Diurnal Growth Trends, Water Potential, and Osmotic Adjustment of Maize and Sorghum Leaves in the Field

The daily cycle of leaf elongation rate, water potential, and solute potential of maize and sorghum, as well as temperature, were monitored in the field. Major climatic features were high radiation and a minimum air temperature of about 12 C. Leaf elongation of both crops was slowest at night, presumably because of low temperature. Peak elongation rates were in daytime when leaf water potential (Psi) was low. Solute potential also decreased during daylight, thus permitting the maintenance of appreciable turgor pressure, a critical parameter for cell expansion.Leaf Psi versus relative water content (RWC) curves were developed by sampling detached leaves at intervals as they dried quickly in the laboratory. At a given RWC, Psi was lower in leaves at midday than early in the morning, which is evidence that the decrease in psi. at midday was caused by an increase in the amount of solute in the tissue. Estimates of psi. at 100% RWC were 4 bars lower at midday than early in the morning in both crops. Soluble sugars, mainly nonreducing, accounted for most of the observed psi. decrease in sorghum leaves. Shading the leaves from light eliminated most of the solute buildup.