The effects of temperature and the Rht3 dwarfing gene on growth, cell extension, and gibberellin content and responsiveness in the wheat leaf

The effects of low temperature and the RM3 dwarfing gene on the dynamics of cell extension in leaf 2 of wheat were examined in relation to gibberellin (GA) content and GA-responsiveness of the extension zone. Leaf 2 of wild-type (rht3) wheat closely resembled that of the Rht3 dwarf mutant when seedlings were grown at 10 C. The maximum relative elemental growth rate (REGR) within the extension zone in both genotypes was lower at 1 0 C than at 20 C, but the position with respect to the leaf base was unaffected by temperature. The size of the extension zone and epidermal cell lengths were similar in both genotypes at 10°C. Growth at 20 °C, instead of 1 0 C , increased the length of the extension zone beyond the point of maximum REGR in the wild type, but not in the Rht3 mutant. Increasing temperature resulted in longer epidermal cells in the wild type. Treating wild-type plants at 10 °C with gibberellic acid (GA3) also increased the length of the extension zone, but the Rht3 mutant was GA-nonresponsive. However, the concentrations of endogenous GA, and GA3 remained similar across the extension zone of wild-type plants grown at both temperatures, despite large differences in leaf growth rates. The period of accelerating REGR as cells enter the extension zone, and the maximum REGR attained, are apparently not affected by GA. It is proposed that GA functions as a stimulus for continued cell extension by preventing cell maturation in the region beyond maximum REGR and that low temperature increases the sensitivity threshold for GA action.