The Role of Water Distribution in Plant Tropisms.

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Abstract
Dark-grown seedlings of Helianthus annuus, Pisum sativum and Phaseolus vulgaris all showed twofold to threefold increases in the rate of shoot elongation when the relative humidity was increased from 50% to 100%. In experiments with H. annuus, the lower half of the geostimulated hypocotyl had a higher water content and dry weight than the upper half, while the shaded side of hypocotyls subjected to phototropic stimulation had a higher water content and dry weight than the side towards the light. These differences occurred even when tropic curvatures were physically prevented. It was therefore postulated that water is normally the limiting factor in shoot elongation and that changes in its lateral distribution are primarily responsible for the differential growth induced by geotropic and phototropic stimulation. Experiments with P. sativum and Zea mays, which showed that root growth was inhibited by immersion of the root apex in water, lent support to the hypothesis that O2 is normally the limiting factor in root growth and that the positive geotropism of roots is caused by a gravity-induced increase in the water content of the lower half of the apex which inhibits growth in this region by reducing the O2 supply.