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Grammatophyllum speciosum orchid plants were grown both on the ground and on trees under high light (HL) and low light (LL) conditions in Pasir Ris Park, Singapore. This paper studied photosynthetic energy utilization investigated by chlorophyll (Chl) fluorescence and photosynthetic pigments; plant water status measured by leaf relative water content (RWC) and stomatal conductance (gs). All plants had midday leaf RWC greater than 80%, indicating no severe water deficit occurred in any plants. Except for plants grown on trees under LL, midday depression of Fv/Fm ratios occurred in all other plants, indicating dynamic photoinhibiton. There was no chronic photoinhibition in any plants as Fv/Fm ratios recovered to healthy level at early morning. Grown on the ground under HL, G. speciosum had higher photochemical quantum yield (ΔF/Fm’), greater electron transport rate (ETR) and non-photochemical quenching (NPQ), indicating their higher efficiency in utilization and dissipation excess energy. Plants grown on the ground under HL had lower Chl and Chl/Car ratio, but higher content of carotenoids and Chl a/b ratio compared to the other plants, implying their acclimation to HL conditions. The highest gs was recorded at 1000 h, suggesting the avoidance of drought stress by widely opening stomata only for a short period of time in the early morning. Higher gs was related to higher leaf temperature, indicating that the role of gs in moderating leaf temperature. Understanding the effects of growth irradiance on photosynthesis of G. speciosum would be an important step towards a sustainable conservation of native orchids in Singapore.

Orchid Conservation in Singapore under Natural Conditions: Responses of Grammatophyllum Speciosum to Growth Irradiances