Contribution of Sugars to Osmotic Adjustment in Elongating and Expanded Zones of Wheat Leaves During Moderate Water Deficits at Two Light Levels

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Abstract
Water was withheld from wheat seedlings grown at two light levels, so that the soil water content was reduced by 50% and the leaf water potential of treated plants was about 0.5 MPa below that of controls. Growth was initially reduced but recovered after about 4 days of the water deficit treatment. Osmotic adjustment (calculated as the difference between the osmotic potential at 100% relative water content of the treated and controls plants) occurred in both the elongating and expanded zones of leaves. In plants grown at a very high irradiance, an increase in sugars accounted for 70-100% of the osmotic adjustment of 0.12-0.34 MPa in the elongating and expanded zones of leaves sampled both at dawn and at midday. The increased amounts of sugars were approximately equivalent to the decreased consumption of carbohydrate during the initial reduction in growth. Glucose was the main component of the increase in sugars in the elongating regions, while sucrose was more important in the expanded laminae. During the morning, sugars increased to similar extents in controls and treated plants. In plants grown at a moderate irradiance, sugars did not account for the osmotic adjustment of 0.10-0.21 MPa in the expanded laminae at dawn or at midday. At dawn, sugars were much lower in treated than in control plants but, during the morning, sugars accumulated more in the treated plants and sugar levels of the two groups at midday were similar. In contrast, sugar levels in the youngest elongating leaf of the treated plants were the same as in the controls at dawn, and were double those in the controls at midday.