Pear plantlets cultured in vitro under lime-induced chlorosis display a better adaptive strategy than quince plantlets

In many fruit trees species lime-induced chlorosis causes serious economic damage. In the last few years, the in vitro culture technique has been applied to test rootstocks for susceptibility to iron chlorosis and to study biochemical and molecular aspects of the syndrome. In this study in vitro shoot cultivation of quince rootstocks MA, BA29 and pear cv. 'Conference' was used to unravel the relationship between iron deficiency, presence of bicarbonate, and growth development and chloroplast pigment behaviour. Stresses were obtained by using MS medium supplemented with various concentrations of FeNaEDTA and/or KHCO 3. Substrate pH, shoot growth and development parameters, total leaf iron and photosynthetic pigment contents of plantlets were independently determined after 24 days of culture. The medium was acidified by the species under all stress conditions. Iron deficiency and bicarbonate condition led to different growth patterns and modular development among the genotypes. This reflects a different sensitivity and plastic adaptation to the elements of this stress. Iron concentration in leaves and the shoot apex of plantlets decreased in all genotypes. However, chloroplast pigments only decreased in quince plantlets under iron deficiency and bicarbonate condition while they did not change in pear plantlets. Our results demonstrate firstly that a plantlet without a root system, which is cultivated in vitro, is able to sense iron deficiency and bicarbonate enriched conditions and, consequently, activates biochemical and physiological responses. Secondly, acidification appears to be related to iron concentration in tissues of quince rootstocks and pear cultivar. Finally, chloroplast biochemical elements are strongly and differently regulated under both stress conditions and between genotypes.