The cost of stress: Dry matter partitioning changes with seasonal supply of water and nitrogen to dryland wheat

Spring wheat cv. ‘Gutha’ was grown in continuous wheat (W/W) and narrow-leafed lupin (L. angustifolius L. cv. Yandee)-wheat (L/W) rotation on a yellow earth over mottled clay (Arenic Fragiudult) in a mediterranean climate for two years. The first year had a higher than average rainfall with adequate soil water until anthesis. The second year was very dry (only 232 mm total rainfall) and soil water contents were low throughout the growing season. Nitrogen fertilizer (+N) treatments were included in both years. In the first year an adjacent experiment compared the effects of loosening a pronounced traffic pan which existed on the site (LS)versus unloosened (US). In the first year roots contained more dry matter than tops in the early vegetative stage in all crops and then declined exponentially to a ratio of 0.1 in the US and LS treatments. In the second year however, the decline was both linear and much less, so that root:shoot ratios at harvest were still between 0.4 and 0.8. There was a consistent trend in root:shoot ratios from the most favourable (LS) to least favourable (W/W-N) treatments over the combined two years’ data, and this was also found in grain yield, with a higher yield in year one from the LS than US, and the lowest yield in year two from the W/W-N treatment. The proportion of total biomass recovered from below ground was substantially higher than is commonly reported from studies carried out in temperate, high fertility soils, but probably still under-estimates of the true amount of dry matter in roots because of inadequacies of sampling, washing and storage techniques. Root length densities were much greater in the drier year, especially in the surface 0.1-m, and based on theoretical considerations, much greater than required for extraction of available water. The effect of environmental conditions on the relative size of cereal crop carbon sinks are discussed in relation to these results.