Leaf structure in southern African species of Salsola L. (Chenopodiaceae)

Abstract

Salsola L. is one of the largest genera within the Chenopodiaceae. It has been suggested that almost 90 species occur in southern Africa where the plants are most conspicuous in karroid areas. Members of Salsola are of considerable importance as pasture plants in the Karoo, especially during winter and periods of prolonged drought. Some species also have medicinal and other valuable properties. However, not all Salsola species are beneficial; some may cause diseases and deficiencies in livestock, leading to high mortalities and severe financial loss. Because of the occurrence of harmful and beneficial species within the same area, correct identification of the different species is of extreme importance. Correct identification is, however, rendered problematic by the great morphological similarity of almost all southern African Salsola species and uncertainties concerning the infrageneric classification of the group. There is a great need for a system of easy identification that can be used in the herbarium as well as in the field. This is especially so since available keys to the group cannot be used to identify sterile specimens. There also exists great uncertainty as to the exact identity of most of the 69 new species described by V.P. Botschantzev (Komarov Botanical Institute, St Petersburg) between 1972 and 1983. For this reason many of the names have hardly been taken up and used by South African botanists. A clear delimitation of the different character states within the genus would greatly facilitate and enhance the process of solving the systematic problems that exist within the genus. A comparative anatomical study of the leaves of southern African Salsola species was conducted using LM and SEM techniques. Leaf anatomy proved to be very useful for delimiting groups within the genus. Of particular importance is the structure of the leaf in transverse section and the type of the indumentum. The investigated species can be primarily divided into two main leaf types, according to the presence or absence of a uniseriate hypodermis underlying the adaxial epidermis. A secondary division can be made by indumentum types. Four main indumentum types have been identified based on the appearance of the abaxial leaf surface. One of these indumentum types can be further subdivided according to the area of the leaf covered by trichomes and the number of elongated cells in the trichomes. There is a weak association between leaf type and subsection, as well as between leaf type and indumentum type. No obvious association could be found between leaf or indumentum type and fruit type or any other macromorphological character. Further investigation in this respect is required. In general the species possessing an adaxial hypodermis tend to have a denser covering of trichomes than those species lacking one. This denser indumentum probably provides the plants with better insulation to help prevent excessive water loss and to protect subtending tissues from extreme heat in their arid environment. When studied in combination with other anatomical and ecological evidence these characteristics might prove to be very useful to help establish a classification system whereby Salsola species can be more easily identified.