Photosynthetic Response to Light and Nutrients in Sun-Tolerant and Shade-Tolerant Rainforest Trees. II. Leaf Gas Exchange and Component Processes of Photosynthesis

Abstract

Species with contrasting shade tolerance were grown under three light by two nutrient treatments. Gas exchange by intact leaves, leaf disk O₂ evolution and chlorophyll fluorescence were measured.

In shade-tolerant evergreen species (Argyrodendron sp., A. trifoliolatum and Flindersia brayleyana) photosynthetic activity of seedlings in air at light saturation (A) was lower under weak (30 pmol quanta m^-2 day-1 ), compared with medium (130) or strong light (535). In Toona australis, a shade-intolerant and deciduous tree, A was reduced 44% from strong to weak light treatment on high nutrients (71 mg N L^-1 nutrient solution). Nevertheless, nitrogen-use efficiency for leaf photosynthesis was highest in Toona under all growing conditions and, with higher specific leaf area, probably contributes towards fast occupancy of sites which underlies early succession in this species.

All species made photosynthetic and respiratory adjustments from strong to medium to weak light, which resulted in a lower light compensation point (Q₀). Such adjustments were accentuated by low nutrient supply (1.0 mg N L^-1 nutrient solution) and were especially pronounced for shade-intolerant Toona. Reduced Q₀ in Toona was accompanied by lower A and light saturation point (Q_A). Both species of Argyrodendron showed no decrease in Q_A despite reduction in Q₀ under weak light.

Contrary to expectation, photosynthetic responses to light × nutrient treatments did not correlate with degree of shade tolerance accorded each species by rainforest ecologists.