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

Diurnal changes in the vertical profiles of irradiance incident upon the adaxial leaf surface (I), stomatal resistance (r(s)), leaf water potential (psi), osmotic potential (pi), and turgor potential (P) were followed concurrently in crops of maize (Zea mays L. var. Pa 602A), sorghum (Sorghum bicolor [L.] Moench var. RS610), and tobacco (Nicotiana tabacum L. var. Havanna Seed 211) on several days in 1968 to 1970 when soil water potentials were high. In all three crops the r(s), measured with a ventilated diffusion porometer, the psi, measured with the pressure chamber, the pi, measured with a vapor pressure osmometer, and the calculated P, decreased from sunrise to reach minimum values near midday and then increased again in the afternoon. The diurnal range of all the variables was greater for leaves in the upper canopy than for those in the lower canopy. P was observed to decrease with decreasing psi, but never became zero. Sorghum had a higher P at a psi of, say -10 bars, than did maize, and maize had a higher P than tobacco at the same psi. Moreover, at the same psi the upper leaves in all canopies had a higher P than the lower leaves. When compared at high irradiances, r(s) did not increase as psi declined to -13, -15, and -10 bars or as P declined to 0.3, 3.5, and 1.2 bars in maize, sorghum, and tobacco, respectively. The relation between r(s) and I in the upper, nonsenescent leaves of all three crops fits a hyperbolic curve, but the response varied with species and leaf senescence. The adaxial and abaxial epidermises had the same response of r(s) to I in maize and sorghum, whereas in tobacco the adaxial epidermis had a higher r(s) than the abaxial epidermis at all values of I. At equal values of I, tobacco had the lowest leaf resistance (r(l)) and maize had the highest r(l). Senescent maize leaves had nonfunctional stomata, whereas the lowermost sorghum leaves had higher stomatal resistances on average than the other leaves.

Water potential gradients in field tobacco

A pressure chamber was used to establish the vertical gradients of leaf water potential (Psi(Leaf)) and stem water potential (Psi(Stem)) in field-grown tobacco (Nicotiana tabacum L. var. Havanna seed 211) at three different times of day. Leaves enclosed in polyethylene bags and aluminum foil the previous afternoon and left to equilibrate overnight were used to determine Psi(Stem). The greatest difference between Psi(Leaf) and Psi(Stem) occurred in the upper part of the plant at 1100 hours Eastern Standard Time and was 5.5 bars. The largest vertical gradient in Psi(Stem) occurred at 1300 hours. The soil water potential (Psi(Soil)), extrapolated from the potential of leaves on a completely enclosed plant, was higher than -1 bar. The vertical gradient in Psi(Stem) and the difference between Psi(Leaf) and Psi(Stem) showed the existence of a resistance to water movement within the stem (r(stem)) and a further resistance between the stem and leaf (r(petiole)). The r(petiole) and root resistance (r(root)) were estimated to be 931 and 102 bars seconds per cubic centimeter, respectively. The r(stem) was low (94 bars seconds per cubic centimeter) at 1100 hours but increased to 689 bars seconds per cubic centimeter at 1300 hours.

Effects of changing stomatal width in a red pine forest on soil water content, leaf water potential, bole diameter, and growth

Spraying a 16 meter tall stand of red pine (Pinus resinosa Ait.) with 10(-3)m phenylmercuric acetate in early June and again in mid-July resulted in the water use between June 1 and October 25 being reduced by almost 10%. It was demonstrated that this was caused by an increase in the leaf resistance with partial stomatal closure, which reduced absolute water potential in the needles by 1 to 3 bars in the middle of the day. Smaller demands were made upon the reserves of water in the bole of the tree as shown by the smaller bole contraction in the treated trees. Although needle length and dry weight were unaffected by the spray, radial growth was reduced by approximately 32%. The dependence of leaf resistance on light intensity is shown, and its independence from leaf water potential discussed.