PHQ 100 characterization of - University of California, Davis

1 November 1999 Perishables Handling Quarterly Issue No. 100 Page 11 characterization of Carotenoid Composition ofCarrots Affected by Light Root Syndrome Suslow, Jiangchun Wu, and Galen Peiser, Department of Vegetable Crops, UCDI ntroductionUniform, bright orange color is a major qualityattribute for fresh market and fresh-cut carrots in bothretail consumer markets and foodservice outlets andinstitutions. Overall poor root color or irregular colordevelopment have been identified as a productiondefect that impacts postharvest carrot production is affected by a variety ofincompletely understood disorders that impact rootcolor development or intensity. Divergent factors arebelieved to be responsible for roots with poor color(carotenoid) development as compared to irregularcarotenoid development.

2 The severity of the problem ismost acute during the production season that transitionsfrom late summer seeding to mid-winter harvest. Rootssymptoms are characterized as having a streaked orblotchy appearance due to the variability in pigmentdevelopment within groups of plant cells. Originallyreported as white root and later light-rootsyndrome (LRS), predominantly in the ImperialValley, this disorder appears to have been grouped witha broad range of sub-optimal production conditions andpractices that effect carotenoid is in progress to distinguish betweenlow color roots and the malady light root syndrome (LRS). Currently, we consider the term LRS to berestricted to roots with irregular, patchy, or streakedcarotenoid development that extends from external tointernal tissues.

3 The cause of this disorder remainsunclear but appears distinct from roots with onlyinternal carotenoid deficiencies or overall poor a marketing and consumer satisfactionperspective, both conditions are undesirable. Progressin developing management strategies, in parallel withopportunities for variety screening, is hampered by theunpredictable occurrence of LRS and the likelytemporal separation of inducing factors and and carrot root developmentTo review, -carotene is the primary contributor toorange color intensity. Total carotenoid content anddistribution of carotenoids at harvest is influenced bygenetic and environmental factors including; Physiologic maturity; peak carotenoid content isan index of maturity See Fig.

4 1 Climatic conditions during the period 3-6 weekspreceding harvest Temperatures below or above the optimal range of10-16 C Micronutrient levels; preliminary researchidentified excess Mo, Zn , and Se as significantlydepressing total carotenoid or relative -carotenecontent (Biacs et. al., 1995) Soil oxygen content, soil water potential, and standdensity are commonly held as a strongcontributory factors to carotene synthesis andorange color intensity. Specific data or referencesto support this observation have not been obtained,at this 1. Carotenoid content of carrots increases ResearchEarly efforts to determine the cause and possiblesolutions to LRS have focused on the possibilities thatLRS is associated with whitefly feeding, atransmissible virus or ds-RNA elements, or a methylbromide sensitive, soil-borne agent (Creamer et.)

5 Experimental results were negative and largelyinconclusive. A corrective program using ethephon(Ethrel ) has been conducted to determine whetherorange color intensity of roots could be improved byTotal Carotene Content of Maturing Carrot Roots01020304050607076839097 Days from Seeding Pe pk ow itz e 1944 {fr om Gr os s , 1991}ug(g fresh wt.)ImperatorNan tesTendersw eetNovember 1999 Perishables Handling Quarterly Issue No. 100 Page 12foliar applications (McGiffen, 1995, 1997). Varietalvariability in orange root color intensity was alsoaddressed, but the opportunities to distinguish betweenlow root color and the originally described LRS werenot efforts to identify a major or solecontributing factor to LRS have not, thus far, beensuccessful.

6 Research progress has been hampered bythe unpredictable incidence of the disorder inproduction fields. The identification of presumptivecontributing factors may have been misdirected by themisidentification of low color roots as ApproachWe have taken two approaches to resolve this situationthat will hopefully determine whether low color roots(a common situation) can be differentiated from LRSroots (a more specific disorder). The first approachseeks to determine whether low orange root color andLRS are chromatically and biochemically distinct. Thesecond approach, described below, takes an applied biotechnology approach by developing novel tools toevaluate production and environmental impacts oncarotenoid development and timing in is popularly held that carrot roots with ahigh -carotene: - carotene ratio will result in a moreintense orange color.

7 The absolute -carotene contentis believed to be less important than a high : ratio inreaching the color potential of a given geneticbackground. This is not apparent from publishedanalytical comparisons of carotenoid content (Simonand Wolff 1987) See Fig. 2. A : -carotene ratio ofFigure 2. A comparatively low : -carotene ratiobut high absolute content was associated with intenseorange is characteristic of the dark orange color lineHCM whereas an intermediate color line had a ratio The distinction between total and proportional -carotene content may prove useful in resolving thebasis for ContentHPLC analysis during the initial phase of this projectshowed that the total carotenoids are depressed in lowcolor root tissue and LRS tissue.

8 A significant shift inthe ratio of key biosynthetic products is not preliminary conclusion would be that a variety offactors that influence root color quality and, morespecifically, LRS results in an overall reduction incarotenoid biosynthesis and not a block in one of thebiosynthetic precursor steps between phytoene and -carotene. No accumulation of compounds leading to - carotene have been detected thus far. Although thetotal concentrations are variable for variety and sampledate, the ratio of the compound phytoene to and -carotene remain proportional and not significantlydifferent between light and dark roots. Disruption ofcarotenoid production may occur at earlier steps in thepathway or at some other point of cellular replicated trials demonstrated that lowcolor deficiencies and poor color development that isrestricted to internal tissue are variety dependent ( and 4).

9 Despite limited field trial results to date, it isapparent that variety selection within periods ofmoderate to severe low color development would bebeneficial to avoiding the problem. From observationsand review of past literature, reaching physiologicmaturity prior to low air and soil temperatures willlikely result in greater color 3. Comparative carotenoid content of carrotroot Carotenoid Content of Low, Intermediate and High Color Lines; Simon and W olff (1987)020040060080010001200B6274B9692 HCMC arrot Lineug / g f re sh w t .betaalphatotal020406080 InnerOuterCoreRoot Tissue LocationComparative Carotenoid Content ofCarrot Root TissueCaro Pak 3 CaroPak 2 CaroPak 1 November 1999 Perishables Handling Quarterly Issue No.

10 100 Page 13 Figure 4. An example of research in progress, thisfigure represents the mean Orange Color rating of 15-20 roots from four replicated plots of each analysis of other subjective and objective colorcomparisons is in Research ToolsNovel research tools are desperately needed tofacilitate the investigation of potentially complexenvironmental, crop management and physiologicinteractions that result in diminished carotenoidbiosynthesis. In conjunction with continuing to explorethe basis for LRS in the field, it is essential to createmethods to make the evaluation of multiple variablesthat impact the production of carotenoids in carrotroots more feasible. The approach we feel hassignificant merit would result in the development of agenetic reporter system for factors that delay orpromote the expression of key enzymes in thecarotenoid production biosynthesis is a widely studiedarea of great interest for photosynthesis, stresstolerance, and as nutrients with cancer-reducingpotential.