Effects of High Temperature on Membrane Stability and Chlorophyll Fluorescence in Glycinebetaine-Deficient and Glycinebetaine-Containing Maize Lines

Pairs of homozygous near-isogenic glycinebetaine-containing (Betl/Bet1) and -deficient (betl/bet1) Fδ lines of Zea maysM L. (maize) were tested for differences in their responses to high temperature. Membrane integrity was evaluated by electrolyte leakage, and the in vivo photochemical activity of PSII was inferred from modulated chlorophyll fluorescence. Both variables were more markedly impaired by thermal stress in glycinebetaine-deficient lines. When whole plants, excised whole leaves or leaf segments were subjected to temperatures above 45�C, Betl/Bet1 lines exhibited less membrane injury than their near-isogenic betl/bet1 sister lines. An average difference of 2�C was observed between Betl/Betl and betl/betl lines in the critical temperature threshold which triggered catastrophic membrane damage in vitro. Betl/Bet1 lines also exhibited greater thermostability of PSII function. The empirical fluorescence parameters Fv/Fm and ΔF/F′m, indices of PSII quantum yield (ΦII) in the dark-adapted and illuminated states, respectively, were each markedly reduced in heat-treated leaves of betl/bet1 lines, relative to their Betl/Bet1 sister lines. These results strongly suggest that a single gene confering glycinebetaine accumulation (and/or a tightly linked locus) confers protection against heat destabilisation of leaf plasma membranes as well as the photochemical reactions of PSII in vivo.