Characterization of Growth, Water Relations, and Proline Accumulation in Sodium Sulfate Tolerant Callus of Brassica napus L. cv Westar (Canola)

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Abstract
Unselected and sodium sulfate tolerant callus cultures of Brassica napus L. cv Westar were grown on media supplemented with mannitol, NaCl, or Na2SO4. In all cases, growth of tolerant callus, measured on a fresh weight or dry weight basis, was greater than that of unselected callus, which was also subject to necrosis on high levels of salt. Tissue water potential became more negative in both unselected and tolerant callus grown in the presence of mannitol or Na2SO4. Water potentials in unselected callus were more negative than those of the tolerant tissues; but over a range of Na2SO4 concentrations both cultures displayed osmotic adjustment, maintaining relatively constant turgor. Proline accumulation in both unselected and tolerant callus was low (15 to 20 micromoles per gram dry weight) in the absence of stress, but increased on media supplemented with mannitol, NaCl, or Na2SO4. Increases in proline concentration were approximately linear in tolerant callus, reaching a maximum of 130 to 175 micromoles per gram dry weight. In unselected callus, concentrations were higher, reaching 390 to 520 micromoles per gram dry weight. Proline accumulation was correlated with inhibition of growth, and there was a negative correlation between proline concentration and culture age for tolerant callus.