Effects of kinetin on tiller bud development in wheat

Tillering in cereals, ecologically important to plants and economically important to agronomists, is the result of the development of axillary buds into tillers. It has been shown to be affected mainly by nutrition, moisture, climate, seed size, depth of planting and species (cf. Gardner, 1942; Mitchell, 1953a, b; Aspinall 1961, 1963; Langer, 1963). Moreover, Leopold (1949) has attempted to show that axillary buds in Gramineae also suffer from correlative inhibition, although there is little evidence of apical control of lateral bud development in Gramineae. Extensive observations predominantly in dicotyledonous herbaceous plants and some tree species have shown that the fate of lateral bud development is commonly, but not necessarily, determined by the actively growing apical bud. This phenomenon, called correlative inhibition, is common in many plant species. However, the degree of sensitivity of laterals to inhibition varies greatly among different plants. Work done over the last few decades on apical dominance in relation to the influence of some hormones, nutrients, genotypes, certain physiological factors, season, age, virus infection and general vigour of plants has revealed that no single factor is adequate to explain the mechanism involved. However, hormones and nutrition are probably the main regulatory agents for tillering or branching in plant species, adapted to the prevailing climate. Very little information is available in monocotyledonous plants in relation to hormonal effects on tillering, whereas considerable work has been done in dicotyledonous plants. This discrepancy is attributable to differences in structure, especially the position of apical and lateral buds, those of dicotyledonous plants being easily accessible for investigation. It has been demonstrated that cytokinins do not only cause the initiation of lateral buds but also release those that are inhibited (cf. Thimann & Wickson, 1957, 1958). The present investigation designed as a result of these considerations with the aim of gaining information on the effects of, kinetin and carbohydrate on the growth of laterals in wheat plants. The investigation was confined to the period between the double-ridge stage and ear emergence, because there is little chance of tiller bud development after the double-ridge stage in Gramineae, as shown by various workers, although tillering may resume when the ear has emerged (cf. Leopold, 1949; Patel and Cooper, 1961; Cooper, 1948; Laude et al. 1968; Langer, 1956). In Hilgendorf '61 wheat no buds were found to grow from axillary positions beyond leaf 4 on the main stem under glasshouse conditions in plants which attained the double-ridge stage at the 5½ leaf stage. The investigation, described here was conducted in growth cabinets in which environmental conditions were controlled. The study was confined to lateral buds on the main shoot. Wheat was chosen to study the influence of kinetin on lateral growth because of the lack of information on Gramineae. The New Zealand wheat cultivar, Hilgendorf '61, was selected, since it is representative of many temperate wheat varieties grown for milling and baking.