papaya: high tech tactics enhance a tropical fruit - agricultural

4 Agricultural Research/January 2004

While you’re

enjoying the sweet

taste and creamy

texture of a fresh

papaya—whether it’s

in a zesty salsa,

colorful fruit salad, or

refreshing shake or

served alone with a

splash of lime juice—

scientists from the Ag-

ricultural Research

Service are busy delv-

ing into the genetic

makeup of this tropi-

cal fruit. Their goal?

To ensure that there

will always be a good,

affordable supply of

papayas at your

market.

High-Tech Tactics Enhance aTropical Favorite

Plant physiologist Maureen Fitchand her colleagues select andmicropropagate outstandingpapayas like this Laie Gold tree,with its plump, sweet fruit, at afarm on Oahu.

Flowers and young fruit ofARS’s Laie Gold papaya, wellsuited for growing on theIsland of Oahu.

PEGGY GREB (K10922-1)


Agricultural Research/January 20044

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5Agricultural Research/January 2004

In laboratories throughout Hawaii,where most of America’s papaya crop isgrown, ARS scientists and their univer-sity and corporate colleagues are work-ing out a science-based strategy tostreamline today’s costly replanting ofpapaya orchards. Other ARS experts aresearching for genes that might boost pa-paya’s natural resistance to disease andinsects. And still other researchers aresafeguarding both familiar and unusualvarieties of papayas in the nation’s offi-cial papaya collection. (See story, p. 7.)

Sidestepping Papaya’s Costly QuirksPapaya trees are remarkably preco-

cious. Unlike apple trees, for example,which typically take at least 4 years tobear fruit, papaya trees offer delicious,ready-to-pick fruit less than a year afterthey’re planted.

But producing perfect papayas—at aprofit to growers—can be problematic.Plant physiologist Maureen M.M. Fitchof the ARS U.S. Pacific Basin Agricul-tural Research Center in Aiea, Hawaii,explains that yields typically taper offonce trees reach 3 years of age. By then,the trees have usually grown so tall thattheir uppermost fruit may be beyond thereach of the picking poles that workersuse to harvest the crop. This means thatmost papaya orchards have to be replant-ed every 3 to 4 years. In addition, papayatrees lack an anchoring taproot, so wholeorchards are vulnerable to being flattenedby the 30- to 40-mile-an-hour winds thatblow in from the Pacific. All this can addto the cost of producing papayas.

Fitch is smoothing out the wrinklesin a high-tech approach to simpler, lesscostly replanting. Her research couldslash costs by eliminating the currentpractice of cultivating five seedlings perplanting hole.

Why do growers plant five seedlingsfor every hole? “Papaya trees,” saysFitch, “can form male flowers, femaleflowers, or perfect flowers, which haveboth male and female flower parts.” Onlyperfect flowers yield the elongated,

somewhat pear-shaped fruit that’s pre-ferred in the markets where Hawaii sellsits papaya crop. “Growers know that theyhave a 97 percent chance of getting atleast one perfect-flowered papaya tree ifthey plant five seedlings per hole,” shesays. “That’s why they plant so manyextra seedlings.

“When seedlings are about 3 to 6months old,” notes Fitch, “they showflowerbuds that reveal the kind of flow-ers the trees will soon bear. Growers cancheck the flowerbuds and then chopdown any trees that won’t form perfectflowers. Typically, growers are left withone seedling per planting hole—or some-times none.”

Growing Up in a Petri DishThe goal of Fitch’s research is to pro-

duce papaya seedlings that are nearlycertain to bear only the perfect, male-female flowers. She starts with plantletsreared in petri dishes in the laboratory, aprocess called micropropagation, andthen genetically tests them to verify thatthey will produce perfect flowers.

The lab test that makes this geneticscreening possible was developed at theUniversity of Hawaii. Fitch says that,with genetically screened plantlets, farm-ers should need to plant only one plant-let per hole, making cultivation cheaperand faster.

Plantlets that pass the screening arecarefully clipped of shoots. Fitch saysthat each petri dish shoot can yield anaverage of seven shoots per month, allof which can be used to start new plant-lets. After a stint in the greenhouse, theplantlets should be ready to go outdoors.

Preliminary results from Fitch’s stud-ies indicate that trees that started aslaboratory plantlets bear market-size fruitabout 1 to 3 months earlier than conven-tionally grown seedlings. They remainnotably shorter in stature, thus keepingtheir fruit within easier reach at harvestand making them less vulnerable to gale-force winds. Conventional seedlings, onthe other hand, have to put on height so

Mature Rainbowpapaya trees on

the Island ofHawaii, where80 percent of

the state’spapayas are

grown.



Horticulturist Aileen Yeh, of theHawaii Agriculture Research Center,checks root development of papayashoots. A healthy root system iscritical to a plant’s success.

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6 Agricultural Research/January 2004

they won’t be shaded out by other seed-lings that share the same planting hole.

Micropropagation of papaya treesisn’t new. But the procedure Fitch is fine-tuning may be the first that will be avail-able, for free, to growers and breedersworldwide.

Papaya’s Disease DefensesIn addition to determining the type of

flower a tree produces, papaya genesmastermind other prized traits that areof keen interest to scientists and breedersalike. For instance, ARS plant physi-ologist Henrik H. Albert at Aiea andUniversity of Hawaii graduate studentXiaohui Qui have pinpointed more thantwo dozen genes that appear to bolsterpapaya’s ability to fend off harmfulmicrobes such as the fungus Phy-tophthora palmivora, which causes fruitand root rot. Some of these genes aresimilar to other disease-resistance genesthat researchers elsewhere have alreadyidentified in other plants, such as rice.“But we also found novel genes, not

Aileen Yeh and U.S. Pacific BasinAgricultural Research Center directorDennis Gonsalves discuss the robust natureof micropropagated Rainbow plants.



previously known to be involved indisease resistance,” says Albert.

Albert and Qui did their work withFitch, ARS plant physiologist Paul H.Moore, and Dennis Gonsalves, directorof the ARS U.S. Pacific Basin Agri-cultural Research Center. Researchersfrom the Hawaii Agricultural ResearchCenter were also involved.

Genetic Profiles ComparedIn related experiments, Hawaii sci-

entists have done an exacting comparisonof the genetic profiles of more than 60kinds of papaya, from the familiarPapaya carica varieties that we eat tolesser known, wild-papaya species. Theanalysis, conducted by Moore and Fitchat Aiea, along with ARS horticulturistFrancis T.P. Zee and several others, wasmore comprehensive than earlier com-parisons. And the team used newer, moreprecise techniques. Their work con-firmed that the varieties we buy at thesupermarket are all genetically very simi-lar to each other. The problem with thisis that they lack the safety of a broadgenetic base that would make them betterequipped to deal with natural adversity.

These new genetic studies also con-firmed that ARS’s papaya collection, ofwhich Zee is curator, apparently encom-passes an impressive 80 percent of allpapaya genetic material, also called thepapaya gene pool. A next step is to usegenes from the unique papayas in thecollection to help breeders enhance andsafeguard today’s most popular papa-yas.—By Marcia Wood, ARS.

This research is part of Plant, Micro-bial, and Insect Genetic Resources, Ge-nomics, and Genetic Improvement, anARS National Program (#301) describedon the World Wide Web at www. nps.ars.usda.gov.

To reach scientists named in this arti-cle, contact Marcia Wood, USDA-ARSInformation Staff, 5601 Sunnyside Ave.,Beltsville, MD 20705; phone (301) 504-1662, fax (301) 504-1641, [email protected]. ★

Fruit from micropropa-gated Laie Gold, a newvariety developed byMaureen Fitch andpatented by ARS, is notedfor its sweet mango-and-coconut flavor, thickorange-yellow flesh,attractive globular shape,and higher market price.

Agricultural Research/January 2004

345490 12/17/03, 2:28 AM6


Imagine a papaya the size of a football! In Thailand, such hefty, fla-vorful papayas, weighing easily 8 or 9 pounds, are a familiar sight atproduce markets.

These mega-papayas can also be found—along with their familiarand other unusual cousins—at the U.S. repository for this exotic tropicalfruit.

The repository’s offices, laboratories, and greenhouses—along withsome of its orchards—are tucked away in the lush Kulani rainforest onthe outskirts of Hilo, Hawaii. Formally known as the ARS National ClonalGermplasm Repository, it is an international treasure trove of cultivatedPapaya carica varieties and their wild relatives.

Why keep these papaya seeds, cuttings, plantlets, and trees?“To make sure that their genes remain in the breeding pool,” says

curator and horticulturist Francis T.P. Zee. Genes could vanish if wildpapayas are evicted from environments where they presently flourish.Genes can also disappear as older commercial varieties fade away, out-sold by newly popular ones.

Commercial papayas in the collection include Hawaii’s single-serving-size “solo” varieties, like Sunrise and Sunset, each pink to salmon inside,and the attractive golden Kapoho. These pear-shaped, 1- to 11/

2- pound

papayas are just the right size for one person to eat at a sitting.The trees in the collection come from Florida, Puerto Rico, Malaysia,

Taiwan, China, and Australia, as well as from Central and South America.“Some unusual species have pink blossoms instead of the typical white,”notes Zee, in commenting on some of the features of the different plantsin the collection. “Some resist plant viruses, while others are extremelyvulnerable to infection.

“One relative, Jacaratia spinosa, makes a nice 12- to 15-foot orna-mental tree. From Paraguay, it bears small orange fruit and is easily pro-duced from seed. At 4 or 5 years, it appears as stately and mature as trees30 or 40 years old. Our visitors from Southeast Asia think it would beideal for their formal gardens.

“Some papaya trees thrive in cold weather, like Carica pubescens,from Colombia, or C. stipulata, from Ecuador. We’ve planted them about30 miles from here, at Volcano, where it’s higher and colder.”

The collection’s primary users are plant breeders and scientists. “Ourpapaya specimens were very important to the team that developedringspot-virus-resistant papayas,” Zee says. Team members DennisGonsalves, now director of the ARS U.S. Pacific Basin AgriculturalResearch Center at Hilo; Maureen M.M. Fitch, plant physiologist withARS at Aiea (see story, p. 4); and their co-researchers are credited withhelping Hawaii’s papaya industry rebound from this devastating plantpathogen.

The Hilo repository is part of a nationwide network of ARS-managedcollections.—By Marcia Wood, ARS.

This research is part of Plant, Microbial, and Insect Genetic Resourc-es, Genomics, and Genetic Improvement, an ARS National Program(#301) described on the World Wide Web at www.nps.ars.usda.gov.

Francis T.P. Zee is at the USDA-ARS U.S. Pacific Basin AgriculturalResearch Center, 920 Stainback Hwy., Hilo, HI 96720; phone (808) 959-5833, fax (808) 959-3539, e-mail [email protected].

Protecting the Planet’s PapayasProtecting the Planet’s Papayas


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papaya: high tech tactics enhance a tropical fruit - agricultural

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