POMEGRANATE TREE: DESCRIPTION AND USE
P. Melgarejo Moreno
Escuela Politecnica Superior – Universidad Miguel hernandez
Oihuela - Spain
BOTANICAL SPECIES: Punica granatum L.
FAMILY: Punicaceae. The genus Punica only has two species: P. granatum L. and P. Nana L., the former mainly cultivated for its fruit and occasionally as an ornamental tree, and the latter only ornamentally as its fruits are inedible.
HABITAT: The pomegranate belongs to Vavilov’s Centre of Origin IV, central Middle East which includes the interior of Asia Minor, the Transcaucasus, Iran and the highlands of Turkmenistan. Pomegranate is a species which adapts to all kinds of soil and climate; it is tolerant of drought, salt, ferric chlorosis and active calcium carbonate. Indeed, in SE Spain it is capable of adapting to the worst cultivation conditions alongside fig, prickly pear and date palm. It is traditionally cultivated in salt marsh areas in a subtropical Mediterranean climate and frequently irrigated with water with an electrical conductivity of more than 4 dS/m. Its degree of salt tolerance is only exceeded by the date palm, Zizyphus vulgari and prickly pear, and is equal to that of fig. Although pomegranate is capable of growing in a great variety of climates ranging from the tropical to temperate, in the latter it does not mature as it should and is better suited as an ornamental, rather than as a fruit producer. In tropical, humid climates the fruit are of lower quality than those obtained in subtropical conditions, where high summer temperatures coincide with the last stages of fruit ripening. The most suitable areas for growing pomegranate are inland, where the summers are warm and dry and where the fruit assume their best colour and taste. Coastal areas and mild summers are limiting areas for commercial production and in such conditions the trees are usually only used for ornamental purposes. It can be grown in areas where the winter temperatures drop to –15ºC and certain acidic and central Asian cultivars can even survive temperatures of –25ºC or –30ºC. The plant´s needs during winter are small, and the length of winter dormancy is correlated with temperatures below 16ºC. The fruits tend to split if rain falls during the harvesting period. Pomegranate is a traditional crop in all the countries which border the Mediterranean Sea.
CULTIVARS: Genetic diversity is very high for most traits, although differences between certain cultivars may be small. Pomegranate may be classified according to the acidity of its fruit into acid, sour-sweet or sweet. Some cultivars produce hard seeds, rendering their fruit inedible and only the varieties classified as soft seed are of commercial interest. Although there are no large germplasm banks in Europe, species diversity is great both in Europe and in other parts of the world. There are a great number of cultivated varieties which are occasionally grouped under the same name despite the disparate nature of their characteristics. The most important varietal group is that denominated Mollar, which provides fruit of the highest quality. The following group in commercial importance is Valencianas, which although providing fruit of a lower quality than Mollar, matures earlier. Many other groups are cultivated on a local basis. Pomegranate varieties can be classified into sweet, sour-sweet and sour. The sweet varieties, which are of the greatest commercial interest, constitute a homogeneous group and can be classified according to their harvesting time into early, mid and late. The sweet varieties are characterised by their low acidity and their soluble solid content is slightly higher than that of the sweet-sour and sour varieties. Their organic acid content is much lower, with malic acid sometimes being more predominant than citric acid in some varieties, while the opposite is the case in others. In the sweet-sour and sour varieties, on the other hand, citric acid is always the dominant acid. The maturity index at harvesting is normally in the 40-100 range in the sweet varieties, between 15 and 30 in the sweet-sour varieties and between 5 and 10 in the sour varieties. Among the sweet varieties, one can point to Mollar, Valencianas, Casta del Reino, Piñón Tierno, and Albar. The sweet-sour varieties are less interesting from a commercial point of view, although their fruit may be useful in the future for obtaining derived products.
PLANT: Pomegranate is a small tree, measuring less than 4 m when cultivated, although it can reach 7 m in the wild. Some trees may live longer than 100 years. The root is knotty, consistent and reddish, well developed and extremely absorbent in saline soils. Numerous suckers grow beside the trunk and have to be eliminated occasionally. The bark, known as pomegranate bark, has traditionally been used for the alkaloids it contains although the trunk and bark of the branches contain similar quantities of the same. The trunk is more or less round, erect, ramified, with alternate open branches, sometimes prickly at the apex. The ageing bark shows cracks and takes on a greyish colour. It appears knotted ant twisted. The tree itself varies in appearance from drooping to erect. The trees usually have only one trunk although in some countries they form various trunks from the suckers which are allowed to grow from the foot of the principal trunk. The plant emits such suckers with great regularity both at the foot of the trunk and from the principal branches, the latter being used to obtain woody cuttings for propagation. The buds are lateral and are found on the axils of the leaves. The terminal bud sometimes becomes a thorn, sometimes grows into a flower or cluster of flowers, or simply falls. Since the plant does not therefore possess real terminal buds, growth has to be from the lateral buds, for which reason the tree is included in the sympodial species.
LEAVES: The leaves in vegetative or mixed clusters measure about 2 to 9 cm in length and 1 to 3 in width. They are entire, smooth, opposed, with no stipule, sometimes verticillate, hairless, oblong, deciduous and with short petioles. Phyllotaxis: crossed; sometimes, and more often in some varieties than in others, with three leaves per knot arranged at 120º and even four leaves buds per knot on the same tree which has the above arrangement (two opposed leaves per knot). The reddish young leaves turn bright green when adult, the face darker than the reverse, while the petiole maintains its reddish colour.
FLOWERS: The flowers appear singly or in small clusters generally of 2-7 flowers, occasionally at the end of the branch but sometimes on the axilary buds. In addition, they may appear on next season shoot with the same arrangement as mentioned, and occasionally only on next season shoot. The staggered flowering of the tree not only gives rise to a long harvesting season, but is also responsible for fruits of a wide range of quality, which are worse the later the flowering time. For this reason thinning out is essential. The flowers may be hermaphrodite (normally) and staminated (with no pistil and poorly developed). They are spectacular, with a pear shaped thallus, concave and fleshy, almost seated, single or in groups of 2-7, with bell-shaped calyx. The petals, 5-9, are wrinkled, alternating with and longer than the sepals and scarlet. The shorter sepals (5-9) alternate with the petals and form a continuous fleshy red crenelation. The stamens around the pistil are inserted into the calyx walls and frequently amount to more than 300 per flower. Filiform, about 1cm long, they have a reddish filament and yellowish bilocular anthers, the remains of which do not disappear but remain inserted into the calyx which crowns the ripe fruit. The carpels (polysperm cavities), variable in number but usually eight are superimposed in two whorls because of the development of the thallus. They form a syncarpic ovary and are arranged in two layers, usually 5 on top and 3 underneath.
FRUIT: The fruit is a fleshy berry denominated balausta, thick-skinned, complex, enclosed by the thallus, with various polyspermal cavities separated by tenuous membranous partitions (carpelar membranes). The interior is filled with many fleshy seeds, prismatic in shape, with pulpy testa and woody tegmen, very juicy. The ripe fruit is greenish yellow or brown with reddish areas which may occasionally occupy the whole surface of the fruit. This often appears attractive to the consumer but there is not necessarily any correlation between the exterior and the quality of the fruit. In normal temperatures the fruits normally ripen 5 to 7 months after flowering, depending on the variety. The best quality fruits are obtained in those areas where the period of development and maturation coincide with high summer temperatures. The fruit is non-climacteric and should be picked after it has reached maturity.
USES: Normally consumed fresh, pomegranate can also be used to obtain a variety of products such as juices, jams, preserves, jellies etc. In some countries not given to eating the fruit are used as decoration in fruit bowls. The plant, too, is of ornamental interest, especially old examples with twisted trunks and branches. Both P.granatum with its edible fruit and the dwarf P.nana, whose fruit is inedible, are used for this purpose.