Seasonal changes in photosynthesis and nitrogen allocation in leaves of different ages in evergreen understory shrub Daphniphyllum humile

Seasonal changes in photosynthetic capacity, leaf nitrogen (N) content, leaf chlorophyll (Chl) content and leaf N allocation patterns in leaves of different ages in the evergreen understory shrub, Daphniphyllum humile Maxim, growing at a forest border and an understory site were studied. In current-year leaves at the understory site, the N and Rubisco contents increased from spring to autumn although their light-saturated photosynthetic rate at 22°C (P _max²²) remained stable, indicating that their mesophyll conductance rates declined as they completed their development and/or that they invested increasing amounts of their resources in photosynthetic enzymes during this period. In contrast, seasonal changes in P _max²² in current-year leaves at the forest border site were correlated with changes in Rubisco content. In 1-year old leaves at the understory site, P _max²² and contents of Chl, leaf N, and Rubisco remained stable from spring to autumn, while these parameters decreased in 1-year-old forest border leaves, indicating that N may have been remobilized from shaded 1-year-old leaves to sunlit current-year leaves. When leaves senesced at the forest border site the Rubisco content decreased more rapidly than that of light-harvesting proteins such as LHCII, suggesting that N remobilization from Rubisco may be more efficient, possibly because Rubisco has greater N costs and is soluble, whereas the light-harvesting proteins are membrane components.