RESPONSE OF CONTRASTING VEGETABLE-COWPEA CULTIVARS TO PLANT-DENSITY AND HARVESTING OF YOUNG GREEN PODS .2. DRY-MATTER PRODUCTION AND PHOTOSYNTHESIS

Harvest of vegetable cowpea (Vigna unguiculata (L.) Walp.) involves periodic removal of pods just prior to the stage of rapid seed-growth, which may have physiological consequences. The influences of pod-picking on shoot biomass production, efficiency of light utilization, photosynthesis, and leaf morphology and composition were studied with two contrasting vegetable bush-type cowpea cultivars under field conditions during the summers of 1986 and 1987 at the University of California, Riverside. A split-split-plot experimental design was used, with four replicates. Cultivars were main plots, plant densities of 100 to 400 thousand plants ha-1 (tph) were sub-plots, and sub-sub-plots consisted of picking immature 'snap' pods at 3-4-day intervals, and an unpicked treatment in which dry pods were picked at the end of the season. Shoot dry-matter production was greater at high plant density than low plant density, and for UCR206 than UCR193. The differences in shoot dry-matter production were attributed to the differences both in amounts of photosynthetically active radiation intercepted by crop canopies and in the efficiency of light conversion into biomass. UCR193 exhibited 54% lower efficiency in light utilization than UCR206 during the reproductive phase of plant development, probably due to the greater number of pods set early and above the canopy by UCR193, which would have reduced light penetration to leaves. This hypothesis was confirmed by the observation that pod-picking resulted in substantial increases in shoot dry-matter production by UCR193 and had little effect on UCR206. Pod-picking also resulted in higher photosynthetic rates by UCR193, but rates of non-picked UCR193 and for both picked and non-picked UCR206 were similar, and declined steadily with progression of leaf senescence. The differences in photosynthetic rates were positively correlated with stomatal conductances, but not with the chemical constituents that were measured. For both cultivars, pod-picking resulted in longer retention of green leaf area, higher chlorophyll, and lower protein contents. Specific leaf weight increased with picking, especially for UCR193, but changes in starch and sugar contents in leaves were too small to account for the large increases in specific leaf weight. It would appear that translocation of assimilates out of leaves was not greatly impaired by the continuous removal of immature pods. The physiological consequences of frequent removal of young pods were substantially different, in that UCR193 produced more pods while UCR206 produced fewer pods compared with non-picked treatments. © 1990.