Shoot growth, root growth and grain yield of bread and durum wheat in South Australia

Summary. In South Australia, durum wheat yields more than bread wheat under well-watered and fertile conditions, but over much of the state’s cereal belt the yields of durum wheat, relative to bread wheat, are low. Three experiments were conducted over 3 years at 2 sites to compare the growth and yield of bread and durum wheat and to investigate some of the reasons for the differences in the relative yields of the 2 cereals. Durum wheat yielded less than bread wheat when annual rainfall was less than about 450 mm or when the site mean yield for bread wheat was less than 250 g/m2. Compared with bread wheat, durum wheat had poorer early vigour, which was associated with fewer tillers/m2, and produced fewer kernels/m2. Under favourable grain filling conditions, durum wheat produced larger kernels than bread wheat but its kernel weight was more variable across sites and seasons and consequently, the relative yields of the 2 cereals depended largely on kernel weight. For example, in a wet year, durum wheat yielded 20% more than bread wheat, despite producing 16% fewer kernels/m2, because of its larger kernels (52 v. 36 mg). In 2 drier years, kernel weights of durum and bread wheat were similar (durum and bread wheat mean kernel weights: 40 v. 37 mg; 30 v. 33 mg) and so durum was unable to overcome the limitation of fewer kernels/m2 and its yields were similar to or less than bread wheat. Root length densities of durum and bread wheat below 30 cm were low. Durum wheat had an equivalent or lower root length density than bread wheat and lower length per gram of root dry matter, indicating less finely divided roots. This suggests that durum wheat may sometimes be less able than bread wheat to utilise moisture and nutrient reserves in the subsoil because of a smaller root system. This is an undesirable characteristic for a crop that appears to be more reliant than bread wheat on producing large kernels for high yields. Efforts to improve the yield of durum wheat, either through genetic improvement or by agronomic means, should focus on reducing the levels of stress during the post anthesis period so that limitations to kernel growth are minimised. Improving the early vigour of the crop, having cultivars of the appropriate maturity and with adequate levels of resistance to root disease, and improving root growth and function in the subsoil are likely to be desirable characteristics.