We examined the net photosynthetic rate (Pn), the maximal photochemical efficiency of PSII (Fv/Fm) and the activities of antioxidant isoenzymes in cucumber (Cucumis sativus) w ith or without photosynthetic bacteria (PSB) application. PSB spraying resulted in a significant increase in fresh weight and leaf area for both genotypes of cucumber plants, accompanied by a marked increase of Pn. However, Fv/Fm showed negligible changes. Exogenous PSB also increased activities of total superoxide dismutase (SOD) and ascorbate peroxidase (APX) , and a general increase of their isoenzymes. The increased level in Cu/Zn-SOD, Fe-SOD
and stroma APX (sAPX) was most significant in chloroplasts. These results implied that PSB played a favorable role in growth and photosynthesis and improved hardiness in cucumber by enhancing antioxidant enzyme activities.