Separation and Measurement of Direct and Indirect Effects of Light on Stomata

Conductance for water vapor, assimilation of CO(2), and intercellular CO(2) concentration of leaves of five species were determined at various irradiances and ambient CO(2) concentrations. Conductance and assimilation were then plotted as functions of irradiance and intercellular CO(2) concentration. The slopes of these curves allowed us to estimate infinitesimal changes in conductance (and assimilation) that occurred when irradiance changed and intercellular CO(2) concentration was constant, and when CO(2) concentration changed and irradiance was constant. On leaves of Xanthium strumarium L., Gossypium hirsutum L., Phaseolus vulgaris L., and Perilla frutescens (L.), Britt., the stomatal response to light was determined to be mainly a direct response to light and to a small extent only a response to changes in intercellular CO(2) concentration. This was also true for stomata of Zea mays L., except at irradiances < 150 watts per square meter, when stomata responded primarily to the depletion of the intercellular spaces of CO(2) which in turn was caused by changes in the assimilation of CO(2).Stomata responded to light even in leaves whose net exchange of CO(2) was reduced to zero through application of the inhibitor of photosynthetic electron transport, cyanazine (2-chloro-4[1-cyano-1-methylethylamino]-6-ethylamino-S-triazine). When leaves were inverted and irradiated on the abaxial surface, conductance decreased in the shaded and increased in the illuminated epidermis, indicating that the photoreceptor pigment(s) involved are located in the epidermis (presumably in the guard cells). In leaves of X. strumarium, the direct effect of light on conductance is primarily a response to blue light.Stomatal responses to CO(2) and to light opposed each other. In X. strumarium, stomatal opening in response to light was strongest in CO(2) free air and saturated at lower irradiances than in CO(2) containing air. Conversely, stomatal closure in response to CO(2) was strongest in darkness and it decreased as irradiance increased. In X. strumarium, P. vulgaris, and P. frutescens, an irradiance of 300 watts per square meter was sufficient to eliminate the stomatal response to CO(2) altogether. Application of abscisic acid, or an increase in vapor pressure deficit, or a decrease in leaf temperature reduced the stomatal conductance at light saturation, but when the data were normalized with respect to the conductance at the highest irradiance, the various curves were congruent.