The date palm (P. dactylifera L.) belongs to the Arecaceae (or Palmae) family and has been cultivated for a long time in the semiarid and desert areas of the Middle East, Pakistan, and India; in California, USA; in the Canary Islands; and in the northern African countries for fuel, shade, fiber, food, and as building material (Nixon, 1951). In addition to date palm, this family also includes other kinds of palm trees such as oil palms, coconut palms, andWashington palms.
Although it is not known exactly where the date palms originated, it is suggested that they first originated in
Babel, Iraq or in Dareen or Hofuf, Saudi Arabia, or Harqan, an island in Bahrain, from where it spread to other places (Marei, 1971). Date palms were first introduced to Andalus by the Arabs, during the 7th and 8th centuries and later spread throughout the deserts of the Middle East and North Africa by the Bedouin tribes of the Arab countries.
It is also believed that date palm trees were introduced to India after the victory over India by Alexander, the Great around 327 B.C. In around 1769, date seeds were introduced to the arid areas in the United States, namely the states of New Mexico, California, and Arizona (Al-Tayeb, 1982).
Evidently, the Middle East and North African countries are the major date-fruit producing countries in theworld, although the United States also produces sizable quantities of date fruits in North America, and Spain in the European Union.
DATE PALM CULTIVATION
For millennia, date fruits have been an important part of the dietary patterns of the people in the Middle Eastern countries. Being a tree of the desert, it is well established in this region. Date trees grow to about 23 m tall; the stems are strongly marked with the pruned stubs of old leaves. The plant has a crown of graceful, shining, pinnate leaves, measuring about 5 m long. Depending on the cultivar and other agricultural practices, 1000 dates may appear in a single bunch and weigh 8 kg or more.
Date trees start bearing fruit in 4–5 years and reach their full potential in 10–15 years, producing 40–80 kg of fruit per tree. Although date trees can live as long as 150 years, due to declining fruit production, commercial cultivators replace them at an earlier age. Propagation of date palm trees through seeds is not successful because of the high genetic variation (not true-to-type) in male and female seedlings.
Thus, date palms are mainly propagated by offshoots produced by younger palm trees, and these offshoots maintain true-to-type make up of the cultivars (Nixon and Farr, 1965).With the recent developments in tissue culture techniques, date palm breeding has been given a boost. Now, this technique is being used extensively to clone a wide range of economically important palms such as coconut palms (Eeuwens and Blake, 1977), oil palms (Rabechault and Martin, 1976), and date palms (Tisserat, 1979). Date cultivation is not mechanized in the Middle Eastern and North African countries; yet most of the operations such as pollination, pruning, and harvesting are carried out manually. Attempts are now being
made to mechanize date production due to the rising cost of production and shortage of skilled labor (Shabana and Mohammad, 1982). The number of palm trees per hectare is an important parameter that determines the yield and quality of date fruits. The number of trees per hectare varies from 100 to 250 depending on the cultivars. For most cultivars, palm trees are usually planted in square patterns with about 9 m gap between the rows. For the initial few years, intercropping of fodder or vegetable crops can be carried out. In the United States, most operations, such as pollination, bunch thinning, pruning, bagging, and harvesting, are carried out with the help of machines (Perkins and Brown, 1966; Brown, 1982).
Harvesting is the most expensive operation and consists of handpicking individual, mature fruits from each bunch.
Pollination is the next most expensive operation. To ensure good yields of quality fruits, pollination must be performed according to the opening of the flowers. One part of pollen is usually mixed with 6–10 parts of wheat flour a day or so before the application to the female flowers (Brown et al., 1969).
Thinning is carried out at that time, bunches are tied down so that optimal amounts of fruits are retained in these bunches to attain an acceptable fruit size. With regard to the irrigation requirements of date trees, an old Arab saying, “The date palm, queen of trees, must have her feet in running water and her head in the burning sky,” has been quoted by Hussein and Hussein (1982a), who recommended moderate irrigation of 12 applications per year with 300 m3/Feddah each time at intervals of about 4 weeks. A few reports on the irrigation requirements of date palm trees in California, USA, are available.
Prolonged periods of severe water restriction during the growing period have been reported to affect the growth of leaves, and the size, grade, and yield of fruits (Aldrich, 1942; Furr, 1956). Unsuitable irrigation practices result in small-sized early ripening fruit of poor quality (Hilgman et al., 1957).
Deep irrigation at long intervals is more beneficial in maintaining higher moisture levels in the root zones compared with shallow irrigation at short intervals (Hilal, 1986). Although the date tree can tolerate the hot, dry conditions of desert climates, sufficient quantities of irrigation water must be provided during the fruiting season to obtain the maximum yield of higher quality fruits. The amount of nitrogen fertilizers used affects the vegetative growth and yield of fruit (Furr and Armstrong, 1959). The addition of nitrogen to palm trees increases the fruit yield but the dry matter, total
soluble solids, total sugars, and sucrose remain lower (Hussein and Hussein, 1982b). When the fruits have higher moisture contents, the fruit maturity is delayed. About 750 g of nitrogen per palm tree is recommended to obtain the highest fruit yield and quality. The Saudi date palm growers are generally satisfied with the use of organic manure only.
Bacha and Abo-Hassan (1982) investigated the effect of seven fertilizer treatments comprised of nitrogen, phosphate, potassium, and organic manure on the yield and fruit quality of the Khudari date variety grown in Saudi Arabia, and showed that chemical fertilizers increased the fruit yield and size but did not affect the mineral contents of the fruits. In addition to organic manure, they recommended 1500 g of nitrogen per tree. Pruning is one of the important cultural practices in palm cultivation, and it affects the fruit-bearing capacity of the date tree.
The number of green leaves present on a date tree depends on the cultivar and determines its photosynthetic potential and ultimately its fruit-bearing capacity (Nixon, 1943). Too many leaves may affect the fruit quality adversely by shading and may increase disease incidence. The presence of an adequate number of leaves on palm trees is important; the most suitable leaf–bunch ratio has been suggested as being 5.4–9.0 by a number of researchers (Miremadi, 1970; Miremadi, 1971; Nixon, 1957; Nixon and Wedding 1956). Abdulla et al. (1982) reported that leaf–bunch ratio affected the fruit yield and other properties except the titratable acidity, tannins, and crude fiber of Hayany dates grown in the Kalubia Province of Egypt. In Saudi Arabia, 12 leaves are usually left in each bunch, to obtain the best yield and fruit quality (Hussein et al., 1977), but farmers in the western province cut off only dry leaves after harvest (Khalifa et al., 1983).
During pruning, the spikes (thorns) are also removed to facilitate pollination and fruit picking during the harvesting operation. Date trees bear flowers that can be bisexual or unisexual and monoecious or dioecious. The occurrence of hermaphroditism in male date palm has been reported (Sudhersan and El-Nil, 1999). Although the date tree normally produces only unisexual flowers, the occurrence of bisexual flowers has also been reported in this species (De Mason and Tisserat, 1980). Because of the unisexual nature of this plant, the female flowers, within a few days of opening, are pollinated manually (a tedious and expensive operation) by sprinkling pollen from male flowers on them. The production, storage, and handling of
pollen, therefore, are very important operations for the success of date cultivation. In the United States, pollination is done mechanically from the ground, thus minimizing the drudgery (Nixon and Carpenter, 1978). To economize on the use of pollen, it is mixed with diluents such as talcum powder or wheat flour. The time of pollination, type of male flowers, and storage history of the pollen are known to affect the fruit set as well as its quality. Use of 20–40% pollen in talcum powder gives satisfactory yield and fruit quality for Zegloul date trees. Use of 20% pollen results in lower bunch weight but the fruit quality is improved (El-Kassas and Mahmoud, 1986).
Better fruit set and quality are obtained if pollination is carried out just before sunset rather than in the morning (Moustafa et al., 1986). Pollen stored either at room temperature or at chilling temperature in a refrigerator produces higher fruit set compared with the pollen stored in a deep freezer (Shaheen et al., 1986).
Male palms having greater (spathe) weight, greater length and number of strands, and more than 15 g of pollen per spathe should be selected for pollination to obtain the best fruit set and quality (Nasr et al., 1986). Mixing one part pollen with nine parts wheat flour or in 10% sucrose solution has been reported to be more effective than the use of pure pollen in smaller amounts (Khalil and Al-Shawaan, 1986).
Wheat flour and sugar solution media are good carriers of date palm pollen grains. Due to overbearing of fruits, alternate bearing is common among many cultivars of date palm, resulting in smaller fruits of inferior quality.
Fruit and bunch thinning usually overcomes this problem and improves fruit quality (El-Hamady et al., 1982). Commercially, thinning operations could be accomplished mechanically by reducing the number of fruits per bunch or the number of bunches per tree.
To reduce alternate bearing and to improve fruit quality, certain chemicals such as 2, 4-D (2,4-dichlorophenoxyacetic acid), 2, 4, 5-T (2,4,5- trichlorophenoxyacetic acid), and ethephon are used as thinner for many fruit crops including the date palm (Ketchie, 1968; El-Zeftawi, 1976; Weinbaum and Muraoka, 1978). Use of 400 ppm of ethephon significantly reduces the biennial bearing behavior in date trees (El-Hamady et al., 1982). Compared with controls, significantly higher fruit weight and total soluble solids but lower tannin contents were obtained in fruits treated with ethephon.
Thinning treatments improve the fruit quality but result in an overall lower yield of fruit per tree. To expose the date fruits to sunlight and to prevent fruit-bearing stalks from breaking, bunches are pulled downward and tied to a nearby leaf stalk at the kimri stage. This practice is particularly important for those cultivars having long fruit stalks.