Light-Induced Enhancement in Proline Levels in Vigna Radiata Exposed to Environmental Stresses.

Four-day-old Vigna radiata (L.) Wilczek seedlings were exposed to water stress (5% mannitol), salt stress (200 mM sodium chloride) and heavy metal stress (2.5 mM cadmium nitrate), in continuous light and darkness. The seedlings exposed to any of these stresses accumulated higher levels of proline in their shoots compared with control seedlings. Irrespective of the type of stress, accumulation of proline was higher in light than in the dark, which suggests that the photosynthetic activity of the seedlings might be responsible for light-induced enhancement of proline levels. In a second set of experiments, seeds were germinated under water stress, salt stress and heavy metal stress in continuous light and darkness. The plumule did not emerge from seeds germinated under stress, even after day 7. However, a 1.0-1.5 cm long radicle did emerge in light and in darkness. In spite of the lack of photosynthetic organs, seedlings raised under stress in light accumulated more proline than those raised in continuous darkness. In a third set of experiments, roots excised from 7-day-old light-grown seedlings were exposed to salt stress in light and in continuous darkness. The roots exposed to stress accumulated considerably more proline in light than in darkness. The results obtained in the last two sets of experiments suggest, for the first time, that some factoris) or process(es) other than photosynthesis plays an important role in light-induced enhancement of proline accumulation.