Higher Leaf Area Per Plant can Compensate the Decline in Net Photosynthetic Rate in Moderately Shade Grown Seedlings of Hevea Brasiliensis in Low Winter Temperature

Photosynthetic performance of Hevea brasiliensis seedlings at different light conditions was studied to assess their adaptability to low light levels. Seedlings were grown under full light as well as low light conditions (65% and 25% of incoming radiation). Leaf photosynthetic rate (P_N) was higher in full sunlight grown plants both during the first week of February (winter) and April (post-winter) than low light grown plants. Under severe low light condition (25% of incident radiation), the photosynthetic rate was as low as 2.0 ?mole CO₂ m⁻² s⁻¹ in winter and 10.31 ?mole CO₂ m⁻²s⁻¹ in post winter period when measured at ambient light. In addition to that, photosynthetic rate (P_N) versus incident radiation (PFD) curves indicated that the light saturated photosynthetic rate in full sunlit plants was higher than shade grown plants. Chlorophyll fluorescence studies carried out during the winter season revealed that dark adapted Fv/Fm was higher in shade grown plants than open grown plants. This result indicated that low temperature-induced photo inhibition was severe in full sunlight grown plants than to shade grown plants which was reflected by low leaf chlorophyll content in open grown plants than shade grown plants. However, a small decline in leaf photosynthesis under 65% light condition was more than compensated by large increase (54%) in leaf area per plant leading to higher net canopy photosynthesis. Thus the plants grown under 65% light condition were taller and also had higher biomass than full sunlit plants during low temperature period. The increased biomass in Hevea seedlings at 65% light condition may be due to higher canopy photosynthesis.