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Water deficits result in large yield reductions on the one-half of the world's rice land that is rainfed. The most severe water deficits tend to occur during the reproductive stage. Field studies were conducted over a three-year period to develop a method to screen rices for reproductive-phase drought stress resistance. Staggered planting date was effective in synchronizing the flowering of test entries during the stress treatment period. Grain yield, relative grain yield, and spikelet fertility were significantly and negatively related to the number of days beyond the beginning of the stress period that a cultivar flowered. A drought index based on the deviation of entry performance from regression of the plant character on flowering date (expressed as days from application of the drought stress treatment) was used to adjust cultivar performance. Rankings were similar for the index values of the grain yield, relative grain yield, and spikelet fertility. Percent spikelet fertility was highly correlated with grain yield in the event of reproductive-stage water stress ( r 2 = 0.64 ∗∗ , 0.84 ∗∗ and 0.66 ∗∗ during three years of tests). Spikelet fertility was judged the most practical character by which to score cultivar performance. A mass screening method is proposed that involves staggered planting, interruption of irrigation during the flowering period, visual scoring for spikelet fertility, and regression analysis to adjust for differences in flowering date.

Cowpea [ Vigna unguiculata (L.) Walp] is grown widely as an inexpensive source of protein in tropical regions. Production is often limited by either waterlogged or dry soils in the wet-dry transition period following rice because periodically saturated soils are followed by progressively severe drought. An optimum fit of cultivar phenology to these situations is crucial to higher, stable yields. The responses of a set of diverse cowpea differing in maturity were compared under a line-source moisture gradient applied during the vegetative stage on isohyperthermic, clayey, Typic Tropudalf with a fluctuating shallow water table, during two dry seasons in the Philippines. In 1986–1987, in saturated soil, the mean yields of the medium-maturing grain (460 kg ha −1 ) and dual-purpose (460 kg ha −1 ) types exceeded yield of the early-maturing grain (250 kg ha −1 ) and vegetable (210 kg ha −1 ) types, with a similar ranking (920, 730, 470, and 390 kg ha −1 ) without irrigation. In 1988, the medium-maturing cultivar group had a mean yield 985 kg ha −1 in saturated soil and 1389 kg ha −1 in unirrigated, again exceeding the early-maturity group with yields of 544 and 985 kg ha −1 , respectively. Saturated soil reduced seed yields by 10 to 71% in 1986–1987 and 10 to 42% in 1988. Pods per plant, seeds per pod, and individual seed mass of the cultivars were reduced 13 to 32%, 12 to 19%, and 3 to 12%, respectively. The results suggest that the best medium-maturing cultivars had superior adaptation to that of the best early-maturing cultivars in a post-rice environment with prolonged high rainfall and a high water table. Contrasting results are expected in post-rice niches with a deep water table and limited soil water reserves.

There is potential in tropical regions to include cowpea ( Vigna unguiculata (L.) Walp.) in rice ( Oryza sativa ) based cropping systems, especially during the post-monsoonal dry season. The plant water relations, and root and shoot growth of two cowpea cultivars (IT82D-889, early maturing, and TVX1948-012F, medium maturing) were compared in saturated and drier soil moisture regimes on an isohyperthermic, clayey, Typic Tropudalf soil by using a line-source sprinkler differential irrigation system. Saturated soil reduced total dry matter accumulation, plant height, crop growth rate and leaf area index by 20 to 50%, 7 to 10%, 20 to 50% and 20 to 30%, respectively. Root length density (RLD) measurements showed that in saturated soil, roots of both cultivars were concentrated in the top 0.1 m during the vegetative stage, but in the reproductive stage, grew down to 0.8 m. Leaf water potential (LWP) was higher by 0.3 MPa, and conductances were higher by 5 to 29% in dry than saturated soil. The cultivar TVX1948-012F had greater RLD, conductance and LWP than did with IT82D-889 under saturated conditions. Results suggest that the medium-maturing cultivar is better adapted than the early-maturing to high rainfall and high water table conditions in the post-rice environment. Its shallow rooting under saturated soil conditions enables more effective oxygen uptake in the vegetative growth phase. Deep rooting provides for effective water uptake and increased plant water status under subsequent drought conditions in the reproductive growth phase.

Recent studies have shown that the grain yields of corn (Zea mays L.) and wheat (Triticum aestivum L.) are related to the degree of water stress they undergo. The purpose of the study reported here was to establish relationships between crop temperature and the grain yields, phenological development, evapotranspiration rates (ET) and leaf water potential (ψl) of two hybrids of grain sorghum (Sorghum bicolor L. Moench) subjected to varying levels of plant water stress. The study was conducted at the University of Nebraska Sandhills Agricultural Laboratory in 1978 on a Typic Ustipsamment (Valentine fine sand) soil. The sorghum hybrids used were RS 626 and NB 505. Four irrigation treatments were applied in order to subject the crops to varying levels of water stress during each of three major growth stages. Soil moisture was monitored with a neutron probe. ET was estimated with the water balance technique. Crop temperature was measured with an IR thermometer and leaf water potential was measured with a Scholander pressure bomb. Grain yields were reduced by water stress occuring at anytime during the growing season. Yield reductions were largest when stress occurred during only the grainfill period and were least when stress occurred during the entire growing season. The percentage reduction in sorghum grain yield can be described by an index involving the seasonal accumulation of the daily mid-day temperature differences between well-watered and stressed crops (Σ TSD). As Σ TSD values increased, ET decreased. However, the correlation of ET with Σ TSD was relatively low (R2 = 0.60) probably due to the limited amount of data available for analysis and inaccuracies in the soil water balance method used to estimate ET. The mid-day temperature of well-watered rows ranged between 18.0 and 32.8 °C with a mid-day temperature range of about 0.5 °C between the well-watered rows in various plots for several days following an irrigation. However, in certain instances, the mid-day temperature range increased to 1–2 °C for a few days before irrigation. This suggests that certain of the rows experienced water stress and should have been irrigated earlier. Yield data support that conclusion. Range in crop temperature within a field appeared to be a sensitive indicator of crop water stress in sorghum. No significant difference in the phenological development of sorghum resulted from water stress except in one NB 505 plot in which plants were stressed throughout the entire season. In that plot, the stressed plants lagged in development behind non-stressed plants by approximately ten days. The differences in mid-day leaf water potentials (Δψl) and crop temperatures (ΔT) between stressed and non-stressed vegetation were examined. As ΔT increased up to about 4 °C, Δψl, also increased. Beyond that point, Δψl decreased while ΔT continued to increase. This behavior was attributed to stomatal closure which permitted an increase in ψl of the stressed plants (hence reducing Δψl) even as ΔT continued to increase.