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 (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. In theory, plants have evolved to maximise the efficiency with which they use light (maximise photosynthesis per unit light), yet there is limited empirical evidence to show that efficiency can have strong effects on plant growth. By exposing plants to different distributions of light in the day, we demonstrated that low efficiency can reduce plant growth by nearly 50%. Thus, we found clear support for the general expectation that light use efficiency is a central determinant of plant performance. [ABSTRACT FROM AUTHOR]