HORTSCIENCE 44(3):651–655. 2009.

Characterization of Adventitious RootDevelopment in SweetpotatoArthur Q. Villordon1

LSU AgCenter, Sweet Potato Research Station, 130 Sweet Potato Road,Chase, LA 71324

Don R. La BonteLSU AgCenter School of Plant, Environment, and Soil Sciences, BatonRouge, LA 70803

Nurit Firon, Yanir Kfir, and Etan PressmanInstitute of Plant Sciences, The Volcani Center, Agricultural ResearchOrganization, P.O. Box 6, Bet Dagan, 50250, Israel

Amnon SchwartzInstitute of Plant Sciences and Genetics in Agriculture, Faculty ofAgriculture, The Hebrew University of Jerusalem, Jerusalem, Israel

Additional index words. storage root development, storage root anatomy, Ipomoea batatas

Abstract. Adventitious roots of ‘Beauregard’ and ‘Georgia Jet’ sweetpotato wereobserved and anatomically characterized over a period of 60 days of storage rootdevelopment. The majority of ‘Beauregard’ and ‘Georgia Jet’ adventitious rootssampled at 5 to 7 days after transplanting (DAT) possessed anatomical features (fiveor more protoxylem elements) associated with storage root development. The majority of‘Beauregard’ (86%) and ‘Georgia Jet’ (89%) storage roots sampled at 60 to 65 DAT weretraced directly to adventitious roots extant at 5 to 7 DAT. The two varieties, however,differed in the timing in which regular and anomalous cambia were formed. Regularvascular cambium development, i.e., initiation and completion, was observed in bothvarieties at 19 to 21 DAT. Formation of complete regular vascular cambium wasnegligible for ‘Beauregard’ (4%) in comparison with ‘Georgia Jet’ (32%) at 26 to 28DAT. However, anomalous cambia development adjacent to xylem elements was greaterin ‘Beauregard’ (30%) in comparison with ‘Georgia Jet’ (13%). Nearly 40% to 50% ofsamples in both varieties showed extensive lignification in the stele region. At 32 to 35DAT, 62% to 70% of the adventitious roots for both varieties had either been initiated(developed anomalous cambium) or were lignified. The remaining adventitious rootsshowed intermediate stages of vascular cambium development. The adventitious rootcount increased up to 19 to 21 DAT and then remained constant up to 32 to 35 DAT.These accumulated results suggest that the initial stages of adventitious root developmentare critical in determining storage root set in sweetpotato.

Sweetpotato producers have high expect-ations every season. What they want is auniform crop consisting of U.S. No. 1 gradestorage roots (elliptical roots 8 to 23 cm inlength and 5 to 9 cm in diameter) and fewsmall or oversized grade (jumbos) roots.What producers lack is the ability to predictyield. Individual plants vary greatly in thenumber of storage roots that they produce.

This extends to entire fields, which can varyin yield by up to 50% (T. Smith, personalcommunication) in a given season. Whyyields vary is perplexing. Production fieldsare often adjacent to one another and plantedjust days or weeks apart. The durable vege-tative cuttings used to establish the cropappears homogeneous and either establishesquickly in moist, warm soils or delays itsestablishment until environmental conditionsimprove. The resulting canopies under thetwo scenarios appear similar; however, rootdevelopment is profoundly different.

Yield variability is likely a function ofvariability in early transplant (cutting) estab-lishment. Previous research attributed yielddifferences to cultivar, propagation material,environmental, and soil factors (Kays, 1985;Ravi and Indira, 1999). Bare-rooted trans-plants produced adventitious roots and someof those roots subsequently underwentchanges in their growth pattern and devel-oped into storage roots (Belehu et al., 2004;Togari, 1950; Wilson and Lowe, 1973).

Adventitious roots originated from root pri-mordia located on the nodes as well as at thecut ends, i.e., ‘‘wound’’ roots (Belehu et al.,2004; Togari, 1950). These roots becamefibrous roots and exhibited regular secondarygrowth and lignification of the stele, orstorage roots, and exhibited proliferation ofcambial cells that formed starch-accumulat-ing parenchyma cells. Early anatomical stud-ies (Artschwager, 1924; McCormick, 1916)documented that storage roots originatedfrom adventitious roots with pentarch andhexarch steles. Togari (1950) suggested thatthe balance between cambium developmentand lignification determined final storageroot count and root yield. It is the combina-tion of a contiguous, active regular vascularcambium and proliferation and growth ofanomalous cambia that enabled storage rootsto add girth (Kokubu, 1973; Togari, 1950;Wilson and Lowe, 1973). Depending on thenumber of fibrous roots that will be inducedto form storage roots, sweetpotato plants willyield either a high number (four to six perplant) of uniform and high-grade roots, e.g.,U.S. No. 1, or a low number of roots that maybe reduced to one very large storage root perplant or no marketable roots at all. It isunclear, however, when during root develop-ment this balance is determined and whetherthis may be a universal mechanism amongdifferent sweetpotato varieties.

The lack of information relating to storageroot set is compounded by apparently con-flicting information in the literature. Forexample, adventitious roots that appearshortly after transplanting have been referredto as ‘‘feeder’’ roots (Hernandez and Hernan-dez, 1969). Edmond and Ammerman (1971)classified the sweetpotato root system intotwo types: absorbing and storage. Such infor-mation does not reflect an accurate relation-ship of those adventitious roots to storageroot development and it is unclear how manyof those adventitious roots actually becamestorage roots. Also, it is unclear whetherstorage roots emanate from these initialadventitious roots. Hence, the objective ofthis study was to characterize the number,anatomical features, and developmental fateof adventitious roots initiated early aftertransplanting in ‘Beauregard’ and ‘GeorgiaJet’ varieties.

Materials and Methods

Plant materials. ‘Beauregard’ and ‘Geor-gia Jet’ are the most important sweetpotatovarieties grown in the United States andIsrael, respectively. ‘Beauregard’ transplantswere produced in the greenhouse (Chase,LA) by bedding storage roots in unfertilized,washed sand. The temperature in the green-house was maintained between 24 and 27 �C.The mean relative humidity was 60%. Sup-plementary lighting was provided using3000-lx white fluorescent light for 12 h perday. ‘Georgia Jet’ transplants were cut fromfield-grown plants in the southern Aravaregion, Israel. Prevailing minimum and max-imum temperatures in the southern Arava

Received for publication 18 Dec. 2008. Acceptedfor publication 11 Feb. 2009.This research was supported by Research Grant No.US-4015-07 from BARD, the United States–IsraelBinational Agricultural Research and DevelopmentFund.Approved for publication by the Director of theLouisiana Agricultural Experiment Station as man-uscript number 2008-260-2316.We thank the anonymous reviewers for theirthoughtful review and helpful critiques of themanuscript.1To whom reprint requests should be addressed;e-mail avillordon@agcenter.lsu.edu.

HORTSCIENCE VOL. 44(3) JUNE 2009 651

CROP PRODUCTION

region were 24 and 35 �C, respectively.Relative humidity was 30%. ‘Georgia Jet’transplants were then transferred to a green-house maintained between 17 and 33 �C withno supplemental light. All transplants used inthe various studies had five fully openedleaves and were �21 cm long. Except other-wise stated, the transplants were used for theexperiments detailed subsequently.

Relationship between adventitious rootsand storage roots. Studies were conducted todetermine the relationship between adventi-tious roots initiated within 5 to 7 d after trans-planting (DAT) for ‘Beauregard’ (UnitedStates) and ‘Georgia Jet’ (Israel) and eventualstorage root formation. Storage roots weredefined as adventitious roots with localizedswelling (0.5 cm or greater) versus ‘‘pencilroots,’’ which are uniformly thickened (Wilsonand Lowe, 1973). Two tests were conductedfor Beauregard; the first test was conducted inthe greenhouse using greenhouse-grown‘Beauregard’ transplants (10 replications).Transplants were planted in pots and care-fully dug at 5 DAT and digital images weretaken to document number, length, and loca-tion of adventitious roots. All adventitiousroots were marked with latex paint appliedwith a No. 4 round brush and plants replantedin sand. After 40 d, the transplants werecarefully harvested and the storage rootswere examined for the presence of paint.The second test was conducted using ‘Beau-regard’ plants from field beds (10 replica-tions). The field bed was fertilized with 5N–20P–20K at a rate of 448 kg�ha–1. The bedswere established on 23 Mar. 2007 and cov-ered with 1-mil black plastic. The blackplastic was removed on 26 Apr. 2007, aroundthe time the sprouts can be seen touching theblack plastic. The cuttings were establishedin pots grown under natural conditions. Theculture medium was unfertilized, washedsand. A sprinkler attached to a timer wasused to irrigate the plants daily (�0.6 cm).The transplants were planted on 29 June2007, and plants were dug and adventitiousroots marked 7 DAT, before replanting. Theplants were harvested 60 DAT and storageroots were examined for presence of paint. InIsrael, the test was conducted in the green-house using field-grown ‘Georgia Jet’ trans-plants (14 replications). Transplants wereplanted in pots and carefully dug at 7 DAT.All adventitious roots were marked withpaint as described previously, before replant-ing. After 65 d, the transplants were har-vested and storage roots were examined forthe presence of paint.

Anatomical features of 5- to 7-d-oldadventitious roots. Thirty ‘Beauregard’transplants were planted (two to three nodesdeep) in 25.4-cm pots filled with unfertilized,washed sand. The pots were watered to fieldcapacity once everyday. At 5 d, adventitiousroots were harvested, placed in distilledwater, and sectioned immediately using arazor blade under a stereoscope. Transversesections of 133 adventitious roots 8 to 15 mmthick were stained with toluidine blue andobserved under a microscope. Digital micro-

graphs were taken with a Motic Cam 352(Motic Instruments Inc., British Columbia,Canada). Twenty ‘Georgia Jet’ transplantswere planted (two nodes deep) in 17-cm potsfilled with unfertilized, washed sand. Thepots were watered to field capacity once eachday with a 1:10 diluted liquid 7N–3P–7Kfertilizer (Shefer No. 3; Fertilizers and Chem-icals Ltd., Haifa, Israel). Adventitious rootswere harvested after 7 d and assayed asdetailed previously for ‘Beauregard’ trans-plants, except that sections were immersedfor 1 min in 10 M NaOH followed by 1 min in50% acetic acid before staining with tolui-dine blue. Protoxylem element number wasclassified into tetrarch, pentarch, hexarch,septarch, octarch, ennearch, and decarch for‘Beauregard’ and ‘Georgia Jet’.

Anatomical featuresofdevelopingadventitiousroots. Thirty ‘Beauregard’ and ‘Georgia Jet’plants were established in pots and 10 plantswere sampled 5, 19, 26, and 32 DAT in theUnited States and 7, 21, 28, and 35 DAT inIsrael. For each sampling date, all adventi-tious roots were serially sectioned and obser-vations were generally recorded from theproximal 3-cm section of the root. Wherenecessary, lignification was confirmed by

examining cross-sections of the rest of theroot sample. The following anatomical fea-tures were observed: protoxylem elementnumber, initial development of the regularvascular cambium, completion of regularvascular cambium, appearance of anomalouscambium, and lignification of more than 50%of the stele. Adventitious root counts in theproximal two nodes were recorded for eachsampling date. For the purpose of this study,we used ‘‘regular vascular cambium’’ fol-lowing the terminology of Wilson and Lowe(1973). We also used ‘‘anomalous cambium’’as a generic reference to ‘‘anomalous primary’’and ‘‘anomalous secondary’’ cambia. Anoma-lous primary cambia originated around thecentral metaxylem cells and each discreteprotoxylem element (Esau, 1965; Wilson andLowe, 1973). Anomalous circular secondarycambia originated around secondary xylemelements (Esau, 1965; Wilson and Lowe,1973).

Results

Relationship between adventitious rootsand storage roots. The paint applied onadventitious roots 5 to 7 DAT was evident

Fig. 1. Distribution of adventitious root protoxylem element number in ‘Beauregard’ (BX) and ‘GeorgiaJet’ (GA Jet) transplants sampled at 5 and 7 d after transplanting, respectively. ‘Beauregard’transplants were obtained from bedded roots, whereas ‘Georgia Jet’ vine cuttings were obtained fromfield-grown plants. The number of transplants sampled was 30 and 20 for BX and Ga Jet, respectively;the error bars indicate the SE of the mean.

Fig. 2. Adventitious root count in ‘Beauregard’ (Bx) and ‘Georgia Jet’ (GA Jet) transplants sampled atvarious dates. ‘Beauregard’ transplants were obtained from bedded roots, whereas ‘Georgia Jet’ vinecuttings were obtained from field-grown plants. The number of transplants sampled was 10 for both BXand Ga Jet; the error bars indicate the SE of the mean.

652 HORTSCIENCE VOL. 44(3) JUNE 2009

up to 9 weeks after transplanting. Resultsshowed that 80% of the greenhouse-grown‘Beauregard’ storage roots had paint mark-ings (33 of 41 total storage roots), whereas86% (36 of 42 storage roots) had paintmarkings when potted plants were grownunder natural conditions. A high proportion(89%) of ‘Georgia Jet’ storage roots had paintmarkings. These results indicated that, inboth ‘Beauregard’ and ‘Georgia Jet’, a highproportion of the storage roots developedfrom adventitious roots that formed withinthe first week after planting.

Anatomical features of 5- to 7–d-oldadventitious roots. In ‘Beauregard’, the num-ber of primary xylem poles varied betweenfive and eight; in ‘Georgia Jet’, the range wasfrom five to 10 (Fig. 1). The outer trachearyelements at each pole represent protoxylem,whereas the inner metaxylem represents twoor three large metaxylem elements in thecentral part of the vascular cylinder.

Root central cylinders with a vascularsystem consisting of pentarch and hexarcharrangements accounted for a combined 90%of the total number of ‘Beauregard’ roots 5DAT (Fig. 1). The arrangements of the centralcylinder in the remaining roots were septarch(5%) and octarch (5%). In contrast, hexarch andseptarch steles accounted for 82% of the totalnumber of ‘Georgia Jet’ samples. The remain-ing roots were pentarch (12%), octarch (5%),ennearch (1%), and decarch (2%) (Fig. 1).

Anatomical featuresofdevelopingadventitiousroots. Adventitious roots were characterizedat 5, 19, 26, and 32 DAT and 7, 21, 28, and 35DAT for ‘Beauregard’ and ‘Georgia Jet’,respectively. The adventitious root countssummed across the proximal two nodes variedfor the two varieties at 5 to 7 DAT; ‘Beau-regard’ averaged 3.5 adventitious roots perplant, whereas ‘Georgia Jet’ had 12.3 (Fig. 2).The adventitious root count was similar in thetwo varieties at 26 to 35 DAT.

‘Beauregard’ and ‘Georgia Jet’ rootssampled at three developmental stagesshowed various anatomical features relatedto root thickening. Root thickening started asthe initiation of the vascular cambium fromundifferentiated procambium cells betweenthe primary xylem and phloem (Fig. 3A). Themeristematic activity of the vascular cam-bium led to the production of secondaryxylem and phloem and the gradual comple-tion of a continuous cambial ring (regularvascular cambium) (Fig. 3B).

The parenchyma cells that resumed mer-istematic activity around the trachea weredefined by Wilson and Lowe (1973) asprimary cambium when the repeated celldivision occurred around trachea of the pri-mary xylem. Secondary cambia were definedas repeated cell divisions around the tracheaof the secondary xylem. The repeated divi-sion of newly formed cambia [defined byWilson and Lowe (1973) as anomalous cam-bium] led to the formation of rows of thin-walled parenchyma cells that formed thestorage tissue of the swelling root.

The formation of primary and secondarycambia (anomalous cambium) encircling

many of the primary and secondary xylemelements, respectively (Fig. 3C), was the firstclear sign of the developmental transition ofthe root to a storage root. Intensive lignifica-tion of the parenchyma cell walls of the pithand pith ray parenchyma usually occurred inroots that did not develop into storage roots(Fig. 3D). Development of the vascular

cambium, i.e., initiation and completion ofa regular vascular cambial ring, was observedin both varieties at 19 to 21 DAT (Fig. 4A).During this period, 49% of ‘Beauregard’ (19DAT) and 83% of ‘Georgia Jet’ (21 DAT)adventitious roots exhibited initiation of regu-lar vascular cambium. Storage root initiation(secondary cambium activity around the

Fig. 3. Transverse sections derived from different locations of ‘Beauregard’ and ‘Georgia Jet’ sweetpotatoadventitious roots showing (A) initiation of vascular cambium, (B) completion of vascular cambiumdevelopment, (C) appearance of anomalous cambium, and (D) lignification of the stele section. Theadventitious root samples were harvested starting at 26 d after transplanting and sections were removed3 cm from the proximal end. RVC = complete regular vascular cambium (following terminology byWilson and Lowe, 1973); MA = meristematic activity; AC = anomalous cambium; LC = lignifiedcells). ‘Beauregard’ transplants were obtained from bedded roots, whereas ‘Georgia Jet’ vine cuttingswere obtained from field-grown plants. Scale bars = 0.5 mm.

HORTSCIENCE VOL. 44(3) JUNE 2009 653

xylem elements) was observed in 8% of‘Beauregard’ samples, whereas none wasobserved for ‘Georgia Jet’. At 26 to 28DAT, formation of complete regular vascularcambium was negligible for ‘Beauregard’(4%) in comparison with ‘Georgia Jet’(32%) (Fig. 4B). In this period, the proportionof ‘Beauregard’ samples that developed sec-ondary cambium activity (30%) was twice thatof ‘Georgia Jet’ (13%). In the same period,40% to 48% of ‘Georgia Jet’ and ‘Beauregard’samples were lignified. At 32 and 35 DAT,adventitious roots of nearly 62% of ‘Beaure-gard’ and 70% of ‘Georgia Jet’, respectively,were either lignified or had become initiated

(Fig. 4C). In the same period, the remainder of‘Beauregard’ and ‘Georgia Jet’ adventitiousroots either showed complete regular vascularcambium (‘Beauregard’ = 28%, ‘Georgia Jet’ =17%) or were in various stages of primaryvascular cambium development (‘Beaure-gard’ and ‘Georgia Jet’ = 15%).

Discussion

The majority of ‘Beauregard’ and ‘GeorgiaJet’ sweetpotato adventitious roots that wereinitiated as early as 5 to 7 DAT possessed theanatomical characteristic [five or more pro-toxylem poles (Artschwager, 1924; Belehu

et al., 2004; McCormick, 1916; Togari, 1950;Wilson and Lowe, 1973)] associated withdevelopment into storage roots. Furthermore,our data suggest that adventitious roots extantat 5 to 7 DAT accounted for up to 86% and89% of the storage roots formed in ‘Beaure-gard’ and ‘Georgia Jet’, respectively, 9 weeksafter transplanting. Assuming all the paintmarkings remained on the adventitious roots,the remaining unmarked storage roots likelyoriginated from adventitious roots that wereinitiated after 5 to 7 d. Our findings suggest thatadventitious roots initiated as early as 5 to 7d were not merely ‘‘feeder’’ or ‘‘fibrous’’ roots;rather, these adventitious roots represented apool of potential storage roots that ultimatelyaccounted for 85% to 89% of storage rootsunder our experimental conditions. Previousreports suggested that storage root initiationdid not start until 42 DAT (Onwueme, 1978) oras late as 60 DAT (Edmond and Ammerman,1971).

Our results suggest that transplant-relatedas well as environmental and managementvariables at transplanting could have a sig-nificant impact on early storage root devel-opment. For instance, Kano and Nagata(1999) documented that virus-infected sweet-potatoes initiated fewer but larger roots incomparison with virus-free plants. They alsoobserved that the pith and vascular bundlecylinder at the node were larger in the virus-free plants in comparison with virus-infectedplants. It is well documented that virusinfection can reduce the yield of sweetpota-toes (Carroll et al., 2004; LaBonte et al.,2004). The use of sand culture medium in ourstudies allowed us to extract and examineadventitious root samples with minimal dam-age. This method can be adopted to includeexperimental treatments such as fertilizer,herbicides, insecticides, and plant growthregulators targeting the first week of growth.

Ravi and Indira (1999) reviewed storageroot initiation in sweetpotatoes and concludedthat the timing of this event varied widelyamong cultivars and took place from 7 to 91DAT. Under our experimental conditions,anomalous cambia (storage root initiation)began to appear at 19 and 21 DAT in ‘Beau-regard’ and ‘Georgia Jet’, respectively. Thesetwo varieties differed in the magnitude andtiming of the development of anomalous andregular vascular cambia. ‘Beauregard’ formedanomalous cambia to a greater extent earlierthan ‘Georgia Jet’. In contrast, regular vascu-lar cambium formation was delayed in ‘Beau-regard’ in comparison with ‘Georgia Jet’.Lowe and Wilson (1974a, 1974b) showed thatsome cultivars reduced their dependence onthe formation of anomalous cambia for stor-age root initiation by developing a more activeregular vascular cambium. They suggestedthat storage root shape and size were impactedby the differential activities of the two cambia.In ‘Beauregard’, 47% of adventitious rootswere lignified at 26 DAT. In ‘Georgia Jet’,nearly 60% of adventitious roots were ligni-fied at 35 DAT. This indicates that under ourexperimental conditions, lignification reducedyield potential by nearly half at 26 and 35

Fig. 4. Distribution of adventitious root anatomical features among ‘Beauregard’ (BX) and ‘Georgia Jet’(GA Jet) transplants sampled at the following days after transplanting (DAT): 19 and 21 (A), 26 and 28(B), and 32 and 35 (C), respectively. NC = no cambium activity detected; RVCI = primary cambiumactivity, appearance of cambium initials and initial growth of cambium; RVC = complete regularvascular cambium (following terminology by Wilson and Lowe, 1973); RVC + AC = completed RVCand appearance of anomalous cambium; L = lignified stele. ‘Beauregard’ transplants were obtainedfrom bedded roots, whereas ‘Georgia Jet’ vine cuttings were obtained from field-grown plants. Thenumber of transplants sampled was 10 for both BX and Ga Jet in all sampling dates; the error barsindicate the SE of the mean.

654 HORTSCIENCE VOL. 44(3) JUNE 2009

DAT in ‘Beauregard’ and ‘Georgia Jet’,respectively. Togari (1950) stated that theenvironmental conditions during the first 20DAT were important in determining storageroot initiation. Togari (1950) documented thatexcessive applications of nitrogen fertilizer incombination with reduced soil aerationdecreased cambium activity and increasedlignification. This indicated that managementand environmental variables during the first 35DAT contributed to the proportion of adven-titious roots that were initiated and becamelignified. Other than the published work ofTogari (1950), very little is known about theconditions or mechanisms that cause lignifi-cation in developing sweetpotato adventitiousroots. Wilson and Lowe (1973) cited anunpublished study that documented that pithand cortical cells of storage roots did notexhibit peroxidase activity, a normal require-ment for lignin biosynthesis. Informationabout management and environmental varia-bles that predispose adventitious roots tolignification might be used to develop meth-ods or modify existing management practicesto reduce risk associated with lignificationthereby increasing potential storage root yieldin ‘Beauregard’ and ‘Georgia Jet’.

There were considerable differences in theinitial (5 to 21 DAT) adventitious root count inthe proximal two nodes of ‘Beauregard’ and‘Georgia Jet’. The initial ‘Beauregard’ rootcount was relatively low and peaked at 26DAT; ‘Georgia Jet’ started with a higher countand decreased thereafter. A similar trend wasreported by Lowe and Wilson (1974a) amongWest Indian sweetpotato varieties. Theirreport showed that total root number generallypeaked at 14 to 28 DAT and subsequentlydecreased in five of the six varieties that theystudied. There was no information about thenumber of nodes in the study by Lowe andWilson (1974a). Nakatani and Komeichi(1991) also reported variability in root countsof thick and ‘‘tuberous’’ roots in ‘Koganesen-gan’ and ‘Beniako’ (basal three nodes) varie-ties and I. trifida (basal two nodes). However,their measurements did not begin until thefourth week after planting and the period ofmaximum root count varied among the twovarieties and I. trifida. We are unaware of anypublished work that explains the variability ofadventitious root numbers in sweetpotatoes,especially within the first week of transplant-ing. In Arabidopsis, Konishi and Sugiyama(2003) documented that auxin and tempera-ture influenced the formation of adventitiousroots in temperature-sensitive Arabidopsismutants. Takahashi et al. (2003) also docu-mented sugar-induced adventitious rooting inArabidopsis seedlings. Sorin et al. (2005)

studied Arabidopsis mutants with alteredadventitious root formation capabilities andstated that adventitious rooting was a quanti-tative trait controlled by environmental (light)and endogenous factors. Nakatani and Komei-chi (1991) reported variability in indole aceticacid (IAA) levels over a period of 15 weeksin adventitious roots of ‘Koganesengan’ and‘Beniako’ sweetpotatoes but did not correlatethe variability in the number of roots with thetemporal variation in IAA.

Conclusion

The majority of adventitious roots thatwere initiated as early as 5 to 7 DATpossessed the anatomical characteristicsassociated with storage root development.These newly initiated adventitious rootsaccounted for nearly 86% to 89% of the totalstorage root count at 60 to 65 DAT. At 32 to35 DAT, nearly 60% to 68% of the adventi-tious roots were either lignified or developedanatomical features that are prerequisite tostorage root formation. The data suggest thatin ‘Beauregard’ and ‘Georgia Jet’, the periodspanning 5 to 35 DAT was critical in deter-mining whether adventitious roots becamelignified or initiated as storage roots. Theconfounding environmental and managementvariables in the experimental treatments rep-resented a major limitation of the currentwork. Future studies can focus on location-specific environmental and management var-iables within this period that either reducelignification or promote storage root initia-tion. Such studies could potentially reducefield-level variability associated with storageroot yields in sweetpotatoes and serve as abasis for developing phenological models foroptimizing storage root formation.

Literature Cited

Artschwager, E. 1924. On the anatomy of sweetpotato root with notes on internal breakdown. J.Agr. Res. 23:157–166.

Belehu, T., P.S. Hammes, and P.J. Robbertse.2004. The origin and structure of adventitiousroots in sweetpotato (Ipomoea batatas). Aust. J.Bot. 52:551–558.

Carroll, H., A. Villordon, C. Clark, D. LaBonte,and M. Hoy. 2004. Studies on Beauregardsweetpotato clones naturally infected withviruses. Intl. J. Pest Manage. 50:101–106.

Edmond, J.B. and G.R. Ammerman. 1971. Sweet-potatoes: Production, processing, marketing.AVI Publ. Co., Westport, CT.

Esau, K. 1965. Plant anatomy. 2nd Ed. John Wiley& Sons, Inc., New York, NY.

Hernandez, T.P. and T. Hernandez. 1969. Irrigationto increase sweet potato production, p. 36. In:Tai, E.A., W.B. Charles, and P.H. Paynes

(eds.). Proc. Int. Symp. Trop. Root Crops; St.Augustine, Trinidad; 2–8 Apr. 1967. Vol. 1.

Kano, Y. and R. Nagata. 1999. Comparison of therooting ability of virus infected and virus-freecuttings of sweet potatoes (Ipomoea batatasPoir.) and an anatomical comparison of roots. J.Hort. Sci. Biotechnol. 74:785–790.

Kays, S.J. 1985. The physiology of yield in thesweet potato, p. 79–132. In: Bouwkamp, J.(ed.). Sweetpotato products: A natural resourcefor the tropics. CRC Press, Boca Raton, FL.

Kokubu, T. 1973. Thremmatological studies on therelationship between the structure of tuberousroot and its starch accumulating function insweet potato varieties. Bull. Fac. Agr. Kagosh-ima Univ. 22:1–126.

Konishi, M. and M. Sugiyama. 2003. Geneticanalysis of adventitious root formation with anovel series of temperature-sensitive mutants ofArabidopsis thaliana. Development 130:5637–5647.

LaBonte, D., C. Clark, A. Villordon, J. Cannon, M.Hoy, M. Sistrunk, E. Freeman, and G. Roberts.2004. Yield of four generations of virus-testedsweetpotato. HortTechnology 14:320–322.

Lowe, S.B. and L.A. Wilson. 1974a. Compara-tive analysis of tuber development in sixsweet potato [Ipomoea batatas (L.) Lam.]cultivars. 1. Tuber initiation, tuber growthand partition of assimilate. Ann. Bot. (Lond.)38:307–317.

Lowe, S.B. and L.A. Wilson. 1974b. Comparativeanalysis of tuber development in six sweetpotato [Ipomoea batatas (L.) Lam.] cultivars2. Interrelationships between tuber shape andyield. Ann. Bot. (Lond.) 38:319–326.

McCormick, F.A. 1916. Notes on the anatomy ofthe young tuber of Ipomoea batatas Lam. Bot.Gaz. 61:388–398.

Nakatani, M. and M. Komeichi. 1991. Changes inthe endogenous level of zeatin riboside, absci-sic acid and indole acetic acid during formationand thickening of tuberous roots in sweetpotato. Jpn. J. Crop. Sci. 60:91–100.

Onwueme, I.C. 1978. Tropical tuber crops. JohnWiley and Sons Ltd., Chichester, NY.

Ravi, V. and P. Indira. 1999. Crop physiology ofsweet potato, p. 277–316. In: Janick, J. (ed.).Horticultural reviews. Vol. 23. John Wiley &Sons, Inc., New York, NY.

Sorin, C., J.D. Bussell, I. Camus, K. Ljung, M.Kowalczyk, G. Geiss, H. McKhann, C. Garcion,H. Vaucheret, G. Sandbert, and C. Bellini. 2005.Auxin and light control of adventitious rootingin Arabidopsis require ARGONAUTE1. PlantCell 17:1343–1359.

Takahashi, F., K. Sato-Nara, K. Kobayashi, M.Suzuki, and H. Suzuki. 2003. Sugar-inducedadventitious roots in Arabidopsis seedlings. J.Plant Res. 116:83–91.

Togari, Y. 1950. A study of tuberous root forma-tion in sweet potato. Bul. Nat. Agr. Expt. Sta.Tokyo 68:1–96.

Wilson, L.A. and S.B. Lowe. 1973. The anatomy ofthe root system in West Indian sweet potato[Ipomoea batatas (L.) Lam.] cultivars. Ann.Bot. (Lond.) 37:633–643.

HORTSCIENCE VOL. 44(3) JUNE 2009 655