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Tissue culture micropropagation: We produced approximately 3500 liner plantlets of 26genotypes for replicated disease and pest screening tests by Greg Browne, Dan Kluepfel andMike McKenry and for growth and grafting in nurseries for subsequent orchard trials (Table 1).We also produced about 350 liner plantlets of 13 lines transformed for putative resistance tocrown gall plus non-transformed control lines for use in greenhouse screening re-tests for crowngall resistance by Dan Kluepfel (Table 1). This past year’s production, plus hold-over plantletsfrom the previous year’s production, gives us an inventory of about 5000 liner plantlets ready forimmediate use in replicated disease and pest resistance re-tests and in nursery row growth andgrafting for orchard trials (Table 2). Based on an experiment we reported in 2007 which showedimproved root structure of liner plantlets that had been severely root pruned at the time ofplanting in liners, we introduced root pruning into our production process on a trial basis. Table 3shows that root pruning at the time of planting reduced survival of both ex vitro and in vitrorooted plantlets. The reduction in survival was quite small (68 to 63%) for ex vitro rootedplantlets but larger (71 to 51 %) for in vitro rooted plantlets. We don’t have an explanation forwhy survival was reduced more by root pruning of in vitro, as compared to ex vitro, rootedplantlets. Based on these results we will continue root pruning as a standard practice but also tryto figure out why root pruning may reduce survival more for in vitro than ex vitro rootedplantlets.Cold storage of liner size plantlets derived from micropropagation is advantageous for at leastthree reasons: 1) storage conserves greenhouse bench space 2) storage at temperatures of 42-45ºFprovides the chilling necessary for maximum growth potential when plantlets are planted in thenursery row or orchard site 3) storage allows year-around laboratory production while permittingchoice of the most desirable time for nursery row or orchard planting.The length of time plants can be stored has been restricted because plantlets stored attemperatures around 42-45ºF begin to grow after about five months. Because storage occurs withonly very low light, the shoots that form are etiolated and the growth depletes the stored nutrientsrequired for nursery or orchard establishment and growth. In an effort to extend the length ofstorage time possible without shoot growth, we performed an experiment using a lower storagetemperature (33ºF). After an initial eight week storage period at 42-45ºF with short days and lowlight to cause leaf abscission and to put the plants in a dormant condition, the plantlets weretransferred to a cold room with no light at 33ºF. Plants were then transferred to greenhouseconditions after five, seven and 10 months storage at 33ºF to assess survival and growth. Table 4shows that the survival of nine different clones was very high overall after five and seven monthsof storage (89 and 87%, respectively) at 33ºF but dropped off markedly to 35% after 10 monthsstorage. Two clones (Burbank and UX2) appeared to be losing some viability after storage forseven months as their survival in the greenhouse had dropped off to 60 and 40 % respectively,whereas the other clones still had survivals of 80 to 100%. These results indicate that leaflessliner-size plantlets of most clones can retain high viability after nine months of storage, includingtwo months at 42-45ºF and seven months at 33ºF. The reason that plantlets lost viability afternine months is not known but it could be due to desiccation of the leafless shoots, since thehumidity at 33ºF is quite low. We are currently doing an experiment to see if we can reduce lossof viability at 33ºF by storing the plantlets inside plastic garbage bags to reduce water loss anddesiccation.California Walnut Board 64 Walnut Research Reports 2008
Liner plantlets from cold storage were re-potted into avocado pots (1½ gallon) and then grown inthe greenhouse to provide actively growing 3/8-1/2 inch diameter trees for use in screens forcrown gall resistance by Dan Kluepfel. Forty five plants of seven transgenic lines that had showncrown gall resistance in previous screens and control plants were provided. Thirty plants each ofAZ025, RX1, Vlach and VX211 were also provided for an inoculation dosage test along with120 AX1, 80 AX2 and 35 AX3 plants for experiments on methods of wounding and inoculationfor improved crown gall screens. These plants were grown and screening tests were performed ina Plant Science greenhouse specifically managed for crown gall screening. Liner plants were alsogrown on to a size large enough for Phytophthora screening. A total of 670 plantlets of 17genotypes from cold storage were re-potted in Cetap 1½ liter pots and grown further in thegreenhouse to provide actively growing 1/4-3/8 inch diameter trees for use in screens forPhytophthora citricola and P. cinnamomi resistance. Screens were performed by Greg Browne ina USDA greenhouse.Fully chilled liner plantlets of 10 clones (16 to 36 plants each) were used in a nursery experimentto test the effect of root pruning on the quality of root systems produced in the nursery row.Plantlets from 1½” diameter x 7” deep Tree Tubes were planted in a nursery row in May withroot systems intact or the lower 2/3 of the root balls pruned away. Resulting trees will be graftedwith Chandler in 2009 and dug in 2010 at which time the quality of root systems will beevaluated.Hardwood cuttings: Hardwood cutting material of RX032, Juglans cathayensis #21 and UZ229from the original mother trees at Kearney Research and Extension Center was received fromMike McKenry in mid-January 2008. Because these trees had not been pruned to stimulatevigorous shoot growth, the material did not yield many cuttings of the quality that have the bestchance of rooting. In addition we collected cutting material from the J. cathayensis #21 mothertreeat the Wolfskill Experimental Orchard. Hardwood cuttings of all four genotypes were made,treated basally with 8000 mg/l potassium indole butyric acid and placed on a bottom-heating bedat 27C. About 25% of the cuttings of J. cathayensis #21 and UZ229 rooted but none of theRX032 or J. cathayensis mother tree rooted. We were able to provide Mike McKenry with fiverooted-cutting trees of J cathayensis #21 and 10 rooted-cutting trees of UZ229 for a nematoderesistance trial at Kearney. Subsequently the mother trees at Kearney were pruned so that morehigh quality cutting material will be available in 2009.Hardwood cuttings were also used to clonally propagate for re-testing 95 open pollinatedseedlings from mother trees representing nine species from the National Clonal Repository thathad no galls or small gall formation 15 months (two growing seasons) after inoculation with avirulent strain of Agrobacterium tumefaciens. A total of about 300 cuttings were made in earlyFebruary and treated as described above. A total of 89 rooted cuttings were grown on in avocadopots for re-testing by Dan Kluepfel’s laboratory. Rooting of cuttings from seedling trees in potsless than two years old shows promise for a relatively rapid (six months) method for clonallypropagating plants for re-testing for crown gall resistance. Use of hardwood cuttings reduces thenumber of genotypes that need to be propagated by in vitro micropropagation which is initiallyslow to produce plantlets and labor and resource intensive.California Walnut Board 65 Walnut Research Reports 2008
Page 1: CLONAL PROPAGATION OF WALNUT ROOTST
Page 5 and 6: Table 1. Greenhouse survival of roo
Page 7 and 8: Table 2. Inventory of fully acclima
Page 9 and 10: Table 3. Survival of plantlets prod
Page 11 and 12: Table 6. Current Clonal Rootstock F

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