Dependency of Growth, Water Use Efficiency, Chlorophyll Fluorescence, and Stomatal Characteristics of Lettuce Plants to Light Intensity

Abstract

Photosynthesis and stomata dynamically respond to transient changes in light intensity; however, information regarding their long-term responses to the light intensity is limited. In the current study, biophysical properties of photosynthetic apparatus and stomatal characteristics of lettuce plants were investigated in response to long-term exposure to different photosynthetic photon flux densities (PPFDs) [75, 150, 300, and 600 µmol m−2 s−1]. Contrary to leaf growth, SLA decreased with increasing light intensity (i.e., thicker leaves under higher light intensity). Improving effect of higher light intensity on leaf fresh and dry weights was time dependent, in a way that the largest difference in biomass gain was observed following 40 days of exposure to the light treatments. Depending on the leaf developmental stages, exposure to higher light intensities caused faster development of photosynthesis system [in terms of improvement in the maximum quantum efficiency of photosystem II (PSII) and non-photochemical quenching (NPQ)] compared to lower light intensities. PSII performance index on an absorption basis was the highest under 600 PPFD. Small-sized stomata with narrow pore apertures were observed in plants grown under 75 PPFD; however, due to improvement in photosynthetic capacity and also the growth of the plants, water use efficiency (WUE) increased in a light intensity-dependent manner and the highest WUE was detected in 600 PPFD-exposed plants. In conclusion, exposing lettuce plants to higher light intensities (non-stress threshold levels) results in higher accumulation of biomass, faster development of photosynthetic system, and improved WUE.