Crop coefficient (K c) for apple: comparison between measurements by a weighing lysimeter and prediction by CropSyst

Accurate prediction of crop coefficient (K _c) is necessary for proper irrigation management. We explored CropSyst for determining irrigation requirements of apple trees and for accuracy of K _c prediction. Values of K _c were compared to those obtained, over 2002–2010, from lysimeter-grown trees. Over these years, trees had different ratios of height (H) to width (W). CropSyst predicted irrigation requirements using tree light interception and water uptake sub-model components. Parameters of the model were adjusted using data obtained from the lysimeter in 2010. Tree light interception sub-model was verified by 2007 data. After parameterization, good agreement was found between simulated and measured K _c over different seasons. The porosity coefficient of the canopy was related to changes in tree’s H/W ratio and leaf overlapping. Accordingly, different porosity values could be estimated for each year. When yearly changes in canopy porosity was considered, CropSyst improved K _c prediction and generated relevant information for managing irrigation under changing canopy shape for apple trees.     

Wastewater application and water use of Larrea tridentata

Treated wastewater has been applied to agronomic crops, rangelands, forests and recreation areas including parks and golf courses, and to disturbed lands such as mine spoil sites. While land application systems are conventional technology for many communities, there is limited information to guide land managers in arid and semiarid environments where wastewater may be the only source of supplemental irrigation. In order to develop a creosote climate-based water balance irrigation scheduling model to irrigate a desert ecosystem using wastewater, a crop coefficient (K_c) for the creosote bush (Larrea tridentata) must be determined. The objective of this study is to determine the K_c and evapotranspiration rate of L. tridentata in non-water limiting conditions and to use the data for wastewater irrigation scheduling in the Chihuahuan desert. The study site, located in Las Cruces, New Mexico is semiarid with an average annual rainfall of 220 mm. Thirty L. tridentata shrubs were purchased from a commercial greenhouse in 19 l pots. The pots were weighed before an irrigation and 24 h after irrigation. The weight change was converted to depth of Et based on the area of the plots. Reference Et was determined from climate data and a crop coefficient calculated. A third order polynomial described the change in the crop coefficient with both day of year and growing degree days using a base and minimum cutoff temperature of 0 °C, no upper cutoff temperature and only data when the day length was greater than 11 h. The coefficient of determination was 0.76 using day of year and 0.77 using GDD. The crop coefficient was used in a water balance irrigation scheduling model to predict creosote water use under rainfall condition in the Chihuahuan desert.     

Long-term evaluation of yield components of young olive trees during the onset of fruit production under different irrigation regimes

A four-year study was conducted on young Olea europaea L. trees to investigate the effect of deficit irrigation starting from the onset of fruit production. Subsurface drip irrigation was used to supply 100% (FI), 46–52% (DI), or 2–6% (SI) of tree water needs. Tree growth was reduced by deficit irrigation, whereas, return bloom was not. Per tree fruit yield of DI trees was 68% that of FI, but fruit yield efficiency based on tree size was similar between treatments. Fruit set and the number of fruits of FI trees were similar to those of DI trees and significantly higher than in SI trees. No significant differences in fruit fresh weight were found between FI and DI. The oil yield and oil yield efficiency of the DI treatment were 82 and 110% that of FI trees, respectively. A level of about 50% deficit proved sustainable to irrigate trees for oil production.     

Long term responses of olive trees to salinity

Water demand for irrigation is increasing in olive orchards due to enhanced yields and profits. Because olive trees are considered moderately tolerant to salinity, irrigation water with salt concentrations that can be harmful for many of fruit tree crops is often used without considering the possible negative effects on olive tree growth and yield. We studied salt effects in mature olive trees in a long term field experiment (1998-2006). Eighteen-year-old olive trees (Olea europaea L.) cv. Picual were cultivated under drip irrigation with saline water composed of a mixture of NaCl and CaCl₂. Three irrigation regimes (i. no irrigation; ii. water application considering soil water reserves, short irrigation; iii. water application without considering soil water reserves and adding a 20% more as a leaching fraction, long irrigation) and three salt concentrations (0.5, 5 or 10 dS m⁻¹) were applied. Treatments were the result of the combination of three salt concentrations with two irrigation regimes, plus the non-irrigated treatment. Growth parameters, leaf and fruit nutrition, yield, oil content and fruit characteristics were annually studied. Annual leaf nutrient analyses indicate that all nutrients were within the adequate levels. After 8 years of treatment, salinity did not affect any growth measurement and leaf Na⁺ and Cl⁻ concentration were always below the toxicity threshold of 0.2 and 0.5%, respectively. Annual and accumulated yield, fruit size and pulp:stone ratio were also not affected by salts. However, oil content increased linearly with salinity, in most of the years studied. Soil salinity measurements showed that there was no accumulation of salts in the upper 30 cm of the soil (where most of the roots are present) because of leaching by rainfall at the end of the irrigation period. Results suggest that a proper management of saline water, supplying Ca²⁺ to the irrigation water, using drip irrigation until winter rest and seasonal rainfall typical of the Mediterranean climate leach the salts from the first 0-60 cm depth, and growing a tolerant cultivar, can allow using high saline irrigation water (up to 10 dS m⁻¹) for a long time without affecting growth and yield in olive trees.     

Water balance of Swamp Mahogany and Rhodes grass irrigated with treated sewage effluent

Water balance of Swamp Mahogany (Eucalyptus robusta Sm.) and Rhodes grass (Chloris gayana Kunth var. Callide) plantations was studied in large experimental plots, which were irrigated with secondary treated sewage effluent. The tree plots designated as T10, T20, T30 and T40 received four different nitrogen (N) concentrations of 10, 20, 30 and 40 mg/l, respectively. The grass plot designated as G30 received one N level (30 mg/l). The objective of the study was to compare growth and water use of these plantations and the possible effluent losses to the environment.There was little response to N treatment in the first year of tree growth. A significant response to high N concentration was observed in tree treatment plots in the second year of the growth. Thus, at 20 month stage, the T40 trees reached a height of 4.1 m and had a leaf area index (LAI) of 2.5 compared with 2.2 m and 1.6, respectively in T10 trees. As expected, the T40 treatment had the largest interception losses (10%) and the least runoff and interflow. There was a progressive decrease in runoff and interflow with reductions in the level of nitrogen applied.Annual evapotranspiration was calculated to be 982 and 1191 mm, in the first and second year for grass compared with 1126 and 1269 mm, respectively for the T30 treatment. Grass and trees receiving the same concentration of N in effluent (30 mg/l) were transpiring at similar monthly rates, with crop factors of 0.79 for the grass and 0.85 for the trees, which were not statistically different. These results in plots subject to regular effluent irrigation are markedly different from findings of previous studies, which indicated a very large increase in water use of trees compared to grass vegetation under dryland conditions. Although evapotranspiration utilised the largest portion of the incoming water to the plots, the need for irrigation was reduced by the occurrence of frequent rainfall at the site. Runoff comprised the largest off site loss mechanism, especially during high rainfall periods indicating that coastal areas with low irrigation demand provide a limited opportunity for land disposal of effluent. Other site characteristics such as shallow soils increase the risks of environmental pollution through runoff from application site. Increase in area of application and adoption of a filtering technique will reduce risks to the soil and the environment.     

Evapotranspiration of flood-irrigated pecans

Pecan orchards require more irrigation water to maximize yield than any other crop grown in the Southwest US. This paper reports daily evapotranspiration (Et) measurements for 2001 and 2002 in a 5.1 ha, mature pecan orchard on the Rio Grande floodplain, 7 km south of Las Cruces, NM, USA. The 21-year-old stand had an average tree height of 12.8 m, diameter at breast height of 30 cm, and tree spacing of 9.7 m × 9.7 m. Additional pecan orchards surrounded the study orchard. When the tensiometer reached a suction of 65 kPa at the 45 cm depth, the orchard was flood-irrigated. Sparling meters were installed on the pumps and read before and after each irrigation. The total irrigation amount was 1940 mm in 2001 and 1870 mm in 2002. A walk-up tower was placed in the orchard’s center to support flux sensors at 16 m height. The instrument package included a net radiation (Rn), discs for soil heat flux (G), and two sets of one-propeller eddy covariance (OPEC) sensors. OPEC systems measure sensible heat flux (H) with a sensitive, vertically oriented propeller anemometer and a fine-wire thermocouple. Latent heat flux (LE) was obtained as a residual in the surface energy balance LE = Rn − G − H. The maximum daily evapotranspiration was 8 mm/day, and the yearly cumulative evapotranspiration averaged for 2 years was 1420 mm, resulting in a yearly average irrigation application efficiency of 79%. The crop coefficient (daily measured Et/reference Penman Et) ranged from 0.2 to 1.1. Increased evaporation due to irrigation was detected only for the April 9 irrigation in 2001. The seasonal water use was 4% lower in 2001 and 12% lower in 2002 than previously reported values.     

Maximum diurnal trunk shrinkage is a sensitive indicator of plant water, stress in Diospyros kaki (Persimmon) trees

Persimmon tree (Diospyros kaki L.f.) is a deciduous fruit tree included in the so-called group of minor fruit tree species. Worldwide, it is not widely grown but, nowadays, Kaki culture is of some importance in the south-east of Spain because of the high fruit commercial value. Currently, neither it is known about Kaki trees water needs, nor crop responses to the irrigation regime. The objective of the present research was to assess the feasibility of using maximum diurnal trunk shrinkage (MDS) as a plant water stress indicator for Kaki trees. During two drought cycles, in trees under either full or deficit irrigation, the MDS obtained by means of LVDT sensors was compared with a reference indicator of fruit trees water status, the midday stem water potential (Ψ_stem). In addition, stomatal conductance and fruit diameter variations were also followed. As water restrictions began, there was an immediate increase in MDS, in correspondence with a decrease in Ψ_stem. Pooling data from both drought cycles and irrigation regimes, MDS and Ψ_stem were linearly correlated (r² = 0.77***). The magnitude of differences between well watered and deficit irrigated trees was much larger in the case of MDS than for Ψ_stem. However, the tree-to-tree variability of the MDS readings was three times higher than for Ψ_stem; average coefficient of variation of 14% and 38% for Ψ_stem and MDS, respectively. Overall, results reported indicated that MDS is a sensitive indicator of Kaki water status and it can be further used as an irrigation scheduling indicator for optimum irrigation management of this crop. However, the large MDS tree-to-tree variability should be taken into account when selecting the number of trees to monitor within an orchard.     

Improving estimates of <Emphasis Type="Italic">N</Emphasis> and <Emphasis Type="Italic">P</Emphasis> loads in irrigation water from swine manure lagoons

The implementation of nutrient management plans for confined animal feeding operations requires recording N and P loads from land-applied manure, including nutrients applied in irrigation water from manure treatment lagoons. By regulation, lagoon irrigation water nutrient records in Mississippi must be based on at least one lagoon water nutrient analysis annually. Research in Mississippi has shown that N and P levels in lagoon water, and the N:P ratio, vary significantly through the year. Nutrient estimates based on one annual analysis do not account for this variability and may overestimate or underestimate N and P loads. The present study reports an improved method to more precisely estimate N and P loads in irrigation water from swine manure lagoons. The method is based on predictable annual cycles of N and P levels in lagoon water and employs simple curve-fitting of lagoon-specific formulas derived by analyses of historical data. Similarity of curves from analyses of Mississippi lagoons and other lagoon studies suggests that the method can be applied using the often limited nutrient data for a lagoon to more precisely estimate seasonal shifts of N and P and to improve the precision of estimates for N and P in irrigation water. Although the present study focused on swine manure lagoons in the southern US, recognition that the annual N cycle in lagoon water is temperature driven, suggests that additional research incorporating temperature into future models could extend these models to other types of waste treatment lagoons and climates.     

Long-term growth, water consumption and yield of date palm as a function of salinity

Actual measurements of water uptake and use, and the effect of water quality considerations on evapotranspiration (ET), are indispensable for understanding root zone processes and for the development of predictive plant growth models. The driving hypothesis of this research was that root zone stress response mechanisms in perennial fruit tree crops is dynamic and dependent on tree maturity and reproductive capability. This was tested by investigating long-term ET, biomass production and fruit yield in date palms (Phoenix dactylifera L., cv. Medjool) under conditions of salinity. Elevated salinity levels in the soil solution were maintained for 6 years in large weighing-drainage lysimeters by irrigation with water having electrical conductivity (EC) of 1.8, 4, 8 and 12 dS m⁻¹. Salinity acted dynamically with a long-term consequence of increasing relative negative response to water consumption and plant growth that may be explained either as an accumulated effect or increasing sensitivity. Sensitivity to salinity stabilized at the highest measured levels after the trees matured and began producing fruit. Date palms were found to be much less tolerant to salinity than expected based on previous literature. Trees irrigated with low salinity (EC = 1.8 dS m⁻¹) water were almost twice the size (based on ET and growth rates) than trees irrigated with EC = 4 dS m⁻¹ water after 5 years. Fruit production of the larger trees was 35-50% greater than for the smaller, salt affected, trees. Long term irrigation with very high EC of irrigation water (8 and 12 dS m⁻¹) was found to be commercially impractical as growth and yield were severely reduced. The results raise questions regarding the nature of mechanisms for salinity tolerance in date palms, indicate incentives to irrigate dates with higher rather than lower quality water, and present a particular challenge for modelers to correctly choose salinity response functions for dates as well as other perennial crops.     

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.     

Online-monitoring of tree water stress in a hedgerow olive orchard using the leaf patch clamp pressure probe

The need for sophisticated irrigation strategies in fruit tree orchards has led to an increasing interest in reliable and robust sensor technology that allows automatic and continuous recording of the water stress of trees under field conditions. In this work we have evaluated the potential of the leaf patch clamp pressure (LPCP) probe for monitoring water stress in a 4-year-old ‘Arbequina’ hedgerow olive orchard with 1667 trees ha⁻¹. The leaf patch output pressure (P_p) measured by the LPCP probe is inversely correlated with the leaf turgor pressure (>50 kPa). Measurements of P_p were made over the entire irrigation season of 2010 (April to November) on control trees, irrigated up to 100% of the crop water needs (ET_c), and on trees under two regulated deficit irrigation (RDI) strategies. The 60RDI trees received 59.2% of ET_c and the 30RDI trees received 29.4% of ET_c. In the case of the RDI trees the irrigation amounts were particularly low during July and August, when the trees are less sensitive to water stress. At severe water stress levels (values of stem water potential dropped below ca. −1.70 MPa; turgor pressure < 50 kPa) half-inversed or completely inversed diurnal P_p curves were observed. Reason for these phenomena is the accumulation of air in the leaves. These phenomena were reversible. Normal diurnal P_p profiles were recorded within a few days after rewatering, the number depending on the level of water stress previously reached. This indicates re-establishment of turgescence of the leaf cells. Crucial information about severe water stress was derived from the inversed diurnal P_p curves. In addition P_p values measured on representative trees of all treatments were compared with balancing pressure (P_b) values recorded with a pressure chamber on leaves taken from the same trees or neighbored trees exposed to the same irrigation strategies. Concomitant diurnal P_b measurements were performed in June and September, i.e. before and after the period of great water stress subjected to RDI trees. Results showed close relationships between P_p and P_b, suggesting that the pressure chamber measures relative turgor pressure changes as the LPCP probe. Therefore the probe seems to be an advantageous alternative to the pressure chamber for monitoring tree water status in hedgerow olive tree orchards.     

Species differences in transpiration on a saline discharge site

Growth, sap velocity, tree water use and transpiration rates per unit of leaf area were compared between Eucalyptus occidentalis Endl., Eucalyptus spathulata Hook., Eucalyptus leucoxylon F. Muell., and Eucalyptus cladocalyx F. Muell. on a moderately-saline discharge site near Wellington, NSW, Australia. These were four of the best performed species in a 7-year old trial of 36 species and provenances.Even though all trees were the same age and had grown under identical conditions, water use per tree was four–five times greater in E. spathulata than in the other three species. This difference was due to a large difference in tree size. E. spathulata had grown faster than the other species and had a mean tree leaf area four–five times greater than the other species.Species differences in water use per unit of leaf area were smaller, but sometimes statistically significant. During a period of cool dry weather in late winter, there were no significant differences between species in transpiration per unit of leaf area. In early summer, however, when the maximum vapour pressure deficit reached 6–7.5 kPa on some days, E. leucoxylon had a 22% lower rate of transpiration per unit of leaf area than the other three species. This difference was presumably due to a stronger stomatal response to high vapour pressure deficit in E. leucoxylon than the other species. During a period of warm humid weather in late summer, transpiration per unit of leaf area was 75% higher in E. cladocalyx compared with the other three species. The reason for this difference is not known, but it may indicate a species difference in root architecture, and hence a difference in access to ground water or soil water.     

Water relations of field grown Pomegranate trees (Punica granatum) under different drip irrigation regimes

Pomegranate (Punica granatum L.) is a deciduous fruit tree native of central Asia included in the so-called group of minor fruit tree species, not widely grown but of some importance in the south east of Spain. Fruit consumption interest is due to the organoleptic characteristics and to the beneficial effects on health. Pomegranate tree are considered as a culture tolerant to soil water deficit. However, very little is known about pomegranate orchard water management. The objective of this work was to characterize, for the first time in P. granatum, water relations aspects of applied significance for irrigation scheduling. Trees under different irrigation regimes were used and midday stem water potential (Ψ_stem) and midday leaf gas exchange were periodically measured over the course of an entire season. During spring and autumn, Ψ_stem did not show significant differences between irrigation treatments while there were considerable differences in leaf photosynthesis and stomatal conductance, suggesting a near-isohydric behaviour of pomegranate trees. This might explain why the signal intensity of Ψ_stem was lower than those of gas exchange indicators during the experimental period. Thus, leaf photosynthesis rates and stomatal conductance might have a greater potential for irrigation scheduling of pomegranate trees than Ψ_stem measured at solar noon.     

Groundwater uptake and sustainability of farm plantations on saline sites in Punjab province,Pakistan

Productive tree plantations on degraded land within Pakistan’s irrigation areas may help control salinity by extracting shallow groundwater, but their adoption has been limited by a lack of information on tree–water–salt interactions. Tree growth, water use, climate and soil conditions were monitored between 1994 and 1998 in young plantations of Eucalyptus, Acacia and Prosopis at two locations in Punjab province. Eucalyptus camaldulensis on an irrigated, non-saline site near Lahore showed best growth till the age of 5 years, and an annual water use of 1393 mm. Irrigated Eucalyptus microtheca at this site and unirrigated E. camaldulensis dependent on saline groundwater at Pacca Anna also transpired over 1000 mm of water per year. Basal area growth of Acacia ampliceps at the latter site was similar to E. camaldulensis, but its water use was less. Lowest annual water use of 235 mm was shown by an understocked stand of Prosopis juliflora. Canopy conductance decreased with increasing vapour pressure deficit to a species-dependent minimum value. Results of soil sampling, chloride balance modelling and intensive monitoring of soil solution salinity demonstrated accumulation of salt in the root zone of plantations using saline groundwater. The concentration of stored salt varied seasonally as a result of water table fluctuations and redistribution processes within the unsaturated zone. The apparent limitation of salt accumulation by these processes and the continuing satisfactory growth of the plantations justify cautious support of tree growing as a control measure for shallow water tables and salinisation in Pakistan.     

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.     

The effect of deficit irrigation with treated wastewater on sweet corn: experimental and modelling study using SALTMED model

This study investigated the impact of using treated wastewater and deficit irrigation on yield, water productivity, dry matter and soil moisture availability. The experiment included six treatments of deficit irrigation with treated wastewater during the 2010 and 2011 seasons and two deficit irrigation treatments combined with 3 organic amendment levels during the 2012 season. The experimental and SALTMED modelling results indicated that regulated deficit irrigation when applied during vegetative growth stage could stimulate root development, increase water and nutrient uptake and subsequently increase the yield. The organic amendment has slightly improved yield under full irrigation but had relatively small effect under stress conditions. The SALTMED model results supported and matched the experimental results and showed similar differences among the different treatments. The model proved its ability to predict soil moisture availability, yield, water productivity and total dry matter for three growing seasons under several deficit irrigation strategies using treated wastewater. The high values of the coefficient of determination R ² reflected a very good agreement between the model and observed values. The SALTMED model results generally confirm the model’s ability to predict sweet corn growth and productivity under deficit irrigation strategies in the semi-arid region.     

Usefulness of trunk diameter variations as continuous water stress indicators of pomegranate (Punica granatum) trees

Pomegranate trees (Punica granatum L.) is a deciduous fruit tree included in the so-called group of minor fruit tree species, not widely grown but of some importance in the south east of Spain. Pomegranate trees are considered as a culture tolerant to soil water deficit. However, very little is known about pomegranate orchard water management. The objective of this research was to asses the feasibility of using trunk diameter variation (TDV) indexes, obtained by means of LVDT sensors, as a plant water stress indicators for pomegranate trees. The experiment was carried out with mature trees grown in the field under three irrigation regimes: control well watered trees; trees continuously deficit irrigated at 50% of the control regime (SDI); and trees that had a summer water stress cycle being irrigated at 25% of the control rates only in July and August (RDI). The seasonal variations of maximum diurnal trunk shrinkage (MDS) and trunk growth rates (TGR) were compared with midday stem water potential (Ψstem) measurements. During the course of the entire season, control trees maintained lower MDS values than the SDI ones. In the RDI treatment, as water restrictions began, there was a slow increase in MDS, in correspondence with a decrease in Ψstem. When water was returned at full dosage, the RDI quickly recovered to MDS and Ψstem values similar to the control. However, lower MDS for a given Ψstem values were observed as the season advanced. The magnitude of differences between well watered and deficit irrigated trees was much larger in the case of MDS than for Ψstem. However, the tree-to-tree variability of the MDS readings was more than four times higher than for Ψstem; average coefficient of variation of 7.5 and 36% for Ψstem and MDS, respectively. On the other hand, TGR did not clearly reflect differences in tree water status. Overall, results reported indicated that MDS is a good indicator of pomegranate tree water status and it can be further used for managing irrigation. However, the seasonal changes in the MDS-Ψstem relationship should be taken into account when attempting to use threshold MDS values for scheduling irrigation.     

Olive tree response to irrigation with wastewater from the table olive industry

A field experiment was conducted to examine the effect of drip irrigation using wastewater from a table olive industry on physiological, nutritional and yield parameters of olive trees (Olea europaea L.). Very limited information, if any, exists, on the potential of recycling this kind of wastewater in agriculture. Two types of wastewater were used in the experiment, the first with SAR and EC values of 12–56 and 3.5–4.2 dS m⁻¹, respectively, and the second 73–90 and 4.3–6.0 dS m⁻¹. In general, this kind of wastewater has a highly variable composition and SAR values that are too high for agricultural purposes. Olive trees rapidly responded to wastewater application. Compared to the control (fresh water), the more saline wastewater caused important decreases in leaf water potential, stomatal conductance to H₂O and the photosynthesis rate after only 15 days of irrigation, the reduction being more pronounced after 2 months of irrigation. This treatment also caused a rapid, significant reduction in leaf N concentration, as compared with the N level in the trees before irrigation. Both types of wastewater significantly reduced olive yield, compared to that obtained in the control. These results indicate that this kind of wastewater is unsuitable for application to olive orchards under irrigation. Received: 16 August 1999     

Agronomic response and water productivity of almond trees under contrasted deficit irrigation regimes

We investigated the long-term effects of different deficit irrigation (DI) options on tree growth, shoot and leaf attributes, yield determinants and water productivity of almond trees (Prunus dulcis, cv. Marta) grown in a semiarid climate in SE Spain. Three partial root-zone drying (PRD) irrigation treatments encompassing a wide range of water restriction (30%, 50% and 70% of full crop requirements, ET_c) and a regulated deficit irrigation treatment (RDI, at 50% ET_c during kernel-filling) were compared over three consecutive growth seasons (2004–2006) to full irrigation (FI). The results showed that all deficit irrigation treatments have a negative impact on trunk growth parameters. The magnitude of the reduction in trunk growth rate was strongly correlated through a linear relationship with the annual volume of water applied (WA) per tree. Similarly, a significant relationship was found between WA and the increase in crown volume. In contrast, leaf-related attributes and some yield-related parameters (e.g., kernel fraction) were not significantly affected by the irrigation treatments. Except in PRD₇₀, individual kernel weight was significantly reduced in the deficit irrigated treatments. Kernel yield, expressed in percent of the maximum yield observed in the FI treatment, showed a linear decrease with decreasing WA and a slope of 0.43, which implies that a 1% decrease in water application would lead to a reduction of 0.43% in yield. Water productivity increased drastically with the reduction of water application, reaching 123% in the case of PRD₃₀. Overall, our results demonstrate the prevalence of direct and strong links between the intensity of the water restriction under PRD – i.e., the total water supply during the growing season – and the main parameters related to tree growth, yield and water productivity. Noteworthy, the treatments that received similar annual water volumes under contrasted deficit irrigation strategies (i.e., PRD₇₀ and RDI) presented a similar tree performance.     

Response of vegetative growth and fruit development to regulated deficit irrigation at different growth stages of pear-jujube tree

In order to investigate the response of vegetative growth, fruit development and water use efficiency to regulated deficit irrigation at different growth stages of pear-jujube tree (Zizyphus jujube Mill.), different water deficit at single-stage were treated on field grown 7-year old pear-jujube trees in 2005 and 2006. Treatments included severe (SD), moderate (MD) and low (LD) water deficit treatments at bud-burst to leafing (I), flowering to fruit set (II), fruit growth (III) and fruit maturation (IV) stages. Compared to the full irrigation (control), different water deficit treatments at different growth stages reduced photosynthesis rate (P_n) slightly and transpiration rate (T_r) significantly, thus it improved leaf water use efficiency (WUE_L, defined as the ratio of P_n to T_r) by 2.7-26.1%. After the re-watering, P_n had significant compensatory effect, but T_r was not enhanced significantly, thus WUE_L was improved by 31.4-42.2%. I-SD, I-MD, II-SD and II-MD decreased new shoot length, new shoot diameter and panicle length by 8-28%, 13-23% and 10-31%, respectively. Simultaneously, they reduced leaf area index (LAI) and pruning amount significantly. Flowering of pear-jujube tree advanced by 3-8 days in the water deficit treatments at stage I, Furthermore, SD and MD at stage I increased flowers per panicle and final fruit set by 18.9-40.5% and 15.5-36.6%, respectively. After a period of re-watering, different water deficit treatments at different growth stages improved the fruit growth rate by 15-30% without reduction of the final fruit volume. Compared to the control, I-MD, I-SD, I-LD, I-MD and I-SD treatments increased fruit yield by 13.2-31.9%, but reduced water consumption by 9.7-17.5%, therefore, they enhanced water use efficiency at yield level (WUE_Y, defined as ratio of fruit yield to total water use) by 17.3-41.4%. Therefore, suitable period and degree of water deficit can reduce irrigation water and restrain growth redundancy significantly, and it optimize the relationship between vegetative growth and reproductive growth of pear-jujube trees, which maintained or slightly increased the fruit yield, thus water use efficiency was significantly increased.