Effects of potassium deficiency on root morphology, ultrastructure and antioxidant enzyme system in sweet potato (Ipomoea batatas [L.] Lam.) during early growth

Potassium (K+) deficiency is an important abiotic stress which has severe influence on the growth and development of sweet potato. To investigate the difference on root morphology, ultrastructure and antioxidant enzyme system at early growth stage, two representative sweet potato cultivars (Ipomoea batatas [L.] Lam.) with different K+ deficiency tolerance capacities were hydroponically cultivated under normal K+ (control) and K+ deficiency (-K) treatments. The results showed that after 14 days of treatment, the root length, surface area, volume, and average diameter of Ningzishu 1 (sensitive to K+ deficiency) were significantly decreased under -K treatment, by comparison, those corresponding decreases of Xushu32 (tolerant to K+ deficiency) were lower and not significant (except for root length). In addition, the proportion of fine roots (diameter<0.5 mm) and thick root (d>1.0mm) of Xushu 32 seedlings increased significantly under condition of K+ deficiency. Through transmission electron microscopy observations, it can be found that Xushu 32 seedlings showed a more complete root cellular structure and slighter oxidative damage than Ningzishu 1 under -K treatment. Moreover, the hydrogen peroxide content and malondialdehyde content in the root of Xushu 32 seedlings was significantly lower than that of Ningzishu 1, and the variations of superoxide dismutase, peroxidase, and catalase activities were pronouncedly less than those of Ningzishu 1. These differences between the two cultivars indicated that the stronger root morphology and nutrients absorption ability, and the better root cellular structure and physiological role of Xushu 32 seedlings could alleviate the damage of K+ deficiency stress. Thus, it might be a potential mechanism for cultivar tolerance to low K+ in sweet potato.