Experiments were conducted in alpine Kobresia humilis meadow near Haibei Alpine Meadow Ecosystem Research Station (37°29′~37°45′ N, 101°12′~101°33′E;alt.3 200 m). The results measured by CI 301PS portable photosynthetic system indicated that there was marked midday depression of net photosynthetic rate (Pn) in alpine plants of Gentiana straminea and Saussurea superba (Fig.1). Measurements using oxygen electrode demonstrated that both photosynthetic O 2 evolution rate and apparent quantum efficiency (AQY) were depressed around noon (Fig.4). All these phenomena of midday depression could not be fully accounted for by the changes in stomatal conductance (Gs), and there was also no relationship with temperature in canopy. The changes in vapor pressure deficit (VPD) between leaves and air was large enough to affect Pn, but there was no reduction of VPD around noon, so it could not bring about the midday depression of Pn observed in G. straminea and S. superba . When an iron wire net was used to reduce the total solar radiation, PAR and UV B by about 30% at local solar noon (about 13:15 Beijing time), no midday depression of photosynthetic O 2 evolution rate or AQY was observed (Fig.5). All these phenomena demonstrated that midday depression of photosynthesis could be attributed to strong solar radiation, which could also raise leaf and air temperature and thereby VDP around noon in the alpine K. humilis meadow regions.
国家自然科学基金 (3 95 70 12 9
3 9770 13 5 )
国家基础研究规划项目(19980 40 80 0 )
中国科学院特别支持项目 (KZ95T 0 4,KZ95T 0 6,
KZ95 1 A1 2 0 4,KZ95 1 A1 3 0 1)