Symplast Volume, Turgor, Stomatal Conductance and Growth in Relation to Osmotic and Elastic Adjustment in Droughted Sugarcane1

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Abstract
Leaf water relations, stomatal conductance (g) and shoot growth rate (SGR) were monitored during a soil drying cycle in three sugarcane cultivars growing in pots in a greenhouse. The pressure-volume technique was used to evaluate diurnal and droughtinduced variation in leaf water relations characteristics. Leaf solute content and bulk elasticity varied diurnally in both irrigated and droughted plants and were highest at midday. Solute accumulation and increased elasticity were also observed as leaf water deficits developed more slowly during soil drying. This osmotic and elastic adjustment maintained symplast volume essentially constant both diurnally and during soil drying, whereas turgor was only partially maintained. The extent of osmotic adjustment associated with drought was not reflected in the leaf osmotic potential at full turgor because the concurrent increase in tissue elasticity resulted in a larger symplast volume at full turgor. Cultivar responses over the range of leaf water deficits imposed did not provide conclusive evidence for genotypic variation in osmotic and elastic adjustment. It appeared that behavioural differences in rates of water use may have determined the magnitude of osmotic and elastic adjustment in response to drought. In the early stages of soil drying, reductions in SGR and g were not accompanied by significant reductions in bulk leaf water status. This suggested that other factors, presumably signals originating from the roots, may have regulated SGR and g.

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