Effects of water stress on leaf expansion, net photosynthesis, and vegetative growth of soybeans and cotton

Abstract

Soybeans and cotton were subjected to humidities from 40 to 80% at 23 °C and to soil drought during early vegetative growth under controlled conditions. Measurements were made of leaf water potentials, leaf expansion rates, leaf diffusive resistances to water vapor, and whole-shoot net photosynthesis rates. Net assimilation rates were calculated from harvest data. Low humidity resulted in low leaf water potential and low turgor in all cases and resulted in reduced leaf expansion rates in some, but not all, cases. Low humidity reduced dry weight growth only where leaf expansion rates were reduced. Net photosynthesis rates per unit leaf area were unaffected by low humidity, despite up to 1.5-fold increases in diffusive resistance to water vapor. During soil water stress, leaf expansion rates were reduced 1–2 days before net photosynthesis rates per unit leaf area were reduced, but leaf expansion continued at night after net photosynthesis rates were severely reduced by stress. As a result, the relative importance of leaf area expansion and net assimilation rate to growth in dry weight during soil water stress was dependent on the degree and duration of stress.