None
Citrus L. (sensu Mabberley 2004; sec. Bayer et al. 2009); Eremocitrus Swingle (sec. Swingle and Reece 1967; sensu Tanaka sec. Cottin 2002)
Crown compact or open, weeping. First-year twig surface glabrous; second- or third-year twig surface striate; thorns absent or not persistent; prickles absent or not persistent. Petiole glabrous; length short or medium; wings absent. Leaflets one, margins bluntly toothed, shade leaflet blades flat, sun leaflet blades flat or weakly conduplicate. Scent of crushed leaflets sweetly dough-scented. Fruit as broad as long or longer than broad; rind light green with some break to yellow (5), green-yellow (6), or yellow (7-10); rind texture smooth (1-3); firmness membranous; navel absent; flesh green/greenish; taste sour.
Swingle and Reece (1967) provided the following additional notes on the genus:
"Eremocitrus has ovary and fruit characters much like Fortunella, but it has undergone striking xerophytic adaptations that have culminated in the acquisition of leaves very diverse in structure from those of the other genera of the True Citrus Fruit Trees."
"Leaves gray-green, thick and leathery with appressed grayish hairs on both surfaces; spines solitary in the axils of the leaves, sometimes lacking on older branches; flowers single or few in the axils of the leaves, 3-5-merous, petals narrowed at the base, stamens free, 4 times the number of the petals, ovary with 3-5 locules, 2 ovules in each locule, style short; fruit oval or pyriform with a fleshy peel (as in Citrus), having oil glands, pulp acid, with stalked subglobose pulp-vesicles, seeds small with hard wrinkled testa; cotyledons hypogeous, first leaves cataphylls."
"This genus is in many ways the best characterized and most distinct of any of the near relatives of Citrus . It is the only pronounced xerophytic plant in the whole orange subfamily. Both leaves and twigs are gray-green, like sagebrush; the leaves are small, thick, bifacial with palisade parenchyma on both faces, and covered with a thick cuticle. The stomata are deeply sunken in pit-like depressions in the surfaces of both leaves and young twigs. There is a somewhat persistent sparse coating of thick-walled appressed hairs on the leaves and twigs not present in any other citrus fruit tree. In the event of severe drouth, the leaves fall and the gray-green twigs carry on the very restricted photosynthesis possible under such conditions. The young plants have greatly reduced linear leaves that are, in fact, cataphylls. The young seedlings grow very slowly until an extensive root system is formed; then, if adequate moisture is found, the stem grows upward as rapidly as that of any citrus tree and little by little larger leaves are formed until some may measure 50 to 65 by 10 to 20 mm and nearly 2 mm thick! The young plants have single sharp, stiff spines in the axils of the leaves. These are usually 2 to 3 cm long. On certain particularly vigorous wild trees growing in central New South Wales, spines of enormous length have been produced. They are very stout, somewhat up-curved, and reach a length of 6 to 9 cm with a vertical diameter at the base of 5 to 7 mm and a transverse diameter of 3 to 4 mm. The flowers are much like those of Microcitrus but smaller, usually 3- to 5-merous with six to ten entirely free stamens with slender filaments. The ovary has three to five locules with two ovules in each locale, a character not found in any other True Citrus Fruit Tree except Fortunella ! The fruit is small, subglobose, ovoid or obovoid, 15 to 25 by 12 to 15 mm. The subglobose pulp-vesicles have short, slender stalks and are loosely aggregated, not cohering as tightly to each other as in the common citrus fruits. The seeds are small and contain only a single embryo."
Swingle and Reece (1967) additionally noted that:
"Eremocitrus, of semiarid habitat, has the ability to grow with little or no organic nitrogen in the soil; it also withstands relatively high concentrations of salts in the soil moisture."
"The drought resistance of Eremocitrus glauca was discussed by Flanders (1932), who reported seeing small trees growing at Mootwingee (Long. 142° E.), where the annual rainfall averages about 12 inches. He also told Swingle of seeing living but leafless trees near Longreach, Queensland (Long. 144° E.), where no rain had fallen during the preceding 15 months and no green vegetation could be seen!"
Eremocitrus emerged in a strongly supported clade including taxa recognized in Microcitrus by Swingle in the analysis of Bayer et al. (2009).
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.
Flanders, S.E. 1932. Observations concerning a citrus relative native to Australia. California Citrograph 17: 278, 307–308.
Mabberley, D.J. 2004. Citrus (Rutaceae): A review of recent advances in etymology, systematics and medical applications. Blumea 49: 481–498.
Swingle, W.T. 1943. The botany of Citrus and its relatives of the orange subfamily. In: Webber, H.J. and L.D. Batchelor (eds.). The Citrus industry 1: 129–474. University of California Press, Berkeley and Los Angeles.
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.
Search for this genus in NCBI Entrez or NCBI Nucleotide
Citrus ID Edition 2
October, 2011
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