SELECTION FOR NEW PEAR ROOTSTOCKS: IN VITRO SCREENING AND FIELD EVALUATION FOR TOLERANCE TO IRON CHLOROSIS

Abstract

Crosses have been made between the new INRA pear rootstock selection 'Pyriam' and four Mediterranean Pyrus species: Pyrus communis cordata, P. amygdaliformis, P. amygdaliformis persica, and P. elaeagrifolia. An in vitro test to evaluate tolerance to lime-induced chlorosis has been applied to these progenies. Embryos were dissected out of fruits and cultured in a basal medium to ensure proper plantlet development. One week later, plantlets with roots and leaves emerging out of the cotyledons were transferred to a paper bridge in a Magenta flask containing a liquid medium with 2μM Fe3+DTPA and 10mM NaHCO3. After two weeks of culture, different levels of iron deficiency chlorosis on the newly formed leaves were observed. All evaluated plants were acclimated, grown in the greenhouse, and transferred to a field plot with a clay loam soil with 26% of calcium carbonate equivalent. Total chlorophyll content of each seedling was measured twice, during the springs of 2000 and 2001, and once after grafting with 'Conference' during the fall of 2001. No significant correlations between in vitro and field conditions were observed at the individual seedling level of tolerance. However, a significant correlation was found comparing hybrid crosses. The in vitro tolerance to iron deficiency was similar to the known differential tolerance of each Pyrus species, being P. amygdaliformis, Pyrus communis cordata, and P. amygdaliformis persica more tolerant than P. elaeagrifolia and Pyrus communis. The results also confirmed a significant correlation between in vitro seedling evaluation for tolerance to iron deficiency conditions and field tolerance of grafted plants to lime-induced chlorosis. To solve the lack of correlation at the individual seedling level, the next assays will investigate in vitro cloning of each individual prior to the in vitro test. In vitro propagation of some preliminarily selected clones was satisfactory.