Abstract: The effects of reduced UVB radiation (39.59% of natural intensity),which was obtained by covering with Mylar plastic membrane,on leaf water content (LWC),specific leaf weight (SLW),photosynthetic pigments,flavonoid (FV) and malondialdehyde (MDA) of different leaf positions in tobacco variety K326 were studied in different growing stages,eg fast growing period,physiological maturity period and technological maturity period,with field experiment.The results showed that there was a little influence of reduced UVB radiation on LWC in fast growing period,whereas SLW of middle and bottom leaves were decreased UVB radiation reduction in physiological maturity period and technological maturity period reduced LWC of middle and upper leaves,but improved SLW.Chlorophyll a and b (chl a,chl b) of middle leaf were decreased by reduced UVB radiation in fast growing period and physiological maturity period.However,chl a and chl b of middle and upper leaves were increased by UVB radiation reduction in technological maturity period.Reduced UVB radiation in fast growing period and physiological maturity period decreased MDA content and the changes of FV and caroteniod (car) were substantially opposite to MDA.The results indicated that high UVB radiation intensity in fast growing period was in favour of improving SLW of tobacco leaf,and high UVB radiation in fast growing period and physiological maturity period was promoting syntheses of chl and car ,Change of MDA had a certain relation with FV and car.

Key words: Reduced UVB radiation, Flue cured tobacco, Growing stage, Physiological and chemical characteristic