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

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 mols CO2 m-2s-1) than in W. ugandensis (0.3 mols 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 mols m-2s-1and 350 mols m-2s-1respectively) for both species (and ranged between0.4 and 0.5 mols CO 2 m-2s-1). Generally, P. fulva seedlings had significantly (P < 0.05) greater reductions (from 1.0 to0.4 mols CO 2 m-2s-1
) in photosynthetic rates in response to increasing shade than did W. ugandensis (0.6 to 0.4
mols CO 2 m-2s-1). However, W. ugandensis attained significantly (P < 0.05) higher photosynthetic rates under shade
conditions (0.6 mols CO2 m-2s-1) than full sunlight conditions (0.3 mols 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 mols CO2 mmol-1 H2O) was significantly (P <
0.05) higher that for W. ugandensis (1.5 mols 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.