Sugar beet responses to iron nutrition and stress

Sugar beets were grown at four concentrations of added Fe (0.00, 0.02, 0.2 and 2.0 mg L^‐1) in nutrient solutions. The pH increased similarly in solutions at the two higher Fe concentrations and decreased moderately and negligibly respectively at the lowest added Fe and no Fe concentrations. Chlorosis was pronounced in plants of the latter two treatments and was accompanied by marked reductions in the chlorophyll content of apical leaves, in the size of young expanded leaves, and in the fresh and dry mass of roots and shoots. The decreases in these parameters were greater for plants at no Fe than at 0.02 mg Fe L^‐1 The riboflavin content in the nutrient solutions increased only negligibly with maximum iron, increased slightly with 0.2 mg Fe L^‐1, and increased substantially with 0.02 mg Fe L^‐1 and no Fe, but was lower in the latter. The cumulative amounts of riboflavin excreted and the times of maximum excretion varied within replicates of solutions containing both low Fe and no Fe. The maximum riboflavin concentrations were not consistently associated with the lowest solution pH values of single plants within a treatment, nor of plants at different iron concentrations at all sampling times. The release of riboflavin from roots may depend more upon the amount formed than upon root acidification.