Diurnal OJIP chlorophyll a fluorescence changes reveal key Photosystem II parameters associated with yield of siliques in rapeseed-mustard under elevated temperatures

Our understanding of how winter crops would respond to mean temperature rise and warming events is limited. We selected rapeseed-mustard, a prominent winter crop that will likely experience such conditions in the near future. In a greenhouse experiment, 17 rapeseed-mustard genotypes were grown under a simulated elevated-temperature scenario with diurnal fluctuations. The mean day/night-time air temperatures were ~30/24°C, but increased to ~36.9°C at midday. To study photosynthesis changes, we measured the OJIP chlorophyll (Chl) a fluorescence induction of the dark-adapted top wider leaves, at the flowering stage; this was done during three diurnal timepoints: morning (07:30-08:00 h), afternoon (14:00-14:30 h), and evening (16:30-17:00 h). Transient decline in photosystem-II (PSII) maximum quantum efficiency (inferred from variable to maximum Chl a fluorescence, F _V /F _M ) and in the performance indices (PI _ABS and PI _TOT ) was evident in the afternoons, with a higher magnitude of changes in PI _ABS and PI _TOT . However, the fluorescence data suggested no permanent PSII damage, but a strong increase in photoprotective quenching and restricted electron supply from PSII during the afternoons, which extended till the evenings and took overnight to recover. The IP phase of the OJIP kinetics showed robust correlation with PI _TOT but not with F _V /F _M, and identified genotypes that were less or highly sensitive to diurnal changes. Both F _V /F _M and PI _TOT were positively correlated with the number of branches and the dry silique biomass. Lastly, the afternoon PI _TOT displayed better correlation with silique biomass, and it could be used as a trait for identifying potential heat-resilient genotypes with higher yield performance.