Photosynthetic induction in saplings of three shade-tolerant tree species:comparing understorey and gap habitats in a French Guiana rain forest

The photosynthetic induction response under constant and fluctuating light was examined in naturally occurring saplings (about 0.5-2 m in height) of t hree shade-tolerant tree species, Pourouma bicolor spp digitata, Dicorynia guianensis, and Vouacapoua americana, growing in bright gaps and in the sha ded understorey in a Neotropical rain forest. Light availability to sapling s was estimated by hemispherical photography. Photosynthetic induction was measured in the morning on leaves that had not yet experienced direct sunli ght. In Dicorynia, the maximum net photosynthesis rate (A(max)) was similar between forest environments (ca 4 mu mol m(-2) s(-1)), whereas for the two other species, it was twice as high in gaps (ca 7.5) as in the understorey (ca 4.5). However, the time required to reach 90% of A(max) did not differ among species, and was short, 7-11 min. Biochemical induction was fast in leaves of Pourouma, as about 3 min were needed to reach 75% of maximum carb oxylation capacity (V-cmax); the two other species needed 4-5 min. When ind uction continued after reaching 75% of V-cmax, stomatal conductance increas ed in Pourouma only (ca 80%), causing a further increase in its net photosy nthesis rate. When fully induced leaves were shaded for 20 min, loss of ind uction was moderate in all species. However, gap saplings of Dicorynia had a rapid induction loss (ca 80%), which was mainly due to biochemical limita tion as stomatal conductance decreased only slowly. When leaves were expose d to a series of lightflecks separated by short periods of low light, photo synthetic induction increased substantially and to a similar extent in all species. Although A(max), was much lower in old than in young leaves as mea sured in Dicorynia and Vouacapoua, variables of the dynamic response of pho tosynthesis to a change in light tended to be similar between young and old leaves. Old leaves, therefore, might remain important for whole-plant carb on gain, especially in understorey environments. The three shade-tolerant s pecies show that, particularly in low light, they are capable of efficient sunfleck utilization.