Effect of temporally heterogeneous light on photosynthetic light use efficiency, plant acclimation and growth in Abatia parviflora

Individual leaves have a unique instantaneous photosynthetic photon flux density (PPFD) at which net photosynthetic light use efficiency (ϵ_L, the ratio between net photosynthesis and PPFD) is maximised (PPFDϵ_max). When PPFD is above or below PPFDϵ_max, efficiency declines. Thus, we hypothesised that heterogeneous PPFD conditions should increase the amount of time leaves photosynthesise at a PPFD different than PPFDϵ_max and result in reduced growth. To date, this prediction has not been rigorously tested. Here, we exposed seedlings of Abatia parviflora Ruiz & Pav to light regimes of equal total daily irradiance but with three different daily time courses of PPFD: constant PPFD (NoH), low heterogeneity (Low-H) and high heterogeneity (High-H). Mean ϵ_L, leaf daily photosynthesis and plant growth were all significantly higher in No-H and Low-H plants than in High-H plants, supporting our hypothesis. In addition, mean ϵ_L was positively related to final plant biomass. Unexpectedly, High-H plants had more etiolated stems and more horizontal leaves than No-H and Low-H plants, possibly due to exposure to low PPFD in the morning and afternoon. In conclusion, PPFD heterogeneity had an important effect on average ϵ_L, photosynthesis and growth, but also on allocation and plant morphology.