Abstract
Sunflower (Helianthus annuus L.) cv. NSFH-592 was grown in refined sand at graded levels of B ranging from deficiency to excess (0.017 to 0.66 mg B L-1). At low (<0.33 mg L-1) B apart from marked depression in growth at Day 25, deficiency symptoms (0.017 mg B L-1) were basal fading and distortion of young leaves with soaked areas and tissue necrosis. Biomass, weight of capitulum, and seeds were maximum at 0.33 mg B L-1. No seeds were formed in B deficiency (0.017 mg B L-1). In low B sunflower leaves, the content of starch decreased and activity of catalase, peroxidase, acid phosphatase, aldolase, and ribonuclease increased together with an accumulation of sugars (reducing, non-reducing, and total) and nitrogen fractions (protein and non-protein). On the other hand, excess B at 0.66 mg B L-1 increased the concentration of sugars (reducing, non-reducing, and total), nitrogen fractions, and activity of catalase and acid phosphatase in leaves and decreased that of peroxidase, ribonuclease, and starch concentration. Both deficiency (<0.33 mg B L-1) as well as excess (>0.33 mg B L-1) of B deteriorated the quality of sunflower seeds by lowering the sugars, starch, and oil content and increasing the phenol accumulation. The concentration of B was increased in leaves, flowers, and seeds with an increase in B supply but B concentration in seeds was less marked than in leaves and flowers. The values of deficiency, threshold of deficiency, and toxicity were, respectively, 12.5, 27.0, and 89 ?g g-1 in leaves of sunflower.
