Physiology of Cold Acclimation and Deacclimation Responses of Cool-season Grasses: Carbon and Hormone Metabolism

Winter injury of cool-season turfgrasses in northern climates is a significant issue, leading to losses in turf cover and subsequent increased inputs for recovery. Despite the different potential causes for winter injury, the overall level of plant freezing tolerance has been shown to account for a majority of the variation in winter survival of grasses. Freezing tolerance is achieved through cold acclimation, based on a series of physiological and biochemical changes that increase cell stability at freezing temperatures. Winter injury can result from insufficient cold acclimation, or rapid cold deacclimation triggered by temperature fluctuations or freeze-thaw cycles. Previous research has been mostly conducted to investigate the mechanisms associated with cold acclimation, limited research was applied to deacclimation resistance. In order to enhance winter survival, and reduce turf losses and recovery costs, a better understanding of the underlying factors associated with cold acclimation and deacclimation is necessary. Therefore, the objectives of this thesis research are to: (1) evaluate the carbon metabolism factors attributing to different freezing tolerance capacity associated with cold acclimation and deacclimation of annual bluegrass and creeping bentgrass; (2) investigate the differences in hormone regulation of annual bluegrass and creeping bentgrass during cold acclimation and deacclimation; and (3) identify physiological changes in response to cold acclimation and deacclimation among perennial ryegrass genotypes contrasting in freezing tolerance. Overall, our research found that the fast up-regulation of carbon metabolism activities (chlorophyll fluorescence, photosynthesis, respiration) during deacclimation was associated with losses in freezing tolerance. In addition, changes in hormone content, such as abscisic acid, auxin, salicylic acid, and jasmonic acid, at both leaf and crown level, contributed to differences in deacclimation resistance. Lastly, increased crown moisture content during deacclimation was also found to be responsible for the losses in freezing tolerance. Although these factors may aid in a faster recovery in response to temperature increases during late winter and early spring, these physiological changes may also make the plants more susceptible to freezing injuries if plants are once again exposed to freezing temperatures