Temperature dependence of CO2 assimilation and stomatal aperture in leaf sections of Zea mays

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Abstract
CO2 exchange and air flow through the stomata were measured in leaf sections of Zea mays at temperatures between 7 and 52° and under optimal water supply. The results were summarized in polynomials fitted to the data. In leaf samples brought from 16° and darkness into different experimental temperatures and light, CO2 assimilation has a maximum near 30°. Above 37° (in other experiments above 41°), net CO2 uptake stops abruptly and is replaced by CO2 evolution in light. If a 1-hr treatment with 25° and light is inserted between darkness and the experimental temperatures, the threshold above which the assimilatory system collapses shifts 3 degrees upwards, to 40° (or 44°); the decline of CO2 assimilation with high temperatures is less steep than without pretreatment; and the upper compensation point moves upscale by as much as 5 degrees. Stomatal conductance for CO2 does not, in general, follow an optimum curve with temperature. Between 15 and 35° it is approximately proportional to net CO2 assimilation, indicating control by CO2; but above 35°, stomatal aperture increases further with temperature (and so does stomatal variability): the stomata escape the control by CO2 and above 40° may be wide open even if CO2 is being evolved. Stomatal conductance for CO2 below 15° may also be larger than would be proportional to CO2 assimilation. Net CO2 assimilation and stomatal conductance at 25° were reduced if the leaf samples were pretreated with temperatures below approximately 20° and above 30°. Stomata were more sensitive to past temperatures than was CO2 assimilation.