Tradeoffs between irradiance capture and avoidance in semi-arid environments assessed with a crown architecture model

Plants in arid environments cope with stress from excessive irradiance by p hysiological photoprotection of the photosynthetic apparatus and by structu rally reducing the leaf area exposed to the sun (structural photoprotection ). We assessed the ecological relevance of structural photoprotection in tw o plants of contrasting architecture cooccurring in a semi-arid environment , using the three-dimensional canopy model YPLANT. We compared the role of crown geometry in avoiding excessive radiation, analysed the costs of struc tural photoprotection in terms of reduction of potential carbon gain, and c ompared these costs with those due to seasonal constraints of photosynthesi s and tissue ageing. The results of the model simulations indicated that ca nopy architecture of Stipa tenacissima (a tussock grass) and Retama sphaero carpa (a leafless leguminous shrub) minimized the risk of overheating and p hotooxidative destruction of the photosynthetic apparatus with steeply orie nted foliage and moderate self-shading. But this structural photoprotection imposed an increased cost in terms of potential carbon gain. Diurnal and s easonal patterns of light interception by the crown of these plants transla ted into a simulated potential carbon gain only half that of an equivalent, horizontal photosynthetic surface. This reduction in potential carbon gain , due to irradiance avoidance, was similar to that imposed by water shortag e. S. tenacissima, which ceases photosynthetic activity during periods of d rought, exhibited more structural avoidance of irradiance than R. sphaeroca rpa, which remains active throughout the year. This illustrates the influen ce of the capacity of plants to utilize light for carbon fixation on the tr ade-offs between irradiance capture and avoidance. Structural avoidance of excessive radiation efficiently prevents the risk of damage by intense irra diance, has no special maintenance costs, and is biomechanically cheaper th an enhanced light harvesting by a horizontal canopy, which points to struct ural photoprotection as a very effective strategy to cope with high irradia nce stress in poor and adverse habitats. (C) 1999 Annals of Botany Company.