The phenomenon of cross tolerance in osmotically and ionically adapted rice (Oryza sativa L.) cell lines
In the present study, cell lines of local rice (Oryza sativa L.) cv. Swat-1 were incrementally adapted to osmotic [20% Poly ethylene glycol (PEG)] and ion specific (20 mM LiCl) stresses to investigate the phenomenon of cross tolerance. The tolerances of adapted lines were stable up to six generation on stress free medium. Both adapted cell lines were highly tolerant to either osmotic or ionic stresses while, the relative growth rates (RGR) of unadapted cells line decreased to about 95% at 20 mM LiCl and 20% polyethylene glycol, respectively. On subjection to temperature stress, the relative growth rates of unadapted cells line completely ceased, in contrast, both the adapted lines were found to be highly tolerant to low (18°C) and high temperature (36°C) stresses, respectively. At optimum temperature (27°C), proline content of unadapted and LiCl adapted lines were comparable (2 μmoles g-1 fresh weight), while polyethylene glycol adapted cell line had 17 time higher proline (35 μmole g-1 fresh weight) content. Under temperature stresses, the proline level of both the adapted lines increased significantly but the relative rate of increase was greater in LiCl adapted line than PEG adapted line. The tolerance of adapted cell lines to opposite stresses (osmotic or ionic) and to temperature stress (low and high) tangibly revealed that the presence of common stressor in the environment is not simply the cause of induction of cross tolerance, rather it seems to be the adaptations of software that facilitates induction of this phenomenon.
Key words: Cross tolerance, growth, LiCl, polyethyleneglycol (PEG), proline.