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

Summary 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.