Drought Tolerance of Cowpea

2. Comparative study on water relations and photosynthesis among cowpea, soybean, common bean and greengram plants under water stress conditions

Abstract

The effects of water stress on the stomatal aperture and mesophyll photosynthetic activity were investigated in cowpea (Vigna unguiculata (L.) WALP.), soybean (Glycine max (L.) MERR.), common bean (Phaseolus vulgaris L.) and greengram (Vigna radiata (L.) WILCZEK) plants. The water stress applied to the plants was regulated by changing the soil water content, and was represented by the xylem water potential (ψ_x) . Photosynthetic rate, transpiration rate (E) and intercellular CO₂ concentration (Ci) and ψ_x were measured under different water stress conditions. The results were as follows.
The relationship between the ψ_x and E was represented by the following switch function model for all these crops.
E=E_max/ (1 + (ψ_x/ψ_1/2) ⁿ)
where E_max is the maximum value of E, ψ_1/2 is the ψ_x when E= (E_max) /2 and n is a constant.
The photosynthetic efficiency (PE) in common bean began to decrease with a slight deficit in the soil moisture content due to the decrease in the mesophyll photosynthetic activity at high ψ_x level. Furthermore, the transpiration could not cease completely at low ψ_x. Soybean was able to maintain the mesophyll photosynthetic activity at a ψ_x value higher than -1.0 MPa by osmotic adjustment and the ψ_x level markedly decreased with the continuous decrease of the soil moisture content. Transpiration did not cease completely even at -2.5 MPa, and the PE decreased at a ψ_x value lower than -1.0 MPa. Common bean and soybean were considered to waste available soil water under severe drought conditions. On the other hand, although the PE in greengram began to decrease at a high ψ_x level, the E decreased rapidly at a ψ_x level lower than -0.7 MPa. The stomatal closure in cowpea corresponded to the decrease of the mesophyll photosynthetic activity at a low ψ_x. Consequently, a high PE was maintained until the stomata began to close. Furthermore, the E decreased rapidly at a ψ_x value lower than -1.0 MPa and transpiration ceased completely at a relatively high ψ_x level due to the absence of osmotic adjustment. These results suggest that cowpea and greengram can survive for a longer period of time under drought conditions due to the ability to maintain a large amount of water in soil and plant compared with soybean and common bean.