The Influence of Previous Irradiance on Photosynthetic Induction in Three Species Grown in the Gap and Understory of a Fagus Crenata Forest

Photosynthetic induction responses to a sudden increase in photosynthetic photon flux density (PPFD) from lower background PPFD (0, 25, 50, and 100 mol m^–2 s^–1) to 1 000 mol m^–2 s^–1 were measured in leaves of Fagus crenata, Acer rufinerve Siebold & Zucc., and Viburnum furcatum growing in a gap and understory of a F. crenata forest in the Naeba mountains. In the gap, A. rufinerve exhibited more than 1.2-fold higher maximum net photosynthetic rate (P_Nmax) than F. crenata and V. furcatum. Meanwhile, in the understory F. crenata exhibited the highest P_Nmax among the three species. The photosynthetic induction period required to reach P_Nmax was 3–41 min. The photosynthetic responses to increase in PPFD depended on the background PPFD before increase in PPFD. The induction period required to reach P_Nmax was 2.5–6.5-fold longer when PPFD increased from darkness than when PPFD increased from 100 mol m^–2 s^–1. The induction period was correlated with initial P_N and stomatal conductance (g_s) relative to maximum values before increase in PPFD. The relationship was similar between the gap and the understory. As the background PPFD increased, the initial P_N and g_s increased, indicating that the degrees of biochemical and stomata limitations to dynamic photosynthetic performance decreased. Therefore, photosynthetic induction responses to increase in PPFD became faster with the increasing background PPFD. The differences in time required to reach induction between species, as well as between gap and understory, were mainly due to the varying of relative initial induction states in P_N and g_s at the same background PPFD.