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

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 Driver-Kuniyuki 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.