Plant Physiol. (1978) 61, 348-353

Shoot, Stolon, and Tuber Formation on Potato (Solanumtuberosum L.) Cuttings in Response to Photoperiod1

Received for publication April 6, 1977 and in revised form October 10, 1977

ELMER E. EWING2Department of Vegetable Crops, Cornell University, Ithaca, New York 14853PHILIP F. WAREINGDepartment of Botany and Microbiology, University College of Wales, Aberystwyth, Wales S Y23 3DA

ABSTRACT

The intensity of "tuberization stimulus" in potato shoots (Solanumtuberosum L.) can be assessed from cuttings containing one or more leaves.Cuttings maintained in a mist chamber under long days wil form tubersfrom underground buds If prior to taking the cutting the leaves receivedsufficient exposure to photoperiods less than the critical photoperiod. Thegreatest tendency to tuberize was found in cuttings that consisted of asingle, fully expanded leaf and its subtended bud. Grafts showed thatgenetical differences in critical photoperiod resided in properties of theleaf. Short days before cutking tended to shift growth from above groundbuds of two-node cuttings to below ground buds, even if the number ofshort days was insufficient for tuber induction. As few as 6 short daysreduced growth of shoots at the upper bud and increased undergroundgrowth of shoots and stolons.

however, and this trait can be accentuated by breeding (9, 21).In addition to short days, the tuberization stimulus is favored bycool temperatures (3, 11, 22, 24), by low rather than high rates ofN fertilization (24), and by "physiologically old" mother tubers(15, 18).

Grafting experiments (5, 10), especially the demonstration thateven a small piece of stem from an induced plant is capable oftransmitting the tuberization stimulus (14), suggest that the stim-ulus is hormonal in nature. The exact identity of the hormone(s),although extensively investigated, has yet to be established.

In this paper we report on the relationships between the expo-sure of potato foliage to particular photoperiods, the geneticallydetermined CPP, and the levels of tuberization stimulus as re-flected by tuberization of cuttings. We show that the CPP char-acteristic of a given clone is a function of properties of the leaf,not of the underground growing point; and we present evidencethat short days shift growth from above ground to below groundbuds, even if the number of short days is insufficient for tuberinduction.

Although potato tubers ordinarily form on underground stolons,every axillary bud on a potato stem has the potential to developa tuber. Nearly a century ago, Vochting (23) demonstrated thatsessile tubers developed at the covered nodes when cut pieces ofstem were inserted into soil. He also showed that tuber formationwas promoted by darkness and moisture. Recent evidence (16,17) suggests that elevated CO2 levels may also favor tuberizationat below ground rather than above ground buds. However, tubersdo not always develop even from underground buds of cuttings.As with whole plants, tuberization of cuttings is apparently con-trolled by an unknown "tuberization stimulus" (5, 10, 19). Whenthe stimulus is absent or weak, there is no tuberization. Whenthe stimulus is very strong, especially when there is interferencewith its transport to underground growing points, tubers maydevelop in the light (10, 23). This parallels the formation of"aerial tubers" commonly seen on plants in the field when injuryor disease interferes with translocation.The tuberization stimulus is favored by photoperiods shorter

(8) than a critical photoperiod, the length of which varies withgenotype (13, 15). Potatoes cultivated at high altitudes near theequator, Group Andigena, typically have a CPP3 of only 12 to13 hr. In contrast, Group Tuberosum varieties have undergonecenturies of selection for a CPP of 15 or more hr. It is possible tofind within Andigena populations clones of relatively long CPP,

' This study was funded principally by National Science FoundationGrant No. BMS 74-13999 and also received support from the InternationalPotato Center. The work was carried out while the senior author was atthe University College of Wales, on sabbatical leave from Cornell Univer-sity. Paper No. 734, Department of Vegetable Crops, Cornell University.

2 To whom requests for reprints should be addressed.'Abbreviation: CPP: critical photoperiod.

MATERIALS AND METHODS

Clones. Potato ( Solanum tuberosum L., Group Andigena) clonesused in this study were obtained through the Cornell Universityprogram to select from an Andigena population lines that haveTuberosum-like characteristics (6). Andigena accessions werescreened to obtain three clones with CPP of approximately 12 to13 hr. Populations of "Neo-Tuberosum" (9) were examined tofind clones with CPP in excess of 20 hr. Andigena populationssegregating for CPP were utilized to obtain matched pairs ofsibling clones for five replications of the experiment summarizedin Table I. Each matched pair consisted ofa clone with a relativelylong CPP and a clone with a relatively short CPP of the samepedigree. Different pedigrees were used for each replication. Intwo other replications of the same treatments, the long and shortCPP clones were not siblings. Rather, in these two replicationsthe long CPP clones were selected from Neo-Tuberosum and theshort CPP clones were unrelated Andigena types. There werefrom 14 to 36 plants of each CPP in each growth chamber perreplication. Part of each group was used for cuttings, and the restwere examined after 8 days for tuberization.

Initial screening for CPP consisted of sowing true seeds in flatsin March, transplanting the seedlings into 15-cm pots in April,and examining each plant for tuberization in July and September.Pots were maintained in an unheated greenhouse in the absenceof supplemental illumination. Plants that had tuberized by July31 (when the daylength was about 16 hr) were tentatively desig-nated long CPP. Plants that had not yet tuberized by September12 (daylength approximately 13.5 hr) were designated short CPP.All plants tuberized by September 27. The preliminary classifica-tions for long CPP were confirmed by observing the ability of

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POTATO CUTTINGS, PHOTOPERIODIC RESPONSES

the clones to tuberize in growth chambers under 16-hr photope-riods. Further observations were made on the ability of suchclones to tuberize in greenhouses where photoperiods were main-tained at 20 hr.

Clones were established from each seedling plant by makingapical cuttings. The resulting plants were maintained in a green-house with supplemental mercury vapor lighting to give a 20-hrphotoperiod until photoperiod treatments were started. Temper-atures in the greenhouse were approximately 25 C during the dayand 15 C at night.

Photoperiod Treatments. Plants were grown in the greenhouseunder 20-hr photoperiods and were moved to Saxhill controlledenvironment growth chambers for the experiments summarizedin Tables I and II. Plants in the chambers were given 10 days ofeither 8-hr or 16-hr photoperiods. Both treatments received 8 hrof exposure to fluorescent plus incandescent lights, which pro-duced a total irradiance of 299 w m-2, with a physiologicallyactive irradiance (400-700 nm) of 97 w m-2. The temperaturewas 20 C during the 8 hr of full light. For the 16-hr photoperiod,an additional 8 hr of low light intensity was given (19 w m-2 ofphysiologically active irradiance) from incandescent lights only.The temperature during the 8 hr of low intensity lighting andduring the dark periods was 16 C.

I rsohsrrt!in rarai",i"e wvv%arnavite xwar,, Unixvtl1AI1,ong poutoupmaintaining plantsillumination from rhr days in these exa completely darkcaveraged approxinmTemperatures in tgreenhouse.

Cuttings. Three 1cutting consisted cbeneath the fourthas the youngest onleaf was excised, anwith the basal axilcutting contained tleaf was inserted inconsisted of a fullypreliminary experi!was maximal if thewas taken to excispetiole was insertedbud was well cover

Table I. Effects of CPPground buds of

Each percentage is c

199 single leaf cuttingslong CPP distributed amo

CPP Photof 1(

clones befoi

ApicaZCuttings

Short IShortLong ILong

Single leafCuttings

ShortShortLongLong

In the experiment summarized in Table III, stem sectionscontaining the fifth and sixth nodes were cut 6 days, 3 days, orimmediately after exposing plants to 14 8-hr days. A fourthtreatment received continuous 20-hr days. In half of the cuttingsfrom each of these four treatments, the upper bud of the cuttingwas excised before placing the cutting in the mist chamber.The plotting medium was composed of equal volumes of coarse

sand and peat. A compost made of Vermiculite, sand, and peatplus appropriate fertilizers (4) was substituted in some replicationsof several experiments and appeared to give the same results asdid sand and peat. Cuttings were maintained under automaticallycontrolled mist nozzels on a sand bed with heating cables. Thephotoperiod over the mist bench was 20 hr in all experiments,and the air temperature was about 25 C during the day and 15 Cat night. Cuttings were removed and examined after 10 to 14days on the mist bench.

Grafts. Stems were cut at right angles to the main axis, piecesto be grafted were butted together, and the union was wrappedin Parafilm. Grafted cuttings were inserted into potting mediumand maintained in the mist chamber as described for other cut-tings.

RESULTSus1 -remam.g expenmens were acnieveu DJy Eight days after removal of plants from the 16-hr photoperiods,

i in greenhouses that received supplemental most of the long CPP clones had tuberized weakly and the shortmercury vapor lights to give 20-hr days. Eight- CPP clones had failed to tuberize. Both types tuberized well,periments were achieved by moving plants to within 8 days after exposure to the 10 8-hr days.m,ned room. Temperatures in the greenhouses Apical cuttings taken from similar plants at the end of the 10-iately 25 C during the day and 15 C at night. shatelydark room wereisimilath osd inathge day treatments could be divided into four categories: (a) cuttings

with no new growth at the underground bud; m these cuttingsof cuttings were employed. (a) An apical rooting was typically profuse and the apex had grown substan-typeshot ape had (a) An just tially; (b) cuttings with the underground bud developed into either

If the shlootheapex,tha hadntinb heeecisestn an above ground leafy shoot or an underground stolon; (c) cuttingsnode belo atheapx counting Thefisto with tubers at the end of a stolon; or (d) sessile tubers. Tuberized

ie withe autgleafaleaste35 mm lotingg basa cuttings had less new growth of roots and shoot apex than non-td the cutting was inserted in a potting medium tuberized ones.Ilary bud covered. (b) A subapical, two-node tueidons

thr budth c rd.(b)thAd subapic two-nodx.The The frequency of these four types of responses as affected bythe fottiftandsium.xth Asinglenodesbelow e ax.T treatment is presented in Table I. Tuberization on the cuttingsthexpotting medium.n(c)t Asienled nod.Becauti followed a similar pattern to that on intact plants. Apical cuttingsmexpndsshoedleaf tubetende bud. Bcauts taken from short CPP plants exposed to 16-hr days did not

neountsshowattaedthattuberization onsuch cutt tuberize, and only 10%1o of cuttings from long CPP plants formedamount otubers. Exposure of plants to 10 8-hr days prior to excision

e as much stem as possible. The base of the induced tubers on most of the apical cuttings from long CPPI into the potting medium so that the subtended clones and on 15% of those from short CPP clones. Beside thered and the leaf was upright. effects of 8-hr days on tuber induction, there was a striking effect

on growth of the underground bud, especially in the short CPPoapical and singoepeaf cuttings. clones. After 16-hr days, these buds not only failed to form tubers,calculated from at least 104 apical cuttings or at least they remained completely inactive. The 8-hr days stimulated well

There were nine clones of short CPP and nine clones of over half of the nontuberizing buds to grow out as leafy shootsng seven replications.

or stolons.

The responses of single node cuttings were similar to those ofResponse of underground axillary bud, apical cuttings, except that over-all, a greater proportion of the

% in each category

toperiod, Sessile Tubers on Shoots or No bud single node cuttings tuberized (Table I) and a higher percentageO days tubers stolons stolons only growth of nontuberizing buds grew as shoots or stolons.re cutting The tendency for inducing conditions to shift growth from

above ground to below ground buds could be observed mostreadily with two-node, subapical cuttings. Figure I illustrates a

typical progression from the completely noninduced condition

6-hr 0 0 4 96 (Fig. IA) through strong induction (Fig. ID). Changes are appar-8-hr 11 4 48 36 ent not only at the basal bud but also in the decreasing shoot16-hr 10 0 7 84 growth of the upper bud. Table II presents data from an experi-

ment containing 12 clones that varied in CPP from less than 13.54E><; !,* to more than 16 hr. As in previous experiment, half of the plants

received 8-hr and half 16-hr photoperiods prior to cutting. Expo-sure of plants to 10 8-hr days before cutting caused a shift of

16-hr 0 0 grwt grun ron8-hr 52 3 29 16 owth from the above ground to the below ground bud. There18-hr 21 0.5 40 38 was more reduction in growth of the upper bud if tubers formed

in response to the induction, but even the cuttings that failed to

Plant Physiol. Vol. 61, 1978 349

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EWING AND WAREING

FIG. 1. Two-node, subapical cuttings after 10 days in a mist chamber.The lower leaf is excised, and the subtended bud of the excised leaf isinserted into a potting medium. A-D represent increasing intensity of"induction" by exposure of the plants to short days before cuttings weretaken. Note small tuber at end of stolon in C and sessile tuber in D.

Table II. Effects of photoperiod t0 days prior tocutting on bud growth of 2-node, 8ub-apical cuttings.

Lower leaves were excised at time of cutting,and the bottom half of each cutting was insertedinto potting mix. Data (means + SE) are pooledfrom six long and six short CPP clones, dividedamong three replications.

Photoperiod Length of Length ofbefore cutting upper shoot lower shoot

rnn mm16-hr

Non-tuberized (36/36) 26 + 3 11 + 1

8-hrNon-tuberized (23/36) 15 + 3 44 + 5Tuberized (13/36) 8 + 2 (tubers)

tuberize showed a shift of growth from the upper to the lowernode.The shift from above ground to below ground bud growth could

be detected with as few as 6 8-hr days before cutting. Figure 2presents results from an experiment in which plants oftwo clones were exposed to 0, 3, 6, or 9 8-hr days before makingtwo-node cuttings. There were five plants per clone of eachtreatment in each of five replications. One clone had a CPP ofless than 13 hr, and the other had a CPP about 3 hr longer. (It

occasionally formed small tubers under 16-hr photoperiods.) Nineshort days constituted the threshold for tuber induction in theclone with the longer CPP-three of 35 cuttings in this treatmenthad swellings that were intermediate between tubers and stolons.There were no tubers from less than 9 short days, and cuttingsfrom the clone with the shorter CPP had no tubers in anytreatments. Length of buds at each node was measured at cuttingand after 11 long days in the mist chamber. Both clones respondedin a similar manner to the treatments, and results for the two are

averaged together in Figure 2. The total fresh weight of upperplus lower buds remained approximately constant, but 6 shortdays significantly decreased growth above ground and increasedgrowth under the soil (Fig. 2A). Elongation of the buds was

closely related to fresh weight (Fig. 2B).The experiments with two-node cuttings indicated an interde-

pendence between the effects of induction at the two nodes. Itwas therefore of interest to see whether excision of the upperbud at the time of taking the cutting would affect the response toinduction. We also wanted to test whether failure to take cuttingsimmediately after exposure to short days would affect the out-come. There was no detectable difference whether cuttings were

made immediately at the end of the 14 8-hr days or after either 3or 6 20-hr days had elapsed, and data from the three treatmentsreceiving 8-hr days were therefore pooled for comparison withthe long day treatment. There was no effect of upper bud excisionon tuberization (Table III), and excision of the upper bud in-creased growth at the lower bud whatever the daylength treatment.Induced cuttings (8-hr photoperiod) showed greater shoot andstolon growth at the lower nodes than noninduced cuttings even

No. of 8-hr Days Before CuttingFIG. 2. Changes in upper and lower buds on two-node cuttings from

plants exposed to varying numbers of short days (fewer than required fortuber induction). All cuttings received 20-hr days. The effects of numberof short days were all significant at the 1% level, whether measured interms of fresh wt (A) or change in bud length (B). Values for LSD.0o areindicated for upper and lower buds, respectively, by solid and brokenbrackets.

Table III. Effects of photoperiod 14 days prior to cutting and excising upperbud on growth of lower bud in 2-node, sub-apical cuttings.

Above ground buds were excised from half of the 2-node cuttings at time ofcutting. Long CPP clones were used in 3 replications and short CPP clones in 2replications. Percentages are based on 143 cuttings for 8-hr photoperiods(pooled data from 3 treatments) and on 39 cuttings for 20-hr photoperiods. Thelast column presents mean lengths (+ SE) of shoots and stolons at undergroundnodes that did not tuberize.

Response of underground bud

Photoperiod Status of Tubers Shoots or No Mean lengthbefore cutting upper bud (sessile or stolons growth of shoot

on stolons) only or stolon

% % % mm

20-hr Intact 0 8 92 3.6 + 0.420-hr Excised 0 77 23 26.1 + 3.38-hr Intact 56 16 28 13.1 + 2.08-hr Excised 58 4o 2 37.4 + 2.0

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POTATO CUTTINGS, PHOTOPERIODIC RESPONSES

was exposed to 10 8-hr days before removal of the leaf. Allaxillary bud tissue was excised from the base of the petiole attime of cutting. The underground portion of the petiole enlargedin spite of the absence of a bud. Such enlargements occurredonly on cuttings from plants exposed to 8-hr days, never on leavesfrom plants exposed to 20-hr days.

DISCUSSION

Various observations on whole potato plants suggest that thereis an antagonism between shoot growth and tuberization. Thushigh soil N levels and gibberellin, which promote shoot growth,tend to reduce tuber growth, while treatments which inhibit orreduce shoot growth, such as treatment with chlorocholine chlo-ride, promote growth of the tubers. Biran et al (1) proposed thatshort days promote the formation of tuberous roots in dahliathrough inhibition of top growth and consequent diversion ofassimilates to the root. The implication of their hypothesis is thatdaylength controls tuberization only if an alternate "sink" ispresent. This is not the case in potato. Whereas single leaf cuttingsof dahlia tuberize regardless of photoperiod (2), single nodecuttings of potato-like whole plants-tuberized only after expo-sure to days shorter than the CPP (Table I). The response tophotoperiod occurred in cuttings from fully expanded leaves thathad no buds except the one at which the tuber developed. Clonesthat failed to develop roots at the base ofthe cut petiole manifestedthe tuberization response to photoperiod just as did other clones

A

SI

FIG. 3. Plant of clone with exceptionally long CPP, grown under 20-hr days from cuttings. All underground buds were removed to preventtuberization below the soil. Leaves were removed just before photograph-ing to show aerial stolons and aerial tubers.

with excised upper buds (Table III).Through the use of cuttings, we have been able to identify

seedlings with very long CPP. Figure 3 shows plants of a cloneobtained from an examination of 244 Neo-Tuberosum seedlings.So prone were the cuttings from this clone to tuberize that it wasdifficult to keep the clone alive, even when grown under 20-hrphotoperiods. Cuttings tuberized strongly and failed to root unlessall buds below the soil surface were excised. Even then, foliagegrowth was poor unless the photoperiod was long and unlessaerial stolons and tubers were excised as they formed in axillarybuds. Two-node subapical cuttings were taken from the Neo-Tuberosum clone shown in Figure 3 and from an Andigena clonewhich had a CPP of less than 13 hr. All plants from whichcuttings were taken had been grown under 20-hr photoperiods.The lower leaf of each cutting was excised, and reciprocal graftswere made between the two types. Cuttings were kept in a mistchamber with 20-hr days. When the leaf (scion) was from thelong CPP clone, tubers formed at the underground bud of thestock (Fig. 4A). The reciprocal graft produced no tubers (Fig.4B), showing that long CPP is a property of the leaf, not of thereceptor bud. We repeated this experiment with similar results.

Plants that, like the one in Figure 3, are very strongly inducedfrequently develop swellings at the base of the petiole or in theadjacent stem, especially if the axillary bud has been excised.Swellings are especially common in the underground parts ofcuttings taken from strongly induced plants. Figure 5 shows asingle leaf cutting from a plant that had an 18-hr CPP. The plant

B

FIG. 4. Grafted cuttings taken from plants exposed to 20-hr photope-nods. The lower leaf on each stock was excised at time of cutting. Theportion below the graft union was inserted in potting mix. Cuttings weremaintained in a mist chamber under a 20-hr photoperiod for 12 daysafter cutting and grafting. A: scion CPP in excess of 20 hr, stock CPPabout 13 hr. Three of four stocks tuberized. B: reciprocal grafts fromthose shown in A. No tuberization.

351Plant Physiol. Vol. 61, 1978

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352 EWING AND WAREING Plant Physiol. Vol. 61, 1978

FIG. 5. Leaf excised from plant exposed to 10 8-hr days before cuttingand then maintained 9 days in mist chamber with the petiole insertedinto potting medium. Note that the base of the petiole enlarged eventhough no bud was present. Similar cuttings from plants exposed to 20-hr days showed no swelling.

(Ewing, unpublished data). Excision of the above ground bud ontwo-node cuttings had no effect on tuberization of the under-ground bud (Table III). These results indicate that competitionfor assimilates is not the primary factor controlling potato tuberinitiation.The CPP controlling tuberization of whole plants varies greatly

with genotype, and tuberization of cuttings follows the samepattern. Whether based upon tuberization of whole plants orcuttings, some genotypes require dark periods in excess of 11 hr,and others tuberize even when grown under constant illumination(Lazin and Ewing, unpublished data). One might assume that aclone of very long CPP has the ability to initiate tubers whengrown under long days because it has the capacity to form muchmore of the tuberization stimulus under these conditions than doother clones. An alternative hypothesis would be that the receptorbuds tuberize in a response to a very low (or zero) level of thestimulus. The results of the grafting experiment were consistentwith the first of these hypotheses. That is, genetical differencesin CPP apparently were associated with properties of the leafrather than with properties of the receptor bud.Chapman (5) concluded that the stimulus for tuberization was

produced in the terminal leaf cluster (leaves less than 5 cm long).Hammes and Beyers (12) found that the photoperiodic stimuluswas perceived by old leaves as well as young leaves. Whateverits origin, the tuberization stimulus is present in all mature leavesof induced plants. Even cuttings made from a leaf that is startingto senesce will tuberize (Ewing, unpublished data).The ability of a cutting to form tubers is relatively independent

of size of plant from which the cutting is taken (7). This suggeststhat cuttings could be employed in many types of potato researchto indicate the intensity of the tuberization stimulus. The mostobvious example would be in plant-breeding programs where itis desired to screen seedlings for ability to tuberize under variousdaylength and/or temperature regimes. Detailed procedures for

such screening are described elsewhere (7). Another example ofthe application of cuttings would be to assess the intensity of thetuberization stimulus throughout the growing season as affectedby source of seed tubers, date of planting, fertilization, or othervariables studied in field experiments. For all such studies itwould be necessary to maintain uniform conditions of light andtemperature in the mist chamber, since the tuberization on cuttingsis affected by the environment after as well as before cutting (10,20).The effects of photoperiod before cutting on development of

the cutting are not limited to tuberization. From examination ofmany hundreds of cuttings, it is our impression that with increas-ing exposure of plants to short days before cutting, there is afairly regular progression of responses at the underground budof the cutting. A complete lack of induction is associated with nogrowth of this bud. Minimal induction leads to growth of long,thin shoots that are negatively geotropic. Slightly stronger induc-tion produces more plagiotropic growth so that the shoot resemblesa typical stolon. (A frequent variation of this is that the mainaxillary bud grows upward as a shoot, and the smaller budsbeside it develop as stolons.) The next stage is a thickening ofthe stolon, and with still stronger induction the stolon is terminatedby a tuber. Very strong induction leads to a sessile tuber. TablesI and II show the quantitative effects of induction on undergroundbud growth.High levels of the tuberization stimulus seem to promote tuber-

like swelling of succulent tissues that do not contain apical mer-istem and that therefore lack the capacity to develop into truetubers. Like tuberization, such swelling is dependent upon expo-sure to photoperiods shorter than the CPP; and it appears to befavored by dark, moist conditions.The stimulation found in growth of the lower bud was at the

expense of the growth of above ground axillary buds. However,the growth of the lower bud cannot be explained solely as a by-product of the breaking of apical dominance. The excision of theupper bud did not eliminate the underground growth response toshort days, and the response was also observed in single nodecuttings. As few as 6 short days were sufficient to produce theshift from above ground to below ground growth.

If "weak induction" (limited exposure to days shorter than theCPP) stimulates growth of shoots and stolons at the undergroundbud, and if "strong induction" (more short days) stimulates growthof tubers, does this suggest that the stimuli of the two types ofgrowth are basically the same, except in intensity? (We use "in-tensity" to include the concept of a ratio of concentrations amongtwo or more factors as well as concentrationsper se.) Alternatively,do short days trigger two different stimuli, such as a rathernonspecific promoter of growth that responds to weak induction,and in response to stronger induction a more specific inhibitor ofapical meristem growth that might change the plane of celldivisions to produce tubers? Favoring the second hypothesis isthe common observation that clones grown under daylengthsslightly shorter than their CPP will produce prolific numbers ofstolons without tuberization. Definitive evidence is not availablefor choosing between these alternatives.

Acknowledgment-We thank R. L. Plaisted of the Department of Plant Breeding, CornellUniversity, for supplying the potato seeds used in these studies.

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Plant Physiol. Vol. 61, 1978 POTATO CUTTINGS, PHOTOPERIODIC RESPONSES 353

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