Plants of four Plantago species: P. major L ssp. major L, P. lanceolata L, P. media L and P. coronopus L were grown at two light energy fluxes. In all species, except P. major spp major, the growth of the root was more depressed with low light than that of the shoot, resulting in an increased shoot to root ratio. The shoot to root ratio of P. major ssp major was similar at both light energy fluxes.
The Ca2+- and Mg2+-stimulated ATPase activity of microsomal preparations of the roots was also studied. The capacity values (Vmax) of both ATPase activities were usually low in P. major ssp. major and the highest Vmax value for the Ca2+-stimulated ATPase activity was demonstrated in P. media. Both ATPase activities (Ca2+ and Mg2+) were lower in plants grown with low light except in P. major ssp major, which showed the same Vmax values for enzyme activity in plants grown at both light levels.
The lack of response of P. major ssp major to different light energy fluxes is interpreted as a lack of phenotypic plasticity of this species. The data are compared with previous results on plasticity as a response to mineral nutrition. Phenotypic plasticity is discussed as a factor in the plant's strategy for survival.