Node counting in axillary buds of Nicotiana tabacum cv. Wisconsin 38, a day-neutral plant

A mature, quiescent, primary axillary bud on the main axis of a flowering Nicotiana tabacum cv. Wisconsin 38 plant, when released from apical dominance and before forming its terminal flower, produced a number of nodes which was dependent upon its position on the main axis. Each bud produced about one more node than the next bud above it. The total number of nodes produced by an axillary bud was about 6 to 8 greater than the number of nodes present above this bud on the main axis. At anthesis of the terminal flower on the main axis, mature, quiescent, primary axillary buds had initiated 7 to 9 leaf primordia while secondary axillary buds, sometimes present in addition to the primary ones, had initiated 4 to 5 leaf primordia. When permitted to grow out independently, primary and secondary axillary buds located at the same node on the main axis produced the same number of nodes before forming their terminal flowers. In contrast, immature primary axillary buds which had produced only 5 leaf primordia and which were released from apical dominance prior to the formation of flowers on the main axis produced only as many nodes as would be produced above them on the main axis by the terminal meristem, i.e., "extra" nodes were not produced. Therefore, it is the physiological status of the plant and not the number of nodes on the bud at the time of release from apical dominance that influenced the node-counting process of a bud. When two axillary buds were permitted to develop on the same main axis, each produced the same number of nodes as single axillary buds developing at these nodes. Thus, the counting process in an axillary bud of tobacco is independent of other buds. Axillary buds on main axes of plants that had been placed horizontally produced the same number of nodes as identically-positioned axillary buds on vertical plants, indicating that gravity does not play a major role in the counting, by an axillary bud, of the nodes on the main axis. THE OVERALL SHAPE of the aerial portion of a herbaceous plant is primarily a function of the combined activities of the plant's shoot apical meristems, both terminal and lateral. Many plants exhibit some degree of apical dominance, i.e., the growth of lateral buds is reduced or totally inhibited. When lateral buds do grow out, their pattern of growth often depends upon their position on the main axis, e.g., potato (Woolley and Wareing, 1972), pepper (Rylski and Halevy, 1972), Hieracium floribundum (Yeung and Peterson, 1972), Phyllanthus amaReceived for publication 13 May 1988; revision accepted 8 September 1988. We thank Joan Gebhardt for excellent plant care, and Harry Roy (Rensselaer Polytechnic Institute) and Anton Lang (Michigan State University) for critical reviews of the manuscript. This work was supported by grants from the National Science Foundation (PCM83-20407 and DCB 84-09708) and U.S. Department of Agriculture Competitive Grants Program (86-CRCR-1-2053) to CNM, and a National Science Foundation graduate fellowship to SRS. 2 Current address: Department of Biology, Carleton College, Northfield, MN 55057-4025. rus (Bancilhon, 1966) and tobacco (McDaniel and Hsu, 1976). In some instances a lateral bud mimics the growth pattern of the terminal apical meristem, e.g., tobacco and pepper; in other cases lateral buds show dramatically different patterns of growth, e.g., potato axillary buds can produce stolons whose tips may form tubers while the lateral buds of H. floribundum may form rosettes, stolons, or inflorescences. In numerous species and cultivars of the genus Nicotiana axillary buds exhibit two patterns of morphogenesis: they produce a cyme or they produce a complete inflorescence composed of a terminal flower and several cymes. Only axillary buds located in the inflorescence produce cymes while axillary buds which produce inflorescences are always located at the base of or below the inflorescence. An axillary bud meristem located below the inflorescence in Nicotiana tabacum cv. Wisconsin 38, a dayneutral cultivar, produces a normal inflorescence after laying down a number of nodes dependent on its position on the main axis (McDaniel and Hsu, 1976), upper buds pro-