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The higher survival rates of Helicoverpa amigera larvae were usually observed after adverse climate which was related to extreme temperature (T) and relative humidity (RH) stresses in transgenic Bacillus thuringiensis (Bt) cotton.The unstable resistance of Bt cotton to bollworms has been correlated with the reduced expression of CryIAc δ-endotoxin.The objective of this study was to investigate the effects of combined temperature and relative humidity stresses on the leaf CryIAc insecticidal protein expression during critical developmental stages.The study was undertaken on two transgenic cotton cultivars that share same parental background,Sikang 1 (a conventional cultivar) and Sikang 3 (a hybrid cultivar),during the 2007 and 2008 growing seasons at the Yangzhou University Farm,Yangzhou,China.The study was arranged with two factors that consisted of temperature (two levels) and relative humidity (three levels).The six T/RH treatments were 37℃/95％,37℃/70％,37℃/50％,18℃/95％,18℃/70％,and 18℃/50％.In 2007,the six treatments were imposed to the plants at peak flowering stage for 24 h; in 2008,the six treatments were applied to the plants at peak square,peak flowering,and peak boll stages for 48 h.The results of the study indicated that the leaf insecticidal protein expression in CryIAc was significantly affected by extreme temperature only at peak flowering stage,and by both extreme temperature and relative humidity during boll filling stage.The greatest reductions were observed when the stresses were applied at peak boll stage.In 2008,after 48 h stress treatment,the leaf Bt endotoxin expression reduced by 25.9-36.7 and 23.6-40.5％ at peak boll stage,but only by 14.9-26.5 and 12.8-24.0％ at peak flowering stage for Sikang 1 and Sikang 3,respectively.The greatest reduction was found under the low temperature combined with low relative humidity condition for both years.It is believed that the temperature and relative humidity stresses may be attributed to the reduced efficacy of Bt cotton in growing conditions in China,where extreme temperatures often increase up to 35-40℃ and/or decrease down to 15-20℃,and relative humidity may reach to 85-95％ and/or reduce to 40-55％ during the cotton growing season.