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Plantwise Technical Factsheet

lemon (Citrus limon)

Description

C. limon is an evergreen with vigorous growth, it has an upright, rank and spreading growth habit when juvenile, which continues when mature in tropical areas. As trees mature or when grown in Mediterranean climates, the growth habit becomes more spreading. It attains a large size under favourable conditions if not controlled by pruning. Generally, young trees are very thorny with relatively short and slender spines (Hodgson, 1967; Ray and Walheim, 1980; Davies and Albrigo, 1994).

Lemon trees have open growth with relatively few large branches. The larger lateral branches grow in a characteristic, eccentric way that results in a flattened limb. Cambial activity is greater on the lower side of the branches, therefore growth rings are clearly eccentric (Schneider, 1967).

The new shoots originate mainly in the near terminal leaf axils on vegetative growth of the preceding spring or summer. Under cool climatic conditions only two flushes appear annually, but three to five flushes occur in warmer, subtropical regions. Under wet, tropical conditions shoot growth occurs uninterrupted, throughout the year. Lemons retain their tropical nature even in cooler climates and new shoots emerge throughout the year. The spring flush is the most important one, containing both vegetative and reproductive shoots. The midsummer and subsequent flushes are generally vegetative with fewer but longer, vigorously growing shoots and larger leaves (Spiegel-Roy and Goldschmidt, 1996).

The leaves are large and light green. In most Citrus species the petioles are winged, however, in lemon the petioles are reduced and without wings. Leaf blades of lemons are oval to oblong in form (Schneider, 1967). Leaf morphology is quite variable depending on tree vigour. Laminae are large and ovate with pronounced serrations along the apical margins of leaves developing on vigorous shoots. Laminae become ovate to lanceolate again with serrated margins as shoots mature. Newly developing leaves are purple but as the laminae mature they become green (Davies and Albrigo, 1994).

Most Citrus species including the lemon are tap-rooted. During germination the radicle appears first and rapidly grows downward to form a strong well-defined taproot (Spiegel-Roy and Goldschmidt, 1996). Fibrous roots occur in small bunches 20-30 cm long on the taproots of young seedlings and on pioneer roots of older trees. Branches from the main root of the fibrous bunch are of a uniform size that is smaller than the main root; sub-branches from the first branches are further reduced (Schneider, 1967). Not far from the soil surface, a network of strong, lateral roots provides the supporting framework for a dense mat of fibrous roots. A deeper layer of smaller laterals and fibrous roots emerges from the crown in a more or less vertical direction (Spiegel-Roy and Goldschmidt, 1996).

Citrus flowers form on new growth in the early spring. Lemon trees have two major flowering periods in Mediterranean climates but tend to flower continuously throughout the year in cool, coastal climates, where trees will produce several crops each year (Ray and Walheim, 1980). Mature citrus flowers are 1.5-3 cm long, supported by a pedicel. The calyx is cup-like with five sepals. Petals are thick, glossy with interlocking, marginal papillae to keep them reflexed. The stamens appear as 20-40 partially united filaments, each bearing a yellow, four-lobed anther. Anthers surround the pistil at or close to the level of the stigma. The floral disc secretes watery nectar through the stomata. At flowering the ovary is subglobose, distinct from a narrow style, as in orange, or subcylindrical, merging into the style, as in lemon. The ovary with 8-14 carpels, style and stigma comprise the pistil (Schneider, 1967; Spiegel-Roy and Goldschmidt, 1996). Lemon flowers are perfect and complete, having the same general characteristics as other commercial Citrus species. Flower petals are purplish white. Flowers are smaller than those of grapefruit but similar in size to mandarin flowers. They are typically borne in clusters.

The citrus fruit is a special type of berry termed a hesperidium. It is a true fruit and consists of a variable number of united, radially arranged carpels (generally eight and nine in lemon). Citrus fruits are composed of two main, morphologically distinct, regions: pericarp and endocarp. The pericarp is known as the peel or rind and the endocarp, which is the edible portion of the fruit, is called the pulp. Another distinction is made within the peel: the external, coloured portion is the epicarp, usually known as the flavedo, whereas the internal, white layer of the peel is the mesocarp, commonly known as the albedo. The flavedo is composed of the cuticle-covered epidermis and a few compactly arranged parenchyma cell layers adjacent to it. The flavedo has essential oil glands. The pulp consists of segments, the ovarian locules, enclosed in a locular membrane and filled with juice sacs (Spiegel-Roy and Goldschmidt, 1996). Lemon fruits are generally oval to elliptical with characteristic necks and nipples. During early stages of fruit development, the flavedo is a dark green but the peel is yellow at maturity. It varies in thickness and surface texture, and has prominent oil glands. The pulp is pale straw-coloured and very acid (Young, 1986).

Cultivars range from moderately seedy to seedless. Seeds are small with smooth seed coats and pointed micropylar ends (Davies and Albrigo, 1994).

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