Field studies were conducted to determine cowpea [Vigna unguiculata (L.) Walp.] response to different intensities of drought at different stages of growth to guide programs for the development of improved cultivars and improved management practices for semi‐arid zones. Two cowpea cultivars were subjected to different levels of irrigation using a line‐source sprinkler system in the field on a coarse‐loamy, mixed, thermic Haplic Durixeralf soil during 2 years with contrasting weather patterns. Well‐watered plants were irrigated weekly. In other treatments, water was not supplied during either the vegetative, flowering, or pod filling stages. Dry seed yields of well‐watered treatments were negatively correlated with degree days above 35 C during flowering. Yields of both cultivars were not reduced by drought during the vegetative stage, compared with the weekly irrigated treatment, providing subsequent environmental conditions were conducive to rapid recovery of growth and efficient pod set. Drought during the flowering stage substantially reduced yields, but partial yield recovery was observed where drought caused all flowers to abscise and the subsequent environment, after irrigation was resumed, permitted a new flush of flowers to produce pods. Drought during pod filling substantially reduced yields of both cultivars in both years. Variations in yield due to environmental stresses were mainly due to variations in number of pods/m2 except for drought during pod filling which resulted in both low pod density and small seed. Yield with abundant water or drought during pod filling appeared to be partially source limited, whereas intermediate levels of drought at other stages may have resulted in sink limitations on yield because these treatments produced the heaviest seed.
Lanthanum fed to the base of excised leaves of Sesamum indicum L. and Helianthus annuus L. was used as a tracer to investigate by electron microscopy the path of water in the apoplast of leaves. The generally random distribution of lanthanum in cell walls provided no support for the hypothesis that cuticular transpiration may be greater for guard cells than for adjacent epidermal cells. Occasionally, accumulations of lanthanum were observed in anticlinal walls of epidermal cells and at the outer surface of the plasma membrane but lanthanum was not observed in the symplast.The influx of 86Rb to excised roots of sesame and sunflower was inhibited during incubation with 0.5 mM lanthanum or calcium for 15 or for 180 min. Stomata of sunflower partially closed when 2.5 mM lanthanum was supplied to the base of excised shoots in a potometer, whereas this treatment had little effect on stomatal conductance of sesame shoots maintained in a constant environment. Supplying 2.5 mM lanthanum to the base of sesame shoots strongly inhibited stomatal opening response to increase in ambient humidity but had little effect on stomatal opening response to light. It was concluded that stomatal opening response to increased humidity may be dependent upon some process, such as ion influx, that is inhibited by lanthanum, and that opening response to humidity may differ in mechanism from stomatal opening response to increased irradiance.
Synopsis In irrigation‐phosphorus studies the highest fruit yields were associated with a medium moisture treatment and adequate phosphorus. Fruit size, rot, and set were significantly affected by soil moisture treatments but were not significantly influenced by P levels.
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