Plant and Soil 121, 89-98 (1990). © Kluwer Academic Publishers. Printed in the Netherlands. PLSO 8241 Root:shoot ratios of old and modern, tall and semi-dwarf wheats in a mediterranean environment K.H.M. SIDDIQUE, R.K. BELFORD and D. TENNANT Division of Plant Industries, Western Australian Department of Agriculture, Baron-Hay Court, South Perth, 6151, Western Australia Received 11 May 1989. Revised August 1989 Key words: harvest index, old and modern wheats, root:shoot ratio, Rht genes, root dry matter, root length, water use efficiency Abstract A field study tested the hypothesis that modern wheat varieties invest a lesser proportion of the total dry matter (root plus shoot) in the root system compared to old varieties. The study was carried out on a duplex soil (sand over clay) at Merredin, Western Australia in a Mediterranean type environment. We also compared the root:shoot dry matter ratios of near-isogenic lines for Rht dwarfing genes. Root:shoot ratios decreased with crop growth stage and were closely related to the developmental pattern of a variety. All varieties appeared to accumulate more dry matter into shoots than roots after the terminal spikelet stage. For the modern variety Kulin this occurred as early as 55 days after sowing (DAS), but did not occur until 90 DAS in the old variety Purple Straw. For all varieties, root dry matter reached its maximum at anthesis, while shoot dry matter continued to increase till maturity. At anthesis there were no significant differences in shoot dry matter between varieties, but from Purple Straw to Kulin root dry matter and thus root:shoot ratio decreased. The tall and dwarf isogenic lines had similar developmental and root:shoot dry matter accumulation patterns. At anthesis, the old variety Purple Straw had significantly higher root dry matter and root length density in the top 40-cm of the profile than modern variety Kulin. There were no varietal differences in rooting depth, water extraction or water use. At maturity about 30% of the total dry matter was invested in the roots among wheat varieties. Grain yield, harvest index (HI) and water use efficiency of grain (WUEgr) increased from old to modern varieties. The reduced investment of dry matter in the root system and thus the lower root:shoot ratio from early in the growing season may partly explain the increased HI and WUEgr of modern compared to old varieties. Introduction Passioura (1983) provided substantial evidence that roots are a major sink for assimilates, requir- ing twice as much assimilate to produce unit dry matter as do the shoots. He argued that the root system of many crop plants may be unnecessarily large; and if it was smaller, more assimilate could be available for the shoot, to be used for higher grain production, with an increase in water use efficiency of grain. 89 Increase in harvest index (HI) appears to be a consistent result of selection for higher grain yield in wheat, with little change in the above ground biomass (Austin eta!., 1980; Perry and D'Antuono, 1989). Recent studies in Western Australia (Kirby et al., 1989; Loss et al., 1989; Siddique et al., 1989a, b,c) have identified major changes in moving from the old variety, Purple Straw (released 1860) to the modern variety, Kulin (1986). Modern varieties, adapted to the growing environment of the Western Australian wheatbelt, differed from