'Photoinhibition' of Heliconia under natural tropical conditions: The importance of leaf orientation for light interception and leaf temperature

The influence of irradiance on photosynthesis under natural conditions was studied in aseasonal Singapore using three Heliconia taxa: H. rostrata, H. psittacorum x H. spathocircinata cv. Golden Torch and H. psittacorum cv. Tay. When grown under full sunlight, all three heliconias exhibited reduced photosynthetic capacities and lowered chlorophyll content per leaf area as compared with those grown under intermediate and deep shade. A marked decrease in the chlorophyll fluorescence F-v/F-m ratio and an increase in photochemical quenching (1 - q(p)) and nonphotochemical quenching (q(N)) were observed in upper leaves of plants grown under full sunlight. Increases in q(N) suggest that 'photoinhibition' (decreases in F-v/F-m) in Heliconia grown under natural tropical conditions are probably due to photoprotective energy dissipation processes. The quantum yield, the maximum photosynthetic rate, F-v/F-m and the chlorophyll content of upper leaves were lower than those of lower leaves on the same plants grown under full sunlight. Similarly, lower values were obtained for the tip (sun) portion than for the base (shaded) portion of the leaves. The changes in F-v/F-m and in the levels of (1 - q(p)) in leaves grown under intermediate and deep shade were negligible in plants during the course of day. However, there was a steep decrease in F-v/F-m and an increase in the levels of (1 - q(p)), along with an increase in incident light in the sun leaves, The lowest F-v/F-m and the highest level of (1- q(p)) indicated minimum PSII efficiency at midday in full sun. These results indicate that, in Heliconia, the top leaves (particularly leaf tips) experienced sustained decreases in PSII efficiency upon exposure to full sunlight. Although all three taxa exhibited sustained decreases in photosynthetic capacity in full sunlight, the sun leaves of 'Tay' showed higher photosynthetic capacity than those of the other two taxa. This could be due, at least in part, to the vertical leaf angle and smaller lamina area, When the upright leaves of 'Tay' were constrained to a horizontal angle, they exhibited lower PSII efficiency (F-v/F-m ratio), while horizontal leaves of 'Rostrata' and 'Golden Torch' inclined to near-vertical angles showed increased efficiency. Thus, an increase in leaf angle helps to achieve a reduction in the sustained decrease in PSII efficiency by decreasing the levels of incident sunlight and subsequently the leaf temperature.