Stomatal Behavior and Water Status of Maize, Sorghum, and Tobacco under Field Conditions: II. At Low Soil Water Potential.

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), leaf resistance (r(1)), leaf water potential (psi), osmotic potential (pi), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. cv. Pa602A), sorghum (Sorghum bicolor [L.] Moench cv. RS 610), and tobacco (Nicotiana tabacum L. cv. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were low. The r(1), measured with a ventilated diffusion porometer, of the leaves in the upper canopy decreased temporarily after sunrise [ approximately 0530 hours Eastern Standard Time] as I increased, but then r(1) increased again between 0700 and 0830 hr Eastern Standard Time as the psi, measured with a pressure chamber, decreased rapidly from the values of -7, -4 and -6 bars at sunrise to minimal values of -18, -22 and -15 bars near midday in the maize, sorghum, and tobacco, respectively. The pi, measured with a vapor pressure osmometer, also decreased after sunrise, but not to the same degree as the decrease in psi, so that a P of zero was reached in some leaves between 0730 and 0800 hours. The lower (more negative) pi of leaves in the upper canopy than those in the lower canopy gave the upper leaves a higher P at a given psi than the lower leaves in all three species; leaves at intermediate heights had an intermediate P. This difference between leaves at the three heights in the canopy was maintained at all values of psi. The r(1) remained unchanged over a wide range of P and then increased markedly at a P of 2 bars in maize, -1 bar in sorghum, and near zero P in tobacco: r(1) also remained constant until psi decreased to -17, -20, and -13 bars in leaves at intermediate heights in maize, sorghum, and tobacco, respectively. In all three species r(1) of leaves in the upper canopy increased at more negative values of psi than those at the base of the canopy, and in tobacco, leaves in the upper canopy wilted at more negative values of psi than those in the lower canopy.