Distinct Cellular and Organismic Responses to Salt Stress

We have compared metabolic effects of high salinity betweenplants and cell suspension cultures from the facultative halophyteMesembryanthemum crystallinum (common ice plant). This plantshows developmentally-programmed inducibility for a switch fromC₃-photosynthesis to CAM (Crassulacean Acid Metabolism). Themetabolic switch is enhanced by environmental factors such asdrought, low temperature, and, most effectively, soil salinity.CAM induction is dependent on organized leaf tissue and cannotbe elicited by salt stress in suspension culture cells. In contrast,the accumulation of proline Thomas et al. (1992) Plant Physiol.98: 626] is induced by NaCl in cultured cells as well as inplants and must be considered a cellular response to stress.We have extended our observations to include another trait ofsalt- and low-temperature-stress responses in the ice plant,the accumulation of putative osmoprotective sugars and sugaralcohols. In whole plants the cyclic sugar alcohol, pinitol,accumulates to amounts that approach 1 M during stress, whilein suspension cells no increase in sugar alcohols is observed.The distribution of carbon to different sugars is markedly differentbetween cells and plants under stress. Particularly obviousis the distinction between cell types in the different compositionof sugars and polyols, as exemplified by the epidermal bladdercells of ice plants. Ion contents and the content of sugarsand sugar alcohols of bladder cells indicate that Na⁺, Cl^–,pinitol and an unknown carbohydrate compound provide osmoticpressure in these cells, while organic anion concentrationsare low. With the ice plant, we conclude that cells in culturemimic only partly the stress response mechanisms of intact plantsand we hypothesize that communication between different tissuesis required to mount a complete environmental stress response. ⁴ Present address: Department of Botany Oklahoma State University,Stillwater, OK, 74078, U.S.A.