Enhancement of nodulation in bean (Phaseolus vulgaris) by UV-B irradiation

Exposure of bean plants grown in the greenhouse, where UV-B is low, to ambient levels of UV-B light stimulated nodulation more than 2.5-fold. Reduction of UV-B radiation to 3% of ambient levels for outdoor-grown plants through use of Mylar filters consistently reduced nodulation by 45%. The increase in nodulation caused by UV-B was mainly due to an increase in number and size of nodules. The amounts of UV-B-absorbing compounds in roots of UV-B-exposed plants increased almost 5-fold. However, the composition of UV-B-absorbing compounds remained very similar. Exposing leaves to UV-B also significantly increased release of these compounds from roots to the medium. These results suggest that UV-B radiation enhances nodulation, and that a signal may be transported from shoot to roots to play a role in nodulation.