Growth and Hormone Alterations in Response to Heat Stress in Perennial Ryegrass Accessions Differing in Heat Tolerance

Abstract

Plant growth regulators can mediate plant response to stress. The objective of this study was to investigate hormone alterations associated with plant growth and heat tolerance in perennial ryegrass (Lolium perenne). The heat tolerant accession PI265351 and sensitive accession PI225825 were exposed to 35°C/30°C at 7-, 14-, and 21-days in a growth chamber. Heat stress decreased plant height (HT) and leaf water content (LWC) in both accessions, but reductions in HT and LWC delayed one week in PI265351. Chlorophyll fluorescence remained unchanged in PI265351 but decreased in PI225825 under heat stress, compared to their controls. Heat stress increased leaf trans-zeatin and abscisic acid (ABA) and decreased gibberellin (GA3) concentrations, occurring at 14- and 21-days for PI265351 and during the entire stress period for PI225825, respectively, compared to the unstressed controls. Both accessions showed similar trends in kinetin with an increased concentration at 14- and 21-days. An elevated level of indole-3-acetic acid (IAA) was observed in PI265351 at 21 days of heat stress, while indole-3-butyric acid (IBA) increased at 14- and 21-days for PI265351 and only at 14 days for PI225825. Heat stress increased salicylic acid (SA) concentration at 14 days for PI265351 but caused reductions in SA during the stress period for PI225825. The results indicated that the maintenance of IAA, IBA, and SA, delayed increases in ABA, and delayed reductions of GA3 could contribute to heat tolerance of perennial ryegrass.