Stomatal behavior and photosynthetic performance under dynamic light regimes in a seasonally dry tropical rain forest

Rates of photosynthetic induction upon exposure to high light and rates of induction loss after darkening the leaf were measured in the field for four species of tropical shrubs in the family Rubiaceae. During wet season mornings, stomatal conductance (g(s)) in the shade prior to induction was generally high enough so that the time course of induction was determined primarily by rates of activation of biochemical processes. During wet season afternoons, however, g(s) values in the shade tended to be considerably lower and photosynthetic induction following a light increase exhibited a slower time course. In the afternoon, the time course of induction was determined by a combination of stomatal opening time and the rates of activation of light regulated enzymes. Stomatal behavior was also correlated with patterns of induction loss following a transfer from high light to darkness. In the afternoon, maximum g(s) was lower for all species, and for a given time in the darkness, leaves showed a greater loss of induction in the afternoon than in the morning. During the dry season, maximum g(s) and average values for g(s) in the shade were reduced in all species. Along with these shifts in stomatal behavior, reduced rates of photosynthetic induction were observed. in the high-light species. the lower maximum g(s) values observed during the dry season were also correlated with increased induction loss for a given time in the darkness. For all species, stomatal behavior was affected by season and time of day and, with the exception of wet season mornings, stomata appeared to exert significant control over rates of induction and patterns of induction loss. The results of simulation modeling suggest that the observed seasonal and diurnal changes in rates of induction and induction loss can have significant consequences on sunfleck carbon gain under a dynamic light regime.