Comparative Study of the effects of applied indoleacetic acid and horizontal orientation of the primary shoot, upon internode extension and petiole orientation inHelianthus Annuus, and the modifying influence of gibberellic acid

Abstract

1. It is shown that subjecting potted sunflower plants to alternating periods of horizontal and erect orientation, with or without occasional rotation of the primary axis, results in a marked reduction in the rate of stem extension growth, the development of petiole epinasty and increased downward orientation of the leaves. Neither of these symptoms appears to be a direct gravitational response to the horizontal position. 2. It is demonstrated that the petiole epinasty is probably a correlative phenomenon while the inhibition of extension growth apparently results from the development of a factor which takes 24–48 hours to reach maximum intensity after the application of the horizontal stimulus. 3. Similar responses can be induced in permanently erect plants by applying IAA in amounts greater than about 5 μg/plant. 4. The application of GA to either horizontally stimulated or IAA treated plants, alleviates both the petiole epinasty and the inhibition of extension growth. 5. The results obtained are in agreement with the hypothesis that placing the shoot in the horizontal position causes an increase in the concentration of stem auxin and that this induces petiole epinasty and brings about a reduction in stem extension growth. In view of the alleviating influence of GA, it is suggested that the postulated increase in stem auxin may effect extension growth by reducing the natural gibberellin level in the stem to suboptimal values.