Final Report improving cotton yields on sodic soils - a new role for plant growth regulators Sodic soils present a major management constraint for large parts of Australia's cotton producing districts. A combination of structural, chemical and water constraints in these soils leads to reduced plant growth and yields (Yadav et al. 2011). One primary area of concern is the effect of sodicity in reducing the drought tolerance of cotton plants, particularly seedlings. Because much of the soil moisture stored in the top 10cm of soil is lost to evaporation, it is vital that seedlings can quickly access moisture at greater depths. Sodic soils compound this problem in multiple ways. The chemical environment reduces theosmotic potential, and thus the ability of the young plants to draw sufficient water. Also, the dispersive nature of these soils creates crusting and blocks macropores, reducing infiltration and overall water supply under rain-fed conditions (Ghosh et al. 2010). Along with these factors, sodic soils, particularly Vertosols, can demonstrate high soil strength which makes it difficult for seedling roots to penetrate the soil (Odeh and Onus 2008). Cotton plants arewell adapted to water stress and the chemical environment of sodic soils (Dodd et al. 2010). Most growth constraints, therefore, are likely to be attributable to structural limitations in physically preventing root access to water.Mepiquat chloride is a synthetic plant growth regulator widely used in cotton production to maintain a regular crop and prevent excessive vegetative growth under high temperature conditions (Yeates, Constable and McCumstie 2005). It reportedly interacts with natural plant hormones, increasing the levels of auxin and cytokinin hormones in the middle region of the primary root (Duan et al. 2007), and suppressing gibberellin biosynthesis (Yeates, Constable and McCumstie 2005). The suppression of gibberellin production is believed to slow internode lengthening by limiting cell enlargement (Reddy, Reddy and Hodges 1996), which is the agronomic effect intended. Duan et al. (2007) found that mepiquat chloride also increased lateral root primordia development and lateral root growth, apparently in connection with the increased levels of auxins and cytokinins. Other reports have also found increases in root growth from treatment with mepiquat chloride (Xu and Taylor 1992; Iqbal et al. 2005), which can have beneficial effects in terms of drought tolerance, particularly forseedlings (Xu and Taylor 1992).