IN VITRO SCREENING FOR TOLERANCE TO IRON CHLOROSIS AS A RELIABLE SELECTION TOOL IN A PEAR ROOTSTOCK BREEDING PROGRAM

A pear rootstock breeding program aiming to enhance tolerance to iron chlorosis is under development. Eighty-six individuals derived from four inter-specific crosses including Pyrus amygdaliformis, P. amygdaliformis ssp. persica, P. elaeagrifolia and P. communis ssp. cordata, were preselected from a field plot based on a comparison to the open pollinated Pyrus communis ‘Williams’ seedlings used as controls. Once established in vitro, they were micropropagated, and the rooted plantlets were tested in vitro for tolerance to iron chlorosis, using six to eight repetitions per clone, before they were acclimated. Twelve plants per clone were transplanted to the field, grafted with ‘Conference’, and their chlorosis level measured during two years. In comparison with previous assays, the availability of repetitions to perform the in vitro test, as well as for the field evaluation, has considerably increased the significance of the regression between in vitro and field response, although the in vitro test was performed on the rootstock and the field response was evaluated on the scion. The level of prediction of the field performance by the in vitro test differed for each cross. The best prediction was found for crosses with P. amygdaliformis (R²=0.72), P. communis var. cordata (R²=0.44), and P. elaeagrifolia (R²=0.41). Further adaption of the in vitro test and studies on the interactions between ‘Conference’ and some rootstocks might improve the lower levels of correlation. According to the in vitro selection tests, the crosses with the highest iron chlorosis tolerance derived from P. amygdaliformis var. persica and P. amygdaliformis, followed by P. communis var. cordata. These are significantly better (p<0.001) that P. elaeagrifolia and P. communis ‘Williams’ seedling. Up to now, the top performing clones derive from P. amygdali¬formis, P. communis ssp. cordata, and P. amygdaliformis var. persica. This is in agreement with the tolerance observed in the field.