The physiology of "stay-green" in sorghum

Abstract

Developing crops that use water more efficiently is one of the greatest challenges facing crop scientists today. In the face of diminishing water resources, the world is expected to consume twice as much food in the next 50 years as it has in the past 10000 years. To meet this demand, world grain production will have to increase 40% by 2020. Crops such as sorghum and millet are specially adapted to semiarid regions, and contain various mechanisms enabling these crops to escape and resist drought. Understanding the genetic, physiological, molecular and biochemical basis of such drought- resistance mechanisms is fundamental to the development of new strains that are better adapted to dry conditions. Keeping leaves alive longer is a fundamental strategy for increasing crop production, particularly under water-limited conditions. This paper reviews recent physiological studies into the stay-green drought resistance trait in sorghum, including an outline of future research proposed in collaboration with three American universities during the next 3-5 years. Recent studies in Australia have examined two sources of stay-green: B35 and KS19, derived from sorghum lines native to Ethiopia and Nigeria, respectively. Early in crop growth, stay-green hybrids partition more carbon and nitrogen to leaves compared with their senescent counterparts, resulting in higher specific leaf nitrogen (SLN). It is hypothesized that the higher SLN initiates a chain of responses, including enhanced radiation use efficiency (RUE) and transpiration efficiency (TE), which enable the plant to set a higher yield potential by anthesis. After anthesis, higher SLN delays the onset and reduces the rate of leaf senescence and this is associated with stay-green crops taking up more nitrogen from the soil compared with senescent crops. These processes lead to increased grain yield and lodging resistance in stay-green lines under postanthesis drought.