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

In South Australia, durum wheat yields more than bread wheat under well-wat ered and fertile conditions, but over much of the state's cereal belt the y ields of durum wheat, relative to bread wheat, are low. Three experiments w ere conducted over 3 years at 2 sites to compare the growth and yield of br ead and durum wheat and to investigate some of the reasons for the differen ces in the relative yields of the 2 cereals. Durum wheat yielded less than bread wheat when annual rainfall was less tha n about 450 mm or when the site mean yield for bread wheat was less than 25 0 g/m(2). Compared with bread wheat, durum wheat had poorer early vigour, w hich was associated with fewer tillers/m(2), and produced fewer kernels/m(2 ). Under favourable grain filling conditions, durum wheat produced larger k ernels than bread wheat but its kernel weight was more variable across site s and seasons and consequently, the relative yields of the 2 cereals depend ed largely on kernel weight. For example, in a wet year, durum wheat yielde d 20% more than bread wheat, despite producing 16% fewer kernels/m(2), beca use of its larger kernels (52 v. 36 mg). In 2 drier years, kernel weights o f durum and bread wheat were similar (durum and bread wheat mean kernel wei ghts: 40 v. 37 mg; 30 v. 33 mg) and so durum was unable to overcome the lim itation of fewer kernels/m(2) and its yields were similar to or less than b read 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 l ower length per gram of root dry matter, indicating less finely divided roo ts. This suggests that durum wheat may sometimes be less able than bread wh eat to utilise moisture and nutrient reserves in the subsoil because of a s maller root system. This is an undesirable characteristic for a crop that a ppears to be more reliant than bread wheat on producing large kernels for h igh yields. Efforts to improve the yield of durum wheat, either through genetic improve ment or by agronomic means, should focus on reducing the levels of stress d uring the post anthesis period so that limitations to kernel growth are min imised. Improving the early vigour of the crop, having cultivars of the app ropriate maturity and with adequate levels of resistance to root disease, a nd improving root growth and function in the subsoil an likely to be desira ble characteristics.