Stomatal versus biochemical limitations to dynamic photosynthetic performance in four tropical rainforest shrub species

Photosynthetic performance under dynamic light regimes was assessed in four different species of tropical shrubs from the family Rubiaceae via field g as exchange measurements conducted on Barro Colorado Island, Panama. Rates of photosynthetic induction and induction loss were assessed throughout the day in both the wet and dry seasons in order to determine the relative rol es of stomata and biochemistry in limiting photosynthetic performance under transient light conditions. A high degree of coordination was observed bet ween stomatal conductance and biochemical capacity for CO2 assimilation dur ing induction. Rates of biochemical and overall photosynthetic induction sh arply decreased when initial stomatal conductance fell below a narrow range of critical values. Time of day or season did not affect rates of biochemi cal deactivation upon shading, but did influence stomatal closure, which of ten exerted a significant influence over induction loss in the darkness. In measurements of total assimilation due to a 60-s light pulse, both biochem ical activity and stomatal conductance were linearly related to total CO2 u ptake. Only during the mornings of the wet season was stomatal conductance consistently high enough to be non-limiting to dynamic photosynthetic perfo rmance. At all other times, stomatal behavior exercised significant influen ce over induction times, photosynthetic induction loss, and total CO2 uptak e from 60-s light pulses.