To observe the paddy seedlings with 2.5 leaves, sprayed with the solution of Ni(No3)2 respectively at the density of 0.01%,0.03%, 0.04%,and those sprayed with the solution of Tween 20 in contrast at the density of 0.05%.To determine the activation of CAT respectively in four, twenty-four, forty-eight, seventy-two and ninety-six hours. As the result shows, Ni(No3)2 has a repression effect on CAT ,such an effect grows more obvious with the increase of the density of Ni(No3)2 and reaches its peak at seventy-two hours. The activation of CAT...