Contrasting modes of light acclimation in two species of the rainforest understory

In tropical rainforests, the increased light associated with the formation of treefall gaps can have a critical impact on the growth and survivorship of understory plants. Here we examine both leaf-level and whole-plant respo nses to simulated light gap formation by two common shade-tolerant shrubs, Hybanthus prunifolius and Ouratea lucens. The species were chosen because t hey differed in leaf lifespans, a trait that has been correlated with a num ber of leaf- and plant-level processes. Ouratea leaves typically live about 5 years, while Hybanthus leaves live less than 1 year. Potted plants were placed in the understory shade for 2 years before transfer to a light gap. After 2 days in high light, leaves of both species showed substantial photo inhibition, including reduced CO2 fixation, F-v/F-m and light use efficienc y, although photoinhibition was most severe in Hybanthus. After 17 days in high light, leaves of both species were no longer photoinhibited. In respon se to increased light, Ouratea made very few new leaves, but retained most of its old leaves which increased photosynthetic capacity by 50%. Within a few weeks of transfer to high light, Hy banthus had dropped nearly all of i ts shade leaves and made new leaves that had a 2.5-fold greater light-satur ated photosynthetic rate. At 80 days after transfer, the number of new leav es was 4.9-fold the initial leaf number. After 80 days in high light, Hyban thus had approximately tenfold greater productivity than Ouratea when leaf area, photosynthetic capacity, and leaf dark respiration rate were all take n into account. Although both species are considered shade tolerant, we fou nd that their growth responses were quite different following transfer from low to high light, The short-lived Hybanthus leaves were quickly dropped, and a new canopy of sun leaves was produced. In contrast, Ouratea showed li ttle growth response at the whole-plant level, but a greater ability to tol erate light stress and acclimate at the leaf level. These differences are c onsistent with predictions based on leaf lifespan and are discussed within the context of other traits associated with shade-tolerant syndromes.