Breeding long coleoptile, reduced height wheats

Semidwarf wheats have the potential to produce high yields when sown and ma naged under optimal conditions. However, farm yields often fall below this potential because of poor seedling establishment and low early vigour assoc iated with gibberellic acid (GA)-insensitive-reducing height (Rst) genes co ntained in these wheats. Australian and overseas wheats containing major an d minor Pst genes sensitive to GA were intercrossed to develop three popula tions. Seedlings sensitive to GA and therefore lacking Rht-B1b (Rht1) and R ht-D1b (Rht2) plant height genes were selected for further study. GA-sensit ive F-4-derived lines were sown in field and glasshouse environments to det ermine plant height, and then sown at four temperatures to determine coleop tile length. Genetic variation in plant height and coleoptile length was la rge and significant (P < 0.01) among lines within each population with a nu mber of lines identified as producing plant heights as short as current sem idwarf 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 herita bilities (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 pheno typic selection should produce modest genetic gain for both characters. Var iation in coleoptile length was poorly related to differences in plant heig ht (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 gr oups. 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 shorte r height, longer-coleoptile wheats with improved establishment and seedling vigour.