Fig. 3. Tissue cultures of seedling paw-paw. (A) Single nodal explant showing initial shoot elongation. (B) Seedling explant with multiple shoot formation. (C) Pawpaw after several subcultures in the multiplication stage showing shoot clusters and apparent oxidation of phenolics in the basal callus and medium. (D) Multiple shoot formation in A10-11 cultures. Most of these shoots fail to elongate satisfactorily to produce microcuttings.
Grafting is the only current clonal propagation method used commercially and chip budding is usually the method of choice. Layne (1996) provided a detailed description of chip budding techniques used at the Paw-paw Germplasm Repository at KSU. He described spring, chip budding onto container-grown seedling understocks using dormant scion buds from wood stored under refrigeration. Larger buds showed a higher degree of grafting success compared to small buds. Successful buds began to grow within two weeks of grafting. He found that immediate removal of the understock just above the bud inhibited subsequent scion growth. The recommendation was to cut the understock back gradually. Initially the understock should be reduced to 30 to 60 cm (11.8 to 23.6 inches) in height retaining about 6 leaves. Once the scion has reached about 30 cm, all leaves from the stock should be removed and the stem cut back to 20 cm (7.9 inches). When the scion becomes woody, the remaining understock above the scion can be completely removed. Greenhouse produced plants from budding can reach 1.5 m (5 ft) after the first season.
has reached about 30 cm, all leaves from the stock should be removed and the stem cut back to 20 cm (7.9 inches). When the scion becomes woody, the remaining understock above the scion can be completely removed. Greenhouse produced plants from budding can reach 1.5 m (5 ft) after the first season.
There are numerous named cultivars under evaluation in a multistate pawpaw regional variety trial initiated by the Pawpaw Foundation (Frankfort, Ky.) and KSU (Layne 1996). These and other potential cultivars have been budded onto seedling understocks. There are currently no clonal understocks available for pawpaw propagation.
Juvenility is an important factor for cutting and tissue culture propagation. Developing shoots on roots of mature plants appears to be a viable rejuvenation method. However, cultivars are maintained on understocks making roots unavailable for clonal propagation. Layering is currently being investigated as a method to establish cultivars on their own roots (S. Crabtree, unpublished). There may be several important outcomes if layering is successful in propagating pawpaw. First, layering could prove to be a commercially viable option for clonal propagation of cultivars or understocks similar to other fruit crops (Hartmann et al., 2002). In addition, layering would establish cultivars on their own roots providing a source for roots in a rejuvenation strategy for tissue culture as has been successful for establishing shoot cultures (Finneseth et al., 2000). Finally, cultivars on their own roots would be important for maintenance of clonal germplasm at the Germplasm Repository for pawpaw established at KSU (Layne, 1996). Currently clonal germplasm is maintained in a field orchard as budded trees. Since pawpaw can regenerate from suckers developed from the root system, it would be prudent to have all accessions on their own roots. This would be a precaution against damage to the above-ground portion of trees (winter kill) that might still permit the root system to survive and produce clonal shoots. This could prove extremely important to those southern accessions in the germplasm collection that might be susceptible to winter injury.
Pawpaw is currently commercially propagated by seeds or budding. Seeds must be stored moist and require about 100 d of chilling stratification to overcome dormancy. Seed stocks for propagation are limited to yearly production cycles because seed storage life is short and supply is limited. Therefore, seeds are usually submitted to refrigerator stratification and greenhouse sown rather than field sown in the fall as is usually for many nursery crops. Seeds are an appropriate source for propagation of understocks or liners intended for ornamental use. Fruit quality from seed is variable and usually inferior to selected cultivars for orchard use. Liners intended for fruit production should be budded onto seedling understocks. Dormant, vegetative buds are chip budded on over wintered seedlings using standard budding methods. A propagation method to produce pawpaw on its own roots would be advantageous for orchard production because understocks suckering can be a problem. However, clonal propagation of pawpaw from methods other than grafting (i.e., cutting and micropropagation) have proven difficult and remain to be developed for routine propagation of pawpaw.
Baskin, C.C. and J.M. Baskin. 1998. Seeds. Ecology, biogeography, and evolution of dormancy and germination. Academic Press, New York.
Bonner, F.T. and L.K. Halls. 1974. Asimina Adans. —Pawpaw. [Seed production]. Agr. Hndbk. USDA 450:238–239.
Callaway, M.B. 1992. Current research for the commercial development of pawpaw [Asimina triloba (L.) Dunal.]. HortScience 27:90, 191.
Corner, E.H.J. 1948. The annonaceous seed and its four integuments. New Phytol. 48:332–364.
de Vogel, E.F. 1980. Seedlings of dicotyledons. Ctr. Agr. Publ. Doc. Wageningen, The Netherlands.
Dirr, M.A. and C.W. Heuser. 1987. The reference manual of woody plant propagation: From seed to tissue culture. Varsity Press, Athens, Ga.
Farrant, J.M., N.W. Pammenter, and P. Berjak. 1988. Recalcitrance—A current assessment. Seed Sci. Technol. 16:155–166.