Photoregulation and photodamage in Schefflera arboricola leaves adapted todifferent light environments

Leaves of the subtropical understorey shrub Schefflera arboricola Hayata gr owing in full sunlight had higher specific leaf weight, higher chlorophyll a/b ratios, lower total chlorophyll content and a threefold higher xanthoph yll cycle pigment content than leaves growing in a naturally shaded, but su nfleck-punctuated, environment. A number of measurements, all made in situ and during natural day/night cycles, were taken as follows: current photoch emical capacity (F-v/F-m after 10 min dark-adaptation), size and epoxidatio n state of the xanthophyll cycle, CO2 gas exchange and determination of the D1 synthesis rate. In sun leaves the lowest daily F-v/F-m was found to be approximately 0.6, the change from maximum correlating with an increase in zeaxanthin. Daily changes in zeaxanthin were partly due to de novo synthesi s and turnover. We suggest that sun leaves can dissipate most of the excess light energy absorbed safely via the photoprotective xanthophyll cycle. D1 synthesis rates did not correlate with photosynthetic photon flux density or F-v/F-m. The shade leaves had high F-v/F-m values and constant photosynt hetic rates throughout the day except during sunflecks, when photosynthetic rates increased and D1 synthesis accelerated, all without a substantial de crease in F-v/F-m. It seems that leaves of S. arboricola adapted to natural shade conditions can use sunflecks to contribute significantly to their pr oductivity. The third leaf type investigated was from greenhouse-grown plan ts of S. arboricola after exposure to full sunlight. These leaves showed a rapid and large reduction in F-v/F-m (to 0.3), which neither correlated wit h zeaxanthin formation nor recovered within the same day. From long-term ef fects following full sunlight exposure of greenhouse-grown plants we sugges t that this F-v/F-m reduction actually reflects photodestruction.