Abstract

The hypothesis that phototropism is caused by the effect of light on transpiration seemed at variance with the observation that Avena coleoptiles respond phototropically while submerged in water. Further investigations, however, provided evidence that phototropism of submerged coleoptiles may be caused by the light-induced promotion of stomatal guttation. Evidence of guttation by submerged coleoptiles was provided by measuring tritium accumulation in a water jacket surrounding the coleoptile when tritiated water (THO) was supplied to the roots. It was shown that the application of nail polish to the coleoptiles induced almost immediate guttation from the stomata. This response, measured quantitatively by image analysis, occurred predominantly on the irradiated side of coleoptiles previously stimulated phototropically with blue light while submerged in water, and was much reduced and more uniformly distributed when the phototropic response had been eliminated by saturating the water with CO₂. Small curvatures exhibited by coleoptiles kept in the dark at high humidity were almost invariably towards the side from which guttation occurred from the apical (hydathode) stomata. Application of petroleum jelly (Vaseline) to the stomata on the illuminated side of coleoptiles stimulated phototropically in moist air induced a negative curvature in the apical half of the coleoptile and reduced the positive curvature in the basal half. In contrast, vaseline applied to the middle of one of the broad sides of the coleoptile, where no stomata occur, had relatively little effect. These results are consistent with the hypothesis that phototropism of the Avena coleoptile is caused by light-induced loss of water from the stomata. It is also postulated that the mechanism is essentially the same whether water is lost by transpiration or guttation.