Photosynthetic Responses to Lightflecks of Fagus crenata Seedlings Grown in a Gap and Understory of a Deciduous Forest

Abstract

Photosynthetic responses to a series of 1-min lightflecks (1,000 μmol m−2 s−1) superimposed on a background with different duration (1, 5, and 10 min) and intensity (25 and 50 μmol m−2 s−1) of low background photosynthetic photon flux density (PPFD) were measured in the leaves of Fagus crenata grown in a gap and understory of a Fagus crenata forest in the Naeba Mountains. The two background PPFD intensities most frequently occurred in understory and gap sites respectively. The maximum net photosynthetic rate (PNmax) and maximum stomatal conductance (gsmax) were higher in the gap seedlings than in the understory seedlings. However, when the background PPFD was 25 μmol m−2s−1, the net photosynthetic rate (P25) and stomatal conductance (gs25) were almost the same between the gap and understory. When the background PPFD duration was 1-min, the net photosynthetic rate (PN) at the end of each lightfleck increased progressively. When the background PPFD duration was 5- and 10-min, the increase in PN at the end of each lightfleck was less. This indicates that background PPFD duration is important to photosynthetic responses to lightflecks. The higher ratios of P25/ PNmax and gs25/ gsmax in the understory seedlings indicate that the understory seedlings can maintain relatively lower levels of biochemical and stomatal limitations than the gap seedlings under low light conditions. The ratios of PN/ PNmax at the end of each lightfleck (IS) and light utilization efficiency of single lightflecks (LUEs) that showed the influence of lightflecks on carbon gain were higher in the understory seedlings than in the gap seedlings when the background PPFD was 25 μmol m−2 s−1. This means that understory seedling are capable of utilizing fluctuating light more efficiently under low light conditions than the gap seedlings although the net carbon gain of single lightflecks (CGs) in the understory seedlings was not higher than that in the gap seedlings. There were no significant differences in IS and LUEs between understory seedlings at a background PPFD of 25 μmol m−2 s−1 and gap seedlings at a background PPFD of 50 μmol m−2 s−1. However, CGs in gap seedlings was higher than in understory seedlings. These results provide more evidence that F. crenata acclimate to a natural light environment in respect to relative induction state at low background PPFD and can capture the fluctuating light at the same efficiency in both the gap and understory seedlings under natural light environments.