Abstract
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.
Highlights
In addition to structural and biochemical changes, cold acclimation in plants is associated with photosynthetic acclimation at low growth temperatures
In the resistant Flinston cultivar after 3 weeks of cold acclimation, EBR induced the non-photochemical quenching (NPQ) mechanism, which coincided with a sharp decrease in φPSII and qP compared with plants before cold acclimation and those that were cold acclimated for 6 weeks
After 6 weeks of cold acclimation, φPSII and qP values returned to pre-acclimation levels; this change was accompanied by only a slight decrease in NPQ, which was still markedly higher than that of non-acclimated plants
Summary
In addition to structural and biochemical changes, cold acclimation in plants is associated with photosynthetic acclimation at low growth temperatures. Low temperature and light intensity affect the relative redox state of photosystem II (PSII), thereby leading to an imbalance between the energy absorbed in the light phase of photosynthesis and its consumption for photochemistry (Huner and others 1998). 1993); activation of both mechanisms is genotype dependent, plants with higher tolerance towards cold-induced photoinhibition are usually more tolerant to frost, (Huner and others 1993; Rapacz and others 2004). Cold acclimation is a time-dependent response, with freezing tolerance increasing along with acclimation time (Fowler and others 1996). The recovery of photosynthesis after long-term growth at low, non-freezing temperatures is supported by increases in the activities of photosynthetic carbon reduction cycle enzymes, such. Soluble phenolics contribute to scavenging reactive oxygen species (ROS), a common cause of damage to the photosynthetic membranes (Blokhina and others 2002; Nogues and Baker 2000)
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