Organographic and anatomical evidence that the edible storage organs of Plectranthus esculentus N.E.Br. (Lamiaceae) are stem tubers

Abstract

Plectranthus esculentus N.E.Br. is a member of the family Lamiaceae, to which many aromatic plants such as the mints and sages belong. It differs from most members of the family in that it produces tuberous subterranean organs at the base of the stem, which can weigh as much as 1.8 kg per tuber. These organs are often used by local people as a food source. Storage organs in plants can be modified roots (carrot and sweet potato), stems (potato), or composed of leave bases (onion). In the case of P. esculentus, there is a difference of opinion as to the exact ontogenetic origin of these organs, and whether they are roots, rhizomes, underground stems or root tubers. According to our observations, these tuberous, positively geotropic organs develop from axillary buds at the base of the stem, and even axillary buds along aerial parts of the stem. Their branching pattern follows the decussate phyllotaxis of the species, and flowering of some tubers has been observed. Anatomical studies show the presence of both primary and secondary vascular bundles arranged in a ring of vascular tissue. Therefore, these organs are true stem tubers.

Introduction

Plectranthus esculentus N.E.Br., also known as Coleus dazo and Coleus esculentus, is classified in the family Lamiaceae to which many aromatic plants such as the mints and sages belong. It is an indigenous African plant that is widespread throughout the continent, including South Africa. This seasonal, perennial, herbaceous plant differs from most members of the family in that it produces tuberous subterranean storage organs (Fox and Norwood-Young, 1982, Tindall, 1983, Codd, 1985). The plant is a minor food crop in many areas of Africa (Kyesmu, 1994), but has the potential to be developed into a commercial crop.

Biennial and perennial plants such as carrots, beetroot and potato need to store metabolites in order to facilitate growth of the overwintered buds the following spring (Van der Schijff, 1976, Von Denffer et al., 1976). To this end, energy-rich materials are often deposited in subterranean food storage organs during the preceding growing season. These storage organs are among the most valuable sources of vegetable foodstuffs for both animals and man because of their richness in organic materials (Von Denffer et al., 1976).

P. esculentus produces edible, thickened subterranean organs at the base of the stem. In the wild, the aerial parts die during the winter period while the tuberous organs remain in the ground and serve as propagules for regrowth in the next season. The life cycle of this plant is, therefore, very similar to that of potato. The edible organs can either be harvested and stored for later consumption, or they can serve as propagules, with the mass produced by a single plant being able to produce approximately 100 new plants (Dalziel, 1937).

African people have used tubers of P. esculentus as a food source for many centuries, and a certain degree of selection must have taken place. Now that the importance of this plant has been revived, more emphasis will have to be placed on selection. As the tuber is the portion of the plant that is of interest to the consumer, it is necessary to discuss the ideal form that the tubers should take in order to be acceptable for marketing. As the plant has a limited number of axillary buds below ground level, branching of the tubers is required in order to produce a larger mass of tubers. This branching should not be haphazard, but limited to a few branches close to the base of each main tuber, which producers a tuber cluster. Branching near the tip will lead to a tuber bunch that consists of a great number of fairly short branches. These tubers will be more difficult to prepare as a foodstuff than tubers with branches limited to the base.

P. esculentus tubers differ from potato tubers in the sense that they are directly attached to the stem of the mother plant, they are branched, and have a positively gravitropic growth pattern similar to roots. It is no wonder, therefore, that Codd (1975) referred to these organs as “edible roots”. Dalziel (1937) stated that these storage organs of P. esculentus are regarded as rhizomes in the nature of stolons, while Smith (1966) and Codd (1975) described the plants as perennials that have an edible tuberous rootstock. Fox and Norwood-Young (1982) stated that the plant arises from many tuberous roots. Codd (1985) appeared to agree with that statement as he noted that the plant arises from a tuberous-rooted base.

These organs have also been referred to as thickened underground stems that grow like the fingers of a hand from a central point (Williamson, 1974). The description given by Tindall (1983) differs in that these organs were referred to as thickened, long, cylindrical root tubers or rhizomes formed at the base of the stem. Tregold (1986) also referred to the organs as branching tubers but neglected to mention if they are root or stem tubers. It is therefore clear that there is, as yet, no clarity or agreement as to the precise origin and morphology of these organs. The majority of authors seem to be of the opinion that they are tuberous roots, but according to others they could also be stem tubers, rhizomes or even stolons. It was, therefore, deemed necessary to investigate the storage organs of P. esculentus in order to solve this problem by applying organographic criteria based on the descriptions provided by Bell (1991), as well as anatomical criteria based on descriptions by Raven et al. (1992).