The metabolism of Carbon dioxide in maize leaf epidermis

Abstract

Though the maize leaf has a large proportion (more than 30%) of its volume occupied by largely non-photosynthetic (yet probably respiring) epidermal tissue it has been reported that no carbon dioxide (CO2) escapes from it in the light, even into a CO2 -free air stream. The aim of this investigation was to determine if the epidermis was active and if it was to discover the fate of the CO2 produced in it by respiration. Light and electron microscopy revealed that all epidermal cells had an apparently viable cytoplasm containing a full complement of organelles. The long epidermal cells, despite their large size, contained less cytoplasm than the cells of the stomata, an arrangement suggesting an uneven distribution of respiratory activity. Respirometry, despite considerable problems involved in the isolation of viable tissue, tended to confirm this view. Appreciable respiration was recorded in preparations containing only active stomatal cells and little or no increase in respiratory gas exchange could be detected in epidermal strips having a larger proportion of intact cells. Respiration was apparently unaffected by light. Since CO2 was released front isolated epidermal tissue in both the light and dark it was thought necessary to investigate potential mechanisms by which it could be retained in the light. A number of carboxylating and associated enzymes were assayed in extracts prepared from the lower epidermis of maize leaves. Evidence for appreciable activities of both phosphoenolpyruvate (PEP) carboxylase and malic enzyme were obtained, both were localised in the stomata. Some evidence for the presence of ribulosebisphosphate (RuBP) carboxylase in long epidermal cells was also found, though it was felt to be too circumstantial for a definite conclusion to be reached. The cuticular resistance to water vapour loss indicated that the resistance to CO2 escape was probably very high. However, whilst a high cuticular resistance would reduce direct CO2 escape it would also increase the potential advantage to the leaf of preventing the release of gaseous CO2 within it. Overall it was felt that though there was no single mechanism for the retention of CO2 in the epidermis of maize several factors acting together reduced its release to a level undetectable by the techniques used up to now. These factors included a reduced respiration rate in long epidermal cells, a bunching of mesophyll chloroplasts close to the epidermal cells and an appreciable carboxylase activity in the active stomatal cells.