Leaf Orientation, Incident Sunlight, and Photosynthesis in the Alpine Species Suassurea superba and Gentiana straminea on the Qinghai-Tibet Plateau

Abstract
The extremely high level of solar radiation on the Qinghai-Tibet Plateau may induce photoinhibition and thus limit leaf carbon gain. To assess the effect of high light, we examined gas exchange and chlorophyll fluorescence for two species differing in light interception: the prostrate Saussurea superba and the erect-leaved Gentiana straminea. In controlled conditions with favorable water and temperature, neither species showed apparent photoinhibition in gas exchange measurements. In natural environment, however, their photosynthetic rate decreased remarkably at high light. Photosynthesis depression was aggravated under high leaf temperature or soil water stress. Relative stomatal limitation was much higher in S. superba than in G. straminea and it remarkably increased in the later species at midday when soil was dry. Fv/Fm as an indicator for photoinhibition was generally higher in S. superba than in the other species. Fv/Fm decreased significantly under high light at midday in both species, eve...