Stay-green enhances lodging resistance under terminal water deficity in sorghum

Lodging associated with terminal drought has been one of the major constraints facing sorghum growers and breeders since the advent of hybrids in the US and Australia more than 50 years ago. Water stress during grain filling causes the plant to use resources from leaves and stems to fill grain. If the stress is severe enough, this process causes stem death and lodging. In addition, if circumstances are favourable, the tissue death enables stalk rotting pathogens to colonise and degrade stem tissue, further increasing the severity of lodging. In this paper we report on a large number of experiments undertaken in Australia and India to determine the impact of the stay-green trait on lodging resistance and grain yield under varying levels of water supply. Stay-green, as measured by the relative rate of leaf death during grain filling, was positively correlated with lodging at maturity in the Australian and Indian studies using experimental populations. In addition, stay-green was highly correlated with reduced lodging in environments where post-flowering drought stress occurred in trials of sorghum hybrids conducted by the GRDC funded sorghum core breeding program over many years. In another GRDC funded project, genetic lines were constructed where specific stay-green genes were introduced into a senescent line. When these lines were tested under post-flowering drought, the stay-green characteristic was shown to substantially reduce lodging. Evidence from a large body of research trials indicates the overwhelming value of stay-green in increasing grain yield, increasing grain size, and reducing lodging when drought occurs during grain-filling, which is commonly the case in Australia. Sorghum lines produced by the GRDC sorghum core breeding program with the stay-green trait are widely used in Australian commercial hybrids and in commercial breeding programs