Hormonal Changes Associated with Freezing Tolerance of Two Cool-Season Grasses.

Annual bluegrass (Poa annua) (AB) and creeping bentgrass (Agrostis stolonifera) (CB) are cool-season grasses that exhibit variability in winter survival, which may in part be attributed to intraspecific differences in cold acclimation and deacclimation capacities. In order to reduce winter injury, a greater understanding of the physiological factors that contribute to freezing tolerance losses for these two species is necessary. Therefore, the objective of this study was to quantify changes in major hormones in leaf and crown tissues during cold acclimation and deacclimation in relation to freezing tolerance of AB and CB. Plants were established in greenhouse and then moved to a growth chamber for cold acclimation and deacclimation treatments, including: (1) non-acclimated control at 20 °C; (2) cold acclimated at 2 °C; (3) cold acclimated at -2 °C; (4) deacclimated at 8 °C for 1 d; and (5) deacclimated at 8 °C for 5 d. Following each treatment, changes in photochemical efficiency (Fv/Fm) were monitored using chlorophyll fluorescence, and freezing tolerance was calculated based on lethal temperature resulting in 50% kill (LT₅₀).  Leaf and crown tissues were harvested for hormone analyses, including indole-3-acetic acid (IAA), zeatin, abscisic acid (ABA), salicylic acid (SA) and jasmonic acid (JA). Overall, CB exhibited higher freezing tolerance (lower LT₅₀) following cold acclimation and deacclimation. The Fv/Fm of AB was significantly higher than CB during deacclimation, indicating the carbon metabolism of AB was more active than CB. Increases in ABA, IAA, and SA contents of leaves and crowns were observed during cold acclimation. In contrast, the levels of these hormones significantly declined in response to deacclimation, and to a greater extent for AB compared to CB.