Photosynthetic induction and stomatal oscillations in relation to the light environment of two dipterocarp rain forest tree species

1 This paper reports on changes in photosynthetic induction and stomatal conductance (g(s)) in response to (i) continuous saturating light and (ii) darkness, for seedlings of two climax tree species (Shorea leprosula and Dryobalanops lanceolata, Dipterocarpaceae) growing in three differing rain forest light environments (ranging from 2.4 to 21.8 mol photon m(-2) day(-1)) in Malaysian Borneo. 2 For some species in understorey environments, a high proportion of diurnal carbon gain is attained during sunflecks (transient periods of high light), and D. lanceolata is slower growing than S. leprosula but its seedlings can survive under deeper shade conditions. Our aim was to determine whether interspecific differences in dynamic photosynthetic responses to light could further explain observed differences in seedling ecology. 3 In contrast to the expected trend for shade-tolerant species, D. lanceolata showed faster induction in higher light environments and S. leprosula showed no relationship between light environment and induction rate. However, both species showed greater potential sunfleck utilization efficiency in low light environments through slower rates of both induction loss and stomatal closure. 4 Shorea leprosula attained greater rates of maximal photosynthesis and g(s), had faster rates of induction and retained a higher level of induction in prolonged darkness (> 30 min) than D. lanceolata in all light environments. However, S. leprosula showed faster induction loss and stomatal closure in the short term (l0 min) than D. lanceolata, which, together with a potentially negative carbon balance between sunflecks, may limit its distribution to microsites of higher sunfleck frequency. 5 The balance between photosynthesis (A) and g(s) during induction resulted in a constant intercellular CO2 concentration of c. 270 p.p.m. after c. 11 min, which may represent a physiological optimum for both species. 6 In some circumstances transient peaks in A and g(s) were observed during induction, rather than a rise to a stable maximum, which we attribute to overcompensation of the stomatal response to light. In some cases for S. leprosula this initiated synchronized damped oscillations in g(s) and A that continued for c. 1 h in both continuous and discontinuous (80-s light/80-s dark) light. 7 Shorea leprosula plants with the ability to 'peak' or oscillate had the potential to increase both the rate and magnitude of response to sunflecks in comparison with simple sigmoidal induction.