THE ROLE OF TRANSPIRATION IN PHOTOTROPISM OF THE AVENA-COLEOPTILE - EVIDENCE OF STOMATAL CONTROL OF THE PHOTOTROPIC RESPONSE

The hypothesis that phototropism is caused by the effect of light on t ranspiration seemed at variance with the observation that Avena coleop tiles respond phototropically while submerged in water. Further invest igations, however, provided evidence that phototropism of submerged co leoptiles may be caused by the light-induced promotion of stomatal gut tation. Evidence of guttation by submerged coleoptiles was provided by measuring tritium accumulation in a water jacket surrounding the cole optile when tritiated water (THO) was supplied to the roots. It was sh own that the application of nail polish to the coleoptiles induced alm ost immediate guttation from the stomata. This response, measured quan titatively by image analysis, occurred predominantly on the irradiated side of coleoptiles previously stimulated phototropically with blue l ight while submerged in water, and was much reduced and more uniformly distributed when the phototropic response had been eliminated by satu rating the water with CO2. Small curvatures exhibited by coleoptiles k ept in the dark at high humidity were almost invariably towards the si de from which guttation occurred from the apical (hydathode) stomata. Application of petroleum jelly (Vaseline) to the stomata on the illumi nated side of coleoptiles stimulated phototropically in moist air indu ced a negative curvature in the apical half of the coleoptile and redu ced the positive curvature in the basal half. In contrast, vaseline ap plied to the middle of one of the broad sides of the coleoptile, where no stomata occur, had relatively little effect. These results are con sistent 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.