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 morn ings, stomatal conductance (g(s)) in the shade prior to induction was gener ally high enough so that the time course of induction was determined primar ily by rates of activation of biochemical processes. During wet season afte rnoons, however, g(s) values in the shade tended to be considerably lower a nd photosynthetic induction following a light increase exhibited a slower t ime course. In the afternoon, the time course of induction was determined b y a combination of stomatal opening time and the rates of activation of lig ht regulated enzymes. Stomatal behavior was also correlated with patterns o f induction loss following a transfer from high light to darkness. In the a fternoon, maximum g(s) was lower for all species, and for a given time in t he darkness, leaves showed a greater loss of induction in the afternoon tha n in the morning. During the dry season, maximum g(s) and average values fo r 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 giv en 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, sto mata appeared to exert significant control over rates of induction and patt erns 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 dyna mic light regime.