Ontogenetic Development of Crassulacean Acid Metabolism as Modified by Water Stress in Peperomia
Peperomia species (Piperaceae) may show C3 photosynthesis, CAM photosynthesis, or CAM-cycling in which stomata and gas exchange occur primarily during the day with a diurnal fluctuation of acidity typical of CAM (Holthe et al. 1992). In CAM-cycling, enzymes of CAM are present (Sipes and Ting 1985; Ting 1985; Ting and Sipes 1985). In Peperomia species, there is a division of photosynthetic labor among the various tissues of the leaf (Nishio and Ting 1987, 1993). The upper multiple epidermis functions as a window leaf filtering out much of the incident radiant energy and has about 5% of the leaf chlorophyll (Nishio and Ting 1987). The middle palisade mesophyll has most of the C3 photosynthetic activity including ribulose 1,5-bisphosphate carboxylase/oxygenase(RUBISCO) and the light-dependent CO2 fixation. The lower spongy mesophyll has the appearance of CAM tissue, and along with the multiple epidermis contains most of the phosphoenolpyruvate carboxylase (PEPC) and dark CO2 fixation (Nishio and Ting 1987). The lower spongy mesophyll is enriched in PSI relative to PSII, evidently related to the extra ATP requirement of this tissue (Nishio and Ting 1993). Stomata are restricted to the abaxial epidermis. Malic acid accumulates during the night. During the subsequent day period, decarboxylation keeps CO2 high in the vicinity of the photosynthetic palisade cells, thereby minimizing photorespiration (Nishio and Ting 1987).
KeywordsSalt Stress Water Stress Crassulacean Acid Metabolism Phosphoenolpyruvate Carboxylase Crassulacean Acid Metabolism Plant
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