Root Signals Control Leaf Expansion in Wheat Seedlings Growing in Drying Soil

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Abstract
Wheat plants were grown with their roots and soil in pressure chambers, so that the leaves could be kept highly turgid, even when the soil dried, by applying a pneumatic pressure to the roots. The relative leaf expansion rate (RLER) of plants in drying soil eventually fell behind that of well-watered plants, but, remarkably, the fall in RLER was the same whether or not the leaves were kept highly turgid. It is argued that the roots sensed the drying of the soil and sent signals to the leaves that controlled their behaviour, overriding any effects of turgor on the leaves. It is likely that the roots were sensing not only the water potential of the soil but also its hardness, which increased substantially as the soil dried.