Canopy temperature depression (CTD = air temperature [Ta] – canopy temperature [Tc]) has been used to estimate crop yield and to rank genotypes for tolerance to heat and drought, but when to measure CTD for breeding selection has seldom been addressed. Our objectives were to evaluate the suitability of CTD for the Texas High Plains environment and to determine optimal measurement times in relation to growth stage, time of day, and weather. Three years of CTD and weather data were used to assess regression models of grain yield in three wheat (Triticum aestivum L.) lines. Under dryland agriculture, long‐term mean CTD at noon and yield were correlated in 2000 and 2001. The relation of short‐term CTD readings to grain yield was highly variable. Poor correlation was associated with days of low solar irradiance, high wind speed, and rain events. Genotype effects on CTD were detected for all hours of day and night. Genotype × hour interaction was insignificant at night, suggesting that nighttime measurements may provide more stable conditions for CTD comparison among genotypes. In general, tree regression assessed grain yield from short‐term CTD measurements better than linear regression and suggested that the best times to measure CTD were 0900, 1300, and 1800 h. Tree regression models provided a heuristic interpretation of crop water status under different scenarios of soil water availability.

Wheat (Triticum aestivum L.) cultivars with high canopy temperature depression (CTD) tend to have higher grain yield under dry, hot conditions. Therefore, CTD has been used as a selection criterion to improve adaptation to drought and heat. The CTD is a result of the leaf's energy balance, which includes terms determined by environment and physiological traits. We hypothesized that one or more of several physiological traits contributed to consistent CTD differences among three closely‐related winter wheat lines grown under dryland conditions. For three years we measured several leaf traits, including CTD, leaf dimension, gas exchange rates, and carbon‐13 isotope discrimination (Δ). Soil water content was also monitored. Data showed that daytime CTD was related to the leaf size in these wheat lines. The most drought‐tolerant line, TX86A8072, had consistently smaller and narrower leaves than TX86A5606, the least drought tolerant. For TX86A8072, dryland and irrigated average noon CTD was −0.8°C, and average flag leaf area (LA) 11 cm2, for TX86A5606, values were −1.7°C and 12.5 cm2, respectively. However, TX86A8072 also had higher CTD (i.e., lower temperatures) than TX86A5606 at night, despite a theoretically greater sensible heat transfer coefficient, suggesting greater nighttime transpiration (T). Implications of these traits on nighttime leaf energy balance and possible adaptive roles of nighttime T are discussed.

Increased yields with conservation tillage have been attributed to the conservation of soil water (Rao and Sustainable cropping systems are essential for agronomic, eco-Dao, 1996; Papendick and Miller, 1977) due to decreased nomic, and environmental reasons. Data from a winter wheat (Tritievaporation and cooler soil temperatures (Gauer et al., cum aestivum L.)/summer fallow rotation experiment, in eastern Oregon, was used to evaluate long-term effects of tillage, N, soil depth, 1982) and increased infiltration (Good and Smika, 1978; and precipitation on yield. The soil is a Walla Walla silt loam (coarse-Unger and McCalla, 1980; Allmaras et al., 1985; Schilsilty, mixed, mesic Typic Haploxeroll). The experiment consisted of linger, 1992; Tucker et al., 1971). Papendick and Miller three tillage treatments (moldboard plow, offset disk, and subsurface (1977) reported that wheat yield had the potential to sweep) and six N treatments. Four main time periods (1944-1951,

Several studies conducted under high input conditions have indicated little susceptibility of pearl millet to water deficit untill early grain filling, because the losses in main shoot production were fully compensated by increased tiller fertility. The present study assessed the impact of water deficits at three development stages: prior to flowering (S30), at the beginning of flowering (S45), and at the end of flowering (S60) in pearl millet grown in experimental conditions similar to Sahelian farming conditions. It included a control irrigation treatment simulating the natural distribution of rainfall throughout the cropping season. Both biomass production and grain yield were severely reduced by S30 and S45, while S60 had no effect. In S30 and S45, the flowering of tillers was delayed or totally inhibited. In both of these treatments, the low number of productive tillers did not compensate for damage to panicle initiation and flowering of the main shoot. All treatments maintained green leaves on the main shoot during the grain filling period, and in S30 leaf growth recovered from mid-season drought. These results illustrate how pearl millet mostly escapes drought by matching its phenology to the mean rainfall distribution in the Sahel. In the case of mid-season drought, some late productive tillers and the maintenance of green leaf biomass of the main shoots limited, but did not overcome, the yield losses. This study stresses the importance of agro-ecological conditions in control treatments, particularly the water regime and crop density, when assessing crop drought resistance.

Decreased water availability and increased food demand worldwide require development of more water‐efficient crops. This study was conducted to examine preflower transpiration ratio (CO2 assimilation rate [A]/transpiration rate [E], A:E) in four inbred sorghum [Sorghum bicolor (L.) Moench] lines and 12 of their F1 hybrids in terms of phenotypic and genetic variation, inheritance (i.e., additive and nonadditive genetic effects), and its relationship to water use efficiency (WUE) (i.e., the ratio of whole‐plant biomass to cumulative transpiration). Lines were selected with contrasting A and A:E in a field trial and then crossed in a full diallel pattern. All hybrids and lines were further analyzed for A, E, A:E, biomass, and WUE in a greenhouse. Variation among hybrids and lines was significant for all traits evaluated. In both environments, the most contrasting lines for A:E were Tx430 and Tx7078. Average A:E was 3.10 mmol CO2 mol−1 H2O for Tx430 and 2.91 for Tx7078. Both Tx430 and Tx7078 had highest A. Genetic effects resulting from general (GCA) and specific combining ability (SCA) for A, E, and A:E were significant for all traits. Tx430 had positive and Tx7078 negative GCA effects on A:E. Tx430 hybrids had the highest A:E values, and Tx7078 hybrids the lowest. The A:E ratio was correlated with total biomass and WUE for all crosses. Data suggest that concomitant selection for high A:E and A at preflowering could result in improved WUE and biomass of mature sorghum.

Pearl millet [Pennisetum glaucum (L.) R. Br.] production in the West African Sahel is constrained by low, erratic rainfall and low soil nutrient (particularly P) availability. Outdoor pot and growth chamber experiments tested the hypothesis that increasing soil P supply increases transpirational water‐use efficiency (WUET), under waterstressed and non‐water‐stressed conditions. Pearl millet was grown outdoors under semiarid conditions in covered pots containing 85 kg of acid, P‐deficient Betis sand (sandy, siliceous, thermic Psammentic Paleustalf). Plants were treated with four P levels and two water treatments, and harvested at 14‐d intervals. Significant main and interactive effects on WUET due to P level, water treatment, and time of harvest were found. The slope of the curve relating DM to cumulative transpiration (Tcum) increased with P level and water stress when data from all harvests were pooled. In the growth chamber, WUET of nonwater‐stressed plants ranged with increasing P level from 3.22 to 9.12 g kg−1 at 29 days after sowing (DAS) in pots containing 6 kg soil, and from 0.84 to 9.24 g kg−1 at 49 DAS in pots containing 18 kg soil. The ratio of leaf net photosynthetic rate to transpiration 0,WUEGMS,) at 500 μmol m−2 s−1 photosynthetic photon flux density (PPFD) ranged from 1.88μg mg−1 for plants receiving no P to 10.25μg mg−1 for those receiving 0.310 g P 6 kg−1 soil. Between PPFD levels orS00 and 2000 μmol m−2 s−1, plants receiving no P increased WUEGAS to only 3.60 μg mg−1, whereas those receiving higher levels of P increased WUEGAS to as much as 18.2μg mg−1. Our finding that increasing soil P avail° ability increases WUET under water‐stressed and non‐water‐stressed conditions reinforces previous conclusions that water supply in the Sahel and similar semiarid environments cannot be effectively managed for improved crop production without addressing soil fertility constraints.

Drought occurs often in the West African Sahel, but studies have shown that soil water availability is not usually the limiting factor to pearl millet [Pennisetum glaucum (L.) R. Br.] production, and that field water‐use efficiency (WUE)—i.e., the ratio of yield to evapotranspiration (ET)—is almost always very low. The purpose of this study was to determine management effects on yield and water use of pearl millet for a range of climate conditions in the Sahel. Grain and aboveground dry matter yield, daily vapor pressure deficit, and soil water data were taken during four years of contrasting rainfall. Within any given year, genotype, plant population, and fertilizer had relatively small to no effect on ET, but large effects on yield. When high plant population (≥20 000 hill ha−1) was combined with high fertilizer application (≥40 kg N ha−1 and ≥18 kg P ha−1) during the wettest year, total ET was increased by ≈50 mm. High fertilizer application tended to slightly increase ET and thereby deplete soil water reserves, but this was not associated with yield decline. Yield and water‐use data refute the view that, by maintaining fields at low fertility and low plant populations, farmers reduce risk of crop failure during drought by reducing crop water use. Compared with traditional practices that use plant populations as low as 5000 hill ha−1 and zero fertilizer input, moderate plant population (10 000 hill ha−1) and fertilizer application (20 kg N ha−1 and 9 kg P ha−1) substantially increased yield and approximately tripled WUE even during 1984, the driest year on record. In general, grain yield was better predicted from ET within different management categories when corrections were made for mean daily vapor pressure deficit during the growing season (VPD). The study provides evidence for the need to moderately increase pearl millet plant population and fertilizer application in the Sahel to efficiently use available water without risk of crop failure through depletion of soil water reserves. It also provides a practical, albeit empirical, basis for predicting yield under different management systems from seasonal ET and VPD data.

Several studies have identified low soil P and water availability as major constraints to pearl millet [Pennisetum glaucum (L.) R. Br.] production in semi‐arid West Africa. To evaluate the effects of phosphate and water supply on yield, transpirational water‐use efficiency (WUET), and carbon‐isotope discrimination (Δ), two varieties of pearl millet were cultivated in pots in a glasshouse at the ICRISAT Sahelian Centre, near Niamey, Niger. Phosphate and water supply had significant effects on yield, WUET, and Δ. Compared with the control plants, which had adequate water and P availability, yield was reduced 34% by low water supply and 48% by low P supply. Under high P‐supply, water stress increased WUET by approximately 37%. Under low P‐supply, no effect of water supply on WUET was observed. Water stress increased Δ by approximately 0.6‰ for low P plants, and 0.9‰ for high P plants. Added P increased Δ by 0.3 to 0.4‰. WUET and Δ did not differ significantly between varieties. Differences in Δ between green and necrotic leaves were found within both P treatments under low water supply. We attribute changes in Δ to changes in the ratio of external to internal concentration of CO2, (pi/pa), leakage rates of CO2 out of bundle‐sheath cells, respiration rates, or chemical composition of the plant material.