Acclimation to a Change in Light Regime in Seedlings of Six Australian Rainforest Tree Species

Abstract

Acclimation potential of seedlings of six non-pioneer, North Queensland rainforest tree species (Diploglottis diphyllostegia, Flindersia brayleyana, Dysoxylum schgneri, Prunus turnerana, Neisosperma poweri and Castanospora alphandii) were evaluated in three different light regimes. The seedlings were grown from seeds and raised in 37, 10 and 2.5% photosynthetically active radiation (PAR) of full sunlight in a shade house. Plants were harvested at 1, 4, and 15 months after germination. At 4 months, for each species, one-third of the seedlings in each light environment were transferred to each of the two other light environments. The remaining one-third stayed in the same light environment and formed the control. The control seedlings of all species showed a marked response to increasing PAR: relative growth rate (RGR) and total biomass were low in the 2.5% PAR level, increased in the 10% PAR level and were maximal in the 37% PAR level, except for Neisosperma. The seedlings in the 25% PAR level showed a typical shade plant morphology with a high leaf area ratio, low root-shoot ratio and low specific leaf weight. For seedlings of Dysoxylum and Prunus, a change in light regime from 37% PAR to 2.5% PAR resulted in negative relative growth rates. Eleven months after transfer, many growth characteristics still showed significant initial × final light environment interactions, an indication of incomplete adjustment. Acclimation to increasing light availability was faster than acclimation to decreasing light availability. On the basis of biomass allocation patterns (root-shoot ratio, leaf area ratio and specific leaf weight), the six species could not be differentiated under the three light regimes and their dynamics. However, using RGR as a relative measure of carbon economy, it was possible to differentiate the species in their acclimation ability to decreased but not to increased irradiance. It was concluded that, for non-pioneer species, acclimation potential may be more related to physiological than to morphological plasticity.