Chemical characters in plant taxonomy: some possibilities and limitations.

Chemical plant taxonomy or chemotaxonomy of plants may be defined as a sciertific investigation of the potentialities of chemical characters for the study of problems of plant taxonomy and plant phylogeny. Plant taxonomy is the science of delimiting, describing and naming appropriately taxat and arranging them in a natural system of plants. Principles of chemotaxonomy were elaborated in the past century by A. P. De Candolle1 and by Greshoff2. De Candolle put forward two postulates: (i) Plant taxonomy will be the most useful guide to man in his search for new industrial and medicinal plants; (ii) Chemical chara cteristics of plants will be most valuable to plant taxonomy in the future. While the first postulate of De Candolle proved to be extremely fruitful and has been applied repeatedly when new sources of promising plant constituents are to be detected, his second postulate came to be accepted very slowly. Researchers like Rochleder3, Greshoff4, Rosenthaler5, Baker and Smith6, Wheldale7, Iwanow8, Cohn9, Molisch1°, McNair'1 and Weevers'2 were enthusiastic but rather isolated workers in the field of chemotaxonomy. However, the fact that the first postulate of De Candohle was applied very successfully by generations of phytochemists forms an indirect proof of the validity of his second postulate. Some examples may serve to illustrate just the services plant taxonomy renders to chemists interested in distinct types of plant constituents. When the pharmaceutical industry became interested in plant steroids as starting materials for hormone synthesis, the search for suitable sources was essentially guided by taxonomic concepts. The genus Strophanthus was investigated first for cardenolides and its species proved, without exception, to accumulate members of this category of phytoconstituents. Thousands of species were screened for steroidal sapogenins and in the taxa already known to contain them, i.e. in Agavaceae, Dioscoreaceae and Liliaceae, by far the highest frequency of occurrence was observed. In recent years the pregnane-derived alkaloids have begun to attract attention. Such alkaloid-like substances had been known for several years to be present in the apocynaceous genus Holarrhena. In this instance too, an alliance of genera included by taxonomists in the plant family Apocynaceae proved to be most promising for exploration. Very recently