Apparent rates of photosynthetic O2 evolution and light-dependent pH change (Δ[H+]) were measured in solutions containing leaf slices and bubbled with gas streams of varied composition. Intracellular recycling of O2 and CO2 between photosynthetic and respiratory reaction systems seems to be possible in green cells in the light. As a result of photosynthetic O2 evolution, the tissue and the solution are never strictly anaerobic even after prolonged bubbling with N2. Internal gas exchange is more pronounced in slices from the leaves of maize (a C-4 plant) having the complex Kranz tissue differentiation of mesophyll and bundle sheath, than in the leaves of barley (a C-3 plant).Light-dependent Δ[H+] is observed under all conditions used, i.e. in air, air – CO2, pure N2, and N2 + CO2. During the early stages of greening of etiolated leaves, Δ[H+] is found to be closely related to photosynthetic O2 evolution. After prolonged greening, Δ[H+] is larger in air – CO2 than in air. In pure N2, Δ[H+] is smaller and apparent O2 evolution is larger than in N2 + CO2. These results are discussed in relation to the hypothesis that part of light-dependent Δ[H+] is independent of concomitant photosynthetic CO2 fixation.
Mechanism for photosynthetic O2 evolution.