Scientia Horticulturae, 50 ( 1992 ) ~ 1-33 21 Elsevier Science Pub]ishczs B.V., Amsterdam

Growth of somatic embryos of sweet potato (Ipomoea batatas (L.) Lam) in hydroxyethyl

cellulose gel amended with salts and carbohydrates*

Jona than P,. Schultheis and Danie l J. Cant l i f fe

Vegetable Crops Department. University of Florida, Gainesville, FL 32611. USA

(Accepted 30~.'t ober ! 991 )

ABSTRACT

Schultheis. J.R. and Cantliffe, D.J., 1992. Growth of somatic embryos of sweet potato (lperaoea ba- tatas ( L ) Lam ) hi h yd roxyetoyl cellulose gel amended with salts and carbo~hydrates. Scientra Hor- tic., 50: 21-33.

Rapid plantiet development zs critical for successful direct-field sowing of somatic embryos. Sweet potato somatic embryos were placed in hydroxyethyl cellulose gel ( 2.5% w/v ) that was amended with different sources and concentrations of nutrients and carbohydrates to optimize plant formation. Greater root. shoot, and plantlet formation occurred in gel amended with Murashige and Skoog's (MS) or Gam~o.~ s salts compared with White's or Hoagland's salts. Murashige and Skoog's salt covcet~tration~ ~! one-half and full-strength resulted in 40% plantlet production, while at one-quarter- ~trength 80% of the somatic embryos produced roots but no shoots. Both the macronutrients and mlcronutrients of Murashige and Skoog were nccessar~ for maximum p.ant conversion in the gel. whde the addit ;~on oftbe MS vitamins resulted in no significant improvement. Plantlet formation was greatest in gei amended with fructose, maltose or sucrose as a carbohydrate source. Embryos would not survive without these carbohydrates amended to the get or with galactose. Concentrations of fruc- tose or sucrose in the gel between 23 and 187 mmol resulted in the most rooting, while concentrations ranging from 23 to 93 mmol were best for plantlet formation. By amending a transfer gel for direct fluid sowing, over 90% of the somatic embr'jos commonly formed roots and up to 50% of the embryos formed normal plantlets.

Keywords: arttficial seed; fluid drilling; plantlets; somatzc embryogenesis; synthetic seeding.

Abb:cviations: ~,4-D=2.4-dichlcropheno~yacenc acid; MS~Murashige and Skoog.

Correspor'~ence to: J.R. Sch dtl:eis. North Carolina State University, Department of Horticul- tural Science, Raleigh, NC 2~1695-TOU9,USA. *Florida Agt ~cuhural Experimental Station Journal Series no. R-01389. Mention of proprietary products is for con venlcnce ~f lhe reader only and does itot constitute endorsemcn| by the. U~ii- verity of Fk~rida,

© 1992 EIs~¢ier Science Publishers B V, All rights reserved 0304-4238/92/$05.00

22 J.R SCHULTHEIS AND l~.J. CANTLIFFE

INTRODUCTION

Commercial plantings of sweet potato are established using vegetative propagation. Mass plantings of sweet potatoes fo- f~od or oiomass produc- tion is limited by these planting costs. In some cases, these costs would exce~ d the returns, especially if the bicmass were used for meth~.ne (Cantliffe et ai., 1987). This cost limitation could possibly be overcome by direct-field sowing of propagules produced via somatic embryogenesis.

Use of somatic embryogenesis in a seeding system offers several potential advantages. These are the large quantities of propagules produced in limited space, maintenance of genetic uniformity, rapid propagde multiplication, and direct planting of somatic embryos into the field~ thus eiiminati~,g costly transplanting (Fujii et at., 1987 ), and reduced incidence of disease ( Cantliffe et al., 1987).

Somatic embryos have been routinely obtained in sweet potato (Liu and Cantliffe, 1984; Ch6e and Cantliffe, 1988). Although somahc embryos are nearly identical morphologically to zygotic embryos (Steward, 1963; Weth- eraU and Halperin, 1963 ), the former lack protective seed coats and the stor- age reserves that are available to zygotic embryos during germination. The suspension of somatic embryos in a viscous gel (fluid drilling system) sup- plied with "growth additives" has been suggested as a synthetic seeding method (Drew, 1979; Cantliffe et al., 1987 ).

Most reviews dealing with somatic embryogenesis have concentrated on developmental embryogenesis (Tisserat et al., 1979; Ammirato, 1983) and have given little attention to the efficient rooting and plantlet formation of somatic embryos. Reports which have concentrated on root and planttet re- sponse to various nutrient and carbohydrate formulations ,Jemonstrate var- ied responses among species (Rengel and Jelaska, 1986; Lazzeri e~ at., 1987 ) or even within species (Lazzeri et al., 1987; Redenbaugh et al., 1987). Im- proved plantlet formation via amendments to a gel carrier for direct-field sowing of sweet potato would be an important step towards somatic embryo seeding systems. This was the purpose of this work.

MATERIALS AND METHODS

Embryo production. - Embryogenic callus was initiated from s'~eet potato, eultivar 'White Star', shoot apices and proliferated as d ~cribed previously (Ch6e and Cantliffe, 1988 ) on 20 ml agar-solidified basal me4ium containing

GROWTH OF SUM ~TIC EMBRYOS OF SWEET POTATO 2 3

10/~mol 2,4-dichlorophenoxyacetic acid (2A-D), The basal medium con- tained the inorganic salts of Murashige and Skoog (1962), 500/~mol myo- inositol, 5/~mnl t h i a m i n e - H a , 10 gmnl niconnic acid, 5 pmol pyridoxine- HCI, 87 mM (3% w/v) sucrose, and 0.7% w/v Phytagar (GIBCO, Grand Island, NY). The medium was adjusted to pH 5.8 using 1.0 N NaOH and then autoclaved for 20 rain at 121 ° C. Cultures were maintained in the dark at 27 ° C with unmonitored light interruptions during daily observation.

Embryogenic callus was removed from plates and collected in beakers after 6-8 weeks. The callus "~as fragmented by hand into natural ceJ1 aggregate sub- units by dividing with a glass slide, then separated using copper sieves into a 355-710 #m fraction and a greater than 710 #m fraction. The greater than 710/~m fraction was plated for embryo production on basal medium contain- ing no growth regulators, 10 mmol ammonium nitrate and 0.8% (w/v) Phy- tagar, The erntnyos were incubated for one week in the dark, then placed in a 10/14 h light/dark cycle at 27+2°C. Light intensity was 250/zE m -2 s - k Embryos were selected for experimentation at the torpedo stage of develop- ment after 21 days. All gel additive studies for plantlet formation were con- ducted in vitro. Embryos were suspended in the viscous gel.

N u t r i t i o n a l s t u d i e s . - The treatment media solutions were mixed with 87 mM sucrose and the pH adjusted to 6.5 prior to the addition of 2.5% w/v hydroxy- ethyl cellulose gel ( N-gel, Aqualon, Wilmig~on. DE ) ( Schnltheis ct al., 1990 ). Gel solutions were autoclaved for 20 rain, then 25 mt of each gel treatment was poured into presterilized 25 × 100 mm Petri plates (Lab-TeL Nunc, Na- perville, IL). Gel nutrient concentrations were varied to improve the rate and/ or percentage of root, shoot and plantlet production. The first study com- pared gel amended with Hoagland's salts (Hoagland and Arnon. 1950), Gamborg's B5 salts (Gamborg et aL, 1968 ). Whke's salts as reported by Singh and Kaikorian ( 1981 ), and Murashige and Skoog (MS) salts (Murashige and Skoog, 1962). A gel with no inorganic salts served as a control treatment. In a second study, the strength of MS salts was varied to 0, 0.125, 6.25.0.5 of full s~.rength and full stren~h. In a third study, eomponems of our basal me- dium were sequential!y deleted so that thc requirements for different com- ponents of the media could be compared with regard to plantlet development. These treatmeo~g included: ( 1 ) macronutrients and micronutrients plus vi- tamins; (2) macronutfients and micrunutrients, (3) macronutrients only; (4) NH~NO3, KH2PO4, and KNO3; ( 5 ) no nutrients (control).

C a r b o h y d r a t e s t u d i e s . - F~ctose, galactose, glucose, maltose, and sucrose sugars were compared to determine the most effective kind and concentration of carbohydrate (s) for maximum root, shoot and plantlet prod a~ion. Each carbohydrate source was tested at 47 and 93 mmol and MS basal medium was incorporated in the gel aleng with each carbohydrate treatmem.

2 4 J.R, Sf - iULTHEIS AND D J . C# ~ ~ L'- FI:

Another study was conducted to define the fructose and sucrose concentra- tions most conducive for seedling growth. Carbohydrate concentrations were 0, 23, 47, 93, 186 and 374 mmol sucrose or fructose.

Data collection and statistical analysis. - A root of at least 2 m m in length was counted as normal, while a root plus an epical bud and at least one well-sep- arated leaf primordia (which later formed a plant) was counted as a plantlet (Fig 1 ). The rate and uniformity of root and plantlet development from so- matic embryos were determined by a modification o f the method of Gerson and Honm a (1978)

Root or plantlet forma*.ion

Sum of (days to development) x ( no. developed) Total number developed

Root and plantlet formation were recorded each day through D~y 10, then at 12, 14, 16, 20 and 21 and /o r 28 days after placing the embryos in the gel.

Seedling growth was measured by counting the nodes on each plantlet, and taking fresh and dry weights. Plants were blotted dry with paper towels prior to taking fresh weights, then oven-dried at 70°C for at least 3 days before taking dry weights.

All studies were analyzed as a completely randomized design. Each repli- cate contained at least I 0 embryos ( see tables for specific numbers) . All data

Fig. L Plaatlel conversion of embr3'os. Example of an embryo with a root and developing shoot alx'x which was ccnsidered as able to form am plantlet at this stage of developmem.

~Jw .i~ ~JMATICEMBRYO, SOFSWEEIPO/ATO 25

that are reported as a percentage were arc-sine tranfformed. Least sigMficant el;if..*-. °~ a . d reg~re_s~ion a~'jzcz wzrc ~ d ,o d~.ect differences between treatments which evaluated quantitative measurements. Stodtes which com- pared means from qualitative treatments were separated using Duncan's mut- ilple-range test.

RESULTS AND DISCUSSION

In the studies which compared various nutrient salts, greater shoot and plantlet formations were obtained with both Gamborg's (Bs) and MS salts compared to either White 's salts or Hoagland's salts (Table I ). White's, Hoagland's, Gamborg'~, and MS~s salts contain 2, 15, 30, and 60 mM total N, respectively. Thus, the inclusion o f 30-60 mM N appeared to be of prime importance for sw~'et potato plantlct production from somatic embryos. Root formation was greater in Gamborg's media than Hoagland's or White 's me- dia. The speed and uniformity o f those embryos developing were not affected by nutrient salt differences. Murashige and Skoog salts were used in subse- quent experiments since they promoted planflet formation well in sweet taro, and were standard media in numerous previously published works (Liu and Cantliffe, 1984; Chre and Cantliffe, 1988; Schultbeis et a l , 1990). Mu- rashige and Skoog medium contains 20.6 times more ammonium than Gam- borg's medium, which may be critical for embryo-to-plant conversion. The ammonium form of N has been associated with improved production and quality of alfalfa somatic embryos as measured by improved plaotlet produc- tion (Stuart and Strickland, 1984).

TABLE I

E fleet of various media on the percentage of, and mean days to root. shoot and planflet formatior., of embryos placed in hydroxyethyl cellulose geP

Media z ; Root Shoot Plantlet

%4 Mean da~s Yo Mean days % Mean days

Gamborg (B~) t~Sa s 8.7a 26a 15.3a 25a 13.8a HoagMnd 43b 9.2a 3b 16.On lb 16.On Murashnge rind 59ab 9.5a 29a 13.6a 24a 13.8a Skoog While 48b 7.4a 3b 15.Oa 3b 15+0a

1Data combined from two experiments, 8 replicates. IO embryos per replicate. :Medna formulations contained only inorganic salts. ~Gel treatment contained 87 mmol sucrose ( control ) and embryos yielded no roots, shoots, or plantlets, 4Final percentage roots, shoots, t~r plantlets obtained from embryos were deterrained at 21 days. Sig- nificance determined after arc-sine transformation. SMeans separated within columlls using Duncan's multiple-range test. 5% ~cvcL

20 J g. SCHULTHBIS/~N D D J. CANTLIFFE

The number of embryos which formed roots was similar waen MS basal medium from an eighth to full strength was used (Fig. 2). Gels used in fluid drilling could supply nutriepts for early seedling growth of zygotic seeded spe- cies (Taylor, 1986; Schultheis cta:. , 1988 ). TI~¢ gel alone was not an effeczive source of nutrients in the present work. The final percentage of plantlets pro- duced in gels containing half-strength MS basal medium was comparable to that produced in full-strength medium (Fig. 2"~. However, ~s the concentra- tion of the basal medium was reduced, so was the percentage of plantlets pro- duced. Optimizing N for plantlet conversion again appeared important since full-strength and half-strength MS media contain 60 aad 30 mmol N, respec- tively. The mean days for root and plantlct production were similar regardless of MS concentration ( data not presented).

A third experiment was conducted to better define which constituents in the gel additive media were necessary for general plantlet development. More embryos rooted whea the macro ~a!ts of the MS m e d i u ~ were included in the gel, while root formation was less when salts with only N, P and K w e r e ,'.l-

corporated in the gel (Table 2). Shoot production and plantlct woduct ion were greater when all MS inorganic salts were added to the medium; however, vitamins did not improve plantict formation further. Many protocols used for plantlet production from somatic embryos have used a basal medium which has included vitamins and undefined substances such as nicotinic acid, pyridoxin-HCI, myoinosizoi, and th iaminc-HCl (Lazzeri ct al., 1987 ), and e~.seirl hydrolysate and t-glutamine (Gupta and Durzan, 1986). Our results

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10

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MUnASHIGF. ANO ~;KOOG MEDIUM COHCEN'tRATION

Ft$, 2, Effec! of concentrations of Murashige and Skoog salts+_SE on the percentage tool and plantlct production at 28 days• Data combined fro~ 'J~,'o e~.perlmen s, ~ ~¢plications with 10 embryos per tirst 4 rep cat ons and I I embryos pcl ~econd 4 replications.

G RO'~1"H OF SOMATIC EMBRYOS OF SWEET P O T A T O 27

TABLE 2

Effect of Murashige and ¢,~oog (MS) salts, and vttamins on the percentage of root-.., sh3ots and ptant- !e!s p~d,.'ced from embryos placed in hydroxyethyl cellulose gel'

Media Root Shoot Pl~tl.~i components 2

%3 Mean days % M,:an days % Mean days

No nutrtents 12b* 10.2ab 0c -~ 0c Icontrol)

NHaNO3, KNO3, 4b l 1.3a 0c - 0c KH2PO4 Macro salts 5la 7.0be 10b 8.5a 8b 8.2a Macro and micro 57a 6.3c 29a I 1.4a 22a 113a Macro and micro 6~ 7.5bc 26a l l.Ta 24a I l .~

organics

L Data combined from two experiments, 6 tep|it~t.es. 12 embD'os per replicate. 2Medm components were: macro salts= NI~NO~. KNO3. C~CI: 2H:~. MgSO47H~O. KH~PO(; mi- cro salts= KI. H~BO~, MnSO, H,O, ZnSO, 7H20. Na,MoO, 2H~O, CuSO~ 5H~O. ~-~d CoCh 6H:O Organics= myo-mositol, thtamin¢-HCl, nicotinic add, pyridoxin-HCL 3Final percentage roots, shoots, and plaatlets produced were determined after 28 days. Slgnifitmnce determined after arc-sine t ransformati~n. ~Means separated within columns using Duncan's multiple-range lest. 5% level. SNo shoots or plantlets.

do not support the use of vi tamins and myoinositol. Nutritional reserves that are typically found in zygotic seeds are potentially lower, possibly lacking, in somatic sweet potato embryos since cotyledons are not fully developed and no endosperm is present (Ch~e and Cantliffe, 1988 ). Other seed constituents such as protein were proportionally lower in .~,omatic versus zygotic rapeseed embryos (Crouch and Sussex, 1981 ).

The rate of plantlet formation may bc influenced more by the events that occur during embryo development than by the nutritional additives p l a i d in the gel at planting. Without the formation of either a root or shoot apical meristem during development, the embryos will fail to germinate or grew into a plantiet. Those factors ~eeded for roots may control the uniformity and rate of germination./~ uxin could be a key factor; for example, embryo germina- tion in alfalfa g,l~ influenced by exposure of cultures to 2,4-D (Stuart et al., 1985).

The necessity of producing high-quality somatic embryos cannot be over- emphasized. Redenbaugh et al. (1986) were able to increase embryo-to-plant formation to 32% by modifying embryo development with the addition of proline, maintaining a strict schedule for callus subculture, transfer of callus to regeneration medium half-way through the embryo regeneration period and providing a cold treatment just prior to embryo germination. The quality of sweet potato somatic embryos might be improved during the development

?8 LR. SCHULTHEIS ~ND D.J. CANTLIFFE

phase prior to embryo-to-plantlet formation+ and could result in faster, more efficient rooting and growth into plantlets.

Root formation was greater when fructose, maltose, or sucrose was added to the gel ( data not given). Embryos died in gel amended with galactos¢ or in gel without a carbohydrate source. When germination occurred, germination rate was not affected by the carbohydrate source. Shoot and planflet forma- t ion were greater with fructose, maltose or glucose. This may have been due to a more rapid uptake or metabolism of these carbohydrates by the embryo compared to sucrose (Verma and Dougall, 1977 ). Redenbaugh et at. ( 1987 ) compared a broad range of carbohydrates which included maltose, galactose, lactose, cellobiose, sucrose, glucose and fructose for their effects on plantlet conversion from somatic embryos in soybean (Medicago sativa) (Reden- baugh et al.. 1987); they found glucose was best. However, sucrose has been used as a carbohydrate source for somatic cmhryogenesis and plantlet pro- duction in many species (Raghavan, 1986 ).

To look at the differences b~tween a m~r, osaccharide and disacchafide sugar, a wider ra~.ge of cor, cee, t-ations was cval,~ated. The final percentage of root and plantlct formation was signtficantiy attectco lay the concentration of car- bohydrate, but was not as influenced by the source of carbohydrate at com- parative concentrations (Table 3). Embryos placed in gel with 0 and 374 mmol fructose and sucrose showed reduced root formation compared with concen- trations of 23-187 retool, Plantlet production was greatest when either car-

rFAOLE3

Effect of carbohydratv source and concentration on root and p]anllet f~, ii~dtlOn whei; zmbfyos were placed in hydroxyethyl cellulose gel after 28 days ~

Source Concentration (retool)

0 23 47 93 187 374 Mean(S)

% Root formation Fructose 7 82 85 84 82 8 587 Sucrose 7 86 84 87 78 I I 59 Mean (combined) 73 84 84 85 80 10

% Plantlct for~at ion Fructose 0 46 57 52 35 I 32 + Sucrose 0 53 49 48 32 0 30 Mealt (combined) 0 ~ $0 53 51 34 I

*Dnla c~tmblned from two experiments, 9 replications total; 13 embryos per 4 replications and I 0 embryo# per S r~plieatlons. ~LSD (0,0S) for t~mperins source means for percentage root formation = 4%. ~LSD (0,0J) for comparing concentration ~eans for percentage root formation = 7%, *LSD ( 0.0~ ) for comparin s source means Ior percentage plantlet formation = 4%. SLSD (0.05) [or comparing concentration means for percentage plantlel format ion = 80~.

29

bonydrate was added to gel at 23, 47 or 93 mmol. Concentrations from 23 to 187 mmoi which led to the greatest sweet potato plantlet formation in our studies, was also reported to have the same effect in chicory (Heirwegh et al., 1985 ) and sugarcane (Ahloowa!ia and Maretzki, 1983 ) somatic embryos.

The rate of early plantle, growth was evaluated at 23, 47, 93 and 187 mmol fructose and sucrose concentrations since ~'ew plantlets were produced at the other concentrations (Table 4). The rate of ruot formation was improved using sucrose, compared to fructose; however, the rates of plant!et appear- ance were the same for both sugars.

Fructose and sucrose concentrations less than 187 mmol slightly improved the rate and uniformity of root and plantlet development, but not the weight of plantlet formed. Higher concentrations led to improved plantlet growth ( Figs. 3 and 4 ). Linear trends ( higher carbohydrate ¢oncentratmns promoled increased growth) were significant for both sucrose and fructose for f r~h weight (Fi~. 3A ), dry weight (Fig. 3B), and the number of nodes on a piantlet for fructose only ( rig. 4 ).

The preceding experiments illustrate the need to supply both exogenous carbohydrate and inorganic nutrients to enable sweet potato somatic embryos to become plantlets when the embryos are suspended in hydroxyethyl cellu- lose gel. Specifically, under oar conditions, we found that more than 15 mmol N was required for the best sweet potato plantlet production. The ammonium form of N has been reported to play a significant role i n improving somatic

TABLE 4

Eff~! of carbohydrate source and concentration on mean days to root and plantlet fo .rm.='A~ whe~ embryos were placed in hydroxyelhyl cellulose gel I

Source Concent,aaon (mmol)

23 47 93 187 Mean (S)

Mean days to root formation Fructose I 0.8 I 0.0 10.2 12.9 I 1.0 z Sucrose 8.2 7.2 9.0 I 0. I 8.6 I~'ean (combined) 9.5 ~ 8.6 9.6 I 1.5

Mean days to plamlet formation Fructe~e 14.3 14.7 15.2 ! 83 15.64 Sucrose 16.4 12 5 15.8 14,6 14.8 Meart (combined) 15.3 s 13.6 t 5.5 16.4

I Data combined from two experiments, 9 rephcalions total; 13 embryos per 4 rephcalions and 10 embryos per 5 replications. :LSD ( 0.05 ) for comparing source mean~ for rnea~ days Iv root formation - I. 1 days. 3LSD ( 0.05 ) for comparing concentration means lot mean days to r not formation = 1.6 days. ~LSD (0.05) for comparing source means for mean days to plantlet formation = 1.7 days. LSD ( 0.05 ) for comparing concentration means for mean days to plantlet formation.= 2.4 days.

J.R. SCHULTHEI$ AND D.J CANTLIFFE

700 [ - - - ~ . . . . • ~ -~ ~ - z

6oo A s o o / ~ / i

4ool ~ T / " l

lOO ~ ~

23 47 S3 l a 7

mM CARBOHYDRATE

-- s o

.¢ 4o ¢. $ ¢. 3o

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B i i

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o b . . . . . . . . . J 23 4T 93 " 87

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Fig. 3. Plantlet weight in response to fructose and sucrose concentralions from embryos placed in hydruxycthy! cellulose gel after 28 days. Data combined from two experiments. 9 replica- lions, 10 embryos per first 4 replicattons and 13 embryos per remaining 5 replications. (A) fresh weight per plant. The regression equalions for the linear trends: fructose: y= 4.73 + 7.12x; suero~: £= - 278.93 + 25.497,-. (B) dry weight per plant, The regression equations for the linear t relld:~: ['raetose: ym -5,~g 4- {: ,75A'; ~ucrose: y= -- S 844- I O7~-,

embryo qual i ty and plunLlet p roduc t ion in alfalfa (Walker and Sato, 19g l ; S tuar t and Str ickland, 1984) and carrot (Wetherei l and Dougall , 1976) and its ef fect (s) on somat ic embryo and plant le t de ve lopme n t in sweet po ta to warrants fur ther study.

U n d e r field condi t ions , i t would be advan tageous not to include a earbo-

GROWTH OF SOMATIC EM~,R YOS OF SWEET !POTATO 31

d. e f

sL ~ ,

r l f ~

:t3 4? 9 ~ 1 8 7

mM CARBOHYDRATE

Fig. 4. Number of nodes per plant in response to fructose and sucrose concentrations from cmb~os placed in h)droxyethyl cellulose gel after 28 days. Data combined from two expcri- meres, 9 replicatmns, 10 embryos per first 4 replications and 13 embryos per remaining 5 r e~ licatians. Linear regression equation for fructose: y=0.71 +0.16x. No significant trends were found with sucrose.

hydrate in the gel due to its potential as a food source for pathogens. Reden- baugh et al. (1986) have suggested that a carbohydrate source is not required when working with somatic embryos from an exalbuminous crop which sup- plies carbohydrates primarily from the cotyledons. However, this does no', appear to be the case with sweet potato, as it is an albuminous crop req~ring a carbohydrate sourCe (Hayward, 1938). Redenbaugh et aL (1986) sh~w ~l, gt the endosperm supplies the storage reserve. Since sweet potato somatic em- bryos contain no endosperm, our work suggests that an exogenous source of nutrients and carbohydrates are needed. In addition, the inclusion of hor- mones, beneficial microbes t~ i~nprove embryo growth, or the inclusion of pesticides which could prevent growth of microbes that would inhibit embryo ~evelopment into plantlcts, warrant further consideration.

Placement of sweet potato somatic embryos in a transfer get amended with ~t least one-half strength MS inorgamc salts plus 47-93 mmol sucrose or fruc- tose resulted in over 90% of the ~omatic embryos forming fonts and up to 50% plantlet formation. Fluid drilling somatic embryos provides the oppor- tunity of planting the embryo as it is and bypasses the need to arrest embryo growth. A high percentage plantlet production was possible in gel with the proper amendments and hi ,gh-quality embryos. Syr~chr~i:y and gerrnination rate must still be improved. Production of higher quality zomatic embryos is imperative so that the embryos can withstand the harsh environmental con- ditions encountered when directly seedir~g in the field.

32 J.R. SCHULTHEIS AND DJ. CANTUFFE

ACKNOWLEDGEMENT

T h i s w o r k was s u p p o r t e d by a n I F A S / G a s R e s e a r c h Ins t i t u t e COOl~rative g ran t .

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