Hormone Changes of Annual Bluegrass and Creeping Bentgrass in Response to Cold Acclimation and Deacclimation.

In previous studies, annual bluegrass (Poa annua L.) (AB) and creeping bentgrass (Agrostis stolonifera L.) (CB) differed in their capacity to resist deacclimation, suggesting this may be a factor responsible for interspecific differences in winter injury potential.  To date, there is not a good understanding of the signaling mechanisms that may trigger cold deacclimation in grass species.  Among the different signaling compounds, phytohormones have been shown to play an important role in the regulation of stress responses in plants.  Therefore, the objective of this study was to quantify early changes in phytohormones associated with cold deacclimation of ‘L-93’ CB, a freezing-tolerant ecotype of AB (AB-T), and a freezing-sensitive ecotype of AB (AB-S).  Plants were subjected to cold acclimation at 2°C for 14 d and a sub-zero acclimation period of   -2°C for 14 d.  Following sub-zero acclimation, plants were exposed to 8°C to induce deacclimation for 0.5, 1, and 3 d.  Freezing tolerance (lethal temperature for 50% of population, LT₅₀) was determined during cold acclimation and deacclimation.  In addition, leaves were harvested for quantification of phytohormones including abscisic acid (ABA), gibberellic acid (GA), and indole-3-acetic acid (IAA) using high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS).   Significant changes in phytohormone levels were detected in response to both cold acclimation and deacclimation, which varied according to plant species, ecotype, and duration of deacclimation treatment.