Selective light transmittance of translucent bracts in the Himalayan giantglasshouse plant Rheum nobile Hook.f. & Thomson (Polygonaceae)

Translucent bract transmittance of ultraviolet (UV) to infrared (IR) radiat ion (between 320 and 800 nm) and leaf anatomy were examined in a glasshouse plant, Rheum nobile Hook.f. & Thomson (Polygonaceae) to assess the functio n of avoiding injury by UV radiation while keeping the inflorescence warm b y photosynthetically active (Ph) and IR radiation. Although the translucent bracts and rosulate leaves transmitted little UV radiation, the former alw ays transmit more PA and IR radiation. Additionally, the bracts transmit mu ch more scattered solar radiation than direct radiation. The bracts are als o anatomically different from the rosulate leaves. They have two or three l ayers of mesophyll cells with neither palisade nor spongy parenchymatous ce lls; in addition, the uppermost layer of mesophyll and the epidermis stain easily, and both are thought to play a role in attenuating UV radiation. Th e leaf epidermis of many land plants has UV absorbing pigments such as flav onoids, which absorb almost all UV radiation. Thus the role of the bracts o f R. nobile is to protect the reproductive organs by absorbing UV radiation and to keep them warm by transmitting PA and IR radiation. The bracts are believed to have adapted function and form to the environment, in particula r, to the weather conditions of the eastern Himalaya. (C) 2000 The Linnean Society of London.