Abstract: This paper studied the effects of cobalt on the polyamine content,reactive oxygen level,anti-oxidative enzyme activity,and cell membrane permeability of soybean seedling’s leaf under drought stress,and discussed the mechanisms of these effects.The results showed that under drought stress,cobalt at its concentration of 30 μmol ·L^-1 could significantly inhibit the production of ethylene.This concentration of cobalt had no significant effects on the contents of MDA and chlorophyll when the drought stress lasted 0～12 days,but significantly restrained the increase of reactive oxygen level,hampered the decline of putrescine,spermidine and spermine contents and anti-oxidative enzyme activities,and thus,inhibited the accumulation of MDA and alleviated the reduction of chlorophyll content when the stress lasted 15~21 days.It could be concluded that when soybean seedling’s leaf was deeply stressed and damaged under drought stress,cobalt could alleviate the decline of polyamine content and anti-oxidative enzyme activity,and hence,alleviate the increment of reactive oxygen level and cell membrane’s damage degree through inhibiting the production of ethylene,and exerted its protective effects on the soybean seedling’s leaf under drought stress.

Key words: Watermelon, Grafting seedling, Chilling tolerance, Protective system against active oxygen