Physiological and agronomic responses of two pearl millet (Pennisetum americanum L. K. Schum) parents and their hybrid were evaluated under different water levels on a Brazito sandy loam soil at Tucson, Arizona in 1983 and 1984. A line source sprinkler irrigation gradient was used to create the treatments. Soil moisture and physiological parameters were measured under field conditions at weekly intervals. Growth was analyzed every other week, while yield and yield components were evaluated at the end of the experiment. Harvest Index, drought tolerance index, and water use efficiency (WUE) were calculated. Percent crude protein of seeds was analyzed, and photosynthesis was measured in 1984 only. Regression analysis was used to compare the performance of the cultivars for stability. Water stress significantly affected all parameters measured, and the stress effects were more pronounced in 1983. There was no significant difference between entries at each water level for most parameters. Plant height, dry matter, and total leaf area decreased as watering level decreased. The leaf area of the hybrid was significantly higher at high water level in 1983. Firing ratio was increased significantly by stress in both seasons and was significantly lower for the male at low water level in 1983. Yield and yield components were reduced significantly by stress in both seasons, which was reflected in the reduced harvest index. Relative yield of dry matter was higher for the male in both seasons. The hybrid had the highest drought tolerance index in 1983, while the female had the highest drought tolerance index in 1984. WUE of dry matter increased with stress, while WUE of grain yield decreased with stress in both seasons. The significant reduction in transpiration was 57, 56 and 66% in 1983 and 45, 53, and 52% in 1984 for female, male, and hybrid respectively. Leaf diffusive resistance and leaf temperature were increased by water stress in both seasons. Stability regression analysis provided no adequate differentiation among cultivars. The percent crude protein (weight basis) was increased significantly by stress by 38, 43, and 28% in 1983 and by 47, 33, 46% for female, male, and hybrid, respectively.

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A study was conducted in the field to test whether stomatal sensitivity to water stress can be used as a selection criterion for drought tolerance in two pearl millet genotypes. In both cultivars, stomatal aperture was significantly reduced by the water stress. However, the proportion of reduction per 20 mm decrease in applied water was much higher for the hybrid than for the female parent. This is clearly an indication of a higher stomatal sensitivity in the hybrid, which probably explains the superior performance under water stress that was previously observed in this genotype.

The capability to measure the magnitude of water stress in plants is useful for precision irrigation scheduling and other purposes. This paper reports an evaluation of leaf (TL) and canopy (Tc) temperatures, leaf minus air (TL -Ta) and canopy minus air (Tc -Ta) temperatures, and leaf water stress index (LWSI) and crop water stress index (CWSI) in detecting stress in pearl millet (Pennisetum americanum (L.) Leeke) over two growing seasons. Baselines which were used to compute LWSI and CWSI were obtained. The upper and lower baselines for the Tc data, respectively, were Tc -Ta = 4.10 C and Tc -Ta = 3.87- .2001VPD where VPD is vapor pressure deficit in mbars. For the TL data, the upper and lower baselines, respectively, were TL -Ta = 1.97oC and TL -Ta = 1.308- .03006VPD. Tests against photosynthesis, transpiration, and grain yield showed that LWSI and CWSI are better indices of stress than TL -Ta, Tc -Ta, TL, Tc, or Ta. Average seasonal LWSI and CWSI ranged from approximately 0.03 for non- stressed to 0.80 for stressed plants. The reliability of LWSI and CWSI to detect stress and their relation with grain yield suggested the possibility of using these indices for irrigation scheduling decisions.