Influence of Water Stress on the Vacuole/Extravacuole Distribution of Proline in Protoplasts of Nicotiana rustica

Tobacco (Nicotiana rustica) plants were stressed by addition of polyethylene glycol solution (-20 bar) to the growth medium. The proline contents and concentrations in total protoplasts, vacuoles, and extravacuolar fractions of these plants have been determined and compared with protoplasts and cell fractions of well-watered plants. As compared to the control, the stress treatment of intact plants results in a 7-fold increase of the proline content in the extravacuolar fraction while the vacuolar content was enriched only 2.6-fold. In protoplasts of control plants, a proline concentration ratio (extravacuole to vacuole) of 1 was measured. In protoplasts of stressed plants, this ratio was nearly 3. Thus, water stress seems to have an effect on a tonoplast-localized transport system for proline.