Effects of potassium and water availability on water use efficiency of common bean (Phaseolus Vulgaris L.)

Abstract

Water use efficiency (WUE), i.e. biomass produced per unit of water transpired, is an important criterion that determines crop productivity, especially under drought. As WUE is determined by the ratio between photosynthetic and transpiration rates, stomatal conductance (g s) is expected to play a major role in determining WUE. The effects of two factors that may influence g s, namely, different levels of potassium (K) and water supply on WUE of different varieties of common bean (Phaseolus vulgaris L.) were tested. Two indoor pot experiments were done. Experiment 1 was done with low-K soil using a single variety. Experimental treatments were two levels of K application, i.e. with (+) and without (-) K application, and water regimes (75% and 55% of field capacity). The +K treatment h ad two applications of K 2SO 4at 85 mg/pot. Experiment 2 was done using 5 varieties in nutrient solutions containing two levels of K + (0.4 and 4 mM). Transpiration and biomass accumulation were measured by daily weighing and destructive sampling respectively. Both experiments showed that the different levels of K used did not cause a significant effect on WUE. However, WUE was significantly increased by water stress and significant varietal variation in WUE (4.93 - 5.36 g kg -1) was observed. It is argued that the absence of significant differences in WUE under different K levels occurred because both photosynthesis and transpiration may not have been affected by K. Despite the reduced external K supply, the cytoplasmic K concentration may have been maintained at a high enough level to ensure adequate photosynthesis. This is supported by the absence of significant differences in total biomass between different K treatments. Although higher K levels may have increased g s, transpiration may not have increased because of greater boundary layer resistance and decreased sensitivity of transpiration to g s. Therefore, it is concluded that a factor that may cause a change in g s may not necessarily alter WUE. The need for detailed measurements of g s, transpiration and photosynthetic rates is emphasized.