Effect of Soil pH on Photosynthesis Characteristics and Activity of Protective Enzymes in Flue-cured Tobacco Leaves

In early growth period,the content of chlorophyll and the photosynthesis rate were higher in leaves treated at the pH value between 6.5 and 7.5,then reached maximum at the vigorous growth stage.In early growth period,soil pH had negative correlation with the activity of POD declined when the pH value was up from 4.5 to 8.5;but the activity of SOD and CAD as well as the content of MDA increased with the rising of pH value.At mature stage,with the rising of soilpH,the activity of CAT and POD declined while the activity of SOD increased;the content of MDA had an increasing tendency while the pH value was up from 4.5 to 6.5 and declining tendency while the pH value was up from 7.5 to 8.5 and it reached its minimum value on soil pH 8.5.At mature stage the photosynthesis rate were higher in leaves at various pH value.