Long-term measurements of chlorophyll a fluorescence using the JIP-test show that combined abiotic stresses influence the photosynthetic performance of the perennial ryegrass (Lolium perenne) in a managed temperate grassland

Several experiments have highlighted the complexity of stress interaction on plant response. However the impact in field conditions of combined environmental constraints on mechanisms involved in plant photosynthetic response remains understudied. During a long-term field study in a managed grassland, we investigated the photosynthetic apparatus response to environmental constraints and its ability to recover and adapt. Frequent field measurements of chlorophyll ɑ fluorescence (ChlF) were made in order to determine the photosynthetic performance response of a Lolium perenne L. population. Strong midday decreases in the maximum quantum yield of primary photochemistry (FV/FM) were observed during summer, when a combination of heat and high light increased photosynthetic inhibition. During this period an important photosystem I (PSI) activity was also registered, suggesting an increase in the photochemical pathway for de-excitation in summer. Strong climatic events (e.g., heat waves) were shown to decrease electron transport between PSII and PSI. This reduction in electron transport might have preserved the PSI from photo-oxidation in period of limited PSI activity. Periods of low soil moisture and elevated sun irradiance were found to increase photosystem II (PSII) sensitivity to heat stress, suggesting an increased susceptibility to combined environmental constraints. Despite the multiple inhibitions of the photosynthetic functionality in summer, an increased PSII resistance in L. perenne population to environmental stresses was measured in August. This enhanced PSII tolerance might be a response to previous environmental constraints. It could also be linked to the selection or the emergence of well-adapted individuals.