This chapter attempts to provide a description of the structure of the potato plant, including both external morphology and internal anatomy. Since the existing literature appeared to be particularly deficient on developmental aspects, some re-examination of early stages of development has been made where possible. Notwithstanding the considerable morphological and anatomical literature, it is clear that the structure and development of the potato, admittedly a very complex plant, are by no means fully understood; strangely enough, this applies particularly to the development of the tuber, despite its economic importance. There is considerable scope for further work in this field.

Structure and development of the potato plant

Tuber initiation in the potato plant is accompanied by extensive morphological and biochemical changes above and below ground. Plants that are capable of tuber initiation are said to be “induced” to tuberize. It has long been postulated that the changes leading to induction are mediated hormonally. Before considering the evidence, it is helpful to summarize what is known about the tuberization process, its genetic and environmental control, and the techniques that have been employed to study it. For a more comprehensive treatment of these topics and for literature citations, see the recent review article by Ewing and Struik (8).

The Role of Hormones in Potato (Solanum tuberosum L.) Tuberization

Cuttings as simplified models of the potato plant

Although invitro production of potato tubers or microtuberization was achieved more than 40 years ago, the application of microtubers in reliable model research systems has been slow to develop. Several factors such as the use of growth regulators in microtu-ber induction and growth media, the mixotropic nature of thein vitro system, and cultivar-specific responses have led to interpretive difficulties. A cautionary note is also necessary in view of apparent growth and development differences, metabolic alterations, and soma-clonal variation encountered in microtubers that may not be found in field-grown tubers. Evidence for strong and consistent analogies between microtubers and field-grown tubers for their induction, growth and development, and metabolism often is lacking. However, several components such as the rapid and near-synchronous induction and growth, which can be modified by a range of exogenous compounds or conditions, make the microtuber a valuable model system. Complex problems such as dormancy also appear to be particularly amenable to examination by the microtuber system. In addition, the use of microtubers as experimental research tools has potential in the areas of plant metabolism, germplasm selection and evaluation, genetic transformation, somatic hybridization, and molecular farming.

Potato microtubers as research tools: A review

Uridine-H3 was incorporated into meristem tips of both healthy and PLRV-infected potato plants, of the cultivars, Majestic and Primura. Autoradiograms of tips pretreated with actinomycin D showed that 13 of 14 and 11 of 14 respectively, were virus free in the dome and first 4 leaf primordia, 1 and 3 were infected in the 4th leaf primordium, and 1 of Primura also in its 3rd primordium. The highest plantlet yield from cultures in vitro of meristem tips that included 4 leaf primordia (7 to 9 plantlets per meristem), was obtained by growing them in sequence on 3 sequential media, each based on Murashige and Skoog basic medium complemented with various hormones. The 2nd medium, containing benzylaminopurine (0.5 mg/l) and gibberellic acid (0.5 mg/l), elicited callus and the subsequent formation of adventitious buds. Of the plantlets of the cvs Vivaks, Primura and Majestic, 88, 91 and 100 %, respectively, were PLRV-free. Propagation in vitro, by the single-node cutting technique and tuberlet production, were successful.

Potato leafroll virus distribution in potato meristem tips and production of virus-free plants

Coumarin readily stimulates tuberization of cultured axillary shoots obtained from etiolated potato sprouts. Adequate concentrations of coumarin must be continuously present in the medium to effect tuber initiation. High nitrogen, gibberellic acid, abscisic acid, indole acetic acid, or naphthalene acetic acid reduced or inhibited coumarin-induced tuberization. Gibberellic acid and high concentrations of nitrogen in medium inhibited the uptake of coumarin-3-14C by the. axillary shoot. Based on the data of physiological activities by coumarin in other plant species and our data on tuberization, we propose that coumarin may represent in part the unknown inhibitor responsible for tuber initiation with reference to the inhibitor/gibberellic acid ratio theory of tuberization.

G. F. Stallknecht

Papers

General characteristics of coumarin-induced tuberization of axillary shoots ofSolanum tuberosum L. culturedin vitro