Seedling vigour in wheat-sources of variation for genetic and agronomic improvement

The early growth of wheat is slow compared with that of barley and triticale. This is expected to limit the yield of wheat in environments where greater seedling vigour is advantageous. To overcome the slow growth of wheat, genetic sources of seedling vigour are required for use in breeding programs, and/or ways to increase seedling growth by manipulating seed characteristics. This study reports (i) new sources of variation in wheat seedling vigour, (ii) the plant characteristics most closely associated with seedling growth, and (iii) how seed characteristics may influence early growth. Substantial genetic variation was found in the early leaf area and dry weight of wheat. Australian and CIMMYT-derived wheats were among the least vigorous. In this survey of variation, neither rate of leaf appearance nor tiller number was associated with seedling vigour in wheat. Seedling characteristics most closely associated with greater seedling vigour were a large embryo, large primary leaves on the main shoot, a high specific leaf area (leaf area to leaf weight ratio), and a high frequency of large coleoptile tillers. Some wheats from a global survey and from a composite cross population approached the vigour of barley. Wheat-barley addition lines were used to detect whether any specific barley chromosomes increased the early vigour of wheat. Chinese Spring wheat containing barley chromosome 3 was the only addition line with a significantly greater (P<0.05) leaf area and dry weight than the wheat parent. Barley chromosomes contributing to a greater specific leaf area, coleoptile tiller, and breadth of the early seedling leaf, all of which contribute to greater vigour, were identified. Seed weight was closely related to seedling growth in all studies. A specific study to investigate the effect of seed weight showed that a 10-mg increase in grain weight increased leaf area (15%), plant dry weight (21%), leaf number (2%) and dimensions (4% for leaf length and 8% for leaf width), and tiller number (12%), and decreased emergence time (4%). A linear increase in embryo weight and embryo area was also associated with the increase in grain weight. Seed nitrogen content had a small effect on dry weight in a glasshouse experiment but had no effect on plants grown at lower temperatures outside. In the glasshouse, a 1% increase in seed protein increased seedling dry weight by 3%, primarily through faster emergence. Seeds containing low Zn or Mn levels had poorer emergence and subsequently lower dry weight.