The Date Palm (Phoenix dactylifera L.): Overviewof Biology, Uses, and Cultivation

ChihCheng T. Chao'Department of Botany and Plant Sciences, University qI California-Riverside, Riverside, CA 92521-0124

Robert R. Krueger'United States Department ofAgricultureAgricultural Research Service, National Clonal Germplasm Repositoiy

,for Citrus and Dates, 1060 Martin Luther King Boulevard, Riverside, CA 92507-543 7Additional index ui,ds. genetic relationships. Khalal. Kimri, Rutab, Tarnar, offshoot. ofT type. pollination, tissue culture. metaxenia

Abstract. Date palm (Phoenix dac(j'lifera L.) is one of the oldest fruit crops grown in the arid regions of the ArabianPeninsula, North Africa, and the Middle East. The most probable area of origin of the date palm was in or near what is nowthe country of Iraq, but date cultivation spread to many countries starting in ancient times. Dates are a major food sourceand income source for local populations in the Middle East and North Africa, and play significant roles in the economy,society, and environment in these areas. In addition to serving directly as a food source, dates are packed and processed in anumber of ways, and other parts of the tree are used for various purposes. The date palm is a diploid, perennial, dioecious,and monocotyledonous plant adapted to arid environments. It has unique biological and developmental characteristics thatnecessitate special propagation, culture, and management techniques. Thousands of date palm cultivars and selections existin different date-growing countries. Different genetic marker systems have been used to study genetic relationships amongdate palm cultivars. The long life cycle, long period of juvenility, and dioecism of date palms make breeding challenging.Worldwide date production has grown from 1,809,091 tin 1962 to 6,924,975 tin 2005. Worldwide date production willcontinue to grow, especially in the Middle East, despite current and future challenges.

HISTORY OF DATE PALMS

Date palm (Phoenix dacti'liJi i a L.) haslong been one of the most important fruitcrops in the arid regions of the ArabianPeninsula, North Africa, and the Middle East.During the past three centuries, dates werealso introduced to new production areas inAustralia, India/Pakistan, Mexico, southernAfrica, South America, and the United States.Dates are a main income source and staplefood for local populations in many countriesin which they are cultivated, and have playedsignificant roles in the economy, society, andenvironment of those countries.

Date is one of the oldest known fruit cropsand has been cultivated in North Africa andthe Middle East for at least 5000 years(Zohary and Hopf, 2000). The earliest recordfrom Iraq (Mesopotamia) shows that dateculture was probably established as early as3000 ace. Because of the long history ofdate culture and the wide distribution andexchange of date cultivars, the exact origin ofthe date is unknown, but it most likelyoriginated from the ancient Mesopotamiaarea (southern Iraq) or western India (Wrigley,1995). From its center of origin, datecultivation spread through out the ArabianPeninsula, North Africa, and the Middle East.Date culture had apparently spread intoEgypt by the middle of the second millen-nium ace. The spread of date cultivation lateraccompanied the expansion of Islam andreached southern Spain and Pakistan. TheSpanish were the first to introduce date palmsoutside the Arabian Peninsula, North Africa,and the Middle East/South Asia, carryingthem to America (Nixon, 1951).

'Current address: Thermal Plaza Nursery, 8035-PHighway 86, Thermal, CA 92274.2E-maiI rkrueger@ucr.edu .

Date cultivation has had a very importantinfluence on the history of the Middle East.Without dates, no large human populationcould have been supported in the desertregions. The caravan routes existed for cen-turies mainly for the transportation of dates.Early on, date cultivation became a sacredsymbol of fecundity and fertility. Dates hadgreat spiritual and cultural significance to thepeople of the Middle East. Date palms andculture are depicted in ancient Assyrian andBabylonian tablets, including the famousCode of Hammurabi, which contained lawspertaining to date culture and sales. Refer-ences relating to date palms are also found inancient Egyptian. Syrian, Libyan, and Pales-tinian writings (Nixon, 1951; Popenoe, 1973).

WORLDWIDE PRODUCTIONOF DATE PALMS

Dates can grow in very hot and dryclimates, and are relatively tolerant of saltyand alkaline soils. Date palms require a long,intensely hot summer with little rain and verylow humidity during the period from polli-nation to harvest, but with abundant under-ground water near the surface or irrigation.One old saying describes the date palm asgrowing with "its feet in the water and itshead in the fire." Such conditions are foundin the oases and wadis of the date palm'scenter of origin in the Middle East (Fig. I).Date palms can grow from 12.7 to 27.5 Caverage temperature, withstanding up to 50 Cand sustaining short periods of frost at temper-atures as low as 5 C. The ideal temperaturefor the growth of the date palm, during theperiod from pollination to fruit ripening, rangesfrom 21 to 27 C average temperature. Datesare widely grown in the and regions between15N and 35N, from Morocco in the west toIndia in the east (Zaid and de Wet. 2002a).

Worldwide date production has increasedfrom 1,809,091 t in 1962 to 6,924.975 t in2005 (Food and Agriculture Organization ofthe United Nations, 2006). The top 10 date-producing countries in 2005 were Egypt,Saudi Arabia, Iran, United Arab Emirates(UAE), Pakistan, Algeria, Sudan, Oman,Libyan Arab Jamahiriya. and Tunisia. Thetop five date-exporting countries in 2004were Iran, Pakistan, UAE, Saudi Arabia.and Tunisia. The top five date-importingcountries in 2004 were India, Pakistan, Yemen,Morocco, and UAE. The United States(California and Arizona) produced 16,500of date in 2005 and exported 4202 t (ranked10th worldwide). The United States also im-ported 5584 t of date in 2004. As with manyfruits produced in California. date qualityis high and commands a price advantage forexports. The relatively large volume of im-ported dates is most probably in the formof processed products.

The largest production of dates is inEgypt, where production has increased from439,539 t in 1982 to 1,170,000 t in 2005(16.9% of worldwide production). In UAE,there were about 1.5 million date palmsproducing 8400 t when the country wasfounded in 1971. This has increased morethan 90-fold to an estimated 18 millions datepalms producing 760,000 t in 2005. Cur-rently, UAE has the largest date tissue culture(TC) facility, where more than 100,000 TCdate palms are produced annually, becauseoffshoot production cannot keep up with thedemand for new trees to plant (Date PalmTissue Culture Laboratory, 2006).

REPRODUCTIVE BIOLOGYOF DATE PALMS

Date palm is a diploid (2n = 2x = 36),perennial, and monocotyledonous plant

1077He I SI IIV,e. 42(5) AlL Cl 2007

I. I he Kltattoa Oasis in ()niatt is represelita-tive of the types of conditions in which datepalms originated and are adapted. Traditionaldate palm cultis ation is still practiced in suchoases. (R.R. Krueger)

I is. 2. Male ( lelt) and Iental[52111) lloers o I date paltit. (US DA archival photograph)belonging to Palmaceae (Barrow. 1998). Thename of date palm originates from its fruit;"phoenix" from the Greek means purple orred (fruit), and "dactylifera" refers to thefinger-like appearance of the fruit bunch.Date palm is dioecious, meaning it hasseparate female and male trees. Throughoutthe years, there have been various reports ofhermaphroditic trees developing, or maletrees developing female characteristics(Sudhersan and Abo El-Nil, 1999). Inflores-cences of female and male trees differ inmorphology (Nixon and Carpenter, 1978;Swingle, 1904; Vandercook et al., 1980).Both are enclosed in a hard, fibrous cover(the spathe) that protects the flowers fromheat and sunlight during the early stage offlower development.

Dates flower when the shade temperatureincreases to more than 18 C, and form fruitwhen it is more than 25 C (Zaid and de Wet,2002a). The flowers (and later the fruit onfemale trees) are borne on a flat, taperingpeduncle or rachis, commonly known as the" fruit stalk" in the female trees. The inflo-rescence consists of many unbranched rachil-lae, known as "strands," arranged in spiralson the rachis. Both female and male flowersusually have three sepals and three petals(Fig. 2). Male flowers are usually waxy whiteand female flowers usually are yellowishgreen. Just before flowering, the inflores-cence arises in the axis of the leaves, pushingthrough the sheaths, and the sheaths cracklongitudinally at anthesis. Only the portion ofthe rachilla that bears flowers is exposed.Fifty to 60 d after anthesis, the fruit stalkelongates and pushes out the portion of theinflorescence that does not bear flowers to alength of 60 to 120 cm. The fruit normallydevelops after fertilization from one of thethree carpels within each pistillate flower.

Natural fruit drop occurs 25 to 35 d afterspathe cracking, and some cultivars have asecond fruit drop about 100 d after spathecracking (sometimes referred to as "Junedrop") (Reuveni, 1986).

The growth rate and development ofseeded fruit follow a sigmoid growth curve.The date fruit goes through four distinctripening stages. These four stages are usuallyreferred to in terms derived from Iraqi Arabicas "Kimri," "Khalal" (sometimes referred toas "Bisr"), 'Rutab," and "Tamar" to repre-sent the immature green, the mature fullcolored, the soft brown, and the hard raisin-like stages respectively (Reuveni. 1986).During the Kimri stage, the fruit increase insize and weight rapidly until the Khalal stage.The fruit color changes from green duringKimri to a color characteristic of the cultivarduring Khalal. The fruit remain turgid andastringent during Khalal, and contain a sub-stantial amount of water-soluble tannins.During the Khalal stage, the rate of gain insize and weight decreases slightly and thefruit reach full size and weight. Fruit duringthe Rutab stage are characterized by a dark-ening of the skin to amber, brown, or nearlyblack, accompanied by softening, decreasingastringency, and increasing insoluble tannins.During the Tamar stage, fruit lose much oftheir water and the sugar-to-water ratio ishigh enough to prevent fermentation, similarto raisins or dried prunes. Water content is75% to 80% in young fruit, decreasing to40% to 60% at the beginning of ripening, anddecreasing rapidly later. The sugar content isabout 20% dry matter during early Kimri,increasing steadily to 50% dry matter at thebeginning of Khalal, and then accumulating

at a faster rate until reaching 72% to 88% ofdry matter at maturation (Reuveni, 1986).Date fruit vary in size and shape dependingon cultivar, culture, and environment. Theaverage date palm produces 40 kg fruitannually, with yields of more than 100 kgpossible with intensive management. Whenfarmed with low levels of inputs and man-agement, dates may produce 20 kg fruit orless annually. Female plants start producingdates at 4 to 6 years of age and reach fullproduction within IS to 20 years (Nixon andCarpenter, 1978).

CULTIVATION OF DATE PALMSThe average economic life of a date

garden is 40 to 50 years, but some are stillproductive up to 150 years. There are a fewdate palms that are probably several hundredyears old. Because of the biology of the datepalm, its cultivation has a number of unusualfeatures that are not common in other peren-nial crops. There are a number of culturalpractices that require access to the crown ofthe tree, and in old trees reaching tens ofmeters in height, this can be challenging andsometimes dangerous. The crown of the treeneeds to be accessed for pollination, bunchtie-down, covering, harvesting, and pruning.Although the practice of climbing the treesfor access to the crown in still found in alldate-producing areas, the use of mechanicallifts is common in more advanced or indus-trialized production areas, such as the UnitedStates (Nixon and Carpenter, 1978).

Date is wind pollinated in nature, butinsect pollination is possible. Commercially,few male trees are grown in date gardens, and

1078 lloitrSt it \(t Vol.42(5) At (151 2007

pollen is collected for the artificial pollinationthat is critical for the success of production.Artificial pollination has been practiced inNorth Africa and the Middle East for thou-sands of years. Pollination of 60% to 80% ofthe female flowers results in adequate fruitset. Cultivars differ greatly in their fruit setpercentage. Incompatibility or partial incom-patibility between different female and malecultivars is common but not well understood.Only one ovule per flower is fertilized, pro-ducing a single fruit (Nixon and Carpenter,1978: Zaid and de Wet. 2002c).

Different pollen sources can influence thesize and shape of the seeds ("xenia" effect).Pollen also can have a metaxenia" effect,influencing the tissue outside the embryo andendospenn (Nixon, 1934, 1936: Reuveni,1986). As an example of metaxenic effectsill date palms. Nixon (1934) reported that'Fard No. 4' consistently produced earlier butsmaller fruit than the average of all pollensources studied, whereas 'Mosque' consis-tently produced larger but later fruit than theaverage. Pollination with pollen from differ-ent Phoenix species also exhibited metaxeniceffects (Nixon, 1936). Although similareffects have been reported from various othercountries, in some eases no observableeffects were reported from pollinations withpollen from different males tested. This hasbeen attributed to the fact that metaxeniceffects are less pronounced when climaticconditions are favorable (i.e.. higher heatunits) or the possibility that the specific malestested did not produce metaxenie effects(Reuveni, 1986).

After pollination, bunches are often tied tothe leaf stalks to support the weight of thefruit. Fruit thinning is sometimes practicedin date cultivation. Fruit thinning is used todecrease alternate bearing, increase fruit size,improve fruit quality, advance fruit ripening,and facilitate bunch management. Fruit thin-ning can be carried out three ways: removalof entire bunches, reduction in the numberof strands per bunch, and reduction in thenumber of fruit per strand. Cultivar, climate,and cultural practices influence the appropri-ate levels of fruit thinning. Bunches of datesare usually covered (bagged) in the UnitedStates with brown craft paper, white paper,or cotton or nylon mesh bags. Bagging canprotect fruit bunches from high humidityand rain, minimize damage from sunburn,and decrease losses from birds (Nixon andCarpenter, 1978; Zaid and de Wet, 2002c).

Although dates palms can withstand longperiods of drought under high temperatures,large amounts of water are required forvigorous growth, high yield, and high-qualityfruit. Under typical production conditions inCalifornia, annual water consumption was

1.3 m (Furr and Armstrong. 1956). Themajority of this was from the upper 2 m of thesoil, which coincided with the distribution ofthe roots. It should be noted that this studywas conducted with 'Khadrawy', a small-stature cultivar that most probably wouldhave lower water consumption than largersize cultivars. Under similar conditions,

application of excess water did not increasethe rate of growth of date palms or fruit yield;however, withholding water during the sum-mer months decreased the rate of tree growthand decreased the moisture content of thefruit (Reuther and Crawford, 1945). Floodirrigation is the oldest form of irrigation andis still used in many areas. Sprinkler, microsprinkler, and drip irrigation are often used innewer plantations (Abdul-Baki et al., 2002).

Date palms are usually fertilized,although in many cases only with nitrogen(Nixon and Carpenter. 1978). Responses tofertilization are inconsistent and probablydepend upon cultivar, soil type, and otherfactors. For instance, in the United States,Furr and Cook (1952) sampled 'Deglet Noor'and 'Khaclrawy' and reported that N concen-trations were higher in leaves of fertilizedtrees compared with nonfertil ized trees.However, there was not a consistent correla-tion with yield either of fertilization or of leafN levels. There was an apparent weakresponse from 'Deglet Noor', but none wasobserved with 'Khadrawy'. Labanauskas andNixon (1962) sampled healthy and decliningpalms but noted no significant differences innutrient concentrations. The values reportedfor most elements were lower than thoseestablished for citrus. Date palm leaf nutrientconcentrations published from various coun-tries are inconsistent but generally haveshown low N concentrations compared withother perennial crops. Interestingly, concen-trations from date palms grown in the UnitedStates and receiving high levels of N haveconsistently been lower than those reportedfor date palms in the Middle East and NorthAfrica (Krueger, in press).

Manure has traditionally been used in dateproduction, but in many instances inorganicfertilizers are used. Many producers, even inindustrialized production. consider manureto be superior to inorganic fertilizer. Covercrops are also often used in date production(Abdul-Baki et al., 2002). In and regions,date palms can provide shade for growingother crops. lntercropping of dates withfruits, vegetables, and pasture is common intraditional areas of date production.

There are three methods for date palmpropagation (Nixon and Carpenter. 1978;Zaid and de Wet, 2002b). The most commonmethod is the vegetative propagation of off-shoots, which ensures the genetic identity ofmaternal varieties (Fig. 3). Offshoots developfrom axillary buds on the trunk near the soilsurface during the date palm's juvenile stage.Offshoots, after 3 to 5 years of attachment tothe parental palm, produce roots and can beremoved and planted. This is about the agethat the offshoots will begin to produceflowers and, in female lines, fruit. The secondpropagation method is to use chance seed-lings from sexual crosses. Seedlings are notidentical to the maternal trees and not uni-form genetically. varying greatly in theirproduction and fruit quality. About 50% ofthe seedlings are male although they cannotbe identified until trees began to flower after

4 to 5 years. Production and fruit quality

from seedling-derived groves are greatlyreduced compared with groves developedfrom offshoots.

The third date propagation method isthrough TC (Zaid and de Wet, 2002b). Tissueculture propagation of date palms from shoottips through either embryogenesis or organ-ogenesis was first developed in the 1970 to1980s. Organogenesis can be achieved usingauxiliary buds and apical meristems, whereasembryogenesis can be done through callusstage from various meristematic tissues likeshoots, young leaves, stem, rachilla, and soforth. Cultivars respond to TC differently,and different optimal conditions are neededfor each cultivar. It takes about 6 years toreach production through the TC process and8 years to reach commercial quantities. Ingeneral, TC progenies have similar character-istics as those derived from offshoot propa-gation. However, one of the main problemswith TC propagation is somaclonal variation(off types). These somaclonal variantsexhibit several typical phenotypes, includingvariegation in leaves, variation in leaf struc-ture and overall plant growth patterns, treesthat do not form inflorescences or produceabnormal floral development, and trees thatproduce seedless parthenocarpic fruits. Mostsomaclonal variants can be detected in theearly stages: however, some can only bedetected in the field, several years afterplanting or after flowering, fruit set, andmaturation of the trees. The frequency ofsomaelonal variation in TC-derived datepalms can be very high sometimes, but mech-anisms causing these variations are unclearand are under investigation (Gurevich et al.,2005).

PESTS AND DISEASES OF DATEPALMS

Date palms are afflicted with many dis-eases and pests, but the nature and severityof the problems vary with cultivar, location,weather, and cultural practices (Carpenterand Elmer, 1978; Zaid et al., 2002). Mostreported diseases of date palm that can beassociated with a pathogen are attributed tofungi. However, there are several recentreports of phytoplasma-associated disorders.One of the most serious fungal diseases inNorth Africa is the Bayoud disease incited byFusa,'iuni oxvsporuin f. sp. albedinis. Thisdisease has caused large losses of date palms.and research concerning Bayoud is one of themost important areas of date palm research.Various insects attack date palms, but spe-cific insect problems vary with geographicarea. Chemicals, biological control, phero-mone trapping, quarantine, and sanitationpractices are used to control insect pests ofdate palms (Howard et al., 2001). An emerg-ing pest of date palm during the past decade isthe red palm weevil, which has spread fromIndia to North Africa, the Middle East. andsouthern Europe. Weed control is usuallyachieved by herbicide application or, in lessdeveloped areas, by hand weeding.

H0RrSc1 FN( I Voi. 42(5) Ai Id r 2007 1079

chemicals, controlled atmospheres, and phys-ical control methods (Glasner et al., 2002).Fruit quality may be manipulated throughhydration, dehydration, or curing as appro-priate, and dates are generally stored underrefrigerated conditions in industrial produc-CROWN tion (Glasner et al., 2002; Rygg, 1975).

f

OFLEAVES Sorting and grading of dates is generally

performed by hand rather than by electronicdevices at this time.

Fruit quality of dates can be lowered byphysiological and pathological factors(Rygg, 1975). Physiological defects includeblacknose, associated with high humidityduring the Khalal stage; black scald, associ-ated with abnormally high temperatures; andpuffiness of the dates associated with hightemperature and humidity. Storage condi-tions may also promote fruit defects such as

iNCH darkening of the skin and sugar spots. Inaddition, dates are sometimes attacked byvarious pathogens including Aspergithis,A Iternaria, and Pen icilliuni.

USES OF DATES AND DATE PALMS

SCHEMATIC DIAGRAM OF THE DATE PALM

Fig. 3. Diagrammatic representation of date palm structure, showing attachment of offshoot to motherpalm, among other morphological features. (USDA archival diagram)

In addition to pests and diseases, variousother factors may negatively affect date palmproduction. Carpenter (1981) discussed prob-lems in traditional date cultivation includingcrowding of trees, retention of old or unpro-ductive trees, planting of mixed cultivars orseedlings, salt accumulation, poor drainage,insufficient irrigation, fertilization or tillage,lack of insect and disease control, competi-tion with other crops, soil degradation, andwater scarcity. Jaradat (2001) considereddrought, high salinity, aged trees, Bayouddisease, and genetic erosion as the majorconstrains for future date palm productionworldwide

HARVEST AND POSTHARVESTHANDLING OF DATES

Dates are harvested at or near maturity.Harvest is generally by hand, with access tothe crown of the tree being by way ofclimbing or mechanical lifts. Completelymechanized harvest by shakers, such as those

used in some other perennial crops, is notdeveloped enough for routine commercialuse at this time (Glasner et al., 2002). Inmany traditional areas of date production,where the bulk of production is by smallfarms with limited resources, dates are usu-ally transported directly to open-air markets.Because of their low moisture content, datescan be successfully stored for some timewithout specialized storage conditions.

However, in more industrial forms of dateproduction, packing and storage of dates usespecialized equipment and facilities. Pack-inghouses for dates often use equipment thathas been modified from devices used forother crops. Various processes in the pack-inghouse are used to maintain or improvefruit quality. Fumigation for the eliminationof insect pests is commonly practiced(Glasner et al., 2002; Rygg, 1975). Fumigationhas mostly used methyl bromide; however,because of environmental concerns, methylbromide is being phased out. Alternatives tomethyl bromide treatment include alternative

Date palms produce many products thatare useful to humans. The primary product isthe date fruit, which can be eaten fresh, dried,or in various processed forms. In NorthAfrica and the Middle East, some dates areharvested and consumed during the Khalalstage, when the fruit are still very astringentwith a high tannin content (Dowson andAten, 1962; Glasner et al.. 2002; Kader,1992). However, most dates are harvestedduring the fully ripened Rutab and Tamarstages, when they are high in sugar and low inmoisture and tannin. Cultivars of dates canbe classified into "soft," "semidry," or "dry"dates, depending upon the time of harvest andassociated water content. Some cultivars areused in more than one manner. Fruit quality isinfluenced by size, color, texture, cleanliness,freedom from defects (sunburn, insect dam-age, sugar migration to surface, fermenta-tion), and the effects of decay-causingpathogens. Date fruit are available in differ-ent forms, including whole pitted and unpit-ted, dehydrated pieces, diced, extruded datepieces, and macerated fruit. Dates can beused in cereal, pudding, bread, pressed cakes,cookies, candy bars, ice cream, and dateshakes (a California specialty). Date fruitalso can be made into juice, vinegar, wine.beer, sugar, syrup, honey, chutney, pickle,paste, dip, and food flavoring (Barreveld,1993; Glasner et al., 2002).

Date fruit are high-energy food sourceswith 72% to 88% sugar content at maturity.During the Khalal stage, nearly all (80% to85%) of the sugar is sucrose. As ripeningprogresses, the sucrose is hydrolyzed intoreduced sugars such as glucose and fructose.Date fruit are good sources of iron andpotassium; a fair source of calcium, chlorine,copper, magnesium, and sulfur; and a minorsource of phosphorus. In addition, dates are asource of 16 amino acids and vitamins A, B,and B2 (Ahmed et al., 1995; Vandercooket al., 1980).

1080HORTSC!ENCE VOL. 42(5) AUGUST 2007

As a result of the long history of cultiva-tion and utilization of date palms, almostevery part of the date palm can be used forsome purpose (Barreveld, 1993; Dowson andAten, 1962; Nixon, 1951). It is said that thereare as many uses of dates as there are days ina year. The trunk and wood of date palms canbe used as timber, wood, or fuel. Fiber fromthe trunk and leaves can be made into bags,baskets, camel saddles, cords, crates, fans,food covers, furniture, mats, paper, ropes,trays, and twine. Dried bundles of leaves("Barusti") can be made into shades, roofs,separating walls, and enclosures. Ribs of theleaves can be used to build boats (smallfishing boats called "Shasha") or fishingtraps. The base of the leaves and fruit stalkscan be used for fuel. The pith of the palms canbe made into date palm flour. Terminal buds(palm heart) can be eaten as a salad or as acooked vegetable. Date seeds can be used asfeed for livestock or strung as beads fordecoration. Oil from date seeds can bemanufactured into soap. Date fruit also havemany medicinal uses. They can be used as anastringent for treating intestinal problems;treatment for sore throat and colds; relief offever, cystitis, edema, liver, and abdominalproblems; to counteract hangovers; and manymore uses. in India, the gum or exudate ofdates is used for treating diarrhea and theroots are used to treat toothache.

Groves of date palms are important envi-ronmental niches for local wildlife and play acentral role in the desert ecological system.Date palms have been effective for thecontrol of desertification and land reclama-tion in the Arabian Peninsula, especially inUAE. Feral date trees in southern Australiaare sometimes considered invasive species(Gotch et al., 2006). In recent decades, datepalms also have been used for ornamentaland landscape purposes in southern Europeand the United States (Arizona, California,Nevada, and Florida).

CULTIVARS AND GENETICSOF DATE PALMS

Nomenclature of date cultivars is confus-ing because of the long history of cultivation,wide exchanges of germplasm, dioecism, andseedling propagation. Thousands of nameddate cultivars exist in the Arabian Peninsula,North Africa, and the Middle East, but theexact numbers are unknown. For many culti-vars, large numbers of synonyms and homo-nyms exist between and within countries. Thetransliteration of Arabic names into otherlanguages further complicates the issue, andthere are usually several transliterations of asingle Arabic name.

Although there are many cultivars ofdates, some have become preeminent in theworld market (Krueger, 2001; Nixon, 1950).'Deglet Noor', meaning "date of the light" inArabic, comes from the Algerian Sahara andis one of the leading cultivars grown in NorthAfrica and California. 'Medjool', believedto have originated in the Tafilalt regionof Morocco, produces large fruit widely

accepted by markets around the world. 'Bar-hee' is a soft date with high quality from Iraq,and is often consumed during the Khalalstage. 'Halawy', meaning sweet in Arabic,is a soft, high-quality date with rich flavorfrom Iraq. 'l-Iayany' is a large, early-ripeningdate from southern Egypt. 'Khadrawy', mean-ing green in Arabic, is a short and moderatelyproductive tree with soft fruit from southernIraq. 'Zahidi' is a tree with a compact crownand semidry fruit from northern Iraq. Thereare many other local cultivars from variousareas that may become more important inthe future. However, 'Medjool' and 'Barhee'have become the most important cultivarsproduced via TC. As the production areasin the Middle East and other regionsbecome planted with TC-derived 'Medjool'and 'Barhee', these cultivars will becomeeven more important (Zaid and de Wet,2002b).

The long life cycle, long juvenility, anddioecism of date palms make breeding chal-lenging. A breeding program spanning morethan 30 years was initiated by the U.S.Department of Agriculture Date and CitrusStation at Indio, CA, in 1948 (Carpenter andReam, 1976). One of the goals of this pro-gram was to develop male cultivars derivedfrom successive backcrosses for use in breed-ing programs. A number of male backcrosseswere produced that had morphological sim-ilarities to the named female cultivars. How-ever, no useful progenies were releasedbecause of sterility and low vigor frominbreeding depression. Some female culti-vars derived from crossing these backerosseswith named females were also produced. How-ever, these all had some undesirable charac-teristics and are maintained mostly for useby breeding programs.

Because of these obstacles, modernimprovement programs have turned to bio-technological approaches, including the useof TC for mass production of clones, TCselection, Agrohacterium-based transforma-tion, and identification of molecular markersfor associated with desirable traits(Bouchireb and Clark, 1997; Djerbi, 1991).An important goal of these programs is thedevelopment of Bayoud-resistant cultivars orselections. Different marker systems such asmorphological characters (Jaradat and Zaid,2004). isozymes (Bennaceur et al., 1991),restriction fragment length polymorphism(Corniquel and Mercier, 1997), randomamplified polymorphic DNA (Sedra et al.,1998), and amplified fragment length poly-morphism (Cao and Chao, 2002; El-Assaret al., 2005) have been used to study thegenetic relationships among different eultivarsfrom different regions. Amplified fragmentlength polymorphism (Devanand and Chao,2003; Gurevich et al., 2005) and methyl-ation sensitive amplification polymorphismmarkers (Fang and Chao, unpublished) alsohave been used to study intracultivar ge-netic variation, or variation within offshootsand plants derived from TC. 'Medjool', forexample, has been shown to exist as alandrace cultivar in Morocco (Elhoumaizi

et al., 2006). Because of the market accep-tance of certain eultivars such as 'DegletNoor' and 'Medjool', the success of TC pro-pagation of certain eultivars, populationgrowth and development (dam construction),and desertification, wild date gernplasm andlarge numbers of traditional cultivars in manycountries are diminishing; urgent conserva-tion efforts worldwide are needed.

FUTURE OF DATE PALMINDUSTRIES WORLDWIDE

The area of date production in the ArabianPeninsula and the Middle East has increaseddramatically during the past few decades andis expected to continue to increase. The datepalm is among the very few plants that canthrive in and environments and it can provideenormous resources for nomadic tribes andlocal populations. Moreover, the date palmand date fruit have great cultural and tradi-tional significance in that region and in otherIslamic countries. Populations in many ofthese countries are increasing, and this willresult in a larger demand for dates. As a resultof these factors, cultivation of this ancientcrop is expected to increase in the future.

Literature Cited

Abdul-Baki, A., S. Asian, R. Linderman, S. Cobb.and A. Davis. 2002. Soil, water, and nutritionalmanagement of date orchards in the CoachellaValley and Bard. 2nd ed. California DateCommission, Indio, CA.

Ahmed, l.A., A.W.K. Ahmed, and R.K. Robinson.1995. Chemical composition of date varietiesas influenced by the stage of ripening. FoodChem. 54:305-309.

Barreveld, W.H. 1993. Date palm products. Foodand Agriculture Organization of the UnitedNations, Agricultural Services Bulletin no.101, Food and Agriculture Organization ofthe United Nations, Rome, Italy.

Barrow, S. 1998. A monograph of Phoenix L.(Palmae: Coryphoideae). Kew But. 53:513-575.

Bennaceur, M., C. Lanaud, M.H. Chevalier, andN. Bounaga. 1991. Genetic diversity of thedate palm (Phoenix ductt/i/div L.) from Alge-ria revealed by enzyme markers. Plant Breed.107:56-69.

Bouchireb. N. and M.S. Clark. 1997. The applica-tion of biotechnology to date palm culture, p.183-195. In: K. Watanabe and E. Pehu (eds.).Plant biotechnology and plant genetic resour-ces for sustainability and productivity. R.G.Landes Co., San Diego, CA.

Cao, B.R. and C.T. Chao. 2002. Identification ofdate palm cultivars in California using AFLPmarkers. HortScience 37:966-968.

Carpenter, J.B. 1981. Improvement of traditionaldate culture. Date Palm J. 1:1-16.

Carpenter, J.B. and H.S. Elmer. 1978. Pests anddiseases of the date palm. United States Depart-ment of Agriculture, Agriculture Handbook no.527. United States Department of Agriculture,Washington DC.

Carpenter, J.B. and C.L. Ream. 1976. Date palmbreeding: A review. Date Growers Inst. Rep.53:25-3 3.

Corniquel, B. and L. Mercier. 1997. Identificationof date palm (Phoenix dactvlifera L.) cultivarsby RFLP: Partial characterization of a eDNAprobe that contains a sequence encoding a zincfinger motif. Int. J. Plant Sci. 158:152-156.

HORTSCIENCE VOL. 42(5) AUGUST 2007 1081

Date Palm Tissue Culture Laboratory. 2006.Date Palm Tissue Culture Laboratory. 4 June2007. .

Devanand, P.S. and C.T. Chao. 2003. Geneticvariation within 'Medjool' and 'Deglet Noor'date (Phoenix dactvlifera L.) cultivars in Cal-ifornia detected by fluorescent-AFLP markers.J. Hort. Sci. Biotechnol. 78:405-409.

Djerbi, M. 1991. Biotechnologie du palmier dattier(Phoenix clacrelifera L.): Voies de propagationdes clones resistants au bayoud et de hautequalit dattire. Options Mditerranennes14:31-38.

Dowson, V.H.W. and A. Aten. 1962. Dates: Han-dling, processing, and packing. FAO PlantProduction and Protection Series no. 13. Foodand Agriculture Organization AgriculturalDevelopment, Food and Agriculture Organiza-tion, Rome, Italy.

El-Assar, A.M., R.R. Krueger, P.S. Devanand. andC.T. Chao. 2005. Genetic analysis of Egyptiandate (Phoenix dactyl Ura L.) accessions usingAFLP markers. Genet. Res. Crop Evol.52:601-607.

Elhoumaizi, M.A., P.S. Devanand, J. Fang, andC.T. Chao. 2006. Confirmation of 'Medjool'date as a landrace variety through geneticanalysis of 'Medjool' accessions in Morocco.J. Amer. Soc. Hort. Sci. 131:403-407.

Food and Agriculture Organization of the UnitedNations. 2006. 2005 worldwide dates produc-tion statistics. 19 Mar. 2006. .

Furr, J.R. and W.W. Armstrong. 1956. The sea-sonal use of water by Khadrawy date palms.Date Growers Inst. Rep. 33:5-7.

Fun, J.R. and J.A. Cook. 1952. Nitrogen content ofpinnae, fruit, and seed of Deglet Noor andKhadrawy date palms as related to nitrogenfertilization. Date Growers Inst. Rep. 29:13-14.

Glasner, B., A. Bores, A. Zaid, and J. Emmens.2002. Date harvesting, packinghouse manage-ment and marketing aspects, p. 177-208. In:A. Zaid (ed.). Date palm cultivation. Food andAgriculture Organization Plant Productionand Protection paper no. 156. Food and Agri-culture Organization of the United Nations,Rome, Italy.

Gotch, T., D. Noack, and G. Axford. 2006. Feraltree invasions of desert springs. Abstracts,Third International Date Palm Conference,Abu Dhabi, United Arab Emirates, 19-21

Feb., 2006, p. 40. United Arab Emirates Uni-versity, Al-Am, U.A.E.

Gurevich, V., U. Lavi, and Y. Cohen. 2005.Genetic variation in date palms propagatedfrom offshoots and tissue culture. J. Amer.Soc. Hort. Sci. 130:46-53.

Howard, F.W., D. Moore, R.M. Giblin-Davis, andR.G. Abad. 2001. Insects on palms. CABIInternational, Wallingford, Oxon, UK.

Jaradat, A.A. 2001. Date palm: A tree with a tastefor salt. Biosalinity News 2:5-7.

Jaradat, A.A. and A. Zaid. 2004. Quality traits ofdate palm fruits in a center of origin and centerof diversity. Food Agr. Environ. 2:208-217.

Kader, A.A. 1992. Postharvest biology and tech-nology: An overview, p. 15-20. In: A.A. Kader(ed.). Postharvest technology of horticulturalcrops. 2nd ed. Publication 3311, University ofCalifornia Division of Agriculture and NaturalResources, Berkeley, CA.

Krueger, R.R. 2001. Date palm germplasni: Over-view and utilization in USA, p. 2-37. In: M.A.R.Al Afifi and A.A. Al-Badawy. Proceedings, FirstInternational Conference on Date Palms. UnitedArab Emirates University, Al Am, U.A.E.

Krueger, R.R. 2007. Nutritional dynamics of datepalm (Phoenix dactvli/'ra L.). Acta Hort.736:177-186.

Labanauskas, C.K. and R.W. Nixon. 1962. Con-centrations of nutrients in pinnae of date palmsin relation to an unexplained die-back of leavesin Coachella Valley, California. Date GrowersInst. Rep. 39:14-15.

Nixon, R.W. 1934. Metaxenia in dates. Proc.Amer. Soc. Hort. Sci. 32:221-226.

Nixon, R.W. 1936. Metaxenia and interspecificpollmnations in Phoenix. Proc. Amer. Soc. Hort.Sci. 33:21-26.

Nixon, R.W. 1950. Imported cultivars of dates inthe United States. U.S. Department of Agricul-ture Circular no. 834, U.S. Department ofAgriculture. Washington, DC.

Nixon, R.W. 1951. The date palm: "Tree of Life"in the subtropical deserts. Econ. But. 5:274-301.

Nixon, R.W. and J.B. Carpenter. 1978. Growingdates in the United States. United States Depart-ment of Agriculture Bulletin no. 207, U.S.Department of Agriculture, Washington, DC.

Popenoe, 0. 1973. The date palm. Field ResearchProjects, Miami, FL.

Reuther, W. and C.L. Crawford. 1945. Irrigationexperiments with dates. Date Growers Inst.Rep. 22:11-14.

Reuveni, 0. 1986. Date, p. 119-144. In: S.P.Monselise (ed.). CRC handbook of fruit setand development. CRC Press, Boca Raton, FL.

Rygg, G.L. 1975. Date development, handling,and packing in the United States. AgriculturalResearch Service, United States Depart-

ment of Agriculture Agricultural Handbookno. 482, Agricultural Research Service, U.S.Dept. of Agriculture. Washington, DC.

Sedra, M.H., P. Lashermes, P. Trouslot, M.Combes, and S. Hamon. 1998. Identificationand genetic diversity analysis of date palm(Phoenix c/actvljfera L.) cultivars fromMorocco using RAPD markers. Euphytica103:75-82.

Sudhersan, C. and M. Abo El-Nil. 1999. Occur-rence of hermaphroditism in the male datepalm. Palms 43:18-19, 48-50.

Swingle, W.T. 1904. The date palm and its culti-vation in the south-western states. USDABureau of Plant Industry, bulletin no. 53.United States Department of Agriculture,Washington, DC.

Vandercook, C.E., S. Hasegawa, and V.P. Maier.1980. Dates, p. 506-541. In: S. Nagy and P.E.Shaw (eds.). Tropical and subtropical fruits:Composition, properties, and uses. AVI Pub-lishing Company, Westport, CT.

Wrigley, G. 1995. Date palm. p. 399-403. In: J.Smartt and N.W. Sininionds (eds.). Evolutionof crop plants. 2nd ed. Longman Group,Essex, UK.

Zaid, A. and P.F. de Wet. 2002a. Climatic require-ments of date palm. p. 57-72. In: A. Zaid (ed.).Date palm cultivation. Food and AgricultureOrganization Plant Production and Protec-tion Paper no. 156. Food and AgricultureOrganization of the United Nations, Rome,Italy.

Zaid, A. and P.F. de Wet. 2002b. Date palmpropagation, p. 73-105. In: A. Zaid (ed.). Datepalm cultivation. Food and AgricultureOrganization Plant Production and Protec-tion Paper no. 156. Food and AgricultureOrganization of the United Nations, Rome,Italy.

Zaid, A. and P.F. de Wet. 2002c. Pollination andbunch management, p. 145-175. In: A. Zaid(ed.). Date palm cultivation. Food and Agri-culture Organization Plant Production andProtection Paper no. 156. Food and Agri-culture Organization of the United Nations,Rome, Italy.

Zaid, A., P.F. de Wet, M. Djerbi, and A. Oihabi.2002. Diseases and pests of date palm, p. 227-281. In: A. Zaid (ed.). Date palm cultivation.Food and Agriculture Organization PlantProduction and Protection Paper no. 156. Foodand Agriculture Organization of the UnitedNations, Rome, Italy.

Zohary, D. and M. Hopf. 2000. Domesticationof plants in the old world: The origin andspread of cultivated plants in West Asia,Europe, and the Nile Valley. Oxford UniversityPress, Oxon, UK.

Jt.ir

1082HORTSCIENCE VOL. 42(5) AUGUST 2007