Quantitative evaluation of the counterbalance between photosynthetic stimulation and depression caused by low partial pressure of O sub(2) and CO sub(2) in alpine atmospheres

To evaluate the effects of low atmospheric pressure on leaf photosynthesis, we compared the photosynthesis of identical leaves of Fagus crenata at lowland (0 m a.s.l.) and at highland (2360 m a.s.l.). At the high altitude, the atmospheric pressure and partial pressure of CO sub(2) at intercellular air spaces in the leaf (C sub(i360)) decreased to 77% and 78% of those at the low altitude, respectively. On the other hand, the efficiency of photosynthetic CO sub(2)-utilization was apparently higher at the high altitude because of a mitigation of the O sub(2)-inhibition of ribulose- 1,5-bisphosphate carboxylase/oxygenase (Rubisco) under low ambient partial pressure of O sub(2). This stimulation of photosynthesis partly compensated a photosynthetic depression due to the low C sub(i360), and the net assimilation rate (A sub(n360)) at the high altitude retained 94% of the value at the low altitude. A theoretical model indicated that the stimulation of photosynthesis at high altitudes depend on internal conductance (g sub(i)) and/or on Rubisco content. The model demonstrated that low atmospheric pressure at high altitudes caused severe restrictions of photosynthesis when leaves had a small g sub(i) and/or a large amount of Rubisco, whichever are repeatedly reported in alpine plants.