THE RESPONSE OF XANTHOPHYLL CYCLE-DEPENDENT ENERGY-DISSIPATION IN ALOCASIA-BRISBANENSIS TO SUNFLECKS IN A SUBTROPICAL RAIN-FOREST

The photosynthetic responses of leaves of Alocasia brisbanensis (EM. B ailey) Domin (Araceae) to sunflecks were monitored via chlorophyll flu orescence beneath the canopy of a subtropical rainforest in Australia. Additionally, the size and conversion state of the xanthophyll cycle were determined. Acclimation to understory environments that regularly experienced sunflecks involved small increases in the size of the xan thophyll cycle pool in comparison to understory plants that never rece ived sunflecks. In understory plants that regularly experienced sunfle cks the rate of photochemistry and the level of xanthophyll cycle-depe ndent energy dissipation closely tracked changes in incident PFD. Subs equent to the first sunfleck plants tended to retain their pool of xan thophyll cycle carotenoids as the deepoxidised forms (antheraxanthin a nd zeaxanthin) throughout the day. Retention of these deepoxidised for ms apparently allows the trans-thylakoid membrane proton gradient to e ngage and disengage dissipation rapidly in response to a sunfleck, the reby mitigating photooxidative damage and ensuring a rapid return to e fficient light utilisation via photosynthesis in limiting light. Our r esults were also in agreement with previous studies that demonstrated a requirement for light activation of photosynthesis.