Theory and empirical evidence have demonstrated that 13C discrimination (∆) by leaves of C3 plants may be associated with intrinsic water‐use efficiency or productivity. Consequently, selection for ∆ has potential value in breeding plants with improved adaptation. This study evaluates genotypic differences in ∆ among cowpea [Vigna unguiculata (L.) Walp.] and genotype ✕ drought level interactions, and compares ∆ measured in leaves and grains. Sixty cowpea accessions were subjected to drought by growing them on stored soil moisture in six randomized blocks, and genotypes differed significantly (P < 0.001) in leaf ∆. Seventeen cowpea accessions were grown under weekly irrigation (wet) and stored moisture (dry) conditions in four randomized‐split blocks. Genotypes differed significantly in ∆ under both conditions using either leaves or grains. Plants under dry conditions had lower ∆, which theory predicted could be associated with 62% higher water‐use efficiency. The same plants showed a 62% higher ratio of CO2 assimilation rate to leaf diffusive conductance. Genotypic rankings for ∆ were similar under wet and dry conditions for most genotypes, but a significant (P < 0.05) genotype ✕ drought interaction was observed, which was mainly due to one genotype. Correlations between ∆ in grain and subtending leaves were highly significant (P < 0.001), but two genotypes exhibited substantial differences in ranking for ∆ determined in grain compared with leaves. Genotypic differences were more readily detected in leaves than grains with broad sense heritabilities of 0.76 and 0.35, respectively. Heritabilities were similar under wet and dry conditions.

High temperatures cause reductions in grain yield of cowpea [(Vigna unguiculata (L.) Walp.] that are associated with low pollen viability and pod set. Preliminary controlled environment studies showed differences in proline accumulation in anthers and pollen of heat‐tolerant and heat‐sensitive genotypes under hot and optimal temperatures, but insufficient tissue was available to establish if the differences were significant. The objective was to determine whether heat injury under field conditions is associated with specific patterns of proline accumulation in leaves and reproductive tissue using heat‐sensitive an heat‐tolerant cowpea genotypes. Under moderate and hot temperatures, proline was the most abundant free amino acid in the anthers of both heat‐sensitive and heat‐tolerant cowpea genotypes. No differences in leaf proline concentrations were observed. Under hot conditions, proline levels in anthers decreased faster as pollen matured in heat‐tolerant genotypes as compared with heat‐sensitive genotypes. At pollen maturity, heat‐sensitive genotypes contained more proline in anthers and had lower levels in pollen than the heat‐tolerant genotypes under hot conditions but similar levels under more optimal temperatures. The results suggest that heat injury during floral development of sensitive cowpea genotypes may be due to inhibition of proline translocation from anther walls to pollen.

Water temperature has increasingly become a matter of concern for California rice (Oryza sativa L.) growers due to a need for public water agencies to improve habitat for fish. Prudent management of water resources to balance the needs of environmental and agricultural interests requires the quantification of water temperature effects on rice productivity. Our objective was to evaluate two alternative thermal unit models for the effect of low water temperature on yield. One model was based on the total number of hours below a given threshold water temperature Tb (abbreviated TNHB Tb) and the other was based on the concept of inverse degree days (i.e., degree days below a given threshold water temperature) (abbreviated IDD). We tested these models at a range of values of Tb between 10 and 25°C on data from two commercial fields during 2 yr. Results showed that the effect of low water temperature may be much greater than would be apparent from the visual appearance of the rice plants. Values of IDD and TNHB Tb were very highly correlated for 4 of the 4‐yr field combinations. A logistic curve model based on TNHB 20°C provided the best fit to the aggregated data.

High night temperatures cause excessive floral abscission in cowpea (Vigna unguiculata (L.) Walp.), while similar or even higher temperatures during the day do not impair reproduction. The objectives of this study were to determine if high temperatures during the early part of the night have different effects on cowpea pod set than high temperatures during the late part of the night, and if distinct events of pollen development are associated with the diurnal sensitivity to heat. Two genotypes of cowpea were grown in controlled environments under a 12‐h photoperiod and subjected to high temperatures (33 °C) during the day and either the first or second 6‐h period of the night. Temperatures were optimal (24 °C) during the other 6 h of the night. All plants experienced the same heat units. Floral buds were sampled at different stages of development and the developing pollen were microscopically examined. For the two genotypes, high temperatures during the late‐night period resulted in much lower pod set (7 and 20%) and pollen viability (2 and 35%) than high temperatures during the early‐night period (51 and 76%, and 65 and 69%, respectively). No diurnal synchrony in pollen development was observed during the late‐night period when developing pollen was the most sensitive to high night temperatures. No changes in floral morphology were observed that could impede pollen transfer from the anthers to the stigma and be responsible for the low pod set. We hypothesize that the results can be explained by the existence of a physiological process that is sensitive to heat and under circadian regulation.

High temperatures in tropical and subtropical zones often have detrimental effects on plants. Plants in these zones experience differences in daylength that could influence sensitivity to heat. Contrasting genotypes of cowpea, Vigna unguiculata (L.) Walp., were grown under fluorescent plus incandescent (F) or metal halide plus incandescent (MH) lamps at different daylengths (11, 12, 13, 14, or 16 h) to determine whether the sensitivity of floral development to high night temperatures is influenced by light quality and photoperiod. Floral bud development was suppressed in heat‐sensitive genotype (CB5) at a 14‐h photoperiod under MH, while a 16‐h photoperiod was required to elicit a similar response under F. Spectral analysis showed five times more ultraviolet‐A light (UV‐A) (315‐400 rim) in MH than in F, but F supplemented with UV‐A light did not elicit the same suppression of floral bud development as MH. The reproductive response to long days with hot nights (30 °C) was closer to that of field‐grown plants under MH than under F. Percent pod set of two sensitive genotypes (CB5 and 7964) subjected to high temperatures (33/30 °C day/night) was higher (23 and 19%) at an ll‐h photoperiod than at a 14‐h photoperiod (5%) under F. No pod set occurred in an ll‐h photoperiod with red light (R) during the night. Substantial pod set (41%) was observed a 14‐h photoperiod was followed by far‐red light (FR). The effect was reversed when FR was immediately followed by R. Apparently, pod set at high nightemperatures in heat‐sensitive, day‐neutral cowpea is dependent on photoperiod through a mechanism involving phytochrome.