their values in plants under nonlimiting soil water conditions versus the evaporative demand of the atmosphere (Goldhamer and Fereres, 2001; Martin et al., 1990; Shackel et al., 1997). For the above reasons, the objective of this study was to evaluate the feasibility of obtaining SF, MDS and Ystem measurement baselines in adult lemon trees to be used in irrigation scheduling. This approach involved characterizing the behaviour of SF, MDS and Ystem under non-limiting soil water conditions with respect to changes in several parameters related with the evaporative demand of the atmosphere. The relations between SF, MDS and Ystem were also investigated. Materials and methods Plant material and experimental conditions Experiments were conducted at the CEBASCSIC experimental station in Santomera (Murcia) from 2 April 2004, day of year (DOY) 93, to
10 January 2005, DOY 10. The soil is a Paralithic mollic-calciorthid, and the profile shows only a slight differentiation between horizons (an ochric epipedon on a C horizon). The clay loam soil of the experimental site (1.28% organic matter, 4.92 mmol kg)1 available potassium, 1.81 mmol kg)1 available phosphorus, 51% lime content and pH of 7.8) was characterized by volumetric water content of 0.19 cm3 cm)3 at field capacity and 0.08 cm3 cm)3 at permanent wilting point, 1.5 g cm)3 bulk density, high stone content (43% w/w), and 8.0 cm h)1 saturated hydraulic conductivity, which provided an excellent internal drainage (Trout et al., 1982). The experiment was performed on 24-year-old lemon trees (Citrus limon (L.) Burm. fil.) cv. Fino grafted on sour orange (C. aurantium L.) rootstocks. Tree spacing followed a 6 6 m square pattern, with an average ground cover of about 65%. Micrometeorological hourly data, namely air temperature, precipitation, solar radiation, air relative humidity and wind speed at 2 m above soil surface were collected by a weather station of the Servicio de Informacio´n Agraria de Murcia (SIAM), located 8.4 km from the experimental site. Daily reference evapotranspiration (ETo) was calculated using the Penman–Monteith equation (Allen et al., 1998). Daily mean vapour pressure deficit (VPDm) was calculated from mean daily vapour pressure and relative humidity (Goldhamer and Fereres, 2001). No weeds were allowed to develop within the orchard, resulting in a clean orchard floor for the duration of the experiment. Pest control and fertilization practises were those commonly used by the growers. Irrigation was carried out during the night by drip using two lateral pipes per tree row and 12 emitters per plant, six of which delivered 2 L h)1 (0.06 mm h)1) and six 4 L h)1 (0.11 mm h)1). Plants irrigation requirements were determined according to ETo and a crop factor based on the time of the year and the percent of ground area shaded by the tree canopy (Domingo et al., 1996). During the experimental period, total crop evapotranspiration (ETc) was 566.5 mm. From the beginning of the experimental period, lemon trees were irrigated daily above crop water requirements in order to obtain non-limiting soil water conditions. A total water amount
231 of 843.5 mm, measured with in-line water meters, was applied during the experiment. Even though the amount of irrigation water applied to plants during the experimental period was clearly above crop water requirements, the absence of plant symptoms and the lemon tree water relations indicated the absence of any waterlogging situation (Garcı´ a-Orellana, unpublished data).
Measurements Trunk diameter fluctuations (TDF) were measured throughout the experimental period in four trees, using a set of linear variable displacement transducers (LVDT) (model DF ± 2.5 mm, accuracy ± 10 lm, Solartron Metrology, Bognor Regis, UK) attached to the trunk, with a special bracket made of Invar, an alloy of Ni and Fe with a thermal expansion coefficient close to zero (Katerji et al., 1994), and aluminium. Sensors were placed on the north side and were covered with silver thermoprotected foil to prevent heating and wetting of the device. Measurements were taken every 10 s and the datalogger (model CR10 with AM 416 multiplexer, Campbell Scientific Ltd., Logan, USA) was programmed to report 30 min means. Maximum daily trunk shrinkage (MDS) was calculated as the difference between maximum and minimum daily trunk diameter. Sap flow was measured from 2 June 2004 (DOY 154) using the compensation heat-pulse technique (Swanson and Whitfield, 1981) in the same trees as used for TDF measurements. One set of heat-pulse probes was located above the LVDT sensors on each tree. Each set consisted of two temperature probes inserted 10 mm downstream and 5 mm upstream from a heater needle. Heater and thermocouples had a diameter of 1.8 mm and were installed in parallel holes drilled radially in the trunks. The temperature probes comprised four copper–constantan thermocouples to measure the sap velocity at four radial depths of 10, 20, 30 and 40 mm below the cambium to monitor the sap velocity profile over a radial depth. For the experiments, pulse duration of 1 s was used. Sap velocity was measured following the procedure of Green and Clothier (1988) and taking into account the Swanson correction factors for a wound size of 2.4 mm (Swanson and Whitfield, 1981).
Once the heat-pulse velocity (V) was determined, the next step was to relate it to the actual SF. In our analysis, the sap flow density (J) was related with the heat-pulse velocity (V) using the equation developed by Edwards and Warwick (1984): J ¼ ð0:505Fm þ Fl ÞV where Fm and Fl are the volume fractions of wood and water, respectively. These were calculated at the end of the experiment by taking 5 mm diameter wood samples from all trees with a special auger and measuring the fresh mass, mass of displaced water by immersion in a vessel of distilled water on a balance, and oven dried mass. The factor 0.505 is related to the thermal properties of the woody matrix, and is assumed to be constant within and between species. This equation provides an estimate of the values of J at any point in the conducting sapwood. It is widely recognized that sap flux density is not uniform throughout the sapwood, for which reason our probes measured J at four radial depths. The volumetric measurement of total sap flux (Q) was obtained by the integration of these point estimates over the sapwood conducting area, which was determined at the end of the experiment by cutting the trunks and immersing them in a safranin solution overnight and measuring the radius of sapwood at the point where the heat-pulse sensors were installed. The temperature signals and the corresponding heat-pulse velocities were recorded at a 30 min intervals using compensation heat-pulse instrumentation (MITRA-3.1, Polytechnic University of Cartagena, Spain) controlled by a data logger (CR10, Campbell Scientific Ltd., Logan, USA). Midday (12.00 h solar time) stem water potential (Ystem) was measured from 9 July 2004 (DOY 191), every 2–3 days, except when rainfall prevented measurements, in two mature leaves per plant, taken from close to the trunk. Leaves were covered using a small black plastic bag covered with aluminium foil for at least 2 h before measurement with a pressure chamber.
Statistical design and analysis The design of the experiment was completely randomized with four replications, each replication
232 consisting of three adjacent rows of five trees. Measurements were taken in the inner tree of the central row of each replicate, the other trees serving as borders. All the measurements were taken in the same tree in each replicate. Ystem values for each day and replicate were averaged before the mean and the standard error were calculated.
Results All the parameters concerning the evaporative demand of the atmosphere increased from the beginning of the experiment, reaching maximum values in summer and then decreasing until the end of the experiment (Figure 1). During the experimental period, rainfall was 250.2 mm, mainly occurring during spring and autumn, the usual rainy periods in the experimental site area (Figure 1a). Mean daily air temperature (Tm) and midday air temperature (Tmd) presented a similar seasonal trend, reaching maximum values in late June and early August (Figure 1a). During the experiment, average Tm and average daily minimum temperatures were 25 and 13 C, respectively (Figure 1a), and average mean relative humidity was 66% (data not shown). During the experiment, daily ETo reached maximum values in June and July and decreased afterwards. Total ETo was 1009.9 mm (Figure 1b). Solar radiation (Rs) fluctuated widely during the experimental period, showing maximum values in mid June and minimum values in late November (Figure 1b). Midday VPD (VPDmd) and VPDm values presented similar seasonal trends, and maximum values were obtained in late June and early August (Figure 1c). Ystem values varied between )1.5 and )0.6 MPa, increasing during the observation period (Figure 2a). MDS values ranged between 0.03 and 0.49 mm, increasing from the beginning of the experiment to early July and decreasing afterwards (Figure 2b). Daily SF values were between 64 and 227 L day)1, presenting nearly constant values from the beginning of the measurements to mid August and decreasing afterwards (Figure 2c). Figures 3–6 represent the plant-based water status indicators (MDS, Ystem and SF) as a function of ETo, Rs, VPDm, VPDmd, Tmd, and Tm.
The overall increases in any of the selected environmental variables were associated with increases in MDS and SF and decreases in Ystem (Figures 3–6). The r2 values of the data around the regression lines varied from 0.39 to 0.93. In all cases, r2 was higher for SF than for MDS and Ystem (Figures 3–6). Figure 3 presents the MDS, Ystem and SF as a function of ETo. Of the relationships between the water status indicators and ETo, the correlation between MDS and ETo (Figure 3a) presented the weakest r2 value. A cubic fit of the regression between SF and ETo (Figure 3c) yielded an equation with a determination coefficient higher than those found for the first and second order regressions (data not shown). Increases in ETo were associated with increases in SF until ETo reached values of around 3.5 mm, after which SF levelled off as ETo increased (Figure 3c). Ystem showed a better correlation with ETo than with the other selected evaporative demand parameters (Figures 3–6). The first order regression of Ystem values against Rs was characterized by a tighter correlation than that of MDS against Rs (Figure 4a and b). Moreover, in the case of the relationship between SF and Rs a departure from linearity was evident (Figure 4c) and a second degree regression significantly improved the goodness of the first order fit (data not shown). In this case, increases in Rs values were associated with increases in SF until Rs reached values of around 230 W m)2, after which increases in Rs values were associated with near constant SF values (Figure 4c). The MDS and Ystem regressions against VPDm and VPDmd (Figure 5a and b) showed relatively low correlations. The quadratic fit regression between SF and VPDm and VPDmd (Figure 5c) rendered coefficients of determination higher than that found for the linear fit (data not shown). The relations between SF and VPDm and VPDmd departed from linearity at VPDm and VPDmd values above 2 and 3 kPa, respectively. The relations between MDS, and Tm and Tmd presented tighter correlations than that considering the other environmental variables (Figure 6a), while those of SF and Ystem against both temperatures (Tm and Tmd) correlated well but more weakly than those obtained against ETo (Figures 3b, c and 6b, c).
233
(a) 35 30
30
25 25 20 20 15 15
Tm
Rainfall (mm)
Air temperature (ºC)
35
10
Tmd 10
5
(b)
350
8
RS
ETo (mm)
7
ETo
300 250
6 200
5 4
Rs(Wm-2 )
9
150
3
100
2 50
1
(c)
VPD (kPa)
4
Días Julianos vs Eto (mm/d) Días vs Rad solar
VPDm VPDmd
3
2
1
100 120 140 160 180 200 220 240 260 280 300 320 340 360
DOY Figure 1. Daily mean (Tm, dotted line) and midday (Tmd, solid line) air temperature, and daily rainfall (vertical bars) (a), solar radiation (Rs, dotted line) and reference evapotranspiration (ETo, solid line) (b), daily mean (VPDm, dotted line) and midday (VPDmd, solid line) air vapour pressure deficit (VPD) (c) values during the experimental period.
234 -0.6
(a)
Ψstem (MPa)
-0.8
-1.0
-1.2
-1.4
(b) 0.5
MDS (mm)
0.4
0.3
0.2
0.1
(c) 225
-1
SF (L day )
200 175 150 125 100 75
100 120 140 160 180 200 220 240 260 280 300 320 340 360
DOY Figure 2. Midday stem water potential (Ystem) (a), maximum daily trunk shrinkage (MDS) (b) and daily sap flow (SF) (c) values during the measurements period. Vertical bars are twice the overall mean SE.
235
(a) MDS = 0.06 + 0.05 ETo 2 r = 0.6561
0.5
MDS (mm)
0.4
0.3
0.2
0.1
(b) -0.50
Ψstem (MPa)
-0.75
-1.00
-1.25
-1.50
Ψ stem = -0.60 - 0.13 ETo r 2 = 0.7921
(c)
SF (L day-1 )
200
150
100
SF = 33.28 + 78.75 ETo - 13.56 ETo2 + 0.83 ETo3 r2 = 0.9266
50
1
2
3
4
5
6
ETo (mm) Figure 3. Relationships between ETo and maximum daily trunk shrinkage (MDS) (a), midday stem water potential (Ystem) (b) and daily sap flow (SF) (c) values during the measurement period.
236
(a) 0.5
MDS = 0.06 + 0.001 Rs 2 r =0.4666
MDS (mm)
0.4
0.3
0.2
0.1
(b) -0.50
Ψstem = -0.49 - 0.01 Rs r2 =0.6613
Ψstem (kPa)
-0.75
-1.00
-1.25
-1.50
(c) SF = 21.49 + 1.08 Rs - 0.01 Rs r2 =0.7840
2
-1
SF (L day )
200
150
100
50
50
100
150
200
250
300
350
Rs (W m-2) Figure 4. Relationships between solar radiation (Rs) and maximum daily trunk shrinkage (MDS) (a), midday stem water potential (Ystem) (b) and daily sap flow (SF) (c) values during the measurement period.
237
(a) 0.5
VPDm VPDmd
MDS (mm)
0.4
0.3
0.2 MDS = 0.07 + 0.15 VPDm r 2 =0.5560
0.1
MDS = 0.09 + 0.08 VPD md 2
r =0.5293
(b) -0.50
Ψst em = -0.70 - 0.18 VPD md 2
r = 0.394
Ψstem (MPa)
-0.75
Ψstem = -0.68 - 0.31 VPDm r 2 = 0.4268
-1.00
-1.25
-1.50
(c)
VPD m VPD md
VPDm VPDmd
SF (L day-1)
200
150
100
SF = 26.43 + 163.99 VPD m - 36.61 VPDm 2 r = 0.8464
2
SF = 37.25 + 91.47 VPDmd - 12.09 VPDmd2
50
2
r = 0.7883
1
2
3
4
5
VPD (kPa) Figure 5. Relationships between daily mean (VPDm, open symbols) and midday (VPDmd, closed symbols) air vapour pressure deficit, and maximum daily trunk shrinkage (MDS) (a), midday stem water potential (Ystem) (b) and daily sap flow (SF) (c) values during the measurement period.
238
(a)
MDS (mm)
MDS = -0.07 + 0.02 Tm 2
0.5
r = 0.7591
0.4
MDS = -0.13 + 0.02 Tmd 2 r = 0.7460
0.3
0.2 Tm Tmd
0.1
(b) Tm
-0.50
Tmd
Ψstem (MPa)
-0.75
-1.00
-1.25 Ψstem = -0.37 - 0.03 Tm r 2 = 0.5979
-1.50
Ψstem = -0.22 - 0.03 Tmd 2 r = 0.5492
(c) SF = 31.97 + 6.30 Tm 2
r = 0.8613
200
SF = 1.38 + 6.11 Tmd 2
-1
SF (L day )
r = 0.8609
150
100
Tm Tmd
50
10
15
20
25
30
35
Air temperature (ºC) Figure 6. Relationships between daily mean (Tm, open symbols) and midday (Tmd, closed symbols) air temperature, and maximum daily trunk shrinkage (MDS) (a), midday stem water potential (Ystem) (b) and daily sap flow (SF) (c) values during the measurement period.
239
(a)
(b)
-0.50
Ψstem (MPa)
-0.75
-1.00
-1.25
-1.50
Ψ stem = -0.48 - 2.20 MDS
Ψstem = -0.18 - 0.01 SF
r 2 =0.7796
0.1
r 2 =0.6453
0.2
0.3
0.4
0.5
50
100
150
200
SF (L day-1)
MDS (mm)
Figure 7. Relationships between midday stem water potential (Ystem), and maximum daily trunk shrinkage (MDS) (a) and daily sap flow (SF) (b) values during the measurement period.
0.5
MDS = - 0.11 + 0.01 SF 2 r =0.7586
MDS (mm)
0.4
0.3
0.2
0.1
50
100
150
200
SF (L day-1) Figure 8. Relationship between maximum daily trunk shrinkage (MDS) and daily sap flow (SF) values during the measurement period.
The regressions analysis between Ystem, and MDS and SF, pooling data across the observation period, indicated clear associations (Figure 7). Also, the MDS versus SF relationship showed a high coefficient of determination (Figure 8).
Discussion Since the three plant-based water stress indicators studied (MDS, SF and Ystem) encompass different
dimensions and time scales, it was to be expected that MDS and SF would correlate better with the evaporative demand indicators measured on a whole-day basis, while Ystem would best fit the parameters measured at midday. The regression analysis indicated that the highest coefficients of determination were obtained for the regressions of SF, MDS and Ystem against ETo, Tm and ETo, respectively (Figures 3–6). The fact that SF was more closely associated with changes in ETo can be explained by taking into consideration that SF
240 is directly related with daily transpiration (Alarco´n et al., 2005; Ortun˜o et al., 2004a). Even though temperature is not an accurate indicator of the evaporative demand of the atmosphere (Hatfield and Fuchs, 1990), MDS was closely related with Tm. In this sense, Fereres and Goldhamer (2003) showed that MDS correlated well with both Tmd and Tm in almond trees. However, Ve´lez (2004) indicated that MDS was more directly related with changes in Rs and ETo than VPD and temperature in Citrus clementina trees. According to the characteristics of the data for the different studied regression models (Figures 3–6), it can be concluded that the behaviour of SF can be more adequately predicted (higher r2) by changes in the evaporative demand variables than can MDS and Ystem. Also, the MDS was more adequately described by changes in the environmental variables than was Ystem, except when ETo or Rs were considered. At first sight, the fact that SF levelled off after ETo, Rs, VPDmd and VPDm reached threshold values (Figures 3c, 4c and 5c) would suggests that spatial variability of soil moisture availability and/or the high hydraulic conductivity of the soil constraints plant transpiration even with a high volume of irrigation. However, other logical explanation could be that maximum and constant values of SF correspond to physical constraint of plant transpiration. In this sense, Pataki and Oren (2003) studied factors controlling transpiration of some trees species, and showed that Quercus rubra, Q. alba, Carya tormentosa and Fraxinus americana plants were responsive to vapour pressure deficit and presented saturation in increases in daily sap flux density with vapour pressure deficit, these responses being similar at different soil moisture levels. Moreover, considering that MDS respond sooner to water stress than other continuous and discrete plant and soil water status indicators (Goldhamer et al., 1999) and that MDS is a versatile indicator of transpiration stream intensity when the soil water content is not strongly depleted (Huguet et al., 1992; Ortun˜o et al., 2004a, b), the fact that when SF presented high and constant values (Figure 2c) MDS values progressively decreased (Figure 2b) indicated that lemon trees were not under a water stress situation. In contrast with the observed behaviour in the SF versus ETo, Rs and VPD relationships,
the fact that the relationships between MDS and the environmental variables (Figures 3–6) were linear even at high values of evaporative demand was probably the result of the efficient water recruitment from additional stem tissue capacitances when evaporative demand increased and Ystem decreased below a threshold. In this sense, Zweifel et al. (2001) showed that internally stored water contributed to daily transpiration even in well-watered trees, indicating that stored water plays an important role not only during periods of drought, but whenever water transport occurs within the tree. Previous authors have shown weak correlations between MDS and Ystem on a seasonal basis in almond trees (Fereres and Goldhamer, 2003), in plum trees (Intrigliolo and Castel, 2004) and in peach trees (Marsal et al., 2002), but when they were broken into different time periods the correlation for individual periods clearly improved, suggesting that this may be a general trend for deciduous fruit trees due to trunk growth rate and tissue elasticity changes during the season (Intrigliolo and Castel, 2004). Our results, on the other hand, pointed to a constant relationship between MDS and Ystem (Figure 7a), which means neither postulated cause was important in adult lemon trees during the observation period. Molz and Klepper (1973) indicated that the main factor controlling MDS is the leaf water potential, which determines the driving force for water transport between the bark and the xylem. The goodness of the correlation found between Ystem and MDS (Figure 7a) confirmed this view. Moreover, the fact that there were direct relationships between Ystem and ETo (Figure 3b), Ystem and SF (Figure 7b) and SF and MDS (Figure 8) indicated that the control of MDS by Ystem could be mediated through SF. The mechanism, under non-limiting soil water conditions, would be described as follow: increases in the evaporative demand of the atmosphere induce decreases in Ystem (Figure 3b), which increases the soil–plant water potential gradient, and as a consequence SF increases (Figure 7b) turning into an increase in MDS (Figure 8). The above mentioned results indicated that baselines or reference values for plant-based water status indicators (MDS, SF and Ystem) can be obtained for adult lemon trees, even though there
241 was a certain scattering in the relations between the plant-based measurements and the environmental variables. According to the r2 values of the data around the different regression models, it can be concluded that the SF behaviour more adequately reflected changes in the evaporative demand variables than those of MDS and Ystem. The respective behaviour of SF and Ystem correlated more closely with changes in ETo (r2 = 0.93 and 0.79, respectively), while MDS behaviour was best correlated with Tm (r2 = 0.76). Whatever the case, these correlations must be used within their confidence levels. Increases in the evaporative demand induced decreases in Ystem and, as a consequence, SF increased, which, in turn, was translated into an increase in MDS. This confirmed that SF and MDS were very good predictors of the plant water status during the observation period and their continuous recording offers the promising possibility of their use in automatic irrigation scheduling in lemon trees.
Acknowledgements This research was supported by Ministerio de Educacio´n y Ciencia (MEC), (CICYT/FEDER AGL2003-9387-C05-02 and AGL2004-0794-C0302) and PETRI (PTR1995-0693-OP-02-01) grants to the authors. M.F. Ortun˜o and O. Mounzer were research fellowships from CSIC (Program I3P) and MEC (FPI), respectively.
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