Effects of continuous cropping on physiological and biochemical resistance of cotton to Fusarium wilt

Cotton is an economically important crop worldwide. In the primary cotton- producing regions of China,it is increasingly grown repeatedly on the same land,because of the development of effective methods for control of the cotton bollworm( Helicoverpa armigera) using transgenic insect-resistant cotton varieties. However,consecutive cropping has resulted in continuous declines in yield and quality as a consequence of increasing disease pressure. Cotton Fusarium wilt,a vascular disease caused by soil-borne Fusarium oxysporum f. sp. vasinfectum,is a major constraint on cotton production throughout the world. In this study,a pot experiment was conducted to investigate the physiological and biochemical resistance of cotton to F. oxysporum in different soils under continuous cropping,and to provide a scientific explanation for the increase in cotton disease incidence in continuous cropping schema. Two cotton cultivars( Z-38,N-10) with different levels of resistance to F. oxysporum were used,and continuous-cropping soils were collected from two cotton fields that had been continuously cropped with cotton for 5 a and 15 a,respectively,with the soil from a field not previously used in cotton-cropping used as the control. The pot experiment was conducted in a plant growth chamber at 30 [degrees]C /25 [degrees]C,14 h light /8 h dark,and 60% -90% relative humidity,with three repetitions. After culturing for 20 d,the pots were inoculatedwith F. oxysporum spores( 1. 0 x 10~7 cfu / m L). Plant samples were collected prior to inoculation( day 0) and at 2 d,6d,and 10 d after inoculation. The cotton plants were separated into their root and shoot portions to measure the fresh root and shoot weight,root length,and plant height,and then rapidly frozen at- 80 [degrees]C for use in biochemical analyses. The soluble protein and malondialdehyde( MDA) contents,and the activities of superoxide dismutase( SOD),catalase( CAT),and peroxidase( POD) were measured in the cotton leaves. The results showed that continuous-cropping soils significantly inhibited the growth of cotton seedlings. The fresh shoot weights of the N-10 and Z-38 plants in the 15 y cropping soils were substantially reduced,by 39. 69% and 26. 91%,respectively,compared to the control. Root weights showed a decreasing trend with prolonged cropping time. The root lengths of the Z-38 and N-10 plants were 16. 33% and21. 67% lower,respectively,in the 15 y cropping soils,than in the control. Furthermore,continuous-cropping so