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

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Abstract
Conductance for water vapor, assimilation of CO2, and intercellular CO2 concentration of leaves of five species were determined at various irradiances and ambient CO2 concentrations. Conductance and assimilation were then plotted as functions of irradiance and intercellular CO2 concentration. The slopes of these curves allowed us to estimate infinitesimal changes in conductance (and assimilation) that occurred when irradiance changed and intercellular CO2 concentration was constant, and when CO2 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 CO2 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 CO2 which in turn was caused by changes in the assimilation of CO2.