Moderate Levels Of Water Stress Research Articles

Influence of sustained deficit irrigation and foliar kaolin application on almond kernel composition

Deficit irrigation and kaolin foliar application are agronomic practices that have been demonstrated to improve productivity and physiological response in almond trees but there is no consistent information on the effects on the kernel composition. The objective of this study was to evaluate the effect of different Sustainable Deficit Irrigation (SDI) strategies and kaolin application on some physicochemical composition of the cv Ferragnès almond (Prunus dulcis (Mill.) D.A. Webb). A randomized block design with five treatments was developed: non-irrigation (NI), non-irrigation with foliar kaolin application (NI+K), full irrigation (FI), and two levels of SDI receiving 70% and 35% of evapotranspiration (SDI75 and SDI35). Pomological parameters, volatile profile and oil composition were analyzed. SDI effect on kernel weight was reduced against full irrigation (FI) but substantially improved in comparison with the rainfed treatments (NI and NI+K). Irrigation treatments showed a lower volatile compounds concentration in comparison with rainfed treatment. The fatty acid composition of the oil was significantly affected, with all the irrigated treatments having higher oleic acid contents than the rainfed treatments, and therefore an expected increased shelf life. Kaolin application had no positive effects on the non-irrigated trees, other than an apparent increase in the total volatiles, a tendency also observed in the NI. Moderate water stress levels (SDI35) improved the synthesis of tocopherols compared to NI, FI and SDI70, which could be related to the adaptation of the almond tree to drought. SDI35 is therefore regarded as a very interesting approach, with significant improvements in comparison with non-irrigated trees, and a clear saving on water against full irrigation without significant pomological and chemical alterations.

Exogenous Growth Regulators and Water Stress Enhance Long-Term Storage Quality Characteristics of Onion

Exogenous growth regulators (GRs) play a crucial role in alleviating water stress and sustaining crop yields in water-stressed areas. However, their effects on onions post-harvest quality, particularly post-monsoon onion—often preferred for long-term storage—were never studied. Therefore, this led us to investigate the interaction between water stress and GRs on the physiochemical and functional quality attributes of onions during long-term storage (9 months, at 25 ± 1 °C and 65 ± 5% RH). Onion crop was raised under four water stress levels i.e., 1.00–0.85, 0.84–0.70, 0.69–0.40, and 0.39–0.10 IW: CPE, designated as no, low, medium, and severe water stress, respectively, using a line source sprinkler system (LSS). GR treatments include potassium nitrate (PN, 15 g L−1), sodium benzoate (SB, 100 mg L−1), thio-urea (TU, 450 ppm), and gibberellic acid (GA, 25 ppm). Results reveal that the significant temporal changes in the dry matter, rehydration ratio, total soluble sugar (TSS), protein, and total phenolics content (TP) of the onion bulbs during storage, indicate the cumulative impact of the treatments on overall physicochemical status. Water stress increased onion biochemical attributes, especially pyruvic acid content, superoxide dismutase (SOD), and peroxidase (POD) activity. Storage quality of onions progressively decreased with the increase in storage period. Stressed onions, especially those produced under severe water stress condition, showed high weight losses, presenting poor keeping quality. However, application of GRs, especially SB, TU, and PN, reduced bulb weight losses together with maintaining slightly better bulb physicochemical properties, thereby improving the overall storage quality, particularly with a moderate level of water stress (0.69–0.40 IW:CPE). The exogenous application of GRs with moderate water stress is suggested as a key strategy in improving the keeping quality of onion bulbs and ensuring its availability during the lean season.

A study on the physiological parameters of corn during the jointing stage of growth under soil water stress based on the PSII light quantum yield (QY)

Water stress is a major factor limiting corn production in the semi-arid and semi-humid regions of the world. Well-scheduled irrigation is necessary to increase seed yield and ensure yield stability in these regions. A reasonable and intelligent irrigation system is generally based on the measurement of soil water content or the monitoring of corn water demand. Therefore, it is vital to be able to quickly and accurately determine the optimal threshold of the physiological parameters of corn.In this paper, in order to find the optimal threshold of the physiological parameters of corn, experiments in the open fields were conducted from February to June by measuring the corn growth under different degrees of water stress using remote sensing instruments. In this experiment, sweet corn 18# was chosen as the experimental subject, and several parameters of remote sensing spectral index such as leaf water potentia, Normalized difference vegetation index (NDVI), photochemical vegetation index (PRI), and PSII light quantum yield (QY) were measured at the same time every day-along with the varying moisture status—for the duration of the experiment. For this experiment, it was very important to set the varying moisture statuses as the moisture controlling standard. Leaf water potential and QY were selected as indexes to check the different moisture statuses. The QY proved to be sensitive to the water stress of corn and was chosen as the indicator. Furthermore, the effects on the corn under differing levels of water stress were explored by analyzing the variation trend of different remote sensing spectral indices. The main results and conclusions are as follows:(1) Differing degrees of water stress at the jointing stage of corn growth will cause the inhibition (in varying degrees) of leaf water potential, QY, NDVI, and PRI; and will also inhibit the processes of transpiration and photosynthesis. (2) After halting the water stress, with the different levels of water replenishment, the leaf water potential, QY, NDVI, and PRI will display the characteristics of the compensation phenomena. The compensation levels will depend on the degree of water stress. When the water stress is mild, all of the leaf water potential indexes, QY, NDVI, PRI will recover to normal levels or even succeed normal levels in a very short period of time-known as the super compensation effect. Meanwhile, for moderate levels of water stress, the compensation phenomena is also present; however, the replenishment of water cannot reach normal levels. Further, the water replenishment effect does not appear under conditions of severe water stress. This indicates that it is beneficial in maize growth for the corn to be under conditions of mild water stress.(3) Lastly, from the significance of the QY, water potential, PRI, and NDVI to water stress, it is apparent that the QY is the most reliable. Therefore, QY is the most reasonable choice to guide the water-saving irrigation in irrigated farming areas.

Deficient irrigation of vegetable crops

Water is one of the main factors for the production of vegetable crops due to its crucial role in the absorption and transport of nutrients, temperature regulation and some physiological processes, including photosynthesis. Considering the growing needs of a growing population for food and nutrients, a significant part of agricultural research is focused on improving the use efficiency (WUE - Water use efficiency) and saving water without reducing yields. Given the difficulty of increasing WUE through selection due to the tradeoff between photosynthesis and transpiration, agronomic strategies are needed. Due to the shallow root system and the fresh sale of vegetable products, vegetable crops are relatively more sensitive to moisture than field crops. Deficit irrigation (DI) is a direct approach to saving water by reducing irrigation to increase water productivity (WP - Water productivity). Regulated deficit irrigation (RDI) and partial root drying (PRD) are two widely used DI planning methods along with the classic DI approach. We searched the peer-reviewed literature for studies reporting yield differences in vegetable crops subjected to irrigation deficits. The search was conducted on Google Scholar and Web of Science using various combinations of the following keywords: vegetable crop yield and irrigation deficit or moisture shortage or water shortage or drought. Moderate levels of water stress (<65% FI) adversely affect vegetable yields regardless of external factors such as soil structure, climate and production system. This level of irrigation is justified for regions where water restrictions are already in place or forecast. Based on WP growth, irrigation levels as low as 35–50% FI for tomato and pepper, and 50–65% FI for onions and <35% FI for eggplant and watermelon are optimal for areas with severe water scarcity. However, a higher water deficit may also have an adverse effect on quality in terms of reduced fruit/root/bulb size.

Appropriate Time to Measure Leaf and Stem Water Potential in North-South Oriented, Vertically Shoot-Positioned Vineyards

The diurnal changes in vine water status and the appropriate time of day to measure leaf water potential (LWP) and stem water potential (SWP) were examined in Willamette Valley vineyards with north-south (N-S) oriented rows employing a single curtain, vertically shoot-positioned (VSP) canopy. Measurements of LWP and SWP were performed on Chardonnay and Pinot noir grapevines on seven cloudless days between bloom and harvest over two years. On warm days, LWP reached the daily minimum value by midday (1300 hr) and remained there for a longer duration when vines experienced moderate water stress (LWP -1.20 MPa). However, on cool days, LWP reached the daily minimum later in the day (1400 hr to 1500 hr) in both stressed and unstressed vines. SWP reached the daily minimum level late in the day (1400 hr to 1600 hr) under all conditions and even increased between late morning and midday on two occasions. Thus, measuring SWP at midday consistently underestimates the greatest level of water stress experienced by vines in N-S oriented, VSP canopies. Results of this study show that LWP can be determined over a four-hour period starting at midday on warm sunny days when vines experience a moderate level of water stress: conditions when it is most critical to assess vine water status to schedule irrigation. SWP should be measured in the two-hour period between 1500 hr and 1700 hr under all conditions tested here in N-S oriented, VSP canopies.

Intra-specific variability in deep water extraction between trees growing on a Mediterranean karst

Plant transpiration is a major component of water fluxes in the critical zone, which needs to be better characterized to improve our ability to understand and model the hydrological cycle. In water-limited ecosystems such as those encountered on karst environments, climate-induced changes in transpiration are expected to be strongly influenced by the ability of the vegetation cover to resist or adapt to drought. However, because of the high heterogeneity of karst environments, the amount of water available for trees can change within a stand, which may lead to significant differences in drought vulnerability resistance between trees of the same species. So far it is not known if soil and subsoil environment influence the magnitude of deep water extraction, at the intra-specific scale. Here, we investigate the variability in deep water extraction for six individual Quercus ilex trees growing on a karst substrate in a Mediterranean forest. We combined three approaches: (i) electrical resistivity tomography to determine the variability of soil/subsoil characteristics, (ii) isotope tracing to determine the origin of water transpired by plants, and (iii) predawn and midday leaf water potential (Ψ) to assess the trees’ water stress and transpiration regulation. Along the summer season, deep water extraction increased with drought intensity. Deep water use varies between individuals and according to drought intensity. At moderate water stress levels, we found no significant relationship between the origin of xylem water and soil/subsoil characteristics or individual stress level. However, at the peak of the drought (average predawnΨ < −2MPa), individuals that had the least total available water in soil/subsoil (0–2 m) relied more on deep water and were also subject to less water stress. These results suggest that trees with less favorable soil/subsoil conditions (i.e. low water retention capacity) in the near surface (0–2 m) adapt their root systems to exploit deep water reserves more intensively so as to enhance their drought tolerance, while trees with more favorable surface conditions exhibit greater water stress and may be more vulnerable to extreme droughts because of a lower root development in deeper horizons.

Declining root water transport drives stomatal closure in olive under moderate water stress.

Efficient water transport from soil to leaves sustains stomatal opening and steady-state photosynthesis. The aboveground portion of this pathway is well-described, yet the roots and their connection with the soil are still poorly understood due to technical limitations. Here we used a novel rehydration technique to investigate changes in the hydraulic pathway between roots and soil and within the plant body as individual olive plants were subjected to a range of water stresses. Whole root hydraulic resistance (including the radial pathway from xylem to the soil-root interface) constituted 81% of the whole-plant resistance in unstressed plants, increasing to >95% under a moderate level of water stress. The decline in this whole root hydraulic conductance occurred in parallel with stomatal closure and contributed significantly to the reduction in canopy conductance according to a hydraulic model. Our results demonstrate that losses in root hydraulic conductance, mainly due to a disconnection from the soil during moderate water stress in olive plants, are profound and sufficient to induce stomatal closure before cavitation occurs. Future studies will determine whether this core regulatory role of root hydraulics exists more generally among diverse plant species.

Upscaling Evapotranspiration with Parsimonious Models in a North Carolina Vineyard

Water stress can positively or negatively impact grape yield and yield quality, and there is a need for wine growers to accurately regulate water use. In a four-year study (2010–2013), energy balance fluxes were measured with an eddy-covariance (EC) system in a North Carolina vineyard (Vitis vinifera cv. Chardonnay), and evapotranspiration (ET) and the Crop Water Stress Index (CWSI) calculated. A multiple linear regression model was developed to upscale ET using air temperature (Ta), vapor pressure deficit (VPD), and Landsat-derived Land Surface Temperature (LST) and Enhanced Vegetation Index (EVI). Daily ET reached values of up to 7.7 mm day−1, and the annual ET was 752 ± 59 mm, as measured with the EC system. The grapevine CWSI was between 0.53–0.85, which indicated moderate water stress levels. Median vineyard EVI was between 0.22 and 0.72, and the EVI range (max–min) within the vineyard was 0.18. The empirical models explained 75%–84% of the variation in ET, and all parameters had a positive linear relationship to ET. The Root Mean Square Error (RMSE) was 0.52–0.62 mm. This study presents easily applicable approaches to analyzing water dynamics and ET. This may help wine growers to cost-effectively quantify water use in vineyards.

Water Deficit Synchronizes Berry Color Development in Crimson Seedless Table Grapes

Crimson Seedless is one of the most important table grape cultivars in the world, but often exhibits uneven berry color when grown in warm climates. Deficit irrigation is used extensively by growers during the ripening phase to advance fruit maturity and color, but there is a lack of information about the relationship between irrigation practices and fruit ripening variability. We imposed deficit irrigation in a commercial Crimson Seedless vineyard in the Maipo Valley, Chile, from veraison to harvest in two consecutive seasons. The fruit was tested for uniformity of Brix, firmness, and the color parameters “L”, “a”, “b”, and the Color Index of Red Grapes using analysis of variance on absolute residuals (Levene’s test). Postveraison water stress increased water productivity, Brix, and slightly improved berry coloration, but did not affect berry weight, size, or firmness. Moderate levels of water stress improved color uniformity at harvest, as lower values of leaf water potential were associated with a lower percentage of green berries that never matured. These results confirmed the role of deficit irrigation in the table grape ripening process. In contrast, extending the fruit ripening period by delaying harvest beyond 18 Brix did not increase color accumulation, but did increase berry shatter.

Growth and yield of sugarcane genotypes are strongly correlated across irrigated and rainfed environments

Sugarcane is a high biomass crop grown mostly in rain-fed environments. Water is a key determinant of sugarcane productivity, and in most production regions, varying and sometimes severe water stress occurs. Understanding the extent of genetic variation in response to water stress may allow for better targeted breeding programs. In this study a set of field experiments evaluating 31 sugarcane clones under rain-fed and irrigated conditions was conducted in two sugarcane production areas in Yunnan province, South China. While the water treatments had a large impact on mean cane yield there were little or no genotype×water treatment interactions for cane yield or sugar content at harvest, or for dry biomass sampled about five months (early biomass) after the start of crop growth. The high genetic correlation between well-watered and water stress treatments is consistent with the hypothesis that clone performance under water stress is mostly associated with general vigour rather than traits acting specifically under water stress. From a practical breeding program perspective the results suggest that choice or management of water stress level in selection trials is not a critical consideration for sugarcane breeding programs, and that selection under reasonably well watered conditions will be effective for attaining satisfactory selection gains under at least moderate levels of water stress. Early biomass was positively correlated with yield at final harvest for all water treatments. Resolution of the physiological basis of the high genetic correlations between growth under well-watered and water limited conditions may assist in future breeding efforts.

Life cycle assessment of the City of Atlanta, Georgia’s centralized water system

The main goal of this study was to conduct a life cycle assessment (LCA) for the City of Atlanta’s (CoA) centralized water system in order to understand the sources of the largest environmental impacts in the system. The boundaries of the water system we studied included the water supply system, the wastewater collection and treatment system, and the stormwater collection system. The CoA has separate wastewater and stormwater collection systems; therefore, they are considered separately. We defined the functional unit (FU) of our study to be 1 m3 of water distributed to the point-of-use, and our scope includes infrastructure construction and decommissioning in addition to system operation. We used the US Life Cycle Inventory (USLCI) and ecoinvent databases to inventory all emissions and resource use. We determined the major impacts of the water system and the sources that create these impacts using TRACI v2.1, which is the life cycle impact assessment (LCIA) methodology developed for North America by the Environmental Protection Agency (EPA). In addition, a method to evaluate the impact of freshwater consumption on ecosystem was used in this study because it is excluded in TRACI v2.1. The four major impacts of the CoA’s centralized water system were carcinogenic effects, ecotoxicity, non-carcinogenic effects, and eutrophication; respectively, they contribute 62, 8, 6, and 5 % of the average impacts of a US resident per year (2008) for the annual water use of a single-family home resident. Notably, the input which causes 68 % of the carcinogenic effects, which has the largest normalized impact, was from infrastructure construction which is often ignored in the literature. Freshwater consumption had a relatively low impact, with an evaporative water loss of 0.23 m3 per FU and a moderate level of water stress on the middle Chattahoochee River Basin; however, the impact of freshwater consumption would rapidly increase with population growth due to increased water consumption and expansion of the impervious area in the CoA. This study is distinctly different from other LCA studies because it includes infrastructure construction, which was a major contributor to the water system’s environmental impacts. Our results suggest that infrastructure construction should be considered in LCA studies for centralized water systems. It is worth noting, however, that European datasets were used to determine emissions and resource use related to infrastructure. Results would be more accurate if inventory datasets for US infrastructure were available.

Canopy temperature for peach tree at various soil water contents

Canopy temperature measurements with infrared thermometry have been extensively studied as a means of assessing plant water status for field and row crops. Achieving high quality peach fruit depends on the ability to maintain mild to moderate levels of water stress in the crop during the growing season. The paper examined the spatial distribution of tree canopy temperature (Tc) using thermal images in a peach orchard for irrigation scheduling. The variation of Tc was investigated in three irrigation regime treatments (factor A) that produced various soil moisture content (SMC) values, three cardinal points (factor B): South, North and East-West aspects combined, and five up-down vertical position measurements (factor C: upper, middle upper, middle, middle lower and lower) across the tree canopy thermal images. It was found that Tc was significantly influenced by the irrigation regime. Cardinal point showed a significant Tc difference between South on the one hand and the other aspects. The vertical position within canopy image did not significantly influence Tc.

Water relations of pistachio ( Pistacia vera L.) as affected by phenological stages and water regimes

Pistachio is a drought tolerant fruit tree that can be cultivated in rainfed and irrigated conditions. The water requirements of the tree, however, are considerable so in most of the commercial orchards deficit irrigation is a common practice. Regulated deficit irrigation in pistachio trees has been described in several works, which reported that the phenological stage of shell hardening, so called stage II, is the most drought tolerant. This paper proposes that such drought resistance is related to changes in water relations linked to the phenological stages, even in conditions of no water stress. In order to evaluate such changes, the daily pattern of stem water potential and gas exchange (net photosynthesis, P n, and leaf conductance, g s) was measured, determining also the pressure–volume curves, in three different phenological stages of mature pistachio trees ( Pistacia vera cv Kerman on P. terebinthus L. rootstock.). The daily pattern of stem water potential and gas exchange were performed in three different irrigation treatments: control, regulated deficit irrigation and rainfed. The pressure–volume curves were made only in the control and rainfed treatments. Significant differences were found in the daily pattern of stem water potential in all the phenological stages considered, while only in the last one the net photosynthesis was affected by water stress. The daily pattern of gas exchange at the beginning of the season was not affected by the evaporative demand, with a constant value when radiation was not limiting. Moderate levels of water stress during the last measurement date reduced the maximum values of g s and P n resulting also in a clear change in the pattern of the daily curve, with maximum values only at the beginning of the day. The relationships between stem water potential and gas exchange parameters were different during stage II and almost the same in stages I and III. The parameters drawn from the pressure–volume curves also indicated a change in the elastic modulus of the leaf cells in stage II. In addition, differences in the osmotic adjustment (OA) index suggested different degree of osmotic adjustment of the phenological stages in the response to water stress. The results showed that different mechanisms of drought resistance are operating in the different phenological stages in pistachio trees.

Evaluating water stress in irrigated olives: correlation of soil water status, tree water status, and thermal imagery

Irrigation of olive orchards is challenged to optimize both yields and oil quality. Best management practices for olive irrigation will likely depend on the ability to maintain mild to moderate levels of water stress during at least some parts of the growing season. We examined a number of soil, plant and remote sensing parameters for evaluating water stress in bearing olive (var. Barnea) trees in Israel. The trees were irrigated with five water application treatments (30, 50, 75, 100 and 125% of potential evapotranspiration) and the measurements of soil water content and potential, mid-day stem water potential, and stomatal resistance were taken. Remote thermal images of individual trees were used to alternatively measure average canopy temperature and to calculate the tree’s crop water stress index (CWSI), testing empirical and analytical approaches. A strong non-linear response showing similar trends and behavior was evident in soil and plant water status measurements as well as in the CWSI, with decreasing rates of change at the higher irrigation application levels. No statistically significant difference was found between the analytical and the empirical CWSI, suggesting that the relative simplicity of the analytical method would make it preferable in practical applications.

Use of thermal and visible imagery for estimating crop water status of irrigated grapevine

Achieving high quality wine grapes depends on the ability to maintain mild to moderate levels of water stress in the crop during the growing season. This study investigates the use of thermal imaging for monitoring water stress. Experiments were conducted on a wine-grape (Vitis vinifera cv. Merlot) vineyard in northern Israel. Irrigation treatments included mild, moderate, and severe stress. Thermal and visible (RGB) images of the crop were taken on four days at midday with a FLIR thermal imaging system and a digital camera, respectively, both mounted on a truck-crane 15 m above the canopy. Aluminium crosses were used to match visible and thermal images in post-processing and an artificial wet surface was used to estimate the reference wet temperature (T(wet)). Monitored crop parameters included stem water potential (Psi(stem)), leaf conductance (g(L)), and leaf area index (LAI). Meteorological parameters were measured at 2 m height. CWSI was highly correlated with g(L) and moderately correlated with Psi(stem). The CWSI-g(L) relationship was very stable throughout the season, but for that of CWSI-Psi(stem) both intercept and slope varied considerably. The latter presumably reflects the non-direct nature of the physiological relationship between CWSI and Psi(stem). The highest R(2) for the CWSI to g(L) relationship, 0.91 (n=12), was obtained when CWSI was computed using temperatures from the centre of the canopy, T(wet) from the artificial wet surface, and reference dry temperature from air temperature plus 5 degrees C. Using T(wet) calculated from the inverted Penman-Monteith equation and estimated from an artificially wetted part of the canopy also yielded crop water-stress estimates highly correlated with g(L) (R(2)=0.89 and 0.82, respectively), while a crop water-stress index using 'theoretical' reference temperatures computed from climate data showed significant deviations in the late season. Parameter variability and robustness of the different CWSI estimates are discussed. Future research should aim at developing thermal imaging into an irrigation scheduling tool applicable to different crops.

Suitability of existing and novel spectral indices to remotely detect water stress in Populus spp.

Undetected water stress within poplar plantations can result in high economic losses. Although remotely sensed spectral measures associated with variations in plant water status have been developed for non- Populus species, little is known about the suitability of such spectral indices to identify water stress in Populus species. Our experiments assessed whether such spectral indices acquired at the leaf and canopy level were suitable for detecting water stress in Populus. Relationships between four spectral indices and four common measures of plant water status were analyzed for both low to moderate, and highly water stressed trees. The proposed maximum difference water index (MDWI) exhibited significant relationships ( P < 0.001) with changes in plant water status at both the leaf and canopy levels. At the leaf level, statistically significant, though poorer relationships were obtained between each of the normalized differential water index (NDWI), the red edge inflection point (REIP), the water index (WI), and the plant water status measures. At the canopy level, statistically significant relationships were obtained between MDWI and the measure of relative water content (RWC), and the equivalent water thickness (EWT) ( r 2 > 0.56, P < 0.001), while WI produced reasonable relationships with these water status measures ( r 2 > 0.42, P < 0.001). On exclusion of the highly stressed plants, only MDWI exhibited low sensitivity to RWC ( r 2 = 0.37, slope = 0.64) and leaf water potential at the canopy level ( r 2 = 0.42, slope = 9.89) whereas no other indices exhibited sensitivity to changes in plant water status. MDWI, which incorporates short-wave infrared (SWIR, 1300–2500 nm) spectral bands, is more suitable to detect more changes in plant water status as compared to NDWI, REIP, and WI that incorporate near infrared (NIR, 700–1300 nm) spectral bands. These results suggest that such remotely sensed indices are not a viable option to detect low and moderate levels of water stress at a scale of a Populus spp. plantation, although further research is warranted to assess the broader applicability of MDWI and other SWIR wavelength incorporating indices to detect water stress in other species.

Photosynthetic Responses of a Temperate Liana to Xylella fastidiosa Infection and Water Stress

Abstract Xylella fastidiosa is a xylem‐limited bacterial plant pathogen that causes bacterial leaf scorch in its hosts. Our previous work showed that water stress enhances leaf scorch symptom severity and progression along the stem of a liana, Parthenocissus quinquefolia, infected by X. fastidiosa. This paper explores the photosynthetic gas exchange responses of P. quinquefolia, with the aim to elucidate mechanisms behind disease expression and its interaction with water stress. We used a 2 × 2‐complete factorial design, repeated over two growing seasons, with high and low soil moisture levels and infected and non‐infected plants. In both years, low soil moisture levels reduced leaf water potentials, net photosynthesis and stomatal conductance at all leaf positions, while X. fastidiosa‐infection reduced these parameters at basally located leaves only. Intercellular CO2 concentrations were reduced in apical leaves, but increased at the most basal leaf location, implicating a non‐stomatal reduction of photosynthesis in leaves showing the greatest disease development. This result was supported by measured reductions in photosynthetic rates of basal leaves at high CO2 concentrations, where stomatal limitation was eliminated. Repeated measurements over the summer of 2000 showed that the effects of water stress and infection were progressive over time, reaching their greatest extent in September. By reducing stomatal conductances at moderate levels of water stress, P. quinquefolia maintained relatively high leaf water potentials and delayed the onset of photosynthetic damage due to pathogen and drought‐induced water stress. In addition, chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy that protects the photosynthetic machinery of leaves from irreversible photoinhibitory damage that may occur during stress‐induced stomatal limitation of photosynthesis. However, severe stress induced by disease and drought eventually led to non‐stomatal decreases in photosynthesis associated with leaf senescence.

Influence of Seed Size on Seedling Growth of Albizia procera Under Different Soil Water Levels

Albizia procera Benth. is an early successional leguminous tree species that occurs naturally in dry tropical forests in India. The growth response of seedlings of A. procera from seeds of different sizes was studied under four soil water levels. Seeds were surface sterilized, germinated and grown in a glasshouse for 3 weeks under optimal water supply, and were subsequently maintained at four soil water levels for 4 months. Soil matric potentials for 1,<$>{\ extstyle\\raise1.3pt{1\\over 2}}<$>,<$>{\ extstyle\\raise1.3pt{1\\over 3}}<$>and<$>{\ extstyle\\raise1.3pt{1\\over 4}}<$>water holding capacity (WHC) were −0.01, −0.1, −0.61 and −1.20MPa, respectively. Plant height, stem diameter and leaf area were measured fortnightly. Seedlings were harvested at the end of the experiment. Biomass, relative growth rate (RGR), stem weight ratio (SWR), root:shoot ratio (R:S ratio) and net assimilation rate (NAR) were determined. Seedlings from large seeds had a higher biomass and leaf area and were more tolerant of long-term extreme water stress compared to those from small seeds which were more tolerant of moderate levels of water stress. Seedlings from small seeds did not survive beyond 75d at<$>{\ extstyle\\raise1.3pt{1\\over 4}}<$>WHC. Increase in the R:S ratio along with a decrease in SWR reflected a shift in biomass allocation from above- to below-ground parts as soil water level decreased. SLA declined sharply with increasing water stress, which in turn led to a decrease in RGR. NAR increased with water stress. The study revealed the ability of the species to acclimate to a wide range of moisture stress by morphogenetic (increased R:S ratio and decreased SWR) and physiological (increased NAR) plastic responses.

THE RELATION OF MIDDAY STEM WATER POTENTIAL TO THE GROWTH AND PHYSIOLOGY OF FRUIT TREES UNDER WATER LIMITED CONDITIONS

Reductions in rates of both vegetative and fruit growth, and reductions in other physiological parameters such as leaf conductance and photosynthesis, are all well known responses of plants to water limited conditions. One simple hypothesis to explain the overall integration of these responses at the whole plant level is that a reduction in the availability of soil water to the roots will ultimately lead to a reduction in the water potential of the plant as a whole, and hence that a corresponding reduction in the turgor pressure of cells will occur. A reduction in cell turgor may lead, directly or indirectly, to many of the well known plant physiological responses to water limited conditions. A key observation that does not support this simple hypothesis is that under some water limited conditions, reductions in growth and stomatal conductance can occur in the absence of any detectable reduction in the water potential of fully exposed, transpiring leaves. This observation however, does not take into account the possibility that individual leaf stomatal responses may mask reductions in whole plant water potential. By using midday measurements of water potential made on non-transpiring leaves (midday stem water potential) we have found a clear relation between water potential and a number of physiological responses, such as fruit growth and leaf photosynthesis, across a range of fruit tree species growing under field conditions. In almond, overall tree size after two years of growth was linearly related to the average midday stem water potential measured during the same period. A similar linear relation was observed between fruit size and the seasonal average midday stem water potential measured in pear trees. In prune trees, despite an overall linear relation between water potential and light-saturated photosynthesis at the individual leaf level, evidence was found that whole canopy photosynthesis may be relatively unaffected by moderate levels of water stress (midday stem water potential around -1.5 MPa). We have found that moderate water stress is associated with improvements in fruit quality and economic return in prunes, and are currently using midday stem water potential to guide the management of RDI (regulated deficit irrigation) in this crop.

Deficit Irrigation in Prunes: Maintaining Productivity with Less Water

Prune (Prunus ×domestica L.) trees are believed to be relativelytolerant of water stress, and since the fruit are dried, a low fresh to dryweight ratio (“drying ratio”) of the fruit at harvest will reduce energyrequirements for drying, and improve economic returns to the grower.In previous research we found that, under some orchard conditions,irrigation deprivation was associated with a number of economicallybeneficial effects, including a lower drying ratio of the fruit, andincreased return bloom and final marketable crop yield. Analysis ofthese results was complicated by alternate bearing, and by the fact thattree water stress could differ substantially within the same irrigationregime, depending on soil conditions. After accounting for thesefactors, our analysis indicated that economic yield could be main-tained or increased by managing trees at a moderate level of waterstress. An experiment was established to determine whether middaystem water potential (Ψ