THE EFFECTS OF TEMPERATURE AND THE RHT3 DWARFING GENE ON GROWTH, CELLEXTENSION, AND GIBBERELLIN CONTENT AND RESPONSIVENESS IN THE WHEAT LEAF

the effects of low temperature and the Rht3 dwarfing gene on the dynam ics 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 dw arf mutant when seedlings were grown at 10 degrees C, The maximum rela tive elemental growth rate (REGR) within the extension zone in both ge notypes was lower at 10 degrees C than at 20 degrees C, but the positi on with respect to the leaf base was unaffected by temperature, the si ze of the extension zone and epidermal cell lengths were similar in bo th genotypes at 10 degrees C, Growth at 20 degrees C, instead of 10 de grees C, increased the length of the extension zone beyond the point o f maximum REGR in the wild type, but not in the Rht3 mutant. Increasin g temperature resulted in longer epidermal cells in the wild type. Tre ating wild-type plants at 10 degrees C with gibberellic acid (GA(3)) a lso increased the length of the extension zone, but the Rht3 mutant wa s GA-nonresponsive, However, the concentrations of endogenous GA(1) an d GA(3) 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 zo ne, and the maximum REGR attained, are apparently not affected by GA, It is proposed that GA functions as a stimulus for continued cell exte nsion by preventing cell maturation in the region beyond maximum REGR and that low temperature increases the sensitivity threshold for GA ac tion.