Abstract

The physiology of seedlings of Warburgia ugandensis and Polyscias fulva were studied at varying light levels. Photosynthesis was significantly higher in P. fulva(1.4 mmols CO₂ m^-2 s^-1) than in W. ugandensis (0.3 mmols CO₂ m^-2s^-1) when grown in full sunlight. However, photosynthesis was not significantly (P > 0.05) different under moderate and dense shades (PPFD about 600 mmols m^-2s^-1 and 350 mmols m^-2s^-1 respectively) for both species (and ranged between 0.4 and 0.5 mmols CO₂ m^-2s^-1). Generally, P. fulva seedlings had significantly (P < 0.05) greater reductions (from 1.0 to 0.4 mmols CO₂ m^-2s^-1) in photosynthetic rates in response to increasing shade than did W. ugandensis (0.6 to 0.4 mmols CO₂ m^-2s^-1). However, W. ugandensis attained significantly (P < 0.05) higher photosynthetic rates under shade conditions (0.6 mmols CO₂ m^-2s^-1) than full sunlight conditions (0.3 mmols CO₂ m^-2s^-1). Leaf temperatures were highest (30°C) in full sunlight and lowest (27°C) in dense shade for both species. Generally, stomatal conductance, transpiration and water use efficiency showed significantly (P < 0.05) higher values for shade grown W. ugandensis and full sunlight grown P. fulva.  Water use efficiency for P. fulva (2.3 mmols CO₂ mmol^-1 H₂O) was significantly (P < 0.05) higher that for W. ugandensis (1.5 mmols CO₂ mmol^-1 H₂O) in full sunlight than. It was concluded that P. fulva was also less adapted to growth at low light intensity while W. ugandensis was better adapted to growth at low light levels because of its significantly greater ability to maintain higher rates of photosynthesis in low light intensity. Therefore in East Africa P fulva can be a better agroforestry species than W. ugandensis.

Key words: Net carbon exchange, forest tree seedlings, shade, Warburgia ugandensis, Polyscias fulva