Drought Tolerance of Cowpea

3. Effect of soil water deficit on leaf longevity in cowpea

Abstract

Cowpea (Vigna unguiculata (L.) WALP.) can maintain a high xylem water potential (ψ_x) when the soil water content is very low. The effect of this ability on the leaf function has not been analyzed. In this experiment, in order to clarify the relationship between the maintenance of a high ψ_x in cowpea (Kenya) under drought conditions and the function or longevity of the leaf, these parameters in cowpea were compared with those in soybean (Glycine max (L.) MERR.), common bean (Phaseolus vulgarisL.) and greengram (Vigna radiata (L.) WILCZEK) . Stomatal conductance, photosynthetic activity, transpiration rate and number of leaves after discontinuation of irrigation were determined. Furthermore, in order to investigate the relationship between the ψ_x and water storage in the stem reservoir, the diameter of the lower part of the stem was measured and the cross section was examined under a scanning electron microscope. In a local variety of cowpea from Kenya (var.unguiculata) with a large diameter of the lower part of the stem, the diurnal fluctuations in the water content in the leaf blade and lower part of the stem and the effect of cutting of the stem acting as a reservoir on the ψ_x were examined. The results were as follows. When the soil water content was low with a limited soil volume, the ψ_x values in cowpea and greengram remained high during the daytime and recovered in the late afternoon. The maximum photosynthetic rate and the recovery rate of photosynthesis after rewatering, however, did not change in the species. The number of leaves under the repeated exposure to water stress in soybean and common bean started to decrease immediately after the beginning of the treatment, whereas in cowpea and greengram all the existing leaves were maintained for about two weeks under the drought conditions. In cowpea and greengram the pith parenchymatous cells in the lower part of the stem developed and occupied most of the area in the cross section. The water stored in the lower part of the stem in cowpea (Kenya) was transported to the leaf in the daytime.
As a result, although the high ψ_x in cowpea and greengram did not influence directly the function of leaf, the large volume of the pith parenchyma in the lower part of the stem in cowpea and greengram which induced a high ψ_x by acting as a water reservoir, enabled the leaves to survive for a long period of time under drought conditions.