Pollen development ofRheum nobileHook.f. & Thomson (Polygonaceae), with reference to its sterility induced by bract removal

The Himalayan alpine herb,Rheum nobile, terminates in a stout conical compound raceme concealed by large translucent bracts. It bears many fruits even under hostile conditions such as low temperature or persistent cloudy weather. To clarify the role of the bracts, the structure and the development of the pollen grains were examined after removing the bracts to expose the flowers to the open air for 9 days. Half of the individuals with bracts removed showed 0 to 1% of pollen stainability and the pollen grains were variable in shape and size. It was also observed that the bracts ofRheum nobileincreased the temperature of inflorescence by about 10°C above ambient daytime temperatures. These results suggest that one of the causes for the inhibition of pollen development was low temperature. The remainder, however, indicated high stainability of 70–100%. It is suggested that the extreme difference of pollen stainability between two groups ofR. nobileexposed to the surroundings may be related to the stage of microsporogenesis. Bracts ofR. nobilemight play an important role in normal reproduction under low temperature at high altitudes.