Citrus halimii: A New Species from Malaya and Peninsular Thailand

Abstract

A hitherto undescribed species of Citrus (Rutaceae), indigenous to the Malayan Peninsula and the adjacent peninsular region of Thailand, is identified, named C. halimii, and characterized by data from vegetative, floral, fruit and embryo morphology, chromosome number, ecology, and phytochemical analysis of the fruits, leaves, and stem. ALTHOUGH THE genus Citrus has been studied as intensively as almost any other plant genus, the prevalence of several peculiar features has made species classification singularly difficult. These features are the comparative ease of hybridization, the production of spurious embryos and nucellar polyembryony, the obliteration of many original habitats, the presence of numerous cultivars, including spontaneous mutants, and the often inadequate original descriptions and specimens. A plethora of species names exists: 870 at last count, according to Carpenter and Reece (1969). There are two extremely different schemes of classification. That of Tanaka (1954) recognizes 159 species, while that of Swingle (1943) admits but 16 species. Tanaka clearly has conferred species status on many purely horticultural forms and does not subscribe to a modern biological species concept. On the other hand, Swingle probably recognized too few species. Another investigator (Hodgson 1961) recognized 36 species, which is no doubt closer to the truth, but numerous problems remain. Among these is our poor understanding of the wild tropical taxa. For some of these nothing is known apart from the general data obtainable from herbarium specimens. While extensive breeding and other research programs have been carried out in the past in the U.S.A., Europe, and Japan, the primary emphasis has naturally been on temperate and subtropical taxa. The cultivars of tropical Southeast Asia are much less well known, and agricultural knowledge of their behavior is insufficient. In this context, the recognition of a previously unknown wild Citrus species assumes considerable importance. It is now realised that the great majority of taxa which bear species names are in fact cultivars of various kinds, although they may breed true from seed. Nevertheless it may appear that, owing to the rather chaotic state of Citrus classification and the vast number of available names in the genus, the proposal of a new species would be ipso facto rather suspect. On the other hand it must be pointed out that several other new Citrus species have been discovered in the period since 1930, and have been accepted as natural taxa of species rank even when evaluated on the extremely conservative basis of Swingle (Bhattacharya and Dutta 1956, Corner 1952, Hodgson 1961). Historically, plant exploration and systematic analysis has lagged in Southeast Asia, and parts of this region and the adjacent archipelagoes are poorly or quite unexplored botanically. In most genera, the discovery of new species from Malaya, Thailand, or Indonesia is almost a routine occurrence when revisions are made (van Steenis 1954). Equally relevant is the richness of these regions, in which numerous species are endemic (van Steenis 1954). Citrus is undoubtedly a native here, and several species most probably originated in the Malesian tropics, e.g., limes (C. aarantifolia) and pomelos (C. grandis). It is reasonable to suppose that in Citrus, as in other genera, undiscovered species still exist. Because of this background of unsettled classification, peculiar and taxonomically confusing breeding systems, hybrids, and reproductive behavior, an overabundance of names (sometimes ill-defined), and some reluctance to admit that species remain to be discovered, we have been careful to attempt evaluation of as many characters as possible, including macroand micro-morphological characters, ecological data, and phytochemical analysis of fruits, leaves, and stems. 102 BIOTROPICA 5(2): 102-11