Relative Water Use Research Articles

Relative Growth and Water Use of Seedlings from Three Italian Quercus Species

A species’ water use characteristics and growth habits are important to urban foresters. Seedlings from three species (and two sources)—Quercus cerris L., Q. pubescens Willd., and Q. robur L.—were container-grown and subjected to a three day water use trial under nonlimiting soil moisture conditions. Water use varied among species and between sources within a species. Larger seedlings used more water than smaller seedlings; Q. robur seedlings were the tallest, 70.5 cm (27.8 in), and had the greatest water use seedling, 73.4 g (2.5 oz) water, while Q. pubescens and Q. cerris seedlings were shorter, 46 and 45 cm (18.1 and 17.7 in), and averaged 47.5 and 44.9 g (1.68 and 1.58 oz) water, respectively. Quercus pubescens seedlings had the highest water use cm-2 leaf area (0.111 g); Q. cerris seedlings had the highest height-adjusted water use (1.4 g water cm-1 height). There were significant differences in water use between sources within species. Principal component analysis, using 20 variables, showed that seedlings of Q. robur and Q. pubescens sources clustered while seedlings of the two Q. cerris sources were separate from each other and the Q. robur and Q. pubescens sources. The clustering reflected the proportionally greater distribution of dry weight to shoot growth and correspondingly less to root growth of the Q. robur and Q. pubescens seedlings, than that for Q. cerris seedlings. The results are related to the species’ relative value to nursery producers and to the potential value to urban forest managers and the potential for cultivar development.

Exogenous Glycinebetaine and Salicylic Acid Application Improves Water Relations, Allometry and Quality of Hybrid Sunflower under Water Deficit Conditions

AbstractLimited water availability hampers the sustainability of crop production. Exogenous application of glycinebetaine (GB) and salicylic acid (SA) has been found very effective in reducing the adverse effects of water scarcity. This study was conducted to examine the possible role of exogenous GB and SA application in improving the growth and water relations of hybrid sunflower (Helianthus annuus L.) under different irrigation regimes. There were three levels of irrigation, viz. control (normal irrigations), water stress at budding stage (irrigation missing at budding stage) and water stress at flowering stage (FS) (irrigation missing at FS). GB and SA were applied exogenously at 100 and 0.724 mm respectively, each at the budding and FS. Control plants did not receive application of GB and SA. Water stress reduced the leaf area index (LAI), leaf area duration (LAD), crop growth rate (CGR), leaf relative water contents, water potential, osmotic potential, turgor pressure, achene yield and water use efficiency. Nevertheless, exogenous GB and SA application appreciably improved these attributes under water stress. However, exogenous GB application at the FS was more effective than other treatments. Net assimilation rate was not affected by water stress as well as application of GB and SA. The protein contents were considerably increased by water stress at different growth stages, but were reduced by exogenous GB and SA application. The effects of water stress and foliar application of GB were more pronounced when applied at FS than at the budding stage. Moreover, exogenous GB application was only advantageous under stress conditions.

Physiological responses associated to substrate water availability of Rosa ‘Eurored’ plants grown in soilless greenhouse conditions

The aim of the present study was to provide a better insight into the adaptive processes to water deficit of Rosa hybrid plants grown in soilless cultivation. Rose plants were grown in a greenhouse under two water regimes, corresponding to 100% and 67% of the irrigation needs. Watering was stopped for 0, 72, and 96 h (dry-down period) and photosynthetic parameters, water relations, chlorophyll fluorescence, carbohydrate and ion content were determined. The imposed drought conditions, in both regimes, caused: (i) a negative effect on the photosynthetic rate and stomatal conductance but an increase in the intercellular CO 2, (ii) a decline in chlorophyll fluorescence (iii) a decrease in transpiration rate and leaf water potential (LWP) but had no effect on relative water content (RWC) and water use efficiency (WUE), (iv) an increase in carbohydrate and potassium content but no change in calcium. The data indicated that the adaptive responses of Rosa ‘Eurored’ to water stress combined with the dry-down program involved mechanisms at both stomatal and non-stomatal level. The plants maintained their metabolic and physiological processes not simply through stomatal closure but also through osmotic adaptation (higher carbohydrate and potassium concentration).

WATER FOOTPRINT AND CHALLENGES FOR ITS APPLICATION TO INTEGRATED WATER RESOURCES MANAGEMENT IN ROMANIA

Abstract This paper analyses and discusses the concept of water footprint, developed in order to have an indicator of water use in relation to consumption of people. The water footprint of a country is defined as the volume of water needed for the production of the goods and services consumed by the inhabitants of the country. The use of water resources comprises water use in the agricultural, industrial and domestic sectors. The overall objective of water management, which adopts integrated water resources management (IWRM) approaches is to satisfy the freshwater needs of all countries for their sustainable water management. The concept of water footprint has introduced a new and wider dimension in integrated water resources management. The analysis of water footprint trade is providing new perspectives for global water strategies and policies. Water footprint can provide complementary roles in the context of integrated water resources management (IWRM), due to the fact that the water footprint data raises awareness among the general public, government and stakeholders as to the environmental impact of societal activities. This paper discusses the various water problems such as (a) increasing water shortages, (b) deterioration of water quality and (c) stresses on water supplies and the challenges they represent to integrating the approaches of water management to ensure sustainability of the resource. The water footprint concept is relatively new in Romania and it has not been introduced in relationship to IWRM. In Romania the IWRM concept is associated with the EU Water Framework Directives, although it is not considered as being fully covered by the present water and environmental sector policies. This study is to be included in a complex research assessing the possibilities of application of the water footprint concept in Romania, especially by providing an assessment for its application in the North Eastern part of the country, i.e. to the Prut river basin.

Carbon and nitrogen parasitism by a xylem‐tapping mistletoe (Tapinanthus oleifolius) along the Kalahari Transect: a stable isotope study

AbstractThe present study explores the xylem‐tapping parasitism by mistletoe (Tapinanthus oleifolius) on native tree species along the Kalahari Transect (KT) using the stable isotopes of carbon and nitrogen. Mistletoe‐host pairs were collected at three geographical locations along the KT rainfall gradient in the 2005 and 2006 wet seasons. Foliar total carbon, total nitrogen and their stable isotope compositions (δ13C and δ15N) were measured. Heterotrophy (H) was calculated using foliar δ13C values of mistletoes and their hosts as an indicator of proportion of carbon in the mistletoes derived from host photosynthate. Based on the mistletoe H‐value and relationship between the mistletoe foliar δ15N and their host foliar δ15N, the results showed that mistletoes along the KT derived both nitrogen and carbon from their hosts. Mistletoes may regulate water use in relation to nitrogen supply. The proportion of carbon in the mistletoes derived from host photosynthate was between 35% and 78%, and the degree of heterotrophy was species‐specific with only limited annual variation. The study emphasizes the importance of incorporating parasitic associations in future studies on studying carbon, water and nutrient cycling along the Kalahari.

Response of Young Apple Trees to Grass and Irrigation

ABSTRACT Ground covers and irrigation are important components of orchard floor management systems that affect fruit tree vigor and productivity. Three experiments were conducted in a greenhouse to determine the relative water use of candidate ground covers (roughstalk bluegrass, RB, Poa trivialis), Chewing's fescue (CH, Festuca rubra subsp. commutata Gaudin), creeping red fescue (RF, Festuca rubra L. subsp. rubra), tall fescue (TF, Festuca arundinacea Schreber, Fawn), and perennial ryegrass (PR, Lolium perenne L., ‘Saint’) and the response of apple trees to those ground covers and to drip irrigation applied at two soilless substrate depths. Grass ground covers with large and deep root systems (TF and PR) used more water than a shallow- rooted grass (RB) and leaf water potential decreased more rapidly in apple trees grown with TF than RB when irrigation was withheld. Although apple tree shoot growth was greater with shallow- than deep-rooted grass, photosynthesis, transpiration, and root biomass distribution were not differentially affected by grass type. When grown with RB or TF, irrigation depth affected apple tree growth. During the first season in the greenhouse, deep irrigation at 37 cm depth increased apple root length density near emitters but shoot growth was less in apple grown with deep irrigation compared with apple grown with surface irrigation (0 cm) and with split irrigation at 0 and 37 cm. During the second season in the greenhouse, deep irrigation was beneficial to trees grown with grass that had large, deep root systems (TF) but it did not completely overcome interference effects of grass on apple trees, regardless of grass root system size or distribution. The results indicate that grasses with shallow root systems may be grown beneath apple trees and that split irrigation at two depths can provide flexibility that is necessary for water management of ground covers and apple trees.

High crop productivity with high water use in winter and summer on the Liverpool Plains, eastern Australia

We report exceptional productivity and associated water-use efficiency across seasons for commercial crops of rainfed spring wheat and grain sorghum growing on stored soil water in Vertosols on the Liverpool Plains, central-eastern Australia. Agreement between the independently measured terms of evapotranspiration (ET) and the soil water balance (in-crop rainfall + δsoil water) was achieved within acceptable uncertainty across almost all measurement intervals, to provide a reliable dataset for the analysis of growth and water-use relationships without the confounding influence of water outflow either overland or within the soil. Post-anthesis intrinsic transpiration efficiency (kc ) values of 4.7 and 7.2 Pa for wheat and sorghum, respectively, and grain yields of 8 and 7 t/ha from ET of 450 and 442 mm (1.8 and 1.6 g/m2.mm), clearly demonstrate the levels of productivity and water-use efficiency possible for well-managed crops within an intensive and productive response cropping sequence. The Vertosols in which the crops were grown enabled rapid and apparently unconstrained delivery of significant quantities of subsoil water (34% and 51% of total available) after anthesis, which enabled a doubling of pre-anthesis standing biomass and harvest indices of almost 50%. Durum wheat planted into only 0.30 m of moist soil and enduring lower than average seasonal rainfall, yielded less biomass and grain (2.3 t/ha) with lower water-use efficiency (0.95 g/m2.mm) but larger transpiration efficiency, probably due to reduced stomatal conductance. We argue that crop planting in response to stored soil water and management for high water-use efficiency to achieve high levels of average productivity of crop sequences over time can have a significant effect on both increased productivity and enhanced hydrological stability across alluvial landscapes.

Scion photosynthesis and leaf gas exchange in Vitis vinifera L. cv. Shiraz: Mediation of rootstock effects via xylem sap ABA

Limited water resources are placing pressure on the wine grape industry to improve vineyard water use efficiency by continuing improvements in vineyard management. One way to improve vineyard water use efficiency is to improve transpiration efficiency, and judicious use of scion-rootstock combination can play a role. Accordingly, this paper examines the influence of rootstocks on the water use physiology of a common scion variety (Shiraz) and assesses the importance of variation in xylem sap abscisic acid (ABA) as an indicator of relative water use efficiency of different scion-rootstock combinations. Shiraz scions (clone BVRC12) grafted to seven rootstock varieties (along with an ungrafted control), were observed over two consecutive seasons in a field trial located in Adelaide, South Australia. Those two seasons (2001/02 and 2002/03) offered sharp contrasts with respect to soil and atmospheric water stress. Rootstock genotype had a significant impact on scion gas exchange, water status, canopy growth and yield in both seasons. Those impacts were associated with an inverse relationship between xylem sap ABA and stomatal conductance. We suggest that strategic analysis of xylem sap ABA may provide a marker for comparative water-use physiology of different stock-scion combinations, while recognising that irrigation pre-history, and associated degree of vine water stress, will be a further source of variation in grapevine water use.

Water footprints of nations: Water use by people as a function of their consumption pattern

The water footprint shows the extent of water use in relation to consumption of people. The water footprint of a country is defined as the volume of water needed for the production of the goods and services consumed by the inhabitants of the country. The internal water footprint is the volume of water used from domestic water resources; the external water footprint is the volume of water used in other countries to produce goods and services imported and consumed by the inhabitants of the country. The study calculates the water footprint for each nation of the world for the period 1997-2001. The USA appears to have an average water footprint of 2480 m 3 /cap/yr, while China has an average footprint of 700 m 3 /cap/yr. The global average water footprint is 1240 m 3 /cap/yr. The four major direct

Changes in Summer Irrigated Crop Area and Water Use in Southeastern Turkey from 1993 to 2002: Implications for Current and Future Water Resources

Changes in summer irrigated cropland acreage and related water use are estimated from satellite remote sensing and ancillary data in semi-arid Southeastern Turkey where traditionally dry agricultural lands are being rapidly transformed into irrigated fields with the help of water from the Euphrates-Tigris Rivers. An image classification methodology based on thresholding of Landsat NDVI images from the peak summer period reveals that the total area of summer irrigated crops has increased three-fold (from 35,000 ha to over 100,000) in the Harran Plain between 1993 and 2002. Coupled analysis of annual irrigated crop area from remote sensing and potential evapotranspiration based estimates of irrigation water requirements for cotton indicate a corresponding increase in agricultural water use from about 370 million cubic meters to over one billion cubic meters, a volume in accordance with the state estimates. These estimates have important implications for understanding the rapid changes in current agricultural withdrawals in Southeastern Turkey and form a quantitative basis for exploring the changes in future water demands in the region. For example, expansion of irrigated lands have led to a steady decrease in potential evaporation due to increased roughness and decreased humidity deficit in the Harran Plain. Assuming that the changes in future evaporation conditions will be of similar nature, water use for irrigation is expected to decrease over 40 percent in future irrigation sites. Incorporating this decrease in overall planning of the irrigation projects currently under construction should lead to improved management, and by extension, sustainability of water resources in the region.

Predictors of use and consumption of public drinking water among pregnant women

Disinfection by-products (DBPs) in drinking water may be associated with adverse pregnancy outcomes. However, the results from previous epidemiological studies are not consistent, perhaps in part due to individual variation in water use and consumption. This study was performed to evaluate and describe demographic and behavioral characteristics as predictors of ingested water, showering, bathing, and swimming among pregnant women. Water use and consumption data were collected through telephone interviews with 2297 pregnant women from three geographical sites in the southern United States. The data were analyzed according to demographic, health, and behavioral variables expected to be predictors of water use and thus potential confounding factors relating water use to pregnancy outcome. The candidate predictors were evaluated using backward elimination in regression models. Demographic variables tended to be more strongly predictive of the use and consumption of water than health and behavior-related factors. Non-Hispanic white women drank 0.4 (95% confidence interval (CI) 0.2; 0.7) liters more cold tap water per day than Hispanic women and 0.3 (95% CI 0.1; 0.4) liters more than non-Hispanic black women. Non-Hispanic white women also reported drinking a higher proportion of filtered tap water, whereas Hispanic women replaced more of their tap water with bottled water. Lower socioeconomic groups reported spending a longer time showering and bathing, but were less likely to use swimming pools. The results of this study should help researchers to anticipate and better control for confounding and misclassification in studies of exposure to DBPs and pregnancy outcomes.

Controls on transpiration in a semiarid riparian cottonwood forest

Cottonwood ( Populus spp.) forests are conspicuous and functionally important elements of riparian vegetation throughout much of the western U.S. Understanding how transpiration by this vegetation type responds to environmental forcing is important for determining the water balance dynamics of riparian ecosystems threatened by groundwater depletion. Transpiration was measured in semiarid riparian cottonwood ( Populus fremontii) stands along a perennial and an intermittent reach of the San Pedro River in southeastern Arizona. Sap flow was measured using thermal dissipation probes and scaled to the stand level to investigate stand water use in relation to canopy structure, depth to groundwater and climate forcing. The cottonwood stand located at the perennial stream site had higher leaf-to-sapwood area ratio (0.31 ± 0.04 m 2 cm −2), leaf area index (2.75) and shallower groundwater depth (1.1–1.8 m) than the stand at the intermittent stream site (0.21 ± 0.04 m 2 cm −2, 1.75 and 3.1–3.9 m, respectively). Moreover, total annual transpiration was higher at the perennial stream site (966 mm) than at the intermittent stream site (484 mm). The significant positive and linear correlation between transpiration and vapor pressure deficit indicated high hydraulic conductance along the root–shoot pathway of cottonwood trees at the perennial stream site. During the peak dry period prior to the summer rainy season, the trees at the intermittent stream site exhibited greater water stress as transpiration did not increase beyond its mid-morning peak with increasing vapor pressure deficit, which was likely due to leaf stomatal closure. However, this stress was alleviated after significant monsoonal rains and runoff events had recharged soil moisture and raised groundwater levels. Riparian cottonwood forests are exposed to extreme fluctuations in water availability and transpiration demand throughout the growing season, and their access to shallow groundwater sources determines their structural and physiological responses to drought. Spatial and temporal variation in depth to groundwater induces drought stress in cottonwood threatening their productivity and existence along the river systems throughout much of western U.S.

Temperature and water availability effects on radiation and water use efficiencies in alfalfa (Medicago sativa L.)

Alfalfa, the most important forage crop in Argentina, shows considerable variability in forage production caused by variations in inter-annual rainfall and intra-annual radiation and temperature regimes. Such variation may affect radiation use efficiency and water use efficiency. This paper seeks to study the effects of temperature and water availability on radiation use efficiency and water use efficiency. We conducted the experiment in Córdoba, Argentina, under irrigated and droughted conditions. Drought was imposed by mobile rainout shelters during 3 consecutive periods. We measured forage, intercepted photosynthetically active radiation and water use to calculate radiation use efficiency and water use efficiency between cuttings. Under irrigation, radiation use efficiency and water use efficiency normalised by daytime vapour pressure deficit, were not limited by mean temperature above 21.3 and 21.9°C, respectively. Below those critical values, both variables decreased consistently with temperature decrements. Under drought, radiation use efficiency tended to decrease and water use efficiency tended to increase. In addition, the relationship between relative dry matter and relative water use was not linear, as reported in previous studies for annual crops.

PACLOBUTRAZOL AND WATER STRESS INDUCE MORPHOLOGICAL ADAPTATION IN PHILLYREA ANGUSTIFOLIA DURING HARDENING

ISHS VII International Symposium on Protected Cultivation in Mild Winter Climates: Production, Pest Management and Global Competition PACLOBUTRAZOL AND WATER STRESS INDUCE MORPHOLOGICAL ADAPTATION IN PHILLYREA ANGUSTIFOLIA DURING HARDENING

Evaluation of physiological traits as indirect selection criteria for drought tolerance in sugar beet

Drought is the most important limitation to sugar beet ( Beta vulgaris L.) production in the UK and other areas dependent on usually insufficient summer rainfall. As increased irrigation is not a viable answer to the problem, an economically and environmentally desirable solution is new varieties with decreased sensitivity to water deficits. However, there is little genotypic information on drought tolerance in sugar beet, and breeders are not equipped to make these selections. The objectives of this study were to assess the degree of genotypic diversity for drought-related morpho-physiological traits and to measure the strength of association between these traits and indicators of crop performance. 46 sugar beet genotypes with diverse genetic backgrounds were tested in three field experiments from 1999 to 2001. Drought was imposed by covering plots with large polythene tunnels, allowing the crop to grow solely on stored soil moisture. Agronomic characters and drought tolerance indices are described in a companion paper [Ober et al., Assessing the genetic resources to improve drought tolerance in sugar beet. Agronomic traits of diverse genotypes under droughted and irrigated conditions. Field Crops Res., in press]. In this report, we show that there were significant genotypic differences for stomatal conductance, succulence index, specific leaf weight, and osmotic adjustment, but not for photosynthetic rate, relative water content or total water use. Patterns of water use within the soil profile differed between genotypes; some extracted more water from deep soil layers. The maintenance of green foliage cover during drought was positively correlated with drought tolerance index, which was negatively correlated with succulence index and scores for wilting and leaf senescence. Droughted sugar yield was positively correlated with soil water extraction and negatively correlated with relative leaf expansion rate measured in late summer. Under irrigated conditions, there was high positive correlation between transpiration rate and sugar yield. Genotype × trait biplots showed superior genotypes with relatively greater expression of combinations of favourable traits. The results suggest that succulence index and wilting score could be used to cull inferior plants in early stages of breeding programs, and green foliage cover and patterns of water use could help identify superior genotypes in elite germplasm. These data should enable tools to be developed for indirect selection of genotypes suited to drought-prone environments.

Physiological regulation of productivity and water use in Eucalyptus: a review

Eucalyptus species in Australia occur naturally across a wide range of environmental conditions. In native forests, the leaves are typically thick, tough and long-lived with low specific leaf area, and low leaf concentrations of nitrogen and phosphorus. Potential productivity is high but maximum rates are rarely achieved because of limitations due to drought and nutrient availability. In contrast, when Eucalyptus species are grown in conditions with adequate supplies of water and nutrients, or in managed plantations at favourable sites, light-use efficiency and productivity are high. We review the physiological basis for the observed rates of growth and water use in Eucalyptus forests. Our aim is to demonstrate that a process-based approach provides a robust framework to predict productivity and water use by incorporating differences between species, the effects of climate and options for silvicultural management. Concern about decreases in water yield from catchments where Eucalyptus plantations have been established has prompted interest in estimating transpiration rates using a range of techniques. The consequence of the marked sensitivity of decreasing stomatal conductance to increasing air saturation deficit, D, is that, in well-watered conditions, transpiration from Eucalyptus forests can be explained largely by leaf area index and D. Measurements of photosynthesis for many Eucalyptus species over a wide range of conditions have confirmed the potential for high rates of carbon uptake. Measurements of maximum rates of photosynthesis, A max, maximum rate of carboxylation, V cmax, and the maximum rate of electron transport, J max, for Eucalyptus trees are high in relation to other broad-leaved tree species, but actual rates of photosynthesis are often much lower because of water and nutrient limitations. This results in a wide variation in light-use efficiency ranging from 0.7 to 2.7 g (dry matter) MJ −1 (intercepted photosynthetically active radiation between 400 and 700 nm). Several mechanisms for drought avoidance are identified, including low values and large seasonal dynamic changes in leaf area index, near-vertical arrangement of leaves, high stomatal sensitivity to air saturation deficit, deep rooting ability and osmotic manipulation to maintain turgor in leaves. Further evidence from measurements of carbon isotope fractionation at sites along rainfall gradients and estimation of the relationships between leaf area, sapwood cross-sectional area and conductance demonstrate homeostatic adjustment of hydraulic properties in relation to growing conditions. Validation of models for native and managed Eucalyptus stands confirm their usefulness as a management tool to predict productivity and water use in relation to species, climate and silvicultural options.

Climatic and Water Availability Effects on Water‐Use Efficiency in Wheat

In Argentina, wheat (Triticum aestivum L.) is cropped over a wide range of climatic conditions. Considerable variability in the ratio of dry weight produced per unit of transpired water, usually referred to as water‐use efficiency (WUE), is expected as variation in climatic factors affects photosynthesis and transpiration in different ways. Also, previous studies have shown that water supply limitations may affect WUE in wheat. The objective of this study was to quantify the effects of climatic environment and water availability on WUE in wheat crops. Six experiments were conducted at different locations of the Argentine wheat belt and crop dry weight and water use were measured in periods when water use was dominated by transpiration. Three of the experiments included both irrigated and rainfed treatments. Mean daily values of (i) pan evaporation, (ii) relative humidity, (iii) potential water use, and (iv) vapor pressure deficit, were used to find a general relationship that explained effects of the climatic environment on WUE. For experiments with high water availability, daytime vapor pressure deficit was better related to WUE than the other climatic factors. WUE was greater for experiments with water limitation, probably because stomatal closure to restrict transpiration rate occurred around midday when vapor pressure deficit was highest. As a consequence, relative dry weight under water limitation was not linearly related to relative water use as proposed in previous studies. A quadratic relationship that better represented this response was derived.

Locating QTLs controlling field and laboratory predictors of drought tolerance in Agropyron using multiple selection index

To locate the genes controlling field and laboratory predictors of drought tolerance, chromosome addition lines of Agropyron elongatum (donor) into the genetic background of Chinese Spring (recipient) were tested in the field, green house and laboratory conditions. Disomic addition lines revealed significant differences for relative water content (RWC), relative water loss (RWL), water use efficiency (WUE), chlorophyll flourescence (CHF) and stomatal resistance (SR), indicating genetic variation and possibility of selection for improving drought tolerance. Physiological indicators of relative water loss and water use efficiency showed a great contribution in the regression equation of yield under stress. Based on the physiological multiple selection index (MSI) most of the QTLs controlling physiological indices of drought tolerance were located on chromosomes 3E, 5E and 7E. Evaluation of disomic addition lines indicated that most of the genes monitoring stress tolerance index (STI) and germination stress index (GSI) are located on chromosomes 3E, 5E and 7E. It is therefore, suggested that these lines to be used in the breeding programs to transfer useful drought tolerance traits into wheat genetic background.

Climatic and Water Availability Effects on Water-Use Efficiency in Wheat

In Argentina, wheat (Triticum aestivum L.) is cropped over a wide range of climatic conditions. Considerable variability in the ratio of dry weight produced per unit of transpired water, usually referred to as water-use efficiency (WUE), is expected as variation in climatic factors affects photosynthesis and transpiration in different ways. Also, previous studies have shown that water supply limitations may affect WUE in wheat. The objective of this study was to quantify the effects of climatic environment and water availability on WUE in wheat crops. Six experiments were conducted at different locations of the Argentine wheat belt and crop dry weight and water use were measured in periods when water use was dominated by transpiration. Three of the experiments included both irrigated and rainfed treatments. Mean daily values of (i) pan evaporation, (ii) relative humidity, (iii) potential water use, and (iv) vapor pressure deficit, were used to find a general relationship that explained effects of the climatic environment on WUE. For experiments with high water availability, daytime vapor pressure deficit was better related to WUE than the other climatic factors. WUE was greater for experiments with water limitation, probably because stomatal closure to restrict transpiration rate occurred around midday when vapor pressure deficit was highest. As a consequence, relative dry weight under water limitation was not linearly related to relative water use as proposed in previous studies. A quadratic relationship that better represented this response was derived.

Estimation of relative water use among ornamental landscape species

Low water use plantings may enhance water conservation in dry landscapes. However, appropriate plant selection is hindered by the dearth of information available on the water needs of different species. A direct method of classification was tested under the hypothesis that relative water use by woody landscape species growing in 3.8 l containers would be representative of the water use of the same species in the landscape. Four species of distinctly different ecological origin ( Leucophyllum frutescens, Spiraea vanhouteii, Viburnum tinus, Arctostaphylos densiflora) were chosen in order to obtain a wide range of responses, and their water use was measured in plants growing in 3.8 l containers and compared to that of the same species growing in drainage lysimeters, representative of landscape conditions. Half of the plants were subjected to successive cycles of stress by withholding water after irrigation to container capacity in containers, or applying a fraction of the potential evapotranspiration in lysimeters. The good fit of the regression of average daily water use by lysimeter plants on average daily water use by container plants ( R 2=0.87, P<0.01) reflects the consistency of relative water use of the four species. Measurement of water use at the end of nursery production may be useful for predicting the relative water use of various species after establishment in the landscape.