Variation in leaf area development and its effect on water use, yield and harvest index of droughted wheat

Abstract

Genotypes, chosen because of their variation in flowering, tillering and early leaf growth, were evaluated in 1 m long tubes in a glasshouse. These traits were selected for two reasons: firstly, because it was presumed they influence leaf area development and hence the water use, harvest index and yield of droughted wheat; secondly, because they are amenable to genetic manipulation and therefore can be selected in a breeding program. Four drought treatments were imposed in each of two seasons. The treatments and cultural conditions simulated droughts encountered by field-grown crops. Grain yield varied according to the amount of growth and water use before and after anthesis in relation to soil water supply. In the wettest treatments where plants had adequate water up to anthesis but were droughted thereafter (akin to a wet mediterranean-type environment), grain yield was positively associated with the amount of preanthesis growth. In the drier treatments, where droughts were sustained from before anthesis, genotypes having traits that reduced leaf area index saved more water for use after anthesis and had a higher harvest index and grain yield. However, pre-anthesis growth was also important, as it was estimated that in the driest treatments up to 60% of the grain dry weight was formed from reserves present at anthesis. These reserves represented up to 15% of the above-ground biomass at anthesis. Harvest index (y) was a nonlinear function of the proportion of water used after anthesis (x), y = 0.10 + 0.13 ln(x), r2 = 0.82 (P < 0.01).The determinate tillering or 'uniculm' wheats, although having some advantages over conventional tillering lines when sown at the same density, were largely indistinguishable from conventional wheats in their water use pattern when sown at twice the density. However, at both densities they had a higher stem weight per unit length and a higher specific leaf weight than conventional wheats, and this resulted in them having a lower harvest index and grain yield. These pleiotropic effects may reduce the possible advantages of these wheats, that is a reduced maximum leaf area and no sterile tillers, in dry environments.