The effects of hormones and temperature on the initiation of inflorescences and tendrils and the morphology of shoots and roots in grapes

The experiments reported investigated the response of buds, shoots and roots of Vitis vinifera L. cv. Riesling to hormones and temperature. They were done on one-year-old one-node potted cuttings growing in a shadehouse (20% shade) and growth cabinets(20°C day/15°C night, 30°C day/25°C night, 39,000 lux light intensity and 75% humidity), and on cane pruned four-year-old vines growing at St. Helena Estate North of Canterbury. After bud burst, fifty micromoles of IAA, BAP, ABA and GA₃ were applied singly or in combination periodically through the roots in potted vines. While 5O µM IAA, IBA, NAA, GA₃, and 2.5 µM 2,4-0 were applied once through the half-rings cut in the trunk of field-grown vines. Bud scales, leaf primordia, inflorescences, tendrils, internode length in the bud, and bud size were measured. The dry weights of stems, leaves and roots, leaf area, leaf number, main stem length, internode length, node number, lateral length, lateral number, root length, root number, and the relationships between the shoot and root were also determined. Labelled 14C-IAA and 14C-GA₃ were applied through the roots of potted vines to show the movement of hormones in the plant. The results presented show that: 1). The initiation of inflorescence was positively related to the production of leaf primordia in the bud and this response was consistent under different environmental conditions. High temperature (30°C), compared with low temperature (20°C) increased the number of leaf primordia per bud when measured at a similar number of nodes produced by the main stem. The initiation of inflorescence was affected in the same way. 2). IAA promoted the speed of leaf primordia production, initiated earlier and more inflorescences and flowers and significantly reduced the percentage of blind buds. GA₃ prevented the growth of the primary meristem in the bud after which the axillary meristem elongated rapidly, resulting in a split bud. Some of those buds formed necrotic tissues at the base of the primary mersitem and eventually died as the vine matured. The length of internodes and size of scales in the bud was increased, but the size of the bud was reduced. Those buds which survived GA3 treatment formed tendrils instead of inflorescences in potted vines and the number of inflorescences and flowers of field-grown vines was severely reduced. 3). There are many ways in which a bud can achieve a fruitful condition; in the more fruitful ways inflorescences can even be found after four leaf primordia, especially on auxin-treated field-grown vines. Those buds produced one or more inflorescences with or without a tendril. The less fruitful buds initiated inflorescences or tendrils after a higher number of leaf primordia were formed. For example, many were formed after six leaf primordia and some produced no inflorescences or tendrils at all. 4). Temperature affected the vegetative growth of the vine, for example the dry matter production was significantly higher at 20°C than at 30°C. IAA and GA₃ reduced the total leaf number and area per vine, but only GA₃ effects were significant. Both high temperature and GA₃ treatments produced long, but thin stems. However, GA₃ increased the efficiency of the leaves to produce dry matter (dry matter per unit area of leaf) while high temperature and IAA reduced it. 5). The shoot: root ratio was significantly higher at 30°C than at 20°C. GA₃ significantly increased the shoot: root ratio while IAA reduced it. High temperature (30°C) favoured the accumulation of more dry weight in leaves than in stems, whereas, GA3 acted in the reverse way. 6). The growth of roots at 30°C was thin, and the young tip sections (white) were long. The root dry weight was lower and the length was slightly less than at 20°C. GA₃-treated potted vine produced thin, less dense roots with less dry weight, and length, but did not affect the amount of root branching. The roots were brown with only short young (white) sections at the tips. IAA-treated vines showed moderately dense roots with long young (white) sections at the tips. IAA did not show a consistent effect on root dry weight, but it increased root number, and reduced root length for each plant. 7). A significant positive relationship was found between the root length and the total leaf surface area for each vine. Reduction in temperature caused a reduction in total leaf area relative to the length of the roots. Similarly, GA₃ and IAA reduced the total leaf area relative to the length of roots. A hypothesis is presented which states that the switching of the bud apex from the vegetative to the reproductive stage is partly dependent on the development of tissues below the "floral node", i.e. bud scales, leaf primordia, and internodes in the bud. The normal development of these tissues can be modified by changes in hormone balances in the bud induced by exogenous hormones and by temperature. It seems likely that the critical change-over from leaf primordial production to inflorescence production is accompanied by a shortening of the plastochrone in a special relationship to the auxin content of the hormone balance existing at the apex. Further development of the inflorescence is probably dependent on cytokinin and the balance between cytokinin and gibberellic acid.