Semidwarf wheats have the potential to produce high yields when sown and managed under optimal conditions. However, farm yields often fall below this potential because of poor seedling establishment and low early vigour associated with gibberellic acid (GA)-insensitive-reducing height (Rst) genes contained in these wheats. Australian and overseas wheats containing major and minor Pst genes sensitive to GA were intercrossed to develop three populations. Seedlings sensitive to GA and therefore lacking Rht-B1b (Rht1) and Rht-D1b (Rht2) plant height genes were selected for further study. GA-sensitive F-4-derived lines were sown in field and glasshouse environments to determine plant height, and then sown at four temperatures to determine coleoptile length. Genetic variation in plant height and coleoptile length was large and significant (P < 0.01) among lines within each population with a number of lines identified as producing plant heights as short as current semidwarf varieties. Transgressive segregation for coleoptile length produced progenies with coleoptiles significantly (P < 0.05) longer than the longest coleoptile parent in each population. Genotype x temperature interactions for coleoptile length were small thereby resulting in high line-mean heritabilities (h(2) = 85-89) for this character. Larger plant-to-plant variation reduced single-plant estimates of heritability for plant height (h(2) = 29-31) but heritability was increased (h(2) = 68-78) with replication within and over environments. High narrow-sense heritabilities indicate that phenotypic selection should produce modest genetic gain for both characters. Variation in coleoptile length was poorly related to differences in plant height (r(2) = 0.00 to 0.04 ns) while selection differentials for plant height were not associated with any change in coleoptile length of the selected groups. When considered together, height and coleoptile length appeared to be largely under independent genetic control among GA-sensitive wheats. These results suggest that GA-sensitive Rht genes could be used to select shorter height, longer-coleoptile wheats with improved establishment and seedling vigour.
