SENSITIVITY OF LEMNA-MINOR GROWTH TO OSMOTIC POTENTIAL AND RELATIVE TOLERANCE OF ITS CALLUS

Growth of Lemna minor L. plants was blocked by 2.5 mM external proline, but growth of L. minor callus was only 50% inhibited by 5 mM external proline. The growth rates of plants and callus decreased linearly with external Na2SO4, NaCl, mannitol, or PEG-8000 concentration, with the slope of decrease dependent upon the particular osmoticum, and the internal proline content generally increased as growth decreased. The correlations between decreased growth rate and proline accumulation were stronger for plants than for callus, and the callus showed a smaller growth decrease per unit external osmotic pressure. The accumulation of anions and cations as the external Na2SO4 concentration increased was only linear for SO4-2. The tissue content of SO4-2 was almost alone able to account for continued growth of plant or callus as [Na2SO4](ext) increased to 35.2 mM. A callus line adapted to fair growth on 70.4 mM Na2SO4 showed the same rate of growth declination in the presence of the four osmotica as did the unadapted callus: the response curves were merely shifted to higher external osmoticum values. The relative tolerance to osmotic pressure of the Na2SO4-adaptedcallus could not be accounted for in terms of proline or sugar content.