Photosynthesis in mesophyll protoplasts and bundle sheath cells of various types of C4 plants II. Chlorophyll and hill reaction studies

Abstract

Summary Mesophyll protoplasts of C4 plants are capable of photochemical oxygen evolution through the Hill reaction up to 400 to 600 μmoles O2 evolved/mg chl/hr when provided with p-benzoquinone as an oxidant. Dichlorophenolindophenol and ferricyanide were less effective as oxidants in comparison to p-benzoquinone. The Hill reaction with p-benzoquinone was more effective with mesophyll protoplasts than with isolated mesophyll chloroplasts while that with ferricyanide was more effective with mesophyll chloroplasts than with mesophyll protoplasts. Although mesophyll protoplasts had a high potential for the Hill reaction, the natural electron acceptor, HCO3-, did not effectively substitute as the oxidant for the Hill reaction. This correlated with a low CO2 fixation capacity with C4 mesophyll protoplasts. Bundle sheath cells showed a varying capacity for the Hill reaction depending on the species. In C4 species having high NADP-malic enzyme in bundle sheath cells, there was little or no activity with bundle sheath cells or chloroplasts of sorghum and sugarcane when using p-benzoquinone, dichlorophenolindophenol, and ferricyanide as the Hill oxidant, while some activity was found with bundle sheath cells or chloroplasts of Digitaria sanguinalis and Setaria lutescens. All C4 plants having low NADP-malic enzyme in bundle sheath cells in all cases were capable of photochemical oxygen evolution. The maximum potential for the Hill reaction in bundle sheath cells as measured with p-benzoquinone correlated with the capacity for endogenous CO2 fixation by the isolated bundle sheath cells. Independent photosynthesis by mesophyll cells of C4 plants may be limited by the level of carbon assimilating enzymes, while independent photosynthesis by bundle sheath cells, at least in some species, may be limited by the photochemical capacity of Photo-system II. The distribution of chlorophyll between mesophyll cells and bundle sheath cells of various C4 species was estimated by determining the chlorophyll a/b ratio of the two cell types and whole leaves of C4 plants or by complete enzymatic separation of mesophyll from bundle sheath cell tissue. The percentage of leaf chlorophyll in bundle sheath cells ranged from 37 to 74% with the C4 species examined, which suggests that there is substantial photochemical potential in bundle sheath cells.