Abstract: Under specified condition of pollution, the content of endogenous proline was negatively correlative to the K+ efflux in leaves. The relation between the proline content and the K+ efflux may change among species and maturity of leaves. In general, the content of proline in the mature leaves was higher than that in the old leaves and the K+ efflux of the old leaves was higher than that of mature leaves, Spraying proline had a protective effect against sulfurous acid insult.