PHOTOSYNTHETIC CHARACTERISTICS OF TROPICAL TREE SPECIES WITH SPECIAL REFERENCE TO PALMS

Forest ecosystems account for almost 30% of the earth's total land area Among them tropical forests and other tree crop species play a pivoted role in the overall carbon balance of the earth. Perennial trees act as carbon sinks, absorbing CO2 from the atmosphere and storage for longer periods and thereby help to reduce the green house effect. Tropical trees including palms have a 70 to 100 yrs or more life span and are characterized by slow growth and storage of large quantities of carbon in their vegetative components. Though, sunlight is not a limiting factor in the tropical region of the earth, leaves of evergreen perennial trees have low to moderate photosynthetic capacity below 20 mu mol m(2) s(-1). These tree species achieve their optimum rate of photosynthesis at low photosynthetic photon flux densities which are about one third to half that of many annual plant species. Most of such trees have a low light use efficiency and the physiological light response of these tree canopies are modulated by investment of nitrogen and carbon in photosynthetic enzymes in ''sun leaves'' compared to shade leaves. Investment of less nitrogen as leaf protein and low photosynthetic enzymes are some characters which attributed to low photosynthetic capacity of these tree species. The evaluated CO2 transfer conductance from the sub stomatal cavity to the sites of carboxylation within the chloroplast (g(w)) and the chloroplastic partial pressure of CO2 (P-c) by concurrent measurement of leaf gas exchange and on-line C-13 discrimination of palms were found to be substantially low compared to other reported plant species. These observed results further explain low photosynthetic capacity of palm species.