Antioxidant Enzyme Activity Affected by High Boron Concentration in Sunflower and Tomato Seedlings

Agricultural productivity could be limited by toxic levels of boron (B) in soils. However, tolerance to B toxicity varies between plant species. This study investigated the effect of high B concentrations on the growth and antioxidant enzyme activity of relatively B-tolerant sunflower (Helianthus annuus L.) and B-sensitive tomato (Lycopersicon esculentum Miller) seedlings. The seedlings were grown in plastic pots supplemented with 0, 20, 40, and 60 g B g-1 [from boric acid, B(OH)3] for 3 weeks. Root and shoot length together with the dry weight of the harvested plants were measured. Boron analyses were carried out on the soil samples and on root, stem, and leaf samples. The B concentrations were determined to fall in the range of 112-475 g B g-1 leaf tissue in B-sensitive tomatoes and the range of 203-995 g B g-1 in B-tolerant sunflowers. Root length was unchanged in the sunflower but decreased by 38% in the tomato seedlings relative to the control. Shoot length was decreased by 28% in sunflower and by 43% in tomato seedlings at treatment 60 g B g-1 relative to controls. In the tomato leaves, glutathione reductase (GR) and superoxide dismutase (SOD) activity increased but catalase (CAT) activity decreased with greater B concentrations. In sunflower leaves, activities of three enzymes increased but CAT and GR activity began to decrease at applications greater than 40 g B g-1. These results indicate that high B concentrations in leaves have some significant effect on the antioxidant enzyme activity of plants. However, the power of antioxidant defence is not sufficient to remove B toxicity symptoms such as growth decrease.