Photoregulation and photodamage in Schefflera arboricola leaves adapted to different light environments

Leaves of the subtropical understorey shrub Schefflera arboricola Hayata growing in full sunlight had higher specific leaf weight, higher chlorophyll a/b ratios, lower total chlorophyll content and a threefold higher xanthophyll cycle pigment content than leaves growing in a naturally shaded, but sunfleck-punctuated, environment. A number of measurements, all made in situ and during natural day/night cycles, were taken as follows: current photochemical capacity (F-v/F-m after 10 min dark-adaptation), size and epoxidation 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 synthesis 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 photosynthetic rates throughout the day except during sunflecks, when photosynthetic rates increased and D1 synthesis accelerated, all without a substantial decrease 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 productivity. The third leaf type investigated was from greenhouse-grown plants 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 with zeaxanthin formation nor recovered within the same day. From long-term effects following full sunlight exposure of greenhouse-grown plants we suggest that this F-v/F-m reduction actually reflects photodestruction.

Photosynthetic responses of the coral Montipora digitata to cold temperature stress

Coral bleaching events have become more frequent and widespread, largely due to elevated sea surface temperatures. Global climate change could lead to increased variability of sea surface temperatures, through influences on climate systems, e.g. El Nino Southern Oscillation (ENSO). Field observations in 1999, following a strong ENSO, revealed that corals bleached in winter after unusually cold weather. To explore the basis for these observations, the photosynthetic responses of the coral species Montipora digitata Studer were investigated in a series of temperature and light experiments. Small replicate coral colonies were exposed to ecologically relevant lower temperatures for varying durations and under light regimes that ranged from darkness to full sunlight. Photosynthetic efficiency was analyzed using a pulse amplitude modulated (PAM) fluorometer (F-0, F-m, F-v/F-m), and chlorophyll a (chl a) content and symbiotic dinoflagellate density were analyzed with spectrophotometry and microscopy, respectively. Cold temperature stress had a negative impact on M digitata colonies indicated by decreased photosynthetic efficiency (F-v/F-m), loss of symbiotic dinoflagellates and changes in photosynthetic pigment concentrations. Corals in higher light regimes were more susceptible to cold temperature stress, Moderate cold stress resulted in photoacclimatory responses, but severe cold stress resulted in photodamage, bleaching and increased mortality. Responses to cold temperature stress of M digitata appeared similar to that observed in corals exposed to warmer than normal temperatures, suggesting a common mechanism. The results of this study suggest that corals and coral reefs may also be impacted by exposure to cold as well as warm temperature extremes as climate change occurs.