Evaluation of some plum rootstocks in relation to lime-induced chlorosis by hydroponic culture

Lime-induced chlorosis is a limiting factor in peach cultivation when peach seedling rootstocks are utilized. At present, the problem seems to be solved by use of the hybrid 'GF 677', the rootstock most used by fruit-growers, that combines good agronomic and quality characteristics. However 'GF 677' shows some limitations, such as its high vigour in fertile soils, high susceptibility to waterlogged soils and some biotic adversities. In these situations, a valid alternative can be found in the plum rootstock 'Mr.S.2/5', which induces medium vigour and tolerance to waterlogging, but shows some photosynthate partitioning problems, in spite of its good graft-compatibility. Consequently, the selection of other plum rootstocks seems needed, so as to widen the range available as alternatives to ‘Mr.S.2/5’, which are suitable for medium-high plantation densities, tolerant to calcareous soil and also to waterlogging in the autumn-winter period. The aim of this research was to evaluate the tolerance to lime induced chlorosis, by hydroponic culture, of eight clonal plum rootstocks (I.S.G. 1/1; I.S.G. 1/2, I.S.G. 1/4, I.S.G. 1/5, I.S.G. 1/6, I.S.G. 2/3, I.S.G. 2/6, I.S.G. 2/10), obtained by free-pollination of the French St. Julien hybrids #1 and #2 in comparison to the ‘Mr.S.2/5’ and ‘Mr.S.2/8’ clonal rootstocks, whose characteristics are already known. During a month of culture in Hoagland nutrient solution supplemented with 0.0 and 10.0 mM of sodium bicarbonate, vegetative and physiological responses were detected. After two years of investigation, the NaHCO3 treatment reduced weight increment and total shoot growth in every clone, but the most vigorous ones (Mr.S 2/5, I.S.G 1/2, I.S.G 1/5, I.S.G 2/6, Mr.S 2/8, I.S.G. 2/3 and I.S.G 1/4) showed less reduction. On new, fully expanded leaves, a SPAD index was determined periodically and total chlorophyll content at the end of the trials. Results showed different responses of clones to the stress conditions and confirmed that I.S.G 1/2 and I.S.G 1/5 gave similar good behaviour to that of Mr.S 2/5. In the same rootstocks, also the Fe2+ content was high in stress conditions with similar values to the control plants. Root Fe(III)-reducing capacity confirmed the hypothesis of the presence of two iron reduction systems in ‘strategy I’ plants: the clone ‘I.S.G 1/5’ showed the highest activity in nutrient solution with the bicarbonate, so it proved to be the most tolerant among all the rootstocks screened.