Differing Sensitivity of Photosynthesis to Low Leaf Water Potentials in Corn and Soybean

Rates of net photosynthesis were studied in soil-grown corn (Zea mays) and soybean (Glycine max) plants having various leaf water potentials. Soybean was unaffected by desiccation until leaf water potentials were below -11 bars. Rates of photosynthesis in corn were inhibited whenever leaf water potentials dropped below -3.5 bars.The differences in photosynthetic behavior could be attributed solely to differences in stomatal behavior down to leaf water potentials of -16 bars in soybean and -10 bars in corn. Below these potentials, other factors in addition to stomatal closure caused inhibition, although their effect was relatively small.Corn, which has the C(4)-dicarboxylic acid pathway for carbon fixation, generally had a higher rate of photosynthesis than soybean during desiccation. Nevertheless, since inhibition of photosynthesis began at higher potentials than in soybean, and since corn was less able to withstand severe desiccation without tissue death, it was concluded that the C(4) pathway confers no particular ability to withstand low leaf water potentials.