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

Abstract

Individual leaves have a unique instantaneous photosynthetic photon flux density (PPFD) at which net photosynthetic light use efficiency (ϵL, the ratio between net photosynthesis and PPFD) is maximised (PPFDϵmax). When PPFD is above or below PPFDϵmax, efficiency declines. Thus, we hypothesised that heterogeneous PPFD conditions should increase the amount of time leaves photosynthesise at a PPFD different than PPFDϵmax 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 (No_H), low heterogeneity (Low_H) and high heterogeneity (High_H). Mean ϵL, 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 ϵL 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 ϵL, photosynthesis and growth, but also on allocation and plant morphology.