RELEVANCE OF SEED POLYSACCHARIDES AND FLAVONOIDS FOR THE CLASSIFICATION OF THE LEGUMINOSAE - A CHEMOTAXONOMIC APPROACH

This review discusses the usefulness of seed polysaccharides and flavonoids sensu lato for legume classification and other areas of research. As to the seed polysaccharides in the Leguminosae, three types of carbohydrate reserves provide taxonomic characters: starch, galactomannans and amyloid. In contrast to common belief, the great majority of legume species do produce endosperm in their ripe seeds, although the amounts vary. Many taxa in the Papilionoideae store large amounts of starch in mature seeds, e.g. most of the Phaseoleae, Vicieae, Cicereae and Swartzieae. In the other tribes of this subfamily the occurrence of starch is erratic or absent, and in the Caesalpinioideae storage of starch grains is rather an exception. In the Mimosoideae the situation is similar, although storage of small starch grains in seeds occurs slightly more frequently. Members of all three subfamilies which have a thick-walled endosperm store galactomannans, and the presence of this type of reserve cellulose is now considered to be a taxonomically valuable tool. Some caesalpinioids with fleshy, thick-walled cotyledons store amyloid as reserve carbohydrates. Presence of large amounts of amyloid is restricted to the tribes Detarieae and Amherstieae of this subfamily, and can be considered as a key character of these tribes. Physiologically dissimilar amyloid is present in some starch accumulators of the papilionoid tribe Phaseoleae which have thin-walled cotyledons. The family Leguminosae is particularly rich in flavonoids and related compounds; about 28% of all flavonoid and 95% of all isoflavonoid aglycone structures known from the plant kingdom are produced by legumes. The most characteristic feature of these compounds in legumes is the frequent absence of the 5-hydroxyl group. In fact 50% of the flavonoid and 66% of the isoflavonoid structures reported from Leguminosae show this characteristic. Many flavonoids and isoflavonoids in the heartwood, roots and seeds of Leguminosae contain isoprenyl groups or modifications thereof such as dimethylpyrano groups. In contrast to the aglycones, the flavonoid glycosides reported from the Leguminosae do not show many unusual structural features. However, some of the classes of C-glycosylflavonoids are not yet known from other families, e.g. C-glycosylchalcones and C-glycosylisoflavones. Although all three subfamilies are united in containing flavonoid structures lacking the 5-hydroxyl group, the Caesalpinioideae and Mimosoideae on the one hand and the Papilionoideae on the other are distinguishcd by a number of differences in flavonoid metabolism. At lower levels of the taxonomic hierarchy, leaf flavonoids and isoflavonoid phytoalexins can provide useful characters for legume classification.