Abstract: By the method of covering with different transparent plastic films to attenuate UV-B radiation, this paper studied the photosynthetic characteristics of tobacco cultivar K326 at its early maturity stage as affected by the illuminance at the heights 50 and 150 cm when the UV-B transmittance was 100% (CK), 75% (T1), 50% (T2), and 35% (T3). The change of tobacco leaf net photosynthetic rate in treatments T1 and T3 was mainly due to stomatal factor while in treatment T2 was mainly due to non-stomatal factors. Through the comparison of the average water use efficiency (WUE) in the four treatments, it was found that there could be a threshold of UV-B radiation intensity to WUE. The illuminance at height 150 cm had definite inhibitory effects on the photosynthetic parameters except the transpiration rate in all treatments and the stomatal conductance in treatment T2. For all treatments, the responses of tobacco leaf photosynthetic parameters to the illuminance at heights 50 cm and 150 cm were consistent but differed in sensitive degree.

Key words: Zooplankton, Changjiang Estuary, Tide