UV-B increases the harvest index of bean (Phaseolus vulgaris L.)

The effects of small changes in natural UV-B on the photosynthesis, pigmentation, flowering and yield of bean plants (Phaseolus vulgaris L. var. Label) were studied. To obtain a relatively natural growth environment, the plants were grown in small, half-open greenhouses of UV-transmitting Plexiglas of different thickness (3 and 5 mm), resulting in an 8% difference in the weighted UV-B reaching the plants. Although the UV-B doses used did not significantly influence photosynthesis on a leaf area basis during vegetative growth, important changes in biomass allocation were noted. A UV-B-O induced reduction in leaf area during the period of vegetative growth resulted in decreased dry weight after 57 d. During the flowering and pod-filling stages (57–79.d after planting), however, plants grown at high UV-B retained their photosynthetic capacity longer: maximal photosynthesis, chlorophyll and N content of the leaves were higher under the higher UV-B dose at a plant age of 79 d. Combined with an increased allocation under the higher UV-B dose of both N and biomass to the pods, this resulted in a small increase in yield and an important increase in harvest index with increased UV-B.