Photosynthesis and photorespiration in mangroves

Abstract

It has been shown in our earlier work that in many mangroves aspartate is synthesized within a period of less than five seconds and to avoid intense light and high temperatures of midday, the stomata remain wide open in the early morning up to 10–11 a.m. In the present investigation this has been confirmed in Lumnitzera and Ceriops. A further probe into the intricate process of photosynthesis in Rhizophora mucronata Lamk, Rhizophora apiculata B1., Ceriops tagal (Perr) C.B. Rob, Bruguiera gymnorrhiza (L) Lamk, Avicennia officinalis L., Avicennia marina (Forsk.) Vierh. Aegiceras corniculatum (L) Blanco, Lumnitzera racemosa (Willd) and Acanthus ilicifolius L. by supplying uniformly labelled aspartate and studying enzymes of carbon fixation has also been attempted. In these mangroves the activity of PEP Case is considerably higher than that of RuBP Case and justifies synthesis of aspartate as the initial product. The activity of pyruvate-Pi-dikinase can be detected in all these mangroves which provides continuous supply of PEP for carbon assimilation. The decarboxylating enzyme in the form of NAD-malic enzyme is also detected. CO2 compensation point in Lumnitzera and Ceriops are low. The pulse chase experiment with 14CO2 and labelled aspartate, the photosynthetic enzyme studies and photorespiration in Lumnitzera and Ceriops suggest operation of a modified C4 path in the mangroves. It is argued that such a pathway is a physiological response to ecological conditions which impose stress on the ecosystem.