Abstract: By the method of canopy film-covering to reduce UV-B radiation, this paper studied the responses of the morphology and photosynthetic physiology of Nicotiana tabacum cultivar K326 at its physiological, technical, and physiological-technical transitional maturity stages in high -elevation tobacco-growing area of Yunnan. Three treatments were installed, i.e., reducing 75.74% (T₁), 70.08% (T₂), and 30.39% (T₃) of natural solar UV-B radiation. Reducing UV-B radiation increased the stem height and the internode distance of K326 significantly, with the larger values in T₂. Comparing with those under natural UV-B radiation condition, the leaf net photosynthetic rate (P_n), assimilation capacity (AC), water use efficiency (WUE), intrinsic water
use efficiency (WUE_i), photosynthetic pigments (PP) and flavonoids (FL) contents, and specific leaf mass (SLM) in T₁ and T₂ all decreased, with larger decrement in T₂. The factors affecting the P_n in T₁ and T₂ were stomatal and non-stomatal, and the latter was the main one. The major reason of the lower WUE in T₁ and T₂ was due to the increase of transpiration rate (T_r) caused by low stomatal regulation capability. In T₃, the P_n,AC, WUE, WUE_i, and PP increased but the FL and PP decreased to the lowest levels at physiological and transitional maturity stages, and the PP degradation rate was faster at the technical maturity stage.

Key words: photosynthesis, reduced UV-B radiation, tobacco, morphology, burning index, remote sensing, crop residue, Landsat 8.