Analysis of Spatial Variation in CO$_2$ Uptake Within the Intact Leaf and Its Significance in Interpreting the Effects of Environmental Stress on Photosynthesis [and Discussion]

The responses of CO$_2$ uptake to environmental variables and to stress effects in vivo have been largely limited to measurements of whole leaves or 'representative' portions of the leaf. These responses therefore reflect an average for the photosynthetic cells within a leaf, which may differ in environmental pre-history, stage of development and ontogeny. These differences may also result in variation in responses to stress. Improvements in the resolution and accuracy with which gaseous exchanges can be measured in open and closed systems allow the use of small chambers that facilitate a separation of the contributions made by different parts of the leaf to total CO$_2$ uptake. In amphistomatous leaves with large internal resistances simultaneous measurements of CO$_2$, N$_2$O and H$_2$O vapour allow separation of the contributions made by the upper and lower mesophyll. These techniques are applied to Zea mays leaves to examine (1) heterogeneity in the responses of CO$_2$ uptake to light and to internal air-space CO$_2$ pressure fraction, and (2) heterogeneity in the susceptibility to photoinhibition during chilling.