Towards the selection of a new pear rootstock : in vitro and field evaluation for tolerance to iron chlorosis, low vigor and micropropagation of selected clones

In 1998 a pear rootstock breeding program was initiated, at INRA Angers, to enhance tolerance to iron chlorosis. Crosses were made between the new INRA pear rootstock selection ‘Pyriam’ and four “Mediterranean” Pyrus species: Pyrus communis cordata hybrid, P. amygdaliformis, P. amygdaliformis persica and P. elaeagrifolia. An in vitro test, to evaluate tolerance to lime-induced chlorosis was applied to these progenies. This test was designed to quantify the individual tolerance to iron chlorosis of each individual hybrid. After three weeks of culture, different levels of iron deficiency chlorosis, on the newly formed leaves, was quantified with a chlorophyll meter. All evaluated plants were acclimated and transferred in 2000 to a field plot with a clay loam soil, for chlorosis evaluation by visual scale rating. Simultaneously, trunk diameter and maximum height were measured to evaluate vigor of these plants. In 2001, trees were budded with ‘Conference’ and measurements of iron chlorosis and vigor were repeated during the following seasons till 2005. Field evaluation for their tolerance to iron chlorosis, vigor lower than quince rootstock ‘BA-29’, and growth behavior, resulted in the selection of 71 clones out of 619 individuals. Compared to the initial population, selected individuals have an average chlorosis rating 42.3% lower and vigor 34.3% lower. Selected trees were cut under the graft point and sprouts used to establish new in vitro cultures. Seventy of the selected clones were micro-propagated with an average multiplication rate of 11.2 and an average rooting of 69%. Rooted plants were evaluated in vitro for tolerance to lime-induced chlorosis, measuring their level of chlorosis and their capacity to reduce Fe3+ under Fe-deficiency conditions. Compared with the unselected initial population, the selected clones have increased their in vitro chlorophyll content to an average 29.4%. In vitro evaluation of the selected individuals resulted in highly significant differences between the four progenies in total chlorophyll content and Fe3+-reduction capacity. Considering the chlorophyll level, individuals derived from P. amygdaliformis persica and P. amygdaliformis were more tolerant to Fe-deficiency chlorosis than P. communis cordata hybrid and P. elaeagrifolia. In addition, individuals derived from P. amygdaliformis, P. elaeagrifolia and P. amygdaliformis persica had the highest Fe3+-reduction capacity. The cloned selected hybrids have been acclimated and transplanted to two separated sites for a second field evaluation with replications. Further selection of individuals for use as future pear rootstocks will be made following considerations of field visual chlorosis ratings and vigor after grafting with ‘Conference’. Tests of in vitro multiplication and rooting capacity, in vitro chlorophyll content and Fe3+-reduction capacity under Fe-deficiency conditions will also be made.