Long-term study of the photosynthetic performance and acclimation potential of the perennial ryegrass Lolium perenne L. facing combined environmental constraints in a managed temperate grassland

Increases in heat wave occurrence, mean precipitation and longer dry spells are expected in Europe by 2100. The impact of co-occurring environmental stresses on the photosynthetic apparatus of grassland populations, its acclimation and its resilience remain understudied. In the present study, frequent measurements of chlorophyll ɑ fluorescence (ChlF) of a Lolium perenne L. population in a managed grassland were made in order to assess the photosynthetic response to environmental constraints over 2 successive years. Strong midday decreases were observed in the maximum quantum yield of primary photochemistry (FV/FM) during summer, when environmental constraints were combined. Periods of low soil moisture and elevated sun irradiance were found to increase photosystem II sensitivity to abiotic stresses such as heat and vapour pressure deficit. High values of RE/TR, representative of the photosystem I activity, measured in summer also indicate an increase in the photochemical pathway for de-excitation at this period. The lowest ET/TR values were observed during particular stressful climatic events (e.g., heat waves). The different photosynthetic processes were not equally impacted by the different abiotic stresses and their interactions. Despite the repeated inhibitions of the photosynthetic functionality, increased PSII resistance to environmental stresses was measured in August, suggesting a possible acclimation in the grassland. FV/FM returned to its basal level at the end of both measurement periods, showing a good resilience of the photosynthetic apparatus in the grassland.