IN VITRO TISSUE CULTURE OF PEAR: ADVANCES IN TECHNIQUES FOR MICROPROPAGATION AND GERMPLASM PRESERVATION

Abstract

Micropropagation techniques for over 20 pear (Pyrus) cultivars belonging to seven species have been reported. While most published methods use Murashige and Skoog (MS) basal nutrient medium, or slight modifications thereof, Lepoivre (LP) and DriverKuniyuki Walnut (DKW) media, which differ from MS in nitrogen concentration or source, and calcium concentration, have improved shoot proliferation rates. Solid media gelled with agar, sometimes in combination with gellan gum, have traditionally been used, but two-phase liquid overlay or intermittent liquid immersion techniques have greatly increased shoot proliferation. In vitro culture methods, including meristem cryopreservation, are important facets of medium-term and long-term germplasm preservation programs. Medium-term (1 to 4 years) storage techniques involve temperatures of 1 °C to 4 °C, usually in reduced light or darkness, in a nutrient medium with no growth regulators. Long-term preservation of meristems can be accomplished by one of three major methods of cyropreservation: slow freezing, vitrification, and encapsulation-dehydration. Slow freezing, combined with pretreatment by either cold acclimation or abscisic acid has proven to be particularly effective for Pyrus germplasm, including cold-tender species. INTRODUCTION The ability to establish shoot tip cultures, proliferate shoots, induce rooting, and acclimatize the resulting plantlets are all elements of in vitro micropropagation. The methods offer an alternative to the propagation of rootstocks by stooling, layerage, or induced rooting of softwood, semi-hardwood, or hardwood cuttings (Hartmann et al., 1997; Howard, 1987). These same methods could also be used to produce self-rooted scion cultivars. General and genotype-specific protocols that have been empirically developed for pear have been the subject of previous reviews (Chevreau et al., 1992; Chevreau and Skirvin, 1992; Hutchinson and Zimmerman, 1987; Singha, 1986). Meristem culture has been used to produce pathogenfree plants following thermotherapy, and for cryopreservation of pear germplasm (Reed, 1990). In vitro micropropagation methods are also general prerequisites to exploiting somaclonal variation and induced mutations, and for the development of transgenic plants.