Antioxidant Metabolism during Acclimation of Begonia × erythrophylla to High Light Levels

Begonias bleached in bright light

The concept of �shade leaves� and �sun leaves� and their adaptations that maximize photosynthesis in specific light environments are well known to botanists. However, in nature, things are rarely this clear-cut. It is possible, for example, that a shade plant may become exposed suddenly to full daylight, and it is this type of situation that has been modelled by Burritt and Mackenzie (University of Otago, New Zealand, pp. 783-794). They have worked with Begonia � erythrophylla, a hybrid begonia, popular as a houseplant. Many begonias show adaptation to growth in the shade, and indeed several wild Begonia species are described as �shade-demanding�. In this study, the authors have grown the hybrid plants in full sunlight or in shade at about 10-15 % of the photosynthetically active radiation (PAR) experienced by the �sun plants�. Some of the shade-grown plants were then transferred to full sunlight. In a very comprehensive analysis, the authors have compared a range of characters from photosynthetic capacity to antioxidant metabolism in the leaves subjected to the experimental treatments. As might be expected, shade leaves show a saturation of photosynthetic electron transport at much lower PAR levels than do sun leaves, and when shade leaves are transferred to sunlight, the excess energy leads to the production of reactive oxygen species, photobleaching of photosynthetic pigments and oxidative damage to membranes and enzymes. In general, sun leaves have a greater capacity to dissipate reactive oxygen species with higher levels of the relevant enzymes. Furthermore, although the shade leaves transferred to full sun show some acclimation over a period of 25 d this is far from complete and it is only the newly formed leaves that are fully adapted to the sunlight. I will think very carefully the next time I am tempted to move a house plant from a shady corner to a brightly lit window-sill!