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Leaves of Japanese oak (Quercus mongolica var. crispula) mitigate photoinhibition by adjusting electron transport capacities and thermal energy dissipation along the intra-canopy light gradient.

We investigated the morphological and physiological acclimation of leaves grown within a canopy of Japanese oak tree (Quercus mongolica var. crispula) in terms of the susceptibility to photoinhibition under various growth light conditions. The maximum rates of photosynthesis (P(max)) and electron transport (ETR(max)) were higher in mature leaves grown under stronger light with higher area-based leaf nitrogen (N) content closely associated with higher leaf mass per area. The net photosynthetic (P(n)) and electron transport (ETR) rates corresponding to the daily peak photosynthetic photon flux density (PPFD(max)) during leaf maturation were almost comparable to P(max) and ETR(max), respectively. Conversely, P(n) and ETR at the daily average PPFD (PPFD(avg)) were substantially low in shade-grown leaves when compared with P(max) and ETR(max). The susceptibility to photoinhibition at PPFD(max), i.e. at sunflecks for the shade-grown leaves, was assessed by the rate of excess energy production. Although sun leaves showed higher rates of electron transport and thermal energy dissipation than shade leaves under PPFD(max) conditions, the rate of excess energy production was almost constant across shade to sun leaves. The shade leaves of the Japanese oak grown within a crown were suggested to adjust their N investment to maintain higher photosynthetic capacities compared with those required to maximize the net carbon gain, which may facilitate the dissipation of the excessive light energy of sunflecks to circumvent photoinhibition in cooperation with thermal energy dissipation.