Effect of salt stress on secondary metabolites of cotton and biological characteristics and detoxification enzyme activity of cotton spider mites

Cotton spider mites (Tetranychus cinnabarinus Boisduvals and T. truncatus Ehara) are important pest of Bt cotton. Abiotic stress, such as salt stress will not only affect the growth and development, but also cause secondary metabolism changes in cotton plants, which may further influence the occurrence of cotton spider mites on the stressed plants. The objective of the present study was to determine the relationships of salt stress with secondary metabolism and cotton spider mite population dynamics. Two transgenic cotton varieties, SCRC28 and K836, currently grown in large areas in saline-alkali land and stressed with varying concentrations of NaCl, were used. The content of gossypol and tannin, two important secondary insect-resistant substances in cotton plants, were determined in cotton leaves stressed along a NaCl gradient. The development, reproduction and detoxification enzyme activity of cotton spider mites were also studied. The results indicated that the gossypol content in SCRC28 and K836 leaves increased by 34.63% and 30.91% with 200 m mol center dot L-1 of added NaCl, respectively. The tannin content increased by 40.63% and 34.87%. With the increase of salt concentration, the number of mites on salt-stressed cotton decreased compared to controls. The developmental period of cotton spider mites in salt stressed plants was significantly extended, while the reproductive rate of cotton spider mites was significantly reduced compared to controls. The development duration of the immature stages under the 200 m mol center dot L-1 NaCl treatments of SCRC28 and K836 increased by 12.84% and 11.94%, relative to the control, respectively. The fecundity of single female feeding on salt-stressed (200 m mol center dot L-1 NaCl) SCRC28 and K836 reduced by 31.96% and 36.16%, respectively. The CarE activities of cotton spider mite feeding on SCRC28 and K836 with 200 m mol center dot L-1 NaCl treated were 3.76 and 2.05 fold of the control, respectively; the P450 activities were 2.55 and 5.48 fold of the control, respectively. Correlation analysis indicated that the gossypol and tannin contents in cotton leaves were positively correlated with the CarE activity or P450 activity. Thus, salt stress enhanced secondary metabolism and increased accumulation of secondary metabolites such as gossypol and tannin, which ultimately inhibited the population dynamics of cotton spider mites. Cotton spider mites metabolized gossypol and tannin by increasing the activity of CarE and P450.