Effect of Supra-Optimal Boron Levels on Respiration and Carbohydrate Metabolism of Helianthus Annuus

Abstract

Month-old sunflower plants growing in Hoagland s solution were treated with 50 ppm B as H3BO3. Four days after the B addition, the rates of O2 uptake of leaves from these plants and from plants growing in [image] Hoagland''s solution were compared. The level in the treated leaves was higher than in the control leaves. Both treated and control leaves had an R. Q. of L Infiltration of normal sunflower leaf discs with borate solutions (150, 300, and 450 ppm B) caused a slight depression of the rate of O2 uptake. Therefore, it is suggested that the elevated pattern of respiration in leaves from high B plants was due to an effect of B on the concentration of respirable substrates and not to a direct effect of B on respiratory reactions. This conclusion is supported by the fact that leaves and roots of high B plants have a higher level of sugar and a lower level of starch than the control plants. Using sunflower starch phosphorylase it was shown that B competitively inhibits the reaction G-l-P starch+ phosphate, and it was concluded that its participation in this reaction can account for its effect on the starch/sugar balance of sunflower. Since B does not complex with G-l-P, the inhibition must be brought about by the element complexing at the active site of the enzyme. The author puts forward the hypothesis that B may perform a protective function in plants by preventing the excessive polymerization of sugars at active sites of synthesis, and thereby maintaining an elevated pattern of metabolism and translocation throughout the plant.