Comparison of Stomatal Action of Orange, Soybean and Wheat Under Field Conditions

Abstract

Diurnal trends in leaf diffusive conductance, C_s, leaf water potential ΨL and rates of evapotranspiration E*T were monitored on orchard-grown orange trees and field-grown crops of soybeans and wheat. Changes in these measurements were observed on soybeans and wheat as the soil water deficit increased.

Maximum values of C_s of well watered plants differed between the three species (soybeans > wheat >> orange) probably as a result of different stomatal sizes and densities. Diurnal trends in C_s were common for all species, with maximum values occurring during midmorning followed by slightly lower midday values. The reduction in C_s around midday became much more pronounced as the soil water deficit increased. Slight increases in C_s values of soybeans and wheat were recorded during late afternoon. This pattern of stomatal aperture change can reasonably be explained in terms of responses to both radiant energy and plant water status. The pattern also seems to comply with the premise that stomates interact to optimize the rate of assimilation while minimizing the rate of transpiration in a given environment.

Stomatal action appeared to have little effect on daily E_T in soybeans under well watered conditions; E_T was closely related to incoming radiant energy. The low midday values of C_s apparently caused a midday plateau in the rate of C_T in wheat while even lower daytime C_s values for orange seemed to cause a low and fairly constant rate of E_T which was relatively insensitive to changes in incoming radiant energy.

The value of ΨL attained during mid morning at which C_s initially began to decline was fairly constant for soybeans (-0.9 to -1.1 MPa) as the predawn ΨL decreased from -0.1 to -0.8 MPa. A similar decline in predawn ΨL for wheat caused a change in the value of ΨL at which initial decreases in C_s were observed from - 1.3 MPa to -2.4 MPa. Thus there appeared to be little adjustment of stomatal action in soybeans but considerable adjustment in wheat.