While the leaves of many relatives of citrus are distinctly compound — ranging from trifoliolate (e.g., Poncirus trifoliata) to palmately compound (e.g., Casimiroa spp.) to pinnately compound (e.g., Citropsis gilletiana) — the leaves of cultivated citrus are described as unifoliolate rather than simple, being the result of a reduction in number of leaflets. Most leaves of cultivated citrus consist of a petiole (variously winged or not winged) and a single, terminal leaflet. Most leaflets are somewhat toothed at the margins, although to varying degrees. The leaves of most cultivated citrus taxa are somewhat to strongly conduplicate when in full sun. Under shade conditions, however, leaves of many of the major groups, including sweet oranges, sour oranges, blood oranges, pummelos, grapefruits, etc., are more or less plane or only very slightly conduplicate. A very few cultivars exhibit leaves that are strongly conduplicate even in shade. In full sun, leaf margins tend to be somewhat undulate or wavy. Only a few cultivars exhibit leaves that are consistently wavy even in the shade.
Petiole wings vary widely in their development, frequently even within individual plants. To evaluate width for use in identification, it is necessary to examine a number of leaves and choose the predominant form. In addition to petiole width, the following characters may be useful to identify certain cultivars or cultivar groups: petiole length (in relation to leaf blade), petiole pubescence, shape of the petiole wing apex (e.g., rounded or obtuse vs. emarginate), degree of overlap of petiole wing apex and the supertending leaflet.
The scent of crushed young leaves is one of the most helpful characters to distinguish broad cultivar groups within citrus. If possible, calibration at a citrus arboretum or collection would be useful prior to field survey. With some exceptions, scents are distinctive to cultivar group (e.g., sweet in sweet oranges, grapefruits, and pummelos; freshly spicy in lemons; spicy/peppery in sour oranges). Scents can also be useful in distinguishing particular crosses with trifoliate orange; for example, mandarin crosses smell like mandarins, and lemon crosses smell like lemons. Calamondins are unusual in exhibiting a scent reminiscent of bread dough.
Citrus ID Edition 2
October, 2011
idtools.org
