PHOTOSYNTHETIC INDUCTION AND STOMATAL OSCILLATIONS IN RELATION TO THELIGHT ENVIRONMENT OF 2 DIPTEROCARP RAIN-FOREST TREE SPECIES

1 This paper reports on changes in photosynthetic induction and stomat al conductance (g(s)) in response to (i) continuous saturating light a nd (ii) darkness, for seedlings of two climax tree species (Shorea lep rosula and Dryobalanops lanceolata, Dipterocarpaceae) growing in three differing rain forest light environments (ranging from 2.4 to 21.8 mo l photon m(-2) day(-1)) in Malaysian Borneo. 2 For some species in und erstorey environments, a high proportion of diurnal carbon gain is att ained during sunflecks (transient periods of high light), and D. lance olata is slower growing than S. leprosula but its seedlings can surviv e under deeper shade conditions. Our aim was to determine whether inte rspecific differences in dynamic photosynthetic responses to light cou ld further explain observed differences in seedling ecology. 3 In cont rast to the expected trend for shade-tolerant species, D. lanceolata s howed faster induction in higher light environments and S. leprosula s howed no relationship between light environment and induction rate. Ho wever, both species showed greater potential sunfleck utilization effi ciency in low light environments through slower rates of both inductio n 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 s howed 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 micros ites of higher sunfleck frequency. 5 The balance between photosynthesi s (A) and g(s) during induction resulted in a constant intercellular C O2 concentration of c. 270 p.p.m. after c. 11 min, which may represent a physiological optimum for both species. 6 In some circumstances tra nsient 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 i nitiated 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 oscil late had the potential to increase both the rate and magnitude of resp onse to sunflecks in comparison with simple sigmoidal induction.