Leaf Physiology of Shade-Grown Cycas micronesica Leaves Following Removal of Shade

The photosynthetic characteristics of Cycas micronesica K.D. Hill were studied from August 1998 until February 1999 using chlorophyll fluorescence and gas-exchange techniques to determine the responses to long-term shade of 35% ambient light transmission, followed by the transfer of shade-grown leaves into full-sun conditions. The shade-grown leaves exhibited increased photosynthetic light use efficiency and effective quantum efficiency of photosystem II (PS II) and decreased photosynthetic light saturation point and dark respiration when compared with leaves grown in full sun. Shade was removed from shade-grown C. micronesica leaves during midday on December 14, 1998, when effective quantum efficiency of shaded leaves was 45% greater than that of sun leaves. Following one hour in full sun, effective quantum efficiency of the shade-grown leaves declined to below that of the sun-grown leaves. After receiving full sunlight for the rest of the photoperiod, maximum quantum efficiency of PS II photochemistry for shade-grown leaves was below that of sun-grown leaves throughout the night. The damage caused by excessive light to shade-grown leaves progressed for the first three days after shade removal. On day 3, effective quantum efficiency during midday was 30%, net photosynthesis was 47%, apparent quantum yield was 65%, and light compensation point was 136% of that for sun-grown leaves. After day 3, the relationship between full-sun leaves and the previously shaded leaves for these response variables was relatively stable. Two months following removal of shade, the previously shaded leaves continued to exhibit damage from high light. These results have application to transplanting cycad plants from a shaded nursery to a field site or, after tropical cyclones, where protective forest canopy cover has been destroyed and cycad plants in the forest subcanopy are abruptly exposed to full-sun conditions.