Ultraviolet screening increases with elevation in translucent bracts of Rheum nobile (Polygonaceae), an alpine ‘glasshouse’ plant from the high Himalayas

Specialized bracts of Himalayan ‘glasshouse’ plants are well known for adapting to diverse stresses in alpine environments, thus ensuring normal sexual reproduction. However, little information is available on how such specialized plants cope with the elevational increase in stress. In this study, we determine the elevational pattern of ultraviolet (UV) protection provided by the translucent bracts of Rheum nobile, a giant ‘glasshouse’ plant species, endemic to the high Himalayas, and examine its effect on pollen germination and pollen tube growth. Both UV-A and UV-B radiation in the open air increased with elevation, but their intensity beneath bracts remained constant with elevation, suggesting that the bracts of R. nobile growing at higher elevations have an increased ability to screen UV radiation. Enhanced UV-B radiation, equivalent to that experienced at higher elevations (4800 m a.s.l.) compared to that at lower elevations (4200 m a.s.l.), significantly reduced pollen germination and pollen tube growth. Pollen grains from plants at higher elevations were not more tolerant to UV-B radiation. UV absorbance and the content of flavonoids in bracts increased with elevation, but a similar trend was not found in either UV reflectance or adaxial trichome density, suggesting that bract UV absorbance may play a more active role than bract UV reflectance in helping plants cope with the increasing UV radiation. Our results indicate that the bracts of R. nobile have the ability to cope with enhanced UV radiation with increasing elevation through increased UV absorbance, thus protecting pollen grains from injury caused by higher levels of UV radiation, and consequently ensuring normal sexual reproduction in stressful high-alpine conditions.