Photosynthetic acclimation of Grammatophyllum speciosum to growth irradiance under natural conditions in Singapore.

Grammatophyllum speciosum, a native species to Singapore, have become extinct mainly due to habitat loss. Recently, Singapore has reintroduced G. speciosum into the natural environment under the orchid conservation programme. In this study, leaves of G. speciosum grown under low light (LL) under natural conditions had faster expansion rate and higher specific leaf area than leaves grown under intermediate light (IL) and high light (HL). All leaves had more than 95% midday relative water content. Although midday F/F ratios were lower in HL leaves than in IL and LL leaves, none of them exhibited chronic photoinhibition. HL leaves had upregulated their light utilization through higher photochemical quantum yield (ΔF/F') and greater electron transport rate. HL leaves also had higher non-photochemical quenching, indicating that they had higher capability to dissipate excess light as heat, which was supported by their lower chlorophyll but higher carotenoids content. Although there was a linear correction between leaf temperature and photosynthetic photon flux density (PPFD), no correlations were found between stomatal conductance (g) and PPFD, g and leaf temperature. Light-saturated photosynthetic CO assimilation rate (A ) was significantly higher in HL leaves than those of IL and LL leaves. However, all leaves had similar light-saturated stomatal conductance. Although LL leaves had higher leaf total reduced nitrogen that those of IL and HL leaves, none of them seemed to suffer from nitrogen deficiency during the experimental period. To conclude, G. speciosum is able to survive under different growth irradiances without watering and adding fertilizers.