PHOTOSYNTHESIS OF TROPICAL TREE SEEDLINGS IN RELATION TO LIGHT AND NUTRIENT SUPPLY

The photosynthetic performances of two tropical tree species, a pioneer Nauclea diderrichii (De Wilde.) Merrill, and a climax species Entandrophragma angolense (Welw.) C. DC., both from West Africa, were compared. Young plants were grown in controlled environments where photon flux density and nutrient supply were varied. A/C(i) curves of the two species were very similar, both showing an increased initial slope and maximal rate of photosynthesis when the leaves had been grown at high photon flux density. Nutrient supply had a similar, but smaller, influence. However, the pioneer Nauclea showed a greater increase in initial slope in relation to both light and nutrient supply. It also showed greater anatomical response to light, developing a second layer of palisade mesophyll when grown at high photon flux density. Light response curves of photosynthesis were also determined. The apparent quantum efficiency was insensitive to growing conditions, but the carboxylation rate, dark respiration and light-saturated photosynthetic rate were sensitive to the light climate of growth. The pioneer Nauclea displayed an increase in assimilation rate as the photosynthetic photon flux density was increased beyond 300-mu-mol m-2 s-1, but Entandrophragma did not, and often showed a decline.