Host: The Japanese Forestry Society
Light on forest floors is an important factor for the survival and growth of understory plants. The floors in deciduous forest has characterized by sharp deduction in irradiance from spring to summer as the overstory canopy developed. However, the forest border and canopy gap are high light environment as compared with the understory. Thus the floors in deciduous forest has various light environment. Therefore, we expect that plants in the floors has adaptable strategy to use various light. It is known that the photosynthetic system consists of many components and their organization varies depending on growth conditions. In particular, it is well known that the organization changes with growth irradiance. In generally, the fraction of chlorophyll-protein complexes increases and the fraction of Calvin cycle enzymes and electron carriers decreases with decreased light availability. In this study, Photosynthetic characteristics and nitrogen partitioning for Daphniphylum humile, evergreen understory shrub, in the different light environment in deciduous forest were examined. Mean daily PPFD incident on leaves within the understory varied from 1.4 to 24.1 mol m-2 day-1 in September 2002. SLA for current and one-year old leaves in the high light condition was lower than in low light condition. The SLA had a close relationship with PPFD. These result indicate that leaves were morphologically acclimation to light environments. The rates of light-saturated photosynthesis (Pmax) in current year leaves varied throughout the understory from 3.7 to 12.2 mol m-2 s-1. The maximum rate of carboxylation (Vcmax) in current year leaves varied throughout the understory from 27.1 to 60.3 mol m-2 s-1. The maximum rate of electron transport (Jmax) in current year leaves varied throughout the understory from 61.1 to 117.6 mol m-2 s-1. These parameters were linearly related with PPFD for all data pooled. These results suggest that current year leaves has suitable nitrogen for light environment and protein compositions of photosynthetic system changes. By contrast, Pmax, Vcmax and Jmax in one-year-old leaves did not vary throughout the understory. Therefore, these parameters were not related with PPFD. The leaf life span for D. humile changes with different light environment and that of D.humile in high light environment shorter than in low light environment. Therefore, we hypothesized that one-year-old leaves in high light environment was senescent leaves and nitrogen content in leaves decreased.
