Inflorescence initiation in grapes - response to plant growth regulators

Young pot-grown vines of Riesling grapes (Vitis vinifera L.) were treated, via roots, with gibberellin A3 (GA3), indole acetic acid (IAA), 6-benzyl-amino purine (BAP) and abscisic acid (ABA) in a factorial experiment. Seven 100 ml applications at 50 μM were applied at regular intervals. In a second experiment GA3, IAA, indole butyric acid (IBA), naphthalene acetic acid (NAA), and 2,4-dichlorophenoxy acetic acid (2,4 - D) were injected into the trunks of field-grown Riesling vines 3 weeks after bud burst. Auxins induced more leaf primordia, tendrils and inflorescences and reduced internode length in the buds of treated vines examined the following winter. The rate of formation (plastochrone) was also increased. Flowers appearing on shoots the following spring were more numerous on treated vines. GA3 had opposite effects to auxins on all the above. ABA and BAP reduced the production of tendrils, inflorescences and leaf primordia in the bud - although much less than GA3 - however, when applied with GA3, they reduced its deleterious effects on these organs. Examination of buds revealed a variety of bud forms. Buds which had more inflorescences and primordial leaves were from mature vines, especially if treated with auxin-like compounds. Young vines had fewer leaf primordia and fewer inflorescences, a situation also found in GA3 - treated older plants. Treatments or conditions which favoured more fruitful buds also tended to reduce the plastochrone and it is suggested that the two phenomena are causally related.