Abstract: With sweet potato"Lu Xuan 1"as test material, this paper studied the effects of drought- and salt stress on the callus soluble protein, soluble sugar and proline contents and SOD activity, aimed to understand the physiological mechanisms of sweet potato in resisting osmotic stress at cell level, and to analyze the difference of callus responses to drought- and salt stress. The results showed that soluble protein content had a slow increase under drought stress, and increased markedly at the early and middle stages of slight and moderate salt stress but decreased then, suggesting that Na⁺ was likely to promote the synthesis of soluble protein under short period salt stress. MDA content was markedly lower und er severe drought stress than under severe salt stress, while SOD activity was i n adverse, indicating that the increase of cytoplasm membrane permeability was resulted from the lipid peroxidation under salt stress and the dehydration by PEG-6000 under drought stress. Under severe drought stress, soluble sugar content had a fast increase within a short period of time and decreased then, while proline content increased fast at the middle and later stages of this stress, indicating that proline could probably compensate the decrease of sugar content.

Key words: Long term fertilization, NO₃^--N distribution pattern, Loess Plateau dry highland