Photosynthetic light response of flooded cherrybark oak (Quercus pagoda) seedlings grown in two light regimes
Two-year-old cherrybark oak (Quercus pagoda Raf.) seedlings raised in full or partial (27 percent) sunlight were flooded for 30 days to study the effects of light availability and root inundation on photosynthetic light response. Compared with seedlings receiving full sunlight, seedlings receiving partial sunlight developed leaves with 90 percent greater blade area, 26 percent less mass per unit volume, and 35 percent lower nitrogen (N) concentration per unit area, leading to a 15 percent reduction in leaf photosynthetic capacity when carbon exchange rates were based on blade area. However, when carbon exchange rates were based on leaf mass, leaves acclimated to partial sunlight exhibited a 15 percent greater photosynthetic capacity realized primarily through an increased initial slope of the photosynthetic light response (A/PPFD) curve and increased net photosynthesis at leaf saturation (Amax). Short-term flooding increased leaf mass per unit area more than 19 percent, reduced foliar N concentrations per unit dry mass by 19 percent, and initiated reductions in Amax and apparent quantum yield (F) of seedlings in both light regimes. Greatest impairment of Amax (56 percent area basis, 65 percent mass basis) and F (40 percent) were observed in leaves receiving full sunlight, and the declines were concomitant with a 35 percent decrease in chlorophyll concentration. Flooding also depressed instantaneous photosynthetic N-use efficiency (PPNUE) such that Amax decreased 54 percent, and the initial slope of PPNUE/PPFD curves decreased 33 and 50 percent for leaves acclimated to partial and full sunlight, respectively. The A/PPFD patterns indicated that the magnitude of flood-induced inhibition of the photosynthetic mechanism of cherrybark oak seedlings is determined partly by the light environment.