Saturation Vapour Pressure Deficit Research Articles

Effects of variety and row spacing on radiation interception, partitioning of dry matter and seed set efficiency in late season sunflower (Helianthusannuus L.) in a humid zone of Nigeria

In the tropics, crops sown in the late season are subjected to concurrent stresses of high air and soil temperatures, large saturation vapour pressure deficits and negligible soil water regimes. The responses of three sunflower cultivars to row spacing was analysed in terms of radiation interception and use, plant to plant variability, biomass accumulation and seed set efficiency during the late seasons of 2002 and 2003 on the field in Akure, a rainforest zone of Nigeria. Sunflower cultivars (Functua local, Record and Isaanka) were cropped at 90 x 30, 90 x 60 and 60 x 30 cm row spacing corresponding to 37,037, 18,519 and 55,555 plant.m-2 Increasing density of plant stand appeared to have promoted differences in resource availability per plant during growing season. Row spacing did not significantly affect IPAR while the effects of spacing and cultivar were similar on RUE. Leaf area and dry matter accumulation are suboptimal but RUE appeared to be conservative in circumstances of high saturation vapour pressure deficit and soil moisture stress of the late season. Crops in wide rows intercepted less radiation than their counterparts in narrow rows, and yield response to narrow rows was significant in all cultivars. Seed yield/ha increased as plant density increased although yield of the individual plants and their components were significantly reduced. Among the cultivars, differences in the growth of individual plant at different intensities of inter plant competition were obtained, narrow row spacing enhanced plant-plant variability (inter-plant variation). Under the limiting soil water situation enhanced soil temperatures of the late season EF relates to plant biomass (r2 > 0.70), dry matter relates to accumulated intercepted radiation IPAR (r2 > 0.90) and fractional intercepted radiation (fIPAR) relates to seed yield (r2 > 0.95) while fIPAR is a function of thermal time (TT) (r2 > 0.80). Key words: Sunflower, cultivars, late season, RUE, extinction coefficient, plant-plant variability, dry matter partitioning, seed set, seed yield

Use of remote sensing for drought stress monitoring, yield prediction and varietal evaluation in castor beans (Ricinus communis L.)

The current study was taken up to investigate the utility of remote sensing tools like infrared thermometer and spectral radiometer for screening of germplasm, stress monitoring and yield prediction in castor beans (Ricinus communis L.). The study was carried out through field experiments conducted for six years (1994–1999) at Hayatnagar Research Farm, Hyderabad, India. In each year, four cultivars of castor beans, viz. VP‐1, 48‐1, GCH‐4 and Aruna, were planted on two different dates maintaining an interval of 6–8 weeks so as to expose the crop to different environments. The infrared thermometric observations like canopy–air temperature differential (T c–T a) explained 50–60% variation in soil moisture status and showed a significant relationship with soil moisture. Yield of castor beans exhibited significant inverse relationship with T c–T a, which explained 59% of variation in yield. The hybrid GCH‐4, registering comparatively lesser mean T c–T a over the entire growing period, established itself as a better cultivar. The spectrometer observations also proved GCH‐4 to be a superior genotype in view of its higher reflectance in near‐infrared region of the spectrum. The significant negative relationship of T c–T a of GCH‐4 with saturation vapour pressure deficit brought out its drought tolerance trait over the other genotypes studied. These findings at field level can be extended to wider spatial level using satellite imageries.

Evapotranspiration from a riparian fen wetland

Evapotranspiration rates were measured in a riparian fen wetland dominated by vascular vegetation and surrounded by open agricultural areas and forests. The wetland is situated on a floodplain in central Denmark. Measurements were taken throughout the growing season (April–September) of 1999. Evapotranspiration rates were higher than those published for most other wetland types, with an average of 3.6 mm d−1 during the growing season and a peak rate of 5.6 mm d−1. Daily average evapotranspiration was 110% of Penman's potential open water evaporation. Evapotranspiration was the dominant sink in the energy balance of the wetland studied. During the day, evapotranspiration accounted for 82% of the available radiant energy, Rn. Due to the presence of deposited fine-grained sediments, soil-water availability was kept high at all times which resulted in moderate canopy resistances, rc (overall mean =32 s m−1). Evapotranspiration was controlled by a combination of driving forces: Rn, saturation vapour pressure deficit, D, and rc. It is hypothesized that the results presented in this study are conditioned by the proximity of the wetland to drier upland areas. During periods with high evaporative demand and low precipitation, warm, dry air is formed over the upland areas and wetland evapotranspiration rates are enhanced by local advection. Indicative evidence for the hypothesis is presented. Although the absolute magnitude of the results reported is only directly relevant to similar sites in Denmark, the processes and controls described are considered to be representative of riparian wetlands subjected to frequent flooding and/or with a high groundwater table, with vascular vegetation, and which are narrow corridors in open agricultural landscapes.

RESPONSE OF PHOTOSYNTHETIC RATE AND STOMATAL CONDUCTANCE OF RICE TO LIGHT INTENSITY AND CO2 CONCENTRATION IN NORTHERN CHINA

The response of photosynthetic rate and stomatal conductance of rice (Oryza sativa var. Japonica) to changes in light intensity and CO2 concentrations was studied using a Li-6400 in Northern China. In general, photosynthetic rates increased with light intensity and CO_(2) concentrations and could be expressed by a Michaelis-Menten function. Apparent quantum yield increased with CO_(2) concentrations but decreased slightly when CO_(2) concentrations exceeded 800 mol·mol~(-1). Similarly, apparent carboxylation efficiency increased with light intensity but decreased slightly when light intensity exceeded 1600 mol·m~(-2)·s~(-1). The response of stomatal conductance to light intensity can also be expressed by a Michaelis-Menten function, whereas the response to CO_(2) concentrations can be expressed by a hyperbola. If the combined effects of light intensity and CO_(2) concentrations are considered, the photosynthetic rate can be estimated by a Michaelis-Menten equation with a maximum photosynthetic rate of 71.74 mol·m~(-2)·s~(-1). Apparent quantum yield was 0.0560 mol CO_(2)·mol~(-1) photons and carboxylation rate was 0.1031 mol·m~(-2)·s~(-1)/mol·mol~(-1). The response of stomatal conductance (G_(sw)) to light intensity can be expressed by a Michaelis-Menten function too, but the response to CO_(2) concentrations (C_(s)) can be simulated by the equation: G_(sw)=G_(max,c)/(1+C_(s)/C_(s0)) where G_(max,c) is maximum stomatal conductance of stomatal response to CO_(2) under a defined light intensity and C_(s0) is a constant, because the stomatal conductance decreases with increases in CO_(2) concentrations, stomatal conductance can be estimated by G_(sw)=G_(max)(PFD/PFD_(c))/[(1+PFD/PFD_(c))(1+C_(s)/C_(s0))]+G_(ct) in response to the combined effects of CO_(2) concentration and light intensity (I). The potential maximum stomatal conductance, G_(max), can reach 0.6709 mol·m~(-2)·s~(-1) under saturated light levels and CO_(2) near 0 mol·mol~(-1). Ball-Berry model and its revised form can still be used to express the coupled relationship of stomatal conductance and photosynthesis. The simulation precision will be improved if saturation vapor pressure deficit, D_(s), at the leaf surface was used in the Ball-Berry model instead of relative humidity.

Predicting the concentration and uptake of nitrogen, phosphorus and potassium by field-grown green beans under non-limiting conditions

As part of a program to develop a weather-driven model to predict yield and quality of green beans that also describes the accumulation and distribution of nutrients by these plants, we required information on the evolution of the N, P and K content and concentration within the crop. A field experiment with three widely grown green bean cultivars ( Phaseolus vulgaris L. cv. Carlo, Cleo and Mutin) was made with eight sowing dates over 2 years. Water and nutrients were supplied to ensure that they would not limit plant growth. Weather data was collected hourly. Dry matter accumulation and the concentrations of N, P and K in the shoots were determined each week. The concentration of N was related to thermal time by a logistic equation, while P concentration was linearly related to thermal time in the early stages of development but became constant later on. A model that combined the effects of developmental stage and temperature, or saturation vapour pressure deficit, explained the concentration of K. Upper and lower boundary lines were adjusted to total uptake of nutrients, representing the maximum or minimum amount of each nutrient present in the above-ground tissues in relation to thermal time. The harvest index of nutrients depended on the partition of dry matter between pods and shoots, according to a logistic function, and the concentration of nutrients in the pods and shoots.

Factors controlling transpiration of mature field-grown tea and its relationship with yield

The objective of this experiment was to determine the factors influencing the transpiration rates of mature, clonal tea ( Camellia sinensis L.) and estimate its transpiration efficiency. The heat pulse technique was used to measure transpiration rates of tea plants growing in the field as part of extensive canopies at Talawakelle, Sri Lanka during the period between 1 January and 19 February 1997. Irrigation and shading treatments were used to determine the influence of soil water content ( S) and irradiance on transpiration rate. The transpiration rate declined only slightly when S decreased from field capacity (44%) to 33%. However, when S declined below 33%, the transpiration rate showed a rapid decline from 1.6 to 0.7 l per plant per day at 15% S. When S was near field capacity, maximum transpiration rates of 0.53–0.93 l plant −1 h −1 occurred between 1000 and 1500 h. The corresponding maxima when the S was near permanent wilting point (i.e. at −1.5 MPa matric potential) were 0.27–0.53 l plant −1 h −1. Transpiration decreased linearly with decreasing irradiance throughout the range of radiation levels tested (i.e. from 100 to 15% of full sunlight) at a rate of 0.031 l per plant per day per % reduction in solar irradiance. The daily transpiration rates of tea plants (0.42–1.07 l per plant per day) under the natural shade of Grevillea robusta were considerably lower than the value of tea plants in the open, 3.511 l per plant per day. Spraying of an antitranspirant, Kaolin, decreased canopy temperature by 2–4°C and especially around mid-day. Kaolin also decreased transpiration slightly during the period between 1000 and 1500 h. Transpiration efficiency ( T E) was 9.637 kg ha −1 (made tea) mm −1 of water transpired. The relationship between total dry matter yield and the ratio between transpiration and mean saturation vapour pressure deficit also was linear with a proportionality constant of 6.9 g kg −1 kPa.

An international cooperative programme indicates the widespread occurrence of ozone injury on crops

The UN/ECE ICP-Vegetation 1 United Nations Economic Commission for Europe International Cooperative Programme on effects of air pollution and other stresses on crops and non-wood plants (formerly ICP-Crops). 1 routinely investigates the effects of ambient ozone pollution on crops throughout Europe. Each year, a series of co-ordinated ambient air experiments are conducted over a large area of Europe and a range of crop species are observed for the occurrence of injury following ozone episodes. In 1995 and 1996, ozone injury was observed at sites throughout Europe from United Kingdom (Nottingham) to the Russian Federation (Moscow) and from Sweden (Östad) to Italy (Naples). The only site participating in the ICP-Vegetation where it was not observed was that at Finland (Jokioinen). Injury was identified on subterranean and white clover, French bean, soybean, tomato, and watermelon at one or more sites. Injury was also detected in gardens and on crops growing in commercial fields. Two short-term critical levels which incorporate ozone dose and air saturation vapour pressure deficit (VPD) were derived from the 1995 data. These were (i) an AOT40 2 The sum of the difference between the hourly concentration (ppb) and 40ppb when the concentration exceeds 40ppb for the hours when global radiation (GR) exceeds 50Wm −2. 2 of 200 ppb.h over 5 days when mean VPD (0930–1630 h) is below 1.5 kPa and (ii) an AOT40 of 500 ppb.h over 5 days when mean VPD (0930–1630 h) is above 1.5 kPa. In general, the 1996 data supported these critical levels although injury did occur on two occasions when the AOT40 was less than 50 ppb.h, and the VPD was less than 0.6 kPa. Thus, ICP-Vegetation experiments have shown that ozone injury can occur over much of Europe and that plants are most at risk in conditions of high atmospheric humidity.

MANAGEMENT PRACTICES TO IMPROVE LOWLAND SUBTROPICAL SUMMER TOMATO PRODUCTION: YIELDS, ECONOMIC RETURNS AND RISK

Four experiments were conducted in southern Taiwan to evaluate the yield and economic benefits of management practices aimed at alleviating the effects of high temperature and heavy rain on tomato production. Following tropical storms, waterlogging caused wilting which was more evident in tomatoes grown in beds 20 cm in height than in those grown in raised beds 40 cm in height. This indication of crop water stress induced by flooding was quantified by measurement of the difference between canopy surface and air temperature at a specified saturation vapour pressure deficit. Canopy heating was less on the raised beds, especially when straddled by rain shelters. Nevertheless, waterlogging stress for the low-bed treatment did not result in loss of stand. In crops which experienced heavy rain, application of a synthetic fruit-set hormone enhanced fruit numbers as did the simple transparent polyethylene rain shelters. While planting on raised beds was without yield benefit the use of fruit-set hormone improved yield by at least 10 t ha−1. A further 4–6 t ha−1 yield was gained through the additional use of rain shelters. Partial budget analysis showed positive returns to the use of fruit-set hormone (US$6600 ha−1) but, unless rain shelters could be constructed with cheaper frame material or used for two tomato crops throughout the summer, their use would be uneconomical. Using existing frame material, net returns to rainshelters reached US$8000 ha−1. The likelihood of high temperatures, tropical storms and typhoons will influence economic returns to summer tomato production. Data from this study may be subjected to economic analyses using values for inputs and tomato prices from other countries with similar climatic conditions.

A simple model of light and water use evaluated for Pinus radiata.

An existing model of light and water use by crops (RESCAP) was adapted and evaluated for trees. In the model, growth on any given day is determined either by the amount of intercepted radiation (by means of the light utilization coefficient, epsilon) or by the maximum rate of water extraction by roots (a function of root biomass and soil water content). In either case, transpiration and growth are related by the water-use efficiency (q), which is inversely proportional to the daily mean saturation vapor pressure deficit (D). The model was applied to two Pinus radiata (D. Don) stands (control (C) and fertilized (F)) growing near Canberra, Australia, using data collected during the Biology of Forest Growth experiment (1983-1988). For both stands, predicted and measured soil water contents were in close agreement (r(2) > 0.9) over a 4-year period involving several wet-dry cycles. The parameter combination epsilon/qD was estimated to be 0.28 and 0.26 kg H(2)O (MJ total)(-1) kPa(-1) for the C and F stands, respectively. Because of the close physiological link between water use and CO(2) uptake, the results suggest that tree growth may be realistically simulated by simple models based on conservative values for epsilon and qD.

Effects of increasing saturation vapour pressure deficit on growth and ABA levels in black spruce and jack pine

Plant responses to saturation vapour pressure deficit (SVPD) were studied by subjecting black spruce [Picea mariana (Mill) B.S.P.] and jack pine seedlings (Pinus banksiana Lamb.) to humid (0.3 – 0.8 kPa) or dry (2.0 – 2.5 kPa SVPD) regimes for 4 weeks using a computer-controlled environmental system to control diurnal variation in SVPD. Dry matter accumulation in needles was not altered by increasing SVPD. However, root growth declined by 60% which increased shoot to root ratio and reduced total seedling dry weight in both black spruce and jack pine. Relative growth rate of jack pine also declined to about half the rate of plants grown under humid conditions. In situ root marking studies showed that the decline in root growth of jack pine under the high SVPD was the result of reduced lateral root initiation, whereas root elongation was unaffected by humidity. A 4-week exposure to dry air increased abscisic acid (ABA) levels in needles, but not roots, of jack pine whereas ABA levels in black spruce were not altered. A short (3-day) exposure failed to increase needle ABA levels in either species. These results suggest that the responses of conifers to dry air were not the result of ABA accumulation.

Radiation and water use efficiencies of rainfed castor beans ( Ricinus communis L.) in relation to different weather parameters

Field experiments on rainfed castor beans ( Ricinus communis L.) were conducted for 4 years (1990–1993) to explain the variability in radiation use efficiency (RUE) and water use efficiency (WUE) in relation to meteorological parameters. In these experiments castor c.v. Aruna was sown on three different dates (treatments) in each year. The results from this study indicate that both RUE and WUE vary from year to year and also were influenced by the planting dates. Variations in RUE and WUE ranged from 0.79–1.19 g MJ −1 and 0.72–1.25 g kg −1, respectively. The variability in RUE and WUE before and after flower initiation were quite contrasting. The variability in RUE and WUE was associated with saturation vapour pressure deficit (SVPD), drought index, temperature and wind velocity. RUE was related positively with SVPD and wind velocity and negatively with drought index and temperature. WUE showed inverse relationships with SVPD and temperature and a direct relation with wind velocity. The study emphasises the need for incorporation of the effect of weather on RUE and WUE in the algorithms for biomass estimation of crop simulation models.

Whole-plant Net CO2 Exchange of Raspberry as Influenced by Air and Root-zone Temperature, CO2 Concentration, Irradiation, and Humidity

The influence of irradiance, CO2, and temperature on whole-plant net CO2 exchange rate (NCER) of Rubus idaeus L. `Heritage' micropropagated raspberries was examined. Within the set of environmental conditions examined, irradiation was the most important factor, accounting for 58% of the whole-plant irradiance/CO2 concentration/temperature NCER model variation, followed by CO2 concentration (28%) and temperature (2.5%). Net photosynthesis (Pn) required irradiance levels >600 μmol·m-2·s-1 PPF for saturation, greatly increased under CO2 enrichment (up to 1500 μL·L-1), and was optimum at a whole-plant temperature of 20 °C. Temperature effects were partitioned in an experiment using varying air and root-zone temperatures (15, 20, 25, 30, and 35 °C) under saturated light and ambient CO2 levels (350 μL·L-1). Air and root-zone temperature influenced Pn, with maximum rates occurring at an air × root-zone temperature of 17/25 °C. The contribution of air and root-zone temperature to the NCER model varied, with air and root-zone temperature contributing 75% and 24%, respectively, to the total model variation (R2 = 0.96). Shoot dark respiration increased with air and root-zone temperature, and root respiration rates depended on air and root-zone temperature and shoot assimilation rate. Humidity also influenced Pn with a saturated vapor pressure deficit threshold >0.25 kPa resulting in a Pn decrease. Quantifying the physiological response of raspberries to these environmental parameters provides further support to recent findings that cool shoot/warm root conditions are optimum for raspberry plant growth.

Transpiration efficiencies of maize and beans in semi-arid Kenya

Maize and bean were grown under varying levels of nitrogen fertilizer, plant population, and irrigation at Kiboko, Kenya in the short rains 1990, 1991, 1992 and the long rains 1991. The production of dry matter was not affected significantly by any treatment, because treatments only had a small impact on the balance between evaporation and transpiration. In all seasons the greatest loss of water from the profile was through direct evaporation from the soil surface. Transpiration was always less than 25% of rainfall. The ratio of transpiration ( T) to evapotranspiration ( E + T) was small (0.23), but increased from 0.15 to 0.40 as rainfall increased from 158 mm in the long rains 1991 to 470 mm in the short rains 1992. Treatments had little impact on the balance between transpiration and evaporation from the soil surface. The average transpiration efficiencies for maize and bean were 89 and 29 kg shoot dry matter ha −1 mm −1, respectively. For each crop there was a 60% change in transpiration efficiency between the short and the long rain season which could be accounted for by differences in saturation vapour pressure deficit.

A reinterpretation of stomatal responses to humidity

ABSTRACTThe stomatal conductance (g) for single leaves and the equivalent canopy conductance for stands of vegetation are often represented in models as empirical functions of saturation vapour pressure deficit or relative humidity. The mechanistic basis of this dependence is very weak. A reanalysis of 52 sets of measurements on 16 species supports the conclusion of Mott & Parkhurst (1991, Plant, Cell and Environment 14, 509–515) that stomata respond to the rate of transpiration (E) rather than to humidity per se. In general, ∂g/∂E is negative and constant so that the relation between g and E can be defined by two parameters: a maximum conductance gm obtained by extrapolation to zero transpiration, and a maximum rate of transpiration Em obtained by extrapolation to zero conductance. Both parameters are shown to be functions of temperature, CO2 concentration, and soil water content. Exceptionally, transpiration rate and conductance may decrease together in very dry air, possibly because of patchy closure of stomata.

Water-use efficiency of willow: Variation with season, humidity and biomass allocation

Information on the water-use efficiency (WUE) of a vegetation cover improves understanding of the interrelationship between the water and carbon cycles, and enables hydrological practices to be related to agricultural and silvicultural planning and management. This study determined seasonal and climatic variations of the WUE of a fertilized and irrigated short-rotation stand of Salix viminalis L. on a clay soil. The WUE was determined as the ratio of above-ground production to transpiration or, alternatively, to transpiration divided by the saturation vapour pressure deficit. Growth was estimated from a combination of destructive and non-destructive measurements for 10 day periods during the growing seasons of 1986 and 1988. Daily transpiration was estimated using a physically based evaporation model, tuned against energy-balance/Bowen-ratio measurements of total stand evaporation. Nutrients were adequate and climate conditions were similar in both years. In spite of irrigation soil-water deficits developed during midsummer and affected growth rates in different ways: in 1986, both stem and leaf growth decreased, while in 1988 only stem growth decreased. Exceptionally high stem growth rates, twice the total potential growth rates, were recorded after the drought of 1988. They were probably caused by root-allocated assimilates that were sent above-ground after the drought. In both years, stem growth ceased 2–3 weeks after the leaf area had reached its maximum. Since light and temperature were still sufficient to maintain assimilation, all growth presumably took place below ground towards the end of the season. Changes in root-shoot allocation caused large variations in the WUE in 1988. The WUE, weighted by the saturation vapour pressure deficit, was fairly constant in 1986. In both years, the WUE was correlated with the vapour pressure deficit. Towards the end of both growing seasons, when all assimilates were sent below ground, the WUE decreased rapidly to zero. The total WUE, estimated as the seasonally accumulated above-ground production divided by accumulated transpiration, was 4.1 g kg −1 in 1986 and 5.5 g kg −1 in 1988, which is relatively high in comparison with other species.

Effects of ozone on managed pasture: I. Effects of open-top chambers on microclimate, ozone flux, and plant growth

Open-top chambers (OTC) were established in a field of managed pasture, and environmental parameters were recorded inside and outside to study the influence of OTCs on radiation, air temperature ( T air), saturation vapour pressure deficit (svpd), and soil water content in relationship to plant growth and yield. Canopy development in OTCs supplied with non-filtered air (NF) and in ambient (AA) plots was followed by measuring leaf area index (LAI). The dry matter yield was determined after three growth periods in each of two consecutive seasons. Boundary layer conductance ( g bw) and wind speed ( u) were measured along a vertical profile, and day-time flux were measured along a vertical profile, and day-time flux of O 3 was estimated throughout the experiment on the basis of a mass balance. The vertical profile of u showed values in the range 1−1·2 m s −1 at the top of the canopy, and maximum g bw was 20–25 mm s −1. Average reduction in global radiation in OTCs was 25%, and volumetric soil water content was reduced by about 5%. Daily mean T air was increased by 1·3°C, mean daily maximum svpd by 0·08 kPa, and the temperature sum (degree days with base temperature of +5°C) by 12%. Fluctuations in the difference in daily mean T air and svpd during the daytime between OTCs and ambient air were related to canopy structure. Differences were largest after each cut and declined with increasing LAI. A small effect of changes in LAI on T air and svpd occurred during periods with low soil water content. The flux of O 3 in OTCs was largest (>100 μg m −2 min −1) before and smallest (<20 μg m −2 min −1) after each cut. Calculated deposition veloties for O 3 ( v d) in the range 0–3 cm s −1 were generally higher than those measured under most field conditions. Overall, in OTCs the deficit in soil and atmospheric moisture was larger than in the open field, and the increase in daily mean T air was strongly influenced by the stage of canopy development. Changes in microclimate and incoming radiation affected pasture development. LAI was slightly reduced in OTCs as compared to AA plots. The total accumulated dry matter yield for all six growth periods was only about 7% lower in OTCs, but the contribution of clover to total forage mass declined during the experiment. OTCs had no significant effect on weeds. The results indicate that OTCs reduced the competitiveness of clover, and that the increase in growth of grasses compensates for the loss in clover yield. The shift in species composition caused by OTCs must be considered when studying the effect of pollutants on pasture.

Dynamics of water use in a dry mediterranean environment II. A test of four evaporation models using microlysimetry under spring wheat

Microlysimeter data were used to evaluate four empirical evaporation models: the radiation models of Ritchie (1972), Cooper et al. (1983) and Rees (1986), and a transpiration efficiency (TE) model of Fischer (1979). The three radiation models produced good agreement with the observed E sc during wet periods, but the model of Cooper et al. greatly underestimated E sc during dry periods. Thus, while the models of Ritchie and Rees gave total estimates of E sc to within 6 mm of the measured, that of Cooper et al. underestimated seasonal E sc by at least 15 mm. In testing Fischer's model, the TE was determined by three methods, all of which overestimated ∑ E sc in the first half of the season and underestimated ∑ E sc during the dry second half, as a result of TE being too high early in the season and too low late in the season. Regardless of how TE was determined, all gave a similar result, with a final ∑ E sc within 6 mm of the measured. Poor fit during the season was associated with the determination of TE using daily means of either Class A pan evaporation ( E 0) or saturation vapour pressure deficit ( e∗-e) during dry periods when transpiration and photosynthesis are likely to have been limited to the early part of the day. Ritchie's model gave the most consistent agreement with the measured E sc throughout the season; this model discriminates between first and second stages of evaporation, and so limited the relevance of crop canopy to the first stage. However, for initial estimates of seasonal E sc, the model of Fischer may be useful since it requires only (dry matter) DM and pan evaporation data, once the TE constant is established for the crop. The results further support the conclusion of Part I, this issue, of this study, that in this dry mediterranean environment, E sc will generally depend more on soil moisture storage, which controls soil hydraulic conductivity. Hence, when modelling the moisture supply limiting stage of E sc the use of a radiation interception model should be avoided.

The exchange of water and carbon by crops in a mediterranean climate

The amount of biomass accumulated by a crop stand per unit of water transpired is referred to as a Biomass Water Ratio (BWR). The paper is mainly concerned with the dependence of BWR on climatic factors, in particular, saturation vapour pressure deficit (D) and precipitation (P) with which D is highly correlated in a mediterranean climate. A method is outlined for estimating the amount of water evaporated from bare soil during the establishment phase of a crop as a fraction of total seasonal water use. A simple model is derived for the BWR as a function of climate and of two stomatal parameters derived from a relation between stomatal conductance and transpiration. Values of BWR are presented for three contrasting stations in Israel and are shown to be strongly correlated with D and therefore with P. The model is combined with an empirical relation between the Priestley-Taylor coefficient and surface resistance in order to establish a new relation between BWR and resistance and to demonstrate that BWR × D is almost independent of resistance above 100 sm−1.

Variability in crop radiation-use efficiency associated with vapor-pressure deficit

The ratio of the amount of crop dry-matter produced per unit of intercepted photosynthetically active radiation is usually referred to as radiation-use efficiency ( rue ). Large within-species variability in rue has been reported for a number of species growing with adequate moisture and nutrients. This variability raises questions concerning the generality of the approach for plant-growth analysis and modeling. Available data suggest that increased vapor-pressure difference between substomatal air and the air surrounding the leaves may lead to reductions in leaf conductance and photosynthetic capacity of well-watered plants, and therefore to rue reductions. The coefficient of determination ( r 2 ) of linear regressions of reported rue values as a function of daily saturation vapor-pressure deficit ( Δe ) were calculated for sorghum ( Sorghum bicolor (L.) Moench) and maize ( Zea mays ). Decreased rue was associated with increased Δe in well-watered conditions, explaining a large portion of the rue variability. Effects of Δe should be considered when the rue concept is used to estimate biomass accumulation.

Evapotranspiration and yield of irrigated chickpea

Field experiments were conducted during the 1986 and 1987 post-rainy seasons to relate total above ground dry matter (TDM) and seed yields of chickpea ( Cicer arietinum L. cv JG 74) with evapotranspiration (ET). The crop was subjected to various degrees of water stress by applying gradient irrigations during three growth phases: emergence to 50% flowering; 50% flowering to 50% beginning pod-fill; 50% beginning pod-fill to physiological maturity. Analysis of the 2-year pooled data showed that across seasons both total dry matter and seed yields were more strongly correlated with normalized ET ( R 2=0.92, P < 0.01 for TDM, and R 2=0.82, P < 0.01 for seed yield), defined as ratio of actual ET to saturation vapour pressure deficit of air, than with actual ET ( R 2=0.56, P < 0.01 for TDM, and R 2=0.40, P < 0.01 for seed yield). Partitioning of total normalized ET observed during the season into normalized ET observed during two or three growth phases of chickpea and regressing seed yield against them did not improve the predictability of models. It is concluded from this study that both total dry matter production and seed yield responses of chickpea to water management can be predicted if the normalized ET during the season is known.