24-Epibrassinolide Pre-Treatment Modifies Cold-Induced Photosynthetic Acclimation Mechanisms and Phytohormone Response of Perennial Ryegrass in Cultivar-Dependent Manner

We investigated the modification of cold-induced mechanisms of photosynthetic apparatus adjustment and phytohormone response by brassinosteroid 24-epibrassinolide (EBR) and its consequences for frost tolerance of perennial ryegrass ( Lolium perenne L.). We recorded the responses of two cultivars with contrasting frost tolerances to foliar hormone application, both in non-acclimated plants and plants cold acclimated for 3 and 6 weeks at 4 °C. In non-cold-acclimated plants of both cultivars, EBR induced increases in carbon fixation and lowered sucrose levels. Temporary suppression in quantum efficiency of PSII of photosystem II and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose phosphate synthase, a consequence of energy dissipation in non-photochemical quenching, was observed in the leaves of the highly frost-tolerant cultivar after 3 weeks of cold acclimation. After 6 weeks of cold acclimation, EBR accelerated recovery of photosynthesis, reflecting adjustment to cold conditions, and increased frost tolerance. As carbohydrate export from leaves is favored during cold acclimation, EBR application did not increase frost tolerance of the moderately tolerant cultivar, reflecting the downregulation of photosynthesis due to high leaf sucrose concentrations. It is also likely that EBR participated in the enhancement of frost tolerance by regulation of stress-related signaling compounds such as JA and ethylene but not SA, in winter ryegrass undergoing cold acclimation. Taken together, our results demonstrate that EBR-induced changes are temperature dependent. The beneficial effect of EBR is not universal under cold conditions, as genetically determined mechanisms are apparently dominant relative to EBR action.