STOMATAL DYNAMICS AND ITS IMPORTANCE TO CARBON GAIN IN 2 RAIN-FOREST PIPER SPECIES .2. STOMATAL VERSUS BIOCHEMICAL LIMITATIONS DURING PHOTOSYNTHETIC INDUCTION

The relative importance of biochemical and stomatal limitations on ass imilation (A) during photosynthetic induction were compared in sun and shade plants of Piper auritum, a pioneer tree, and shade plants of Pi per aequale, a shade tolerant shrub native to a Mexican tropical rainf orest. For non-induced leaves, increases in A during induction depende d on the dynamics of stomatal conductance (g(s)) and ribulose-1,5-bisp hosphate carboxylase (RuBisCO) activation. At high leaf-air vapor pres sure deficit (VPD), more of the limitation during induction was stomat al. Calculations of mesophyll conductance revealed longer time constan ts for shade than for sun plants. However, no differences in the time course of RuBisCO activity between sun- and shade-plants were found. W e conclude on the basis of the similar RuBisCO responses that differen ces in induction can be accounted for by the differences in stomatal b ehavior. Differences in the time course of mesophyll conductance may b e due to an artifact caused by stomatal patchiness. Experiments on ind uction loss of previously induced leaves revealed that under these cir cumstances biochemical limitations can be important. A more rapid indu ction loss was evident in sun as compared to shade. plants. The rapid loss of induction in sun plants was not due to the decreases in g(s) a nd RuBisCO activity, which both occurred slowly. Instead, a limitation , probably in RuBP regeneration capacity, appeared to develop during t he low light periods. This limitation was much smaller or absent in sh ade plants.