Comparative monitoring of temporal and spatial changes in tree water status using the non-invasive leaf patch clamp pressure probe and the pressure bomb

Real-time monitoring of plant water status under field conditions remains difficult to quantify. Here we give evidence that the magnetic-based leaf patch clamp pressure (LPCP) probe is a non-invasive and online-measuring method that can elucidate short- and long-term temporal and spatial dynamics of leaf water status of trees with high precision in real time. Measurements were controlled remotely by telemetry and data transfer to the Internet. Concomitant measurements using the pressure chamber technique (frequently applied for leaf water status monitoring) showed that both techniques yield in principle the same results despite of the high sampling variability of the pressure chamber data. There was a very good correlation between the output pressure signals of the LPCP probe and the balancing pressure values (on average r² = 0.90 ± 0.05; n = 8), i.e. the external pressure at which water appears at the cut end of a leaf under pressure chamber conditions. Simultaneously performed direct measurements of leaf cell turgor pressure using the well-established cell turgor pressure probe technique evidenced that both techniques measure relative changes in leaf turgor pressure. The output pressure signals of the LPCP probe and the balancing pressure values were inversely correlated to turgor pressure. Consistent with this, the balancing pressure values and the cell turgor pressure values could be fitted quite well by the same firm theoretical backing derived recently for the LPCP probe (Zimmermann et al., 2008). This finding suggests that use of the LPCP probe technique in agricultural water management can be built up on the knowledge accumulated on spot leaf or stem water potential measurements.     

Response of Clementina de Nules citrus trees to summer deficit irrigation. Yield components and fruit composition

The effects of mid-summer regulated deficit irrigation (RDI) treatments were investigated on Clementina de Nules citrus trees over three seasons. Water restrictions applied from July, once the June physiological fruit drop had finished, until mid September were compared with a Control treatment irrigated during all the season to match full crop evapotranspiration (ET_c). Two degrees of water restrictions were imposed based on previous results also obtained in Clementina de Nules trees ( [Ginestar and Castel, 1996] and [González-Altozano and Castel, 1999]). During the RDI period, deficit irrigation was applied based on given reductions over the ET_c, but also taking into account threshold values of midday stem water potential (Ψ_s) of −1.3 to −1.5 MPa for RDI-1 and of −1.5 to −1.7 MPa for RDI-2. Results showed that water savings achieved in the RDI-2 treatment impaired yield by reducing fruit size. On the contrary, the RDI-1 strategy allowed for 20% water savings, with a reduction in tree growth but without any significant reduction in yield, fruit size nor in the economic return when irrigation was resumed to normal dose about three months before harvest. Water use efficiency (WUE) in the RDI trees was similar or even higher than in Control trees. RDI improved fruit quality increasing total soluble solids (TSS) and titratable acidity (TA). In conclusion, we suggest that the RDI-1 strategy here evaluated can be applied in commercial orchards not only in case of water scarcity, but also as a tool to control vegetative growth improving fruit composition and reducing costs associated with the crop management.     

Deficit irrigation without reducing yield or nut splitting in pistachio (Pistacia vera cv Kerman on Pistacia terebinthus L.)

An irrigation experiment involving pistachio (Pistacia vera cv Kerman on Pistacia terebinthus L. rootstocks) was performed over a four-year period in central Spain to determine the effect of regulated deficit irrigation (RDI) on nut yield and quality. The growth season was divided into three phenological stages: stage I - from sprouting until the end of rapid nut growth; stage II - from maximum nut size until the beginning of kernel growth; and stage III - from the beginning of kernel growth until harvest. Control trees were irrigated to supply their full water needs throughout the growth season, except for the post-harvest period. Sustained deficit irrigation at 65% (DI₆₅) and 50% (DI₅₀) of control irrigation was provided to two other groups of trees. The RDI provided to a further group was designed to provide a stress period during stages I and II but no water stress during stage III; the aim was to reduce water use and increase the percentage of split nuts. A fifth group of trees was maintained under rain fed conditions. Water potential and leaf conductance were affected in the DI₆₅, DI₅₀ and rain fed treatments mainly during stages II and III, with midday water potentials below −2.0 MPa. The RDI trees were only significantly water stressed during stage II, showing midday water potentials of around −1.4 MPa. On most days, leaf conductance was not significantly affected in any of the irrigation treatments. The nuts of the DI₆₅ and DI₅₀ trees were smaller in diameter and their total yield was reduced compared to the controls. However, no significant differences in kernel dry weight were observed. The RDI trees showed a total yield and percentage of split nuts similar to those of the controls, even though they received around 20% less water. The split nut yield showed a linear relationship with crop evapotranspiration. However, since the percentage of split nuts was similar in all treatments this variation was likely related to the total yield. The RDI trees did not show the normal alternate bearing pattern (which was clearly maintained in the control trees). Early splitting, a process that decreases the quality of the yield, was not related to water status but to temperatures lower than 13 °C. The results suggest that P. terebinthus L. rootstocks confer P.vera scions a degree of drought-resistance, reducing the likelihood of water stress and, therefore, allowing more severe RDI scheduling.     

Yield and fruit development in mango (Mangifera indica L. cv. Chok Anan) under different irrigation regimes

‘Chok Anan’ mangoes are mainly produced in the northern part of Thailand for the domestic fresh market and small scale processing. It is appreciated for its light to bright yellow color and its sweet taste. Most of the fruit development of on-season mango fruits takes place during the dry season and farmers have to irrigate mango trees to ensure high yields and good quality. Meanwhile, climate changes and expanding land use in horticulture have increased the pressure on water resources. Therefore research aims on the development of crop specific and water-saving irrigation techniques without detrimentally affecting crop productivity.The aim of this study was to assess the response of mango trees to varying amounts of available water. Influence of irrigation, rainfall, fruit set, retention rate and alternate bearing were considered as the fruit yield varies considerably during the growing seasons. Yield response and fruit size distribution were measured and WUE was determined for partial rootzone drying (PRD), regulated deficit irrigation (RDI) and irrigated control trees.One hundred ninety-six mango trees were organized in a randomized block design consisting of four repetitive blocks, subdivided into eight fields. Four irrigation treatments have been evaluated with respect to mango yield and fruit quality: (a) control (CO = 100% of ET_c), (b) (RDI = 50% of ET_c), (c) (PRD = 50% of ET_c, applied to alternating sides of the root system) and (d) no irrigation (NI).Over four years, the average yield in the different irrigation treatments was 83.35 kg/tree (CO), 80.16 kg/tree (RDI), 80.85 kg/tree (PRD) and 66.1 kg/tree (NI). Water use efficiency (WUE) calculated as yield per volume of irrigation water was always significantly higher in the deficit irrigation treatments as compared to the control. It turned out that in normal years the yields of the two deficit irrigation treatments (RDI and PRD) do not differ significantly, while in a dry year yield under PRD is higher than under RDI and in a year with early rainfall, RDI yields more than PRD. In all years PRD irrigated mangoes had a bigger average fruit size and a more favorable fruit size distribution.It was concluded that deficit irrigation strategies can save considerable amounts of water without affecting the yield to a large extend, possibly increasing the average fruit weight, apparently without negative long term effects.     

Crop water stress index is a sensitive water stress indicator in pistachio trees

Regulated deficit irrigation (RDI) strategies, often applied in tree crops, require precise monitoring methods of water stress. Crop water stress index (CWSI), based on canopy temperature measurements, has shown to be a good indicator of water deficits in field crops but has seldom been used in trees. CWSI was measured on a continuous basis in a Central California mature pistachio orchard, under full and deficit irrigation. Two treatments—control, returning the full evapotranspiration (ET_c) and RDI—irrigated with 40% ET_c during stage 2 of fruit grow (shell hardening). During stage 2, the canopy temperature—measured continuously with infrared thermometers (IRT)—of the RDI treatment was consistently higher than the control during the hours of active transpiration; the difference decreasing after irrigation. The non-water-stressed baseline (NWSB), obtained from clear-sky days canopy–air temperature differential and vapour pressure deficit (VPD) in the control treatment, showed a marked diurnal variation in the intercept, mainly explained by the variation in solar radiation. In contrast, the NWSB slope remained practically constant along the day. Diurnal evolution of calculated CWSI was stable and near zero in the control, but showed a clear rising diurnal trend in the RDI treatment, increasing as water stress increased around midday. The seasonal evolution of the CWSI detected large treatment differences throughout the RDI stress period. While the CWSI in the well-irrigated treatment rarely exceeded 0.2 throughout the season, RDI reached values of 0.8–0.9 near the end of the stress period. The CWSI responded to irrigation events along the whole season, and clearly detected mild water stress, suggesting extreme sensitivity to variations in tree water status. It correlated well with midday leaf water potential (LWP), but was more sensitive than LWP at mild stress levels. We conclude that the CWSI, obtained from continuous nadir-view measurements with IRTs, is a good and very sensitive indicator of water stress in pistachio. We recommend the use of canopy temperature measurements taken from 1200 to 1500 h, together with the following equation for the NWSB: (T _c − T _a) = −1.33·VPD + 2.44. Measurements of canopy temperature with VPD < 2 kPa are likely to generate significant errors in the CWSI calculation and should be avoided.     

叶片膨压探针诊断植物体水分状况原理及应用进展

叶片膨压探针(LPCP)技术是通过测定在恒定磁压力下叶片输出压力的衰减情况,能够实现原位连续无损监测植物叶片膨压变化,对植物水分状况的指示具有较高的准确性和敏感性.利用LPCP技术进行相关植物气孔生理、水分状况监测,并在此基础上指导灌溉,是近年来相关领域的研究热点.在梳理现有植物水分状况诊断方法的基础上,概括了 LPC...     

Corn yield responses under crop evapotranspiration-based irrigation management

Improving irrigation water management is becoming important to produce a profitable crop in South Texas as the water supplies shrink. This study was conducted to investigate grain yield responses of corn (Zea mays) under irrigation management based on crop evapotranspiration (ET_C) as well as a possibility to monitor plant water deficiencies using some of physiological and environmental factors. Three commercial corn cultivars were grown in a center-pivot-irrigated field with low energy precision application (LEPA) at Texas AgriLife Research Center in Uvalde, TX from 2002 to 2004. The field was treated with conventional and reduced tillage practices and irrigation regimes of 100%, 75%, and 50% ET_C. Grain yield was increased as irrigation increased. There were significant differences between 100% and 50% ET_C in volumetric water content (θ), leaf relative water content (RWC), and canopy temperature (T_C). It is considered that irrigation management of corn at 75% ET_C is feasible with 10% reduction of grain yield and with increased water use efficiency (WUE). The greatest WUE (1.6 g m⁻² mm⁻¹) achieved at 456 mm of water input while grain yield plateaued at less than 600 mm. The result demonstrates that ET_C-based irrigation can be one of the efficient water delivery schemes. The results also demonstrate that grain yield reduction of corn is qualitatively describable using the variables of RWC and T_C. Therefore, it appears that water status can be monitored with measurement of the variables, promising future development of real-time irrigation scheduling.     

Evapotranspiration, crop coefficient and growth of two young pomegranate (Punica granatum L.) varieties under salt stress

Pomegranate (Punica granatum L.) is a drought-hardy crop, suited to arid and semi-arid regions, where the use of marginal water for agriculture is on the rise. The use of saline water in irrigation affects various biochemical processes. For a number of crops, yields have been shown to decrease linearly with evapotranspiration (ET) when grown in salt-stressed environments. In the case of pomegranate, little research has been conducted regarding the effect of salt stress. Our study focused on the responses of ET, crop coefficient (K_c) and growth in pomegranate irrigated with saline water. Experiments were conducted using lysimeters with two varieties of pomegranate, P. granatum L. vars. Wonderful and SP-2. The plants were grown with irrigation water having an electrical conductivity (EC_iw) of 0.8, 1.4, 3.3, 4.8 and 8 dS m⁻¹. Plants were irrigated with 120% of average lysimeter-measured ET. Seasonal variation in ET, crop coefficient (K_c) and growth were recorded. Variation in daily ET was observed 1 month after initiation of the treatments. While significant seasonal ET variation was observed for the EC-0.8 treatment, it remained more stable for the EC-8 treatment. Salinity treatment had a significant effect on both daily ET (F = 131, p < 0.01) and total ET (F = 112.68, p = 0.001). Furthermore, the electrical conductivity of the drainage water (EC_dw) in the EC-8 treatment was five times higher than that of the EC-0.8 treatment in the peak season. Fitting the relative ET (ET_r) to the Maas and Hoffman salinity yield response function showed a 10% decrease in ET per unit increase in electrical conductivity of the saturated paste extract (EC_e) with a threshold of 1 dS m⁻¹. If these parameters hold true in the case of mature pomegranate trees, the pomegranate should be listed as a moderately sensitive crop rather than a moderately tolerant one. Fitting 30-day interval ET_r data to the Maas and Hoffman salinity yield response function showed a reduction in the slope as the season progressed. Thus using a constant slope in various models is questionable when studying crop-salinity interactions. In addition, both of the varieties showed similar responses under salt stress. Moreover, the calculated value of K_c is applicable for irrigation scheduling in young pomegranate orchards using irrigation water with various salinities.     

Evapotranspiration and water use of full and deficit irrigated cotton in the Mediterranean environment in northern Syria

Cotton (Gossypium hirsutum L.) is the most important industrial and summer cash crop in Syria and many other countries in the arid areas but there are concerns about future production levels, given the high water requirements and the decline in water availability. Most farmers in Syria aim to maximize yield per unit of land regardless of the quantity of water applied. Water losses can be reduced and water productivity (yield per unit of water consumed) improved by applying deficit irrigation, but this requires a better understanding of crop response to various levels of water stress. This paper presents results from a 3-year study (2004-2006) conducted in northern Syria to quantify cotton yield response to different levels of water and fertilizer. The experiment included four irrigation levels and three levels of nitrogen (N) fertilizer under drip irrigation. The overall mean cotton (lint plus seed, or lintseed) yield was 2502 kg ha⁻¹, ranging from 1520 kg ha⁻¹ under 40% irrigation to 3460 kg ha⁻¹ under 100% irrigation. Mean water productivity (WP_ET) was 0.36 kg lintseed per m³ of crop actual evapotranspiration (ET_c), ranging from 0.32 kg m⁻³ under 40% irrigation to 0.39 kg m⁻³ under the 100% treatment. Results suggest that deficit irrigation does not improve biological water productivity of drip-irrigated cotton. Water and fertilizer levels (especially the former) have significant effects on yield, crop growth and WP_ET. Water, but not N level, has a highly significant effect on crop ET_c. The study provides production functions relating cotton yield to ET_c as well as soil water content at planting. These functions are useful for irrigation optimization and for forecasting the impact of water rationing and drought on regional water budgets and agricultural economies. The WP_ET values obtained in this study compare well with those reported from the southwestern USA, Argentina and other developed cotton producing regions. Most importantly, these WP_ET values are double the current values in Syria, suggesting that improved irrigation water and system management can improve WP_ET, and thus enhance conservation and sustainability in this water-scarce region.     

Whole-tree water balance and indicators for short-term drought stress in non-bearing ‘Barnea’ olives

Drainage-weighing lysimeters allowed monitoring of water balance components of non-bearing olive (Olea europaea cv Barnea) trees over a 3-month period including short-term events of controlled but severe water stress. The objective of the study was to evaluate a variety of soil and plant-based water status and drought stress monitoring methods on the basis of tree-scale evapotranspiration (ET). As the trees entered into and recovered from water stress, meteorological data, actual ET (ET_a), soil water content and changes in leaf turgor pressure were continuously monitored. Additionally, midday measurements of stem water potential, stomatal conductance, canopy temperature, and quantum yield of PSII photochemistry were conducted. Diurnal (dawn to dusk) measurements of all the above were made hourly on days of maximum stress. Shoot elongation rate was measured for periods of stress and recovery. Quantum yield of PSII photochemistry, stomatal conductance, and stem water potential all successfully indicated reductions in whole-tree water consumption beginning at moderate stress levels. These measured parameters fully recovered to the levels of non-stressed trees soon after water application was renewed. Shoot elongation was reduced 25-30% for the 10-day period during and following drought and recovered thereafter to levels of non-stressed trees. Whole-tree ET_a was reduced by as much as 20% even following full recovery of the leaf level parameters, suggesting reduced canopy size and growth due to the stress period. Non-destructive, continuous (turgor pressure) and remotely sensed (canopy temperature) methods showed promising potential for monitoring effects of water stress, in spite of technological and data interpretation challenges requiring further attention.     

Evapotranspiration components determined by sap flow and microlysimetry techniques of a vineyard in northwest China: Dynamics and influential factors

Large areas of vineyards have been established in recent years in arid region of northwest China, despite limited water resources. Water to support these vineyards is mainly supplied by irrigation. Accurate estimation of vineyard evapotranspiration (ET) can provide a scientific basis for developing irrigation management. Transpiration and soil evaporation, as two main components of ET, were measured separately in a vineyard in this region by heat balance sap flow system and micro-lysimeters during the growing season of 2009. Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. Daily sap flow rate (SR_l) increased linearly with solar radiation (R_s), but showed an exponential increase to its maximum curve as a function of vapor pressure deficit (VPD). Residuals of the two regressions both depended on volumetric soil water content to a depth of 1.0 m (VWC). VWC also significantly influenced SR_l. The relationship of them could be expressed by a piecewise regression with the turnover point of VWC = 0.188 cm³ cm⁻³, which was ∼60% of the field capacity. Conversely, soil evaporation (E_s) increased exponentially with VWC. Thus, we recommended keeping VWC in such vineyards slightly above ∼60% of the field capacity to maintain transpiration while reducing soil evaporation. Vineyard transpiration (T_s) was scaled from sap flow by using leaf area (A_l) as it explained 60% of the spatial variability of sap flow. Vine transpiration was 202.0 mm during the period from April 28 to October 5; while that of E_s was 181.0 mm. The sum of these two components was very close to ET estimated by the Bowen ratio energy balance method (386.9 mm), demonstrating the applicability of sap flow for measuring grape water use in this region.     

Almond agronomic response to long-term deficit irrigation applied since orchard establishment

This study assesses the long-term suitability of regulated (RDI) and sustained deficit irrigation (SDI) implemented over the first six growing seasons of an almond [Prunus dulcis (Mill.) D.A. Webb] orchard grown in a semiarid area in SE Spain. Four irrigation treatments were assessed: (i) full irrigation (FI), irrigated to satisfy maximum crop evapotranspiration (100% ET_c); (ii) RDI, as FI but receiving 40% ET_c during kernel-filling; (iii) mild-to-moderate SDI (SDI_mm), irrigated at 75–60% ET_c over the entire growing season; and (iv) moderate-to-severe SDI (SDI_ms), irrigated at 60–30% ET_c over the whole season. Application of water stress from orchard establishment did not amplify the negative effects of deficit irrigation on almond yield. Irrigation water productivity (IWP) increased proportionally to the mean relative water shortage. SDI_ms increased IWP by 92.5%, reduced yield by 29% and applied 63% less irrigation water. RDI and SDI_mm showed similar productive performances, but RDI was more efficient than SDI_mm to increase fruiting density and production efficiency (PE). We conclude that SDI_ms appears to be a promising DI option for arid regions with severe water scarcity, whereas for less water-scarce areas RDI and SDI_mm behaved similarly, except for the ability of RDI to more severely restrict vegetative development while increasing PE.     

Estimation of dwarf green bean water use under semi-arid climate conditions through ground-based remote sensing techniques

Determination of temporal and spatial distribution of water use (WU) within agricultural land is critical for irrigation management and could be achieved by remotely sensed data. The aim of this study was to estimate WU of dwarf green beans under excessive and limited irrigation water application conditions through indicators based on remotely sensed data. For this purpose, field experiments were conducted comprising of six different irrigation water levels. Soil water content, climatic parameters, canopy temperature and spectral reflectance were all monitored. Reference evapotranspiration (ET₀), crop coefficient K_c and potential crop evapotraspiration (ET_c) were calculated by means of methods described in FAO-56. In addition, WU values were determined by using soil water balance residual and various indexes were calculated. Water use fraction (WUF), which represents both excessive and limited irrigation applications, was defined through WU, ET₀ and K_c. Based on the relationships between WUF and remotely sensed indexes, WU of each irrigation treatments were then estimated. According to comparisons between estimated and measured WU, in general crop water stress index (CWSI) can be offered for monitoring of irrigated land. At the same time, under water stress, correlation between measured WU and estimated WU based on CWSI was the highest too. However, canopy-air temperature difference (T_c − T_a) is more reliable than others for excessive water use conditions. Where there is no data related to canopy temperature, some of spectral vegetation indexes could be preferable in the estimation of WU.     

Plant based indicators for irrigation scheduling in young cherry trees

Using a correlation between trunk diameter fluctuation (TDF) and stem water potential (SWP) it appears possible to determine water deficit threshold values (WDTV) for young cherry trees. This correlation must be based on a significant effect between SWP and at least one variable associated with the vegetative or reproductive growth of the trees. The objectives of this study are: (1) to determine the effect of several irrigation treatments on vegetative and reproductive growth and the SWP of young cherry trees; (2) to determine the correlation between TDF and SWP, and; (3) to propose a first approximation of SWP and TDF water deficit threshold values for young cherry tree plants. The experiment was carried out between September and April of the 2005-2006 and 2006-2007 seasons, in Quillota, in the Valparaiso region, central Chile. The irrigation treatments consisted of applications of 50% (T₅₀), 100% (T₁₀₀) and 150% (T₁₅₀) of potential evapotranspiration (ET₀) over the two growing seasons, using a randomized complete block design (RCB). The effect of irrigation scheduling was observed on: apical shoot growth rate (GR_AS), branch cross-sectional area (BCSA), canopy volume (CV), annual length of accumulated growth (AL_AG) and productivity. This effect showed that the T₅₀ treatment caused lower SWP (measured pre-dawn), vegetative growth and productivity. The fruit quality variables (cracking and size) were not affected by the different treatments. Combining the vegetative growth, productivity and SWP results shows that the water deficit threshold value, as a first approximation, is between 50% and 100% of ET₀, and therefore the critical SWP for defining irrigation frequency should be close to −0.5 MPa. Upon applying a post-harvest drought period (14 days without irrigation), a linear correlation was determined both between SWP and maximum daily trunk shrinkage, MDS (R² = 0.69) and between SWP and trunk growth rate, TGR (R² = 0.57). Using these correlations and the SWP reference value, reference values were obtained for MDS (165 μm) and TGR (83 μm day⁻¹), which would permit automated control of water status in young cherry trees.     

Effect of irrigation uniformity on the profitability of crops

There are numerous models capable of simulating crop behavior under different water stress conditions. However, none of them takes into account the effect of irrigation water uniformity on yield. The model developed simulates the uniformity effect on yield and the repercussion on gross margin (GM). The application of the model to a maize crop in Albacete (Spain) indicates that for the same irrigation depth, an increase in uniformity of water in the soil (CU) corresponds to a 4% increase in yield for the common irrigation strategy in the area, and a 6.8% increase in yield for the optimal irrigation schedule established by the model. Values of percentage of adequately irrigated area (a) between 50 and 80% appear to be adequate for values of CU > 80%. This effect has special relevance on the GM mainly when designing the irrigation strategy of areas with limited water resources. This leads to improvement of CU from 75 to 95% for the common irrigation depth applied to maize and may increase GM up to 27%. For small irrigation depths, the effect of CU on GM is reduced. The maximum GM is reached at ET_a/ET_m < 1 and a <100%. The paper also describes a methodology for determining the most suitable irrigation schedule under regulated deficit irrigation conditions.     

Interactive responses to water deficits and crop load in olive (Olea europaea L., cv. Morisca). II: Water use, fruit and oil yield

To understand the relations between water use and yield in response to crop load, two experiments were conducted in olive (cv. Morisca), during six consecutive years (2002-2007) in an experimental orchard located in Badajoz, Southwest Spain. Experiment 1, assessed the responses during the early years of the orchard (2002-2004) using four irrigation treatments that applied fractions of the estimated crop evapotranspiration (ET_c) (125%, 100%, 75% and 0%) and three crop load levels (100%, 50% and 0% of fruit removal, termed Off, Medium and On treatments). Experiment 2 assessed the response of more mature trees (2005-2007) to three irrigation treatments (115%, 100%, and 60% of ET_c) and the natural crop load which were Off, On, and Medium in 2005, 2006 and 2007, respectively. Yield was reduced by water deficits and so did the estimated tree transpiration which was linearly related to yield (y = 1.2302x − 21.15, R² = 0.8864), showing the high sensitivity of cultivar Morisca to water deficits. The relations between fruit number and fruit weight showed that high crop loads had lower fruit weights and oil yield, a decrease that was more pronounced as water deficits increased. The yield response to water supply in the control and excess treatments, and the observations on the water relations of these two treatments suggest that the calculations made using the FAO method (Doorenbos and Pruit, 1974) with the crop coefficient proposed by Pastor et al. (1998) and the reduction coefficient (Fereres et al., 1982) to apply 100% of ET_c in the control treatment, underestimated the ET_c of the orchard. The results indicate that, although the absence of fruits lead to reduced water use as compared to situations of medium and high crop loads, canopy size was much more determinant of orchard water requirements than crop load.     

Effects of deficit irrigation on the yield and yield components of drip irrigated cotton in a mediterranean environment

A field study on cotton (Gossypium hirsutum L., cv.) was carried out from 2005 to 2008 in the Çukurova Region, Eastern Mediterranean, Turkey. Treatments were designated as I₁₀₀ full irrigation; DI₇₀, DI₅₀ and DI₀₀ which received 70, 50, and 0% of the irrigation water amount applied in the I₁₀₀ treatment. The irrigation water amount to be applied to the plots was calculated using cumulative pan evaporation that occurred during the irrigation intervals. The effect of water deficit or water stress on crop yield and some plant growth parameters such as yield response, water use efficiencies, dry matter yield (DM), leaf area index (LAI) as well as on lint quality components was evaluated. The average seasonal evapotranspiration ranged from 287 ± 15 (DI₀₀) to 584 ± 80 mm (I₁₀₀). Deficit irrigation significantly affected crop yield and all yield components considered in this study. The average seed cotton yield varied from 1369 ± 197 (DI₀₀) to 3397 ± 508 kg ha⁻¹ (I₁₀₀). The average water use efficiency (WUE_ET) ranged from 6.0 ± 1.6 (I₁₀₀) to 4.8 ± 0.9 kg ha⁻¹ mm⁻¹ (DI₀₀), while average irrigation water use efficiency (WUE_I) was between 9.4 ± 3.0 (I₁₀₀) and 14.4 ± 4.8 kg ha⁻¹ mm⁻¹ (DI₅₀). Deficit irrigation increased the harvest index (HI) values from 0.26 ± 0.054 (I₁₀₀) to 0.32 ± 0.052 kg kg⁻¹ (DI₅₀). Yield response factor (Ky) was determined to be 0.98 based on four-year average. Leaf area index (LAI) and dry matter yields (DM) increased with increasing water use. This study demonstrated that the full irrigated treatment (I₁₀₀) should be used for semiarid conditions with no water shortage. However, DI₇₀ treatment needs to be considered as a viable alternative for the development of reduced irrigation strategies in semiarid regions where irrigation water supplies are limited.     

Interactive responses to water deficits and crop load in olive (olea europaea L., cv. Morisca) I. - Growth and water relations

To characterize the interactions between variable water supply and crop load on vegetative growth and water relations of an olive orchard (cv. Morisca) planted in 1998 at 417 trees ha⁻¹, two different experiments were conducted over a six-year period (2002-2007) in Badajoz, Southwest of Spain. Experiment 1, assessed the responses during the early years of the orchard (2002-2004) using four irrigation treatments that applied fractions of the estimated crop evapotranspiration (ET_c) (125%, 100%, 75% and 0%) and three crop load levels (100%, 50% and 0% of fruit removal, termed off, medium and on treatments). Experiment 2, assessed the response of more mature trees (2005-2007) to three irrigation treatments (115%, 100%, and 60% of ET_c) and the natural crop load which were off, on, and medium in 2005, 2006 and 2007, respectively. Although vegetative growth was mainly affected by the level of water supply, crop load also influenced vegetative parameters, especially the interaction between high loads and water deficit. Trunk growth was more sensitive to water deficits than ground cover, and at the branch scale, water deficits reduced branch length and node numbers but only reduced internode length in on trees. Water relations were more affected by the level of water supply than by crop load. Nevertheless, the presence of fruits affected olive tree water status and, particularly, increased the stomatal conductance of on trees during late summer and early fall under all levels of water supply. Interactions between water stress and crop load levels were not very strong, and were more evident in mature than in young olive trees.     

Assessing yield optimization and water reduction potential for summer-sown and spring-sown maize in Pakistan

Agricultural food production in arid and semi-arid regions faces the challenge to ensure high yields with limited supply of water. This raises the question to which extent irrigation supply can be reduced without detriment to yield. Our study focuses on the yield-water uptake relationship for maize in the moderate water stress range in order to determine the onset of stress-induced dry-matter and yield losses. Compensatory plant responses under moderate stress levels are discussed in relation to seasonal climatic conditions.Summer-sown and spring-sown maize were irrigated with a decreasing amount of water in a field experiment in Pakistan. Water supply ranged from 100% water required to maintain soil at field capacity (FC) to 40% of FC. The average dry-matter and yield levels were slightly higher for summer-sown (15.0 Mg ha⁻¹) compared to spring-sown maize (13.1 Mg ha⁻¹). The onset of significant dry-matter and yield reduction started at the least irrigation treatment in both seasons. The amount of water required to avoid production losses was 272 mm in the summer-sown maize during the autumn growing season, and 407 mm for the spring-sown maize in the summer season, when the evaporative demand of the atmosphere was +27% higher. Water use efficiency (WUE_ET), normalized by vapour pressure deficit, of the summer-sown maize which was 10.0 kg kPa m⁻³, was +15% higher compared to the spring-sown crop; while the irrigation water productivity (2.9 kg m⁻³) was +11% more. WUE_ET increased over the whole range of applied water deficits for summer-sown maize, while the spring-sown crop showed a decreasing WUE_ET in the less irrigated treatment. Due to the higher efficiency in summer-sown maize, the potential in irrigation reduction without production losses (129 mm) was higher compared to the spring-sown maize (57 mm). Our results showed that in Pakistan water saving irrigation practices can be applied without yield loss mainly during the cooler growing season when the crop can efficiently compensate a lower total water uptake by increased use efficiency. For spring-sown maize the increasing evaporative demand of the atmosphere towards summer implies a higher risk of yield losses and narrows the range to exploit higher irrigation water productivity under moderate water deficit conditions.     

Irrigation scheduling for traditional,low-density olive orchards: Water relations and influence on oil characteristics

An experiment was performed in a low-density olive orchard (69 trees ha⁻¹) to study the recovery from water stress of olive trees under different irrigation managements. The effect of water stress on oil quality was also examined. The trees were subjected to one of four irrigation treatments: rain-fed conditions, irrigation with either 100% or 125% of the crop evapotranspiration (ET_c) level, or a deficit treatment in which only 60 mm of water were provided (at different times depending on the weather and phenological stage of the crop). The irrigation water in the deficit treatment was some 43% of the water applied in the 125% treatment. Plant water relations were determined periodically by measuring the water potential of covered leaves and the stomatal conductance at midday. The trees in the water deficit and rain-fed treatments rapidly recovered from water stress after receiving irrigation water or autumn rainwater, respectively, reaching the condition of the fully irrigated trees. However, stomatal conductance took longer to recover. Recovery at mid-summer in the deficit treatment was related to the amount of water in the soil; in autumn, however, this relationship was not so clear in rain-fed trees. The effect on oil quality was recorded in terms of the total concentration of phenolic compounds (TP). This was strongly related to the water stress integral, suggesting that the effect of irrigation on this variable occurs year-round and not just during the oil accumulation phase. Thus, even with low doses of water it should be possible to significantly reduce the TP concentration. Since recovery from water stress is rapid when irrigation is concentrated in the second half of summer, such an irrigation regimen might allow efficient use of the limited amounts of water available in central Spain.