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

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.