Effects of supplemental UV-B radiation on the growth and yield of two cultivars of Japanese lowland rice (Oryza saliva L.) under the field in a cool rice-growing region of Japan

An investigation was made of the variations in growth and grain yield in re sponse to increased exposure to W-B radiation of Japanese lowland rice (Ory za saliva L.) in a cool rice-growing region. Two cultivars, UV-resistant cv . 'Sasanishiki' and UV-sensitive cv. 'Norin 1', were examined in a lowland field at Kashimadai (37 degrees 28'E, 141 degrees 06'E) in Miyagi Prefectur e, Japan, for four cropping seasons from 1994 to 1997. The two cultivars we re grown in a lowland field with or without supplemental UV-B radiation, wh ich was provided by UV-B-emitting fluorescent lamps, with a 0.1-mm-thick ce llulose diacetate film as a filter. In both cultivars, significant decrease s in tiller number as the result of supplemental W-B radiation were observe d during the tillering stage in 1994, 1995 and 1997. Furthermore, decreases in grain size from supplemental UV-B radiation were recorded in all season s. The trend towards small grain size was pronounced in 1996. In that year, the mean daily middle temperatures were lower throughout most of the cropp ing season and the mean daily hours of sunshine during the tillering stage and between the end of the panicle differentiation stage and the beginning of the ripening stage were shorter. In 1993 when the temperature and the am ount of sunshine were both lower, the tiller number, the dry mass of aboveg round parts and the panicle number were significantly reduced by supplement al unfiltered UV-B radiation. There was a cultivar difference in the inhibi tory effects of supplemental UV-B radiation on growth between the sensitive cultivar Norin 1 and the resistant cultivar Sasanishiki. These results indicate that supplemental UV-B radiation has a positive effe ct on the growth and grain development of rice, which may be enhanced by un usual climatic conditions such as lower temperature and less sunshine, in c ool rice-growing regions. (C) 2001 Elsevier Science B.V. All rights reserve d.