Photosynthesizing in the rain: beneficial effects of twig wetting on corticular photosynthesis through changes in the periderm optical properties

Abstract

Summary Field observations with young twigs of woody species during rain indicated that their normally grayish color turns darker or even greenish upon wetting. The change persists as long as rain is falling and recovery to the normal grayish tint upon re-drying occurs within minutes. These naked eye observations were confirmed through spectral reflectance measurements on twig surfaces of seven species, invariably showing considerable reduction in reflectance, particularly at the spectral regions of maximum absorbance by photosynthetic pigments, when wet. Since isolated periderms were not permeable to water and wetting of isolated chlorenchymatous strips did not appreciably affect their reflectance, we argued that wetting affects the optical properties of periderms per se , possibly leading to corresponding changes in corticular photosynthesis. Indeed, light transmittance of isolated periderms from Quercus coccifera was increased by ca. 45% due to a corresponding decrease in reflectance. As a result, wetting of intact twigs resulted in higher photosynthetic linear electron transport rates at low but lower at high incident photon fluence rates, the later indicating arrival at potentially photoinhibitory light levels. Since light levels are low during rain, a beneficial effect on corticular photosynthesis is inferred. The above changes were fully reversible during repeated brief wet/dry cycles. Yet, when wetting was artificially prolonged at high light, chronic photoinhibition was evident. It is suggested that the rapid and reversible changes in periderm optical properties upon wetting and drying are of dual adaptive significance allowing more corticular photosynthesis during rain and minimizing the risk of photoinhibition after sudden sunlight.