Effect of theRht3dwarfing gene on dynamics of cell extension in wheat leaves, and its modification by gibberellic acid and paclobutrazol

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The second leaf of wheat was used as a model system to examine the effects of the Rht3 dwarfing gene on leaf growth. Compared to the rht3 wild type, the Rht3allele decreased final leaf length, surface area and dry mass by reducing the maximum growth rates, but without affecting growth duration. Gibberellic acid (GA3) increased final leaf length and maximum growth rate in the rht3 wild type, but was without effect on the Rht3 mutant, which is generally regarded as being non-responsive to gibberellin (GA). Paclobutrazol, an inhibitor of GA biosynthesis, decreased final leaf length and maximum growth rate in the rht3 wild type to values similar to those in the untreated Rht3 mutant. Neither GA3 nor paclobutrazol affected the duration of leaf growth. The decrease in leaf length was produced by reduction of cell length rather than cell number. The maximum relative elemental growth rate (REGR) for cell extension was essentially the same in all treatments, as was the time between the cells leaving the meristem and achieving maximum extension rate. The differences between the genotypes and treatments were all almost entirely due to differences in the time taken from the attainment of maximum REGR of cell extension to the cessation of extension. This was reflected in the length of the extension zone, which was approximately 6–8 per cent of final leaf length. The effects of the Rht3 allele, GA3 and paclobutrazol all appear to be on the processes which promote the cessation of cell elongation.