Photosynthetic characteristics of durian (Durio zibethinus Murray) leaves grown under different light conditions

Pruning is not common for durian, and each leaf grows under different light conditions. Thus, information is required on the photosynthetic responses of leaves grown under different light conditions. In this study, durian trees grown in a greenhouse on Ishigaki Island, Okinawa prefecture, were used to measure photosynthetic responses. The leaves were grown under full sunlight, 50% shade, or 70% shade for 1 month. Daily maximum photosynthetic photon flux density (PPFD) was 1200-2000 µmol m-2 s-1, except for 3 days. Chlorophyll content of the 70%-shaded leaves was the highest among the treatments. The ratio of chlorophyll a/b decreased as light intensity decreased; thus, the light-harvesting chlorophyll-protein complex might increase under low-light conditions, and so shaded leaves might be able to harvest light efficiently. The photosynthetic rate of 70%-shaded leaves was the highest among the treatments under 400-2000 µmol m-2 s-1 PPFD conditions. Moreover, the photosynthetic rate of 70%-shaded leaves under 400-800 µmol m-2 s-1 PPFD was equal to that of sunlight-exposed leaves when measured under 2000 µmol m-2 s-1 PPFD. The maximum photosynthetic rate of the photosynthesis-intercellular CO2 concentration (A/Ci) response curve, which is correlated with ribulose bisphosphate (RuBP) regeneration capacity, i.e., the light reaction capacity, was the highest in 70%-shaded leaves. The initial slope of the A/Ci response curve, which is correlated with RuBP carboxylation activity, i.e., the dark reaction capacity, was not affected by shade treatment. Fresh and dry weights of 70%-shaded leaves per unit leaf area were the lowest, indicating that these leaves were the thinnest among the treatments. The dry weight per fresh weight ratio of shaded leaves was lower than that of sunlight-exposed leaves. In conclusion, leaves grown under strong light conditions had low photosynthetic ability because of their low chlorophyll content.