Functional responses of Acer species to two simulated forest gap environments: Leaf-level properties and photosynthesis

Abstract

Seedlings of eight forest maple (Acer L.) species were grown outdoors through a full season under two irradiation treatments: (a) 'gap edge' with a photosynthetic photon flux density of 30 μmol m-2 s-1 and a red:far-red ratio of 0.55, and (b) 'gap centre' with 400 μmol m-2 s-1 and a red:far-red ratio of 1.12. Area-based leaf nitrogen concentration was greater in gap centre-grown seedlings, whereas, except for A. saccharum, area-based chlorophyll (Chl) (a+b) was higher in gap edge-grown plants. There was also a significantly lower Chl a/b ratio in gap edge-grown plants. Maximum photosynthetic rate (P(max)) was 60% higher in the gap-centre treatment. These results are consistent with the functional expectation that shade(acclimated) plants will increase their radiant-energy harvesting capacity as a result of limited photon input while gap-acclimated plants will operate more efficiently under bright irradiance by increasing their carboxylation capacity. This inverse relationship between the capacity of the light-harvesting component and the carboxylation component is, however, only partially supported by Chl fluorescence measurements of intact leaves. Compared to gap centre-grown plants, the lower total fluorescence quenching in gap edge-grown plants indicated a lower carboxylation capacity that was in accord with the observed P(max). However, edge-grown seedlings did not show the expected improvement in light-harvesting efficiency and reduction in electron transport of photosystem 2 inferred from their marginally greater t(1/2) and lower F(v)/F(m), respectively. Hence while maples acclimated to different irradiation levels by adjusting leaf N and Chl contents, they showed limited acclimation potential at the photosystem level. Variations in the leaf traits examined had only minor effect on low irradiance photosynthesis and sunfleck utilization.