Breeding Vegetables Adapted to High Temperatures:A Case Study with Broccoli

Mark W. Farnham1,3

U.S. Department of Agriculture, Agricultural Research Service, 2700 Savannah Highway,U.S. Vegetable Laboratory, Charleston, SC 29414

Thomas Bjorkman2

Department of Horticulture, NYSAES, Cornell University, Geneva, New York 14456

Additional index words. Brassica oleracea Italica Group, heat tolerance, environmental stress, vernalization

Abstract. Breeding a vegetable crop for adaptation to a temperature regime that is higher than the recognized optimum forthe species in question is an example of breeding for abiotic stress tolerance. Before embarking on a project to breed forsuch stress tolerance, we propose that several critical considerations or questions must be addressed. These considerationsinclude the following: 1) What is the effect of the abiotic stress on the crop to be improved; 2) what will be the conditionsof the selection environment; 3) what germplasm is available that contains the necessary genetic variation to initiateimprovement; 4) what breeding scheme will be used to facilitate improvement; and 5) what will be the specific goals of thebreeding effort? We use a case study with broccoli to breed for adaptation to high-temperature environments to provideexamples of how each of these considerations might be addressed in developing an improvement effort. Based ondocumented success with this case study in which broccoli quality and performance under high-temperature summerenvironments has been improved, insights are provided that should be useful to future attempts to breed vegetables moretolerant of an abiotic stress.

Breeding is one of the most efficient meth-ods for managing abiotic stress. Some abioticstresses directly reduce growth. Others affectdevelopment in a way that reduces or elimi-nates the crop’s value. An example of the latteris superoptimal temperature stress in broccoli.Breeding for tolerance to this stress has greatpotential for meeting increased demand bygrowing in a larger production region.

Most vegetable crops are adapted to amodest range in temperatures with the opti-mum varying considerably from warm- to cool-season species. Cole crop vegetables (Brassicaoleracea L.) are cool-season crops that in-clude broccoli (B. oleracea Italica Group),cauliflower (B. oleracea Botrytis Group), andcabbage (B. oleracea Capitata Group). Theoptimum temperature range in which to pro-duce these vegetables is between 15 and18 �C (Maynard and Hochmuth, 2007). Thus,the cole crops are best adapted to spring andfall in temperate regions or to winter in areaswhere this season is relatively mild (e.g.,Desert Southwest). In favored locales (e.g.,the Salinas Valley in California) where a coldocean abuts a valley and cool, Mediterranean-like conditions occur nearly year-round, these

crops can be grown at almost any time of theyear.

Broccoli has emerged as the most importantcole crop in recent years and U.S. productionhas a farm-gate value of $800 million (UnitedState Department of Agriculture, EconomicResearch Service, 2010). To produce broccolithroughout the year, broccoli growers havetried to grow this vegetable in temperatureregimes that are considered too hot (e.g., 25 to30 �C) for the production of quality heads. Tocomplicate this matter, potentially higher tem-peratures that may be associated with climatechange may make it more difficult for pro-ducers to maintain broccoli production in someareas where it was previously well adapted. Asa result of these circumstances, most broccolibreeders working to develop either germplasmor elite cultivars adapted to North Americahave given some attention to breeding broccolifor adaptation to temperatures regimes abovethe 15 to 18 �C optimal range.

Breeding a vegetable crop to extend therange of temperature adaptation beyond whatexists in current germplasm is one exampleof breeding for improved abiotic stress toler-ance. Breeding for improved abiotic stresstolerance requires addressing several criticalconsiderations:

1) What is the effect of the abiotic stresson the crop to be improved?

2) What will be the conditions of theselection environment?

3) What germplasm is available that con-tains the necessary genetic variation toinitiate improvement?

4) What breeding scheme will be used tofacilitate improvement?

5) What will be the specific goals of thebreeding effort?

Some of these questions must be addressedas an improvement effort starts, some are more

relevant as the effort is well underway, andothers must continuously be re-examined asinsight of the subject matter is gained and asprogress toward objectives is made.

In this presentation, we examine the effortat the U.S. Vegetable Laboratory (USVL) inCharleston, SC, to breed broccoli for adapta-tion to summer conditions of the southeasternUnited States and consider this improvementeffort as a case study for breeding a vegetablefor adaptation to high temperatures. In par-ticular, we focus on how these questions wereaddressed as this breeding effort was initiatedand as it has developed over time.

WHAT IS THE EFFECT OF THEABIOTIC STRESS ON THE CROP

TO BE IMPROVED?

Several studies have evaluated the effectof low temperatures on the initiation of inflo-rescence development in broccoli (Fonteset al., 1967; Fontes and Ozbun. 1972; Gaussand Taylor, 1969). Most horticulturists rec-ognize the importance of cold temperaturesas necessary for vernalization in biennialvegetables like cabbage or kale; however,few realize that many cultivars of broccolihave a quantitative cold requirement neededto induce flowering (Fontes et al., 1967).Whereas cabbage and kale require tempera-tures at or below 5 �C to shift from vegetativeto reproductive growth phases, the critical tem-perature for annual broccoli is significantlyhigher. Indeed, Wurr et al. (1995) developeda temperature model for broccoli and postu-lated an upper temperature limit for vernal-ization at 23.6 �C, above which plants areexpected to remain vegetative. Although it iscommonly thought that broccoli will prema-turely bolt as a result of relatively high tem-peratures, the opposite is actually the case.Premature bolting in broccoli typically oc-curs when cold temperatures induce small

Received for publication 23 Feb. 2011. Acceptedfor publication 19 May 2011.We acknowledge the excellent technical assistanceof Mr. David Couillard and Mr. Joseph W. Shail,Jr., in conducting the field research components ofthis project.This paper was part of the colloquium ‘‘Improve-ment of Horticultural Crops for Abiotic StressTolerance’’ held 5 Aug. 2010 at the ASHS Con-ference, Palm Desert, CA, sponsored by the Veg-etable Breeding (VGBR) Working Group, andco-sponsored by the Environmental Stress Physi-ology (STRS) Working Group.1Research Geneticist.2Associate Professor.3To whom reprint requests should be addressed;e-mail Mark.Farnham@ars.usda.gov.

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plants to initiate reproductive growth (Milleret al., 1985). Thus, during the vegetative phase,temperatures above the upper limit for vernal-ization will slow or impede the development ofthe inflorescence or head.

In broccoli, the main dysfunction inducedby high temperatures occurs later in develop-ment. Once inflorescence or head develop-ment is initiated, relatively high temperatures(�30 �C) arrest head development. Paradox-ically, such temperatures can be optimal forvegetative growth (Bjorkman and Pearson,1998). It is unknown whether genotypes per-forming better in warm climates have a highertemperature threshold for meeting the chillingrequirement or simply a smaller absolute re-quirement. The typical flaws that result in-clude incomplete head development, unevenbead size, bracting in heads, and rough headsurface. It is this sensitivity to warm temper-atures near harvest, e.g., during heading, thatlowers head quality in field production andthus pushes U.S. broccoli production to cli-mates with an extended cool growing season.

Few studies have been conducted to ex-amine variation among broccoli varieties intheir development under high temperatures.Heather et al. (1992) evaluated a collection ofbroccoli hybrids for heat tolerance and hold-ing ability in summer productions on LongIsland, NY. These authors identified certainhybrids that yielded more marketable headsand that held their quality for a longer periodof time. They found that heat stress was mostdamaging �3weeks before harvest, at whichtime they calculated the immature inflores-cence to be 5 to 10 mm in diameter. Bjorkmanand Pearson (1998) studied temperature arrestin the broccoli hybrid ‘Galaxy’ and deter-mined that the critical heat-sensitive stagewas during floral initiation, which occurs whenthe shoot tip has a diameter less than 5 mmand the apical meristem is �2 mm. Thosesame authors found that the heat stress hadlittle effect before or after the floral initiationstage. The timing of heat stress is very impor-tant in this vegetable and makes it difficultto do field selection for heritable variation insensitivity.

WHAT WILL BE THE CONDITIONSOF THE SELECTION

ENVIRONMENT?

We decided early on that the best way toensure the necessary superoptimal heat stressconditions needed for selecting stress-tolerantplants was to grow plants in a setting withprolonged heat stress. Thus, our selectionenvironment during summer in Charleston,SC, is continuously hot with plants grownfrom seedling stages to maturity during rela-tively high temperatures in the field (Fig. 1).After transplanting in late May, moderatetemperatures last only a few weeks duringearly vegetative growth. However, from lateJune on, daily high temperatures are 30 �C orhigher and average low temperatures seldomgo lower than 20 to 23 �C through the timewhen trials are ending in late August (Fig. 1).Ultimately, our abiotic stress environment

has a characteristic temperature range that istoo hot for commercial production of qualitybroccoli heads while being conducive to rapidgrowth. The relatively high night temperaturesmay be especially important because they maynot be low enough for many broccoli cultivarsto meet the chilling requirement for flowerdevelopment.

It is very important that any abiotic stressbe consistent so it is always possible to dis-criminate genetic variation for any physio-logical defects. Therefore, when evaluatingthe effects of high temperature, like in thiscase study with broccoli, temperatures of thetest environment must be consistently hot. Ingeneral, by conventional standards, the sum-mer environment we select at Charleston istoo hot for broccoli, but it is made relativelyconsistent by the moderating effect of themaritime location. We remain cognizant ofthe possibility of higher or lower than normaltemperature conditions, but lower tempera-tures that would be more optimal for broccoliproduction really do not occur in late Junethrough August in Charleston. In years whentemperatures are lower than normal, wetypically make more selections with the un-derstanding that some may not prove ade-quately tolerant in subsequent years. In yearswhen temperatures are higher than normal,we take advantage of the situation, makefewer selections than normal, and scrutinizethem carefully knowing they will likely per-form better in subsequent years. In general,hotter than normal years allow one to selectthe most tolerant plants likely to occur.

WHAT GERMPLASM IS AVAILABLETHAT CONTAINS THE NECESSARYGENETIC VARIATION TO INITIATE

AN IMPROVEMENT EFFORT?

When our program to breed broccoli foradaptation to high temperature was initiatedin the early 1990s, all commercial broccolihybrids available at the time were evaluatedin summer field trials in South Carolina(Table 1). In addition to the commercialhybrids, two hybrids obtained from Michael

Dickson (Cornell University), bred for adap-tation to summer conditions of New YorkState, were also evaluated. Based on perfor-mance over 2 years, the hybrids that per-formed best in the initial summer trials wereidentified (Dufault and Farnham, 1996). Al-though more than 60 hybrids were examined,only five were deemed to have potential meritfor breeding purposes. These included ‘Bac-cus’, ‘Early Dawn’, ‘Green Comet’, ‘Para-gon’, and ‘NY4502 · 6703’. They were allsubsequently selfed for several generationsto create inbreds. These inbreds were in turnselected and then cross-bred to develop newsegregating populations.

Since making the original selections, thisproject has now advanced through severalcycles of recombination. This recurrent se-lection program has remained mostly closed.However, a few new lines have been inputinto the germplasm pool by crossing them withone or more inbred selections. An additionalline (NY1142) was obtained from Dickson in1996, selfed for several generations, and thencrossed with a number of selected inbreds. Adoubled haploid line from the hybrid Everestwas crossed with selections in 2005. Our cur-rent program to breed broccoli adapted to high-temperature stress is based on a relatively smallgermplasm pool consisting mostly of earlymaturity germplasm.

WHAT BREEDING SCHEME WILLBE USED TO FACILITATE

IMPROVEMENT?

A project to breed for an abiotic stresstolerance trait, similar for a project focusedon nearly any trait, must consider the natureof new cultivars of the crop in question. Vir-tually all new commercial cultivars of broc-coli are F1 hybrids. Although the parents ofindividual hybrids are trade secrets kept bythe seed company or other institution thatmakes it, most are a single cross of two highlyinbred lines. In some cases, the inbred paren-tal lines are produced by successive genera-tions of selection and selfing. In other cases,the inbreds may be doubled haploid lines

Fig. 1. Average daily high and low temperatures during selection trials conducted at the U.S. VegetableLaboratory in Charleston, SC. Julian date 145 is 25 May and date 240 is 28 Aug.

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arising from anther or microspore cultures(Gray, 1993).

Our broccoli improvement effort uses apedigree method of breeding, in which weevaluate segregating populations in the targetenvironment, select individuals with goodperformance, remove the plants from thefield, and self them in an insect-free green-house to obtain seed for the next generation.Throughout the process, clear pedigree re-cords have been maintained. With each suc-cessive generation of inbreeding, many linesare eliminated. However, lines that persist inthe program through successive generations ofselfing (e.g., from F6 to F8), as a result of theirgood horticultural phenotype and head qualityunder summer conditions, are used as parentsto create new segregating populations. In ad-dition, they are also used in test crosses toevaluate the potential of a given line to pro-duce cultivar-quality hybrids. Some type ofpedigree breeding method is commonly usedby broccoli breeders in both the public andprivate sectors. Single seed descent is seldomused in this crop during early generations ofselection. Without prior knowledge of howtolerance to high temperatures might be in-herited in broccoli, we deemed it logical tofollow the more standard pedigree approach.

In general, it is also a common practice ofbroccoli breeders to cross good inbreds bygood inbreds when developing new hybrids.Combining ability in broccoli has not beenadequately characterized. Significant generalcombining ability has been detected for a

number of horticultural traits (Abercrombieet al., 2005; Hulbert and Orton, 1984). How-ever, we are unaware of any reports citingevidence for significant specific combiningability for any trait. The few observationsreported tend to align with the common prac-tice of many broccoli breeders who stringentlyselect inbreds that perform well (e.g., producehigh-quality heads lacking defects) beforemaking test crosses with other inbreds. As webegan our program to improve adaptation tosummer environments, we had no evidencethat would cause us to veer from the generalapproach of crossing ‘‘good by good’’ thatmany broccoli breeders have used successfullyto improve numerous traits of this crop.

WHAT WILL BE THE SPECIFICGOALS OF THE BREEDING EFFORT?

Breeding for abiotic stress is most suc-cessful when the goal is well defined at theoutset of the program. At the same time, theplan should permit refining the goals as thebreeding effort ensues, progress is made, andas the breeders learn more about their exper-imental subject as they gain added experi-ence. Balancing a flexibility of approach withan abiding purpose is the hallmark of themost productive programs.

Our primary goal was to improve broccolifor performance under summer conditionstraditionally deemed too hot. Ultimately, wehoped to select for broccoli that did notexpress the typical flaws such as incomplete

head development, uneven bead size, bractingin heads, and rough head surface that typicallyoccur when conditions are too hot. We ex-pected that progress would likely be incre-mental and slow; therefore, we anticipated aprogram that would operate over a long pe-riod with advances occurring over numerousgenerations. Such a program can only be un-dertaken in certain institutional settings; theUSDA-ARS provides such conditions.

FINDINGS, INSIGHTS, ANDPROGRESS

Relatively early in our program, it becameevident that full-season commercial broccolihybrids developed for Western conditions willnot head in our selection environment. In thecourse of our summer trials, we repeatedly seethat common broccoli hybrids like ‘Marathon’and ‘Arcadia’ fail to make the normal shiftfrom vegetative to reproductive growth thatultimately results in a primary broccoli headand instead remain vegetative (Table 2). Theapical meristem loses dominance, perhaps ina failed reproductive transition, and lateralgrowth becomes excessive. Near the end ofthe summer, plants of ‘Marathon’, ‘Arcadia’,and many other hybrids resemble leafy greenBrassicas more than they do normal broccoliplants. These observations can be explained bythe fact that once transplants are 2 to 3 weeksold in our selection environment, daily lowtemperatures seldom go below 23 �C, the criti-cal temperature reported necessary to vernalizebroccoli and stimulate heading.

Commercial hybrids selected for Westernadaptation, but widely grown in the East,exemplified by ‘Gypsy’ and ‘Packman’ (Table2), will head in our target summer environ-ment, but head quality is poor. Cultivars in thisgroup do not appear to have a lower chillingrequirement to stimulate heading. However,the chilling requirement for subsequent flowerdevelopment is inconsistently met. The resultis uneven bead size, roughness and discolor-ation of the head, and other traits that loweroverall quality (Table 2).

We have been successful at increasingadaptation to high-temperature summer envi-ronments based on performance of our selectedinbreds and, more importantly, hybrid combi-nations derived from them (Table 2). Many ofthe hybrids we have developed mature earlierthan most commercial broccoli hybrids, pro-ducing heads in �60 d after transplanting.Although these USVL hybrids are early, theydo not escape high temperatures because dailyhighs and lows already exceed 30 and 23 �C by3 to 4 weeks post-transplant.

Head quality is not as high for ourexperimental USVL hybrids compared withthat typically observed with modern com-mercial hybrids grown in an optimal envi-ronment. They have larger flower buds andheads are not as dense as the modern com-mercial standard. However, at high tempera-tures, they do not have the flaws that makecommercial material unmarketable (Table 2).Because improved performance has progressedslowly over several generations, and because

Table 1. Cultivars and experimental germplasm (along with the original sources of seed for each) includedin 1993 and 1994 field evaluations aimed at identifying germplasm to use in the U.S. VegetableLaboratory program effort to breed for adaptation to high temperatures.

Cultivar or germplasm Original source Cultivar or germplasm Original source

Arcadia Sakata Green Valiant SakataAtlantic Asgrow Greenbelt SakataBaccus Asgrow Headline TakiiBarbados Ferry Morse Hi Caliber Harris MoranBonanza Burpee High Sierra AsgrowBuccaneer Petoseed Landmark TakiiBig Sur Asgrow Lancelot PetoseedBrigadier Petoseed Legend SakataCitation Harris Moran Marathon SakataCharade Takii Mariner PetoseedClaudia Ferry Morse Mercedes GoldsmithCommander Peto Seed Musket PetoseedCruiser Royal Sluis Ninja SakataDandy Early Park Seed Orion AsgrowEarly Dawn Asgrow Pinnacle TakiiEarly Emerald Goldsmith Packman PetoseedEmperor Sakata Paragon StokesEmerald City Sakata Patriot SakataEmbassy Asgrow Pirate PetoseedEverest Rogers Premium Crop TakiiEureka Sakata Sabre AsgrowExcelsior Sakata Saga Johnny’sFlorette Asgrow Shogun SakataFutura Asgrow Skiff Royal SluisGalaxy Asgrow Southern Comet TakiiGalleon Petoseed Sprinter SakataGem Asgrow Sultan SakataGoliath Stokes Symphony AsgrowGreen Comet Takii Viking PetoseedGreen Duke Sakata Zeus TakiiGreen Goliath Burpee NY2399 · 2403 Cornell UniversityGreen Hornet Stokes NY4502 · 6703 Cornell University

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hybrids perform better than the inbreds usedto create them, the abiotic stress tolerance weare breeding for is likely to be quantitative,i.e., controlled by many genes. Future inves-tigations in our program will confirm the modeof inheritance by examining segregating pop-ulations derived from our best inbreds.

It is likely that other efforts to breed forabiotic stress tolerance will also involvequantitative traits. In some of these cases, itis possible that marker-assisted selection maybe used to improve selection efficiency. Werecently developed a doubled haploid pop-ulation derived from an F1 formed by cross-ing one of our most tolerant inbreds with

a non-tolerant inbred (doubled haploid) thatproduces poor-quality heads in our summerconditions. In the summer of 2011, we willevaluate 150 doubled haploid lines from thiscross for adaptation to summer conditionsbased on head quality characteristics and wewill simultaneously develop a genetic mapfor this population. Assuming the adaptationto relatively high temperatures is inheritedquantitatively, this marker-assisted approachmay identify key quantitative trait loci thatwe might directly select for in the future,possibly used in conjunction with a singleseed descent program that might acceleratethe inbreeding process.

Although we have selected for stress tol-erance in a single environment, our intent is toidentify broccoli hybrids widely adapted towarm growing seasons. To get an indicationof wide adaptability, we tested experimentalUSVL hybrids in Geneva, NY, where aver-age summer temperatures do not necessarilylimit broccoli productivity, but high temper-ature spikes often cause head quality defects.In this testing, all broccoli hybrids producedheads, but most conventional hybrids (e.g.,‘Marathon’ or ‘Packman’) had lower qualityin the middle of summer, whereas the exper-imental USVL hybrids exhibit similar qualitytraits throughout the summer (Fig. 2). Theseinitial tests indicate that our selected geno-types may be widely adapted to summer en-vironments of the type found in the easternUnited States.

It is noteworthy that although there isanecdotal evidence that broccoli producedat high temperatures may exhibit off-flavors,we are unaware of any published evidencethat indicates this is actually so. At the outset,we did not expect, nor have we observed, thatbroccoli produced under our relatively high-temperature conditions has poor flavor. Onthe contrary, our own non-empirical testingindicates broccoli harvested during hot pe-riods of summer can have good taste.

CONCLUSIONS

Breeding for abiotic stress tolerance re-quires persistence and good observation.Traits are multigenic with complex and easilymisinterpreted phenotypes. Progress is oftenincremental, environmental variation large,and heritability low. However, progress can

Fig. 2. Representative heads of broccoli harvested from field plots of ‘Packman’, ‘Marathon’, and U.S. Vegetable Laboratory Experimental Summer Hybrid 4transplanted 2 June, 15 June, 30 June, and 15 July at Geneva, NY, in 2007. Heads harvested from the June plantings matured when temperatures were still hot duringAugust and early September, whereas heads harvested from the 15 July planting matured when temperatures were cooler in late September and early October.

Table 2. Trait means for days from transplant to harvest (DTH) and head characteristics including headsmoothness (smooth), overall quality (quality), bead size, bead uniformity, and fresh weight for sevenbroccoli hybrids grown in three summer environments in Charleston, SC, in 2007, 2008, and 2009.

Hybrid DTHz (d)

Head characteristics

Smoothy

ratingQualityx

ratingBead sizew

(mm)Bead unif.v

ratingHead mass

(g)

Marathon No heads No heads No heads No heads No heads No headsArcadia No heads No heads No heads No heads No heads No headsGypsy 61.1 a 3.4 a 6.9 a 1.52 3.3 a 176.7 aPackman 56.5 b 3.5 a 7.0 a 1.64 3.0 b 154.2 bUSVL HY1 55.8 bc 2.7 c 5.3 b 1.59 2.4 c 160.2 abUSVL HY4 55.8 bc 2.3 d 4.9 b 1.78 2.2 c 155.4 bUSVL HY5 52.6 c 2.3 d 4.6 c 1.75 2.3 c 146.1 bzTrait means (for hybrids that formed heads) followed by the same letter are not significantly (P < 0.05)different from one another based on analysis of variance and protected least significant difference.yHead smoothness is rated 1 to 5 with 1 = perfectly smooth bead surface, 2 = smooth, but not perfect, 3 =some irregularity, 4 = very bumpy surface, and 5 = very distorted surface.xHead quality is rated 1 to 9 with 1 = highest quality head, 2 = excellent quality, 3 = very good head quality 4 =good quality, 5 = acceptable quality, marketable, 6 = poor quality, nonmarketable, 7 = very poor quality, 8 =nearly unrecognizable as broccoli, and 9 = not recognizable as broccoli (e.g., more cauliflower-like).wAn absence of letters indicates that no significant differences for that trait were observed among hybrids.vBead uniformity is rated 1 to 5 with 1 = all beads perfectly uniform, 2 = uniform, but not perfectly, 3 =some lack of uniformity, 4 = irregular uniformity, and 5 = highly irregular.

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be consistent by setting a clear target, keenlyobserving the plant response to select themore casual expression of resistance, andproviding the most selective environment.

Literature Cited

Abercrombie, J.M., M.W. Farnham, and J.W.Rushing. 2005. Genetic combining ability ofglucoraphanin level and other horticultural traitsof broccoli. Euphytica 143:145–151.

Bjorkman, T. and K.J. Pearson. 1998. High tem-perature arrest of inflorescence development inbroccoli (Brassica oleracea var. italica L.).J. Expt. Bot. 49:101–106.

Dufault, R.J. and M.W. Farnham. 1996. Broccolivarieties for spring, summer and fall in coastalSouth Carolina. 1995–96 Clemson UniversityVegetable Report. p. 11–12.

Fontes, M.R. and J.L. Ozbun. 1972. Relationshipbetween carbohydrate level and floral initiationin broccoli. J. Amer. Soc. Hort. Sci. 97:346–348.

Fontes, M.R., J.L. Ozbun, and S. Sadik. 1967.Influence of temperature on initiation of floralprimordial in green sprouting broccoli. J. Amer.Soc. Hort. Sci. 91:315–320.

Gauss, J.F. and G.A. Taylor. 1969. Environmentalfactors influencing reproductive differentiationand the subsequent formation of the inflores-cence of Brassica oleracea L. var. italica,Plenck, cv. ‘Coastal’. J. Amer. Soc. Hort. Sci. 94:275–280.

Gray, A.R. 1993. Broccoli: Brassica oleracea(Italica Group). In: Kalloo, G. and B.O. Bergh(eds.). Genetic improvement of vegetablecrops. Pergamon Press, Oxford, UK.

Heather, D.W., J.B. Sieczka, M.H. Dickson, andD.W. Wolfe. 1992. Heat tolerance and holding

ability in broccoli. J. Amer. Soc. Hort. Sci. 117:887–892.

Hulbert, S.H. and T.J. Orton. 1984. Genetic andenvironmental effects on mean maturity dataand uniformity in broccoli. J. Amer. Soc. Hort.Sci. 109:487–490.

Maynard, D.N. and G.J. Hochmuth. 2007. Knott’shandbook for vegetable growers. 5th ed. JohnWiley and Sons, Inc., Hoboken, NJ.

Miller, C.H., T.R. Konsler, and W.J. Lamont. 1985.Cold stress influence on premature flowering ofbroccoli. HortScience 20:193–195.

United States Department of Agriculture, Eco-nomic Research Service. 2010. Vegetables andmelons yearbook.

Wurr, D.C.E., J.R. Fellows, K. Phelps, and R.J.Reader. 1995. Vernalization in calabrese (Bras-sica oleracea var. italica)—A model for apexdevelopment. J. Expt. Bot. 46:1487–1496.

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