Steady state proline levels in salt-shocked barley leaves

Excised barley (Hordeum vulgare var Larker) leaves were treated with salt solutions or wilted. After the treatment period, the leaves were allowed to recover in a 50 millimolar sucrose and 1 millimolar glutamate solution, and proline, Na(+), and K(+) were measured at intervals. Na(+) and K(+) concentrations stayed at a constant high level after the salt treatments, and proline increased to a steady state concentration in response. The relationship between the maximum rate of proline accumulation and the Na(+) concentration reached in each experiment was linear. The final steady state proline concentration reached was also directly proportional to the Na(+) concentration. For a given Na(+) concentration in the leaves, the steady state proline level was greater when 410 millimolar NaCl was added to the leaves than when 205 millimolar NaCl was added. These results are consistent with proline acting as a compatible cytoplasmic solute, balancing an accumulation of salts outside of the cytoplasm.In contrast to the proline levels in salt-shocked leaves, the concentrations in wilted leaves decreased to near control levels within 24 hours of relief of stress.