Pseudaegle Miq. (sec. Swingle and Reece 1967)
Citrus L. (sec. Mabberley 2004, Bayer et al. 2009); Poncirus Raf. (sec. Swingle and Reece 1967; Cottin 2002)
Crown compact or dense, not weeping. First-year twig surface glabrous; second- or third year twig surface striate; thorns absent or not persistent, recurved or straight; prickles absent or not persistent. Petiole glabrous, length medium, long or very long; wings narrow or medium, adjoining the blade. Leaflets three, margin bluntly toothed or serrate/serrulate, rachis wings absent, shade leaflet blades flat or weakly conduplicate, sun leaflet blades flat, weakly or strongly conduplicate. Leaflets not scented when crushed. Fruit broader than long, as broad as long, or longer than broad; rind yellow (7-10), yellow-orange (11), orange (12), or red-orange (13); rind texture slightly rough (4-5); medium rough (6-7) or rough (8); firmness leathery; navel absent; flesh green/greenish; orange or yellow; taste sour.
Swingle and Reece (1967) provided the following additional notes on the genus:
"Poncirus trifoliata has somewhat similar dimorphic branches and leaves but shows much less variation in the size of the leaves produced on the leaf spurs."
"Poncirus stands alone in [the true Citrus fruit trees] in having trifoliolate, deciduous leaves and winter buds well protected by bud scales. It has pleiomerous ovaries with six to eight locules, with many ovules in each locule. In these ovarial characters it agrees with Microcitrus and Citrus."
"Small trees with single, stout, axillary spines and palmately 3-foliolate, deciduous leaves, twigs dimorphic; (a) normal twigs with internodes as long or longer than the petioles, (b) foliage spurs, which develop from dormant buds on twigs of previous year, with extremely short internodes (less than 0.5 mm long) bearing 1-5 normal foliage leaves but no spines; flowers developing singly at the nodes in spring from scale-covered flower buds formed early in the previous summer on last year's twigs that lose their leaves during the winter; sepals 5, petals 5, long, slender, spathulate or clawed; stamens 4 or more times the number of petals, with free, glabrous, slender filaments; ovary subglobose, pubescent, seated on a short, shallow, cup-shaped disk broader than the ovary; ovary with 6-8 (sometimes fewer or more), usually 7 locules; ovules 4-8 in 2 rows in each locule; style short, thick, merging into the slightly clavate stigma; fruits globose or short-pyriform, subsessile, 3-5 cm diam., finely pubescent, with lemon-yellow, soft, orange-like peel with abundant oil glands; pulp-vesicles elongate-conical, slender-stalked, bearing scattered, secretory, hair-like organs, and filled with acid pulp containing numerous droplets of acrid oil (or oily wax) in the center; seeds oval, plump, often with several supernumerary nucellar embryos; germination hypogeous; young seedlings developing at first bract-like cataphylls, then intermediate forms that soon merge into normal 3-foliolate leaves.
This remarkable genus, although evidently closely related to Citrus, Fortunella, Microcitrus, and Eremocitrus, has many strikingly aberrant characters. In the first place, it differs from all the other True Citrus Fruit Trees, which are found in only tropical or subtropical regions, in having penetrated far into the temperate zone in northeastern Asia; in so doing it has become a deciduous tree with small leaf buds and larger scale-covered flower buds (formed in early summer) that pass the winter on the leafless terminal twigs and open before (and sometimes with) the leaves early in the following spring. The trifoliolate leaves are doubtless a survival of the foliage of some remote ancestral plant. All the other True Citrus Fruit Trees have unifoliolate leaves (except Clymenia, which has simple leaves) that persist on the tree for two or more years and cannot endure severe cold.
The pith of the stem shows transverse plates composed of thick-walled cells, not as yet found in any other genus related to Citrus (see Swingle, 1909). The pulp-vesicles carry scattered hair-like organs that bear at their expanded tips a number of rounded, thick-walled cells with numerous small, oblique fissures. These organs secrete a viscous fluid that allows the pulp-vesicles to slip past one another. These organs are unknown in any other genus of the orange subfamily. The immature fruits contain ponciridin, a glucoside analogous to hesperidin, but not found in Citrus.
Poncirus hybridizes freely with Citrus . Such hybrids, called citranges, citrandarins, etc., are usually ovule-sterile but occasionally produce some fertile pollen grains. Poncirus has also been hybridized (after overcoming many difficulties) with Fortunella and citranges have been hybridized with Eremocitrus.
The pulp-vesicles of Poncirus contain numerous droplets of oil. Penzig (1887, pp. 161-62) discovered these inclusions and found that some of these oil globules were semisolid and more or less brittle. Oil droplets are also found in the pulp-vesicles of Citrus, being very abundant in the species of Citrus which belong to the subgenus Papeda.
Poncirus is not easy to place in a phylogenetic series along with Citrus and other related genera arranged so as to show the course of evolutionary progress. Doubtless many "missing links" between it and the ancestors of Citrus have perished, leaving Poncirus as perhaps the most isolated and aberrant genus of all the True Citrus Fruit Trees."
Swingle and Reece (1967) additionally noted that:
"Poncirus undoubtedly represents an ancient offshoot from the True Citrus Fruit Trees that pushed north into China and in so doing acquired deciduous leaves and bud scales and developed great resistance to winter cold."
"Poncirus has become deciduous and is able to endure very severe cold in winter, a condition fatal to Citrus and even to Fortunella. It also has developed well-protected flower buds that form during the early summer and push into bloom from old twigs the following spring."
"There is every indication that Eremocitrus, Microcitrus, Poncirus, and Fortunella, because of their ancient and deeply inbred adaptations to special climatic and soil conditions, will prove important in breeding new types of citrus fruits and new rootstocks able to resist disease and able to endure unfavorable climatic and soil conditions that no Citrus species can withstand."
Bayer, R.J., D.J. Mabberley, C. Morton, C.H. Miller, I.K. Sharma, B.E. Pfeil, S. Rich, R. Hitchcock, and S. Sykes. 2009. A molecular phylogeny of the orange subfamily (Rutaceae: Aurantioideae) using nine cpDNA sequences. American Journal of Botany 96: 668–685.
Cottin, R. 2002. Citrus of the World: A citrus directory. Version 2.0. France: SRA INRA-CIRAD.
Mabberley, D.J. 2004. Citrus (Rutaceae): A review of recent advances in etymology, systematics and medical applications. Blumea 49: 481–498.
Penzig, O. 1887. Studi botanici sugli agrumi e sulle piante affini. Tip. Eredi Botta, Rome. Ministero di Agricoltura, Industria e Comercio. Annali di Agricoltura, No. 116. 596 pp. and atlas of 58 pls.
Swingle, W.T. 1909. The limitation of the satsuma orange to trifoliate-orange stock. US Department of Agriculture Bureau of Plant Industry Circular 46: 10 pp. 1 pl.
Swingle, W.T. and P.C. Reece. 1967. The botany of Citrus and its wild relatives. In: Reuther, W., H.J. Webber, and L.D. Batchelor (eds.). The Citrus industry. Ed. 2. Vol. I. University of California, Riverside. http://lib.ucr.edu/agnic/webber/Vol1/Chapter3.html.
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Citrus ID Edition 2
October, 2011
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