Influence of varying light regimes on photosynthesis and related variables on tree seedlings of Warburgia ugandensis Sprague and Polyscias fulva (Hiern) harms

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 µmols CO2 m-2 s-1) than in W. ugandensis (0.3 µmols 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 µmols m-2s-1 and 350 µmols m-2s-1 respectively) for both species (and ranged between 0.4 and 0.5 µmols CO2 m-2s-1). Generally, P. fulva seedlings had significantly (P < 0.05) greater reductions (from 1.0 to 0.4 µmols CO2 m-2s-1) in photosynthetic rates in response to increasing shade than did W. ugandensis (0.6 to 0.4 µmols CO2 m-2s-1). However, W. ugandensis attained significantly (P < 0.05) higher photosynthetic rates under shade conditions (0.6 µmols CO2 m-2s-1) than full sunlight conditions (0.3 µmols 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 µmols CO2 µmol-1 H2O) was significantly (P < 0.05) higher that for W. ugandensis (1.5 µmols CO2 µmol-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 fulv