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The sensitivity of photosynthetic and respiratory functions to supraoptimal temperature stress was compared after heating of leaves, protoplasts and membrane systems of spinach (Spinacia oleracea L. cv. Monatol) and lettuce (Valerianella locusta [L.] Betcke) in situ and in vitro.After heating of whole leaves or protoplasts, endogenous respiration was not or only slightly affected at temperatures which caused a marked decrease of photosynthesis. This was manifested when mitochondria and thylakoids were isolated from heat-treated leaves. In the presence of exogenous substrates, mitochondrial electron transport and phosphorylation were even somewhat stimulated compared to the controls.Inactivation of net CO(2) uptake of whole leaves following heat stress and of the photochemical activities of chloroplast membranes isolated from heat-treated leaves of the same origin occurred nearly simultaneously. In protoplasts, photosynthesis was inactivated at temperatures far below those which caused drastic changes in the integrity of the tonoplast and the plasmalemma. This indicates that damage occurring within the chloroplasts rather than alterations in the compartmentation of the cell is responsible for the high sensitivity of photosynthesis to supraoptimal temperature stress.Mitochondria and thykaloids isolated from the same preparation of intact leaves under comparable conditions and subjected to heat treatment in vitro, however, were inactivated nearly in the same temperature range. Thus, mitochondria are much more stable within their cytoplasmic environment.

Effects of High-Temperature Stress on Various Biomembranes of Leaf Cells In Situ and In Vitro