Influence of variety and iron-chelate photodegradation on iron deficiency and on iron and manganese toxicity in marigold (Tagetes erecta L.)

The objectives of this research were (1) to assess the susceptibility of seven marigold varieties to Fe toxicity; (2) to determine if Fe-chelates incorporated into nutrient solutions are vulnerable to photodegradation; and (3) to determine the consequences on plant growth and physiology such photodegraded nutrient solutions would have.; Marigold varieties included were ‘Orange Jubilee’, ‘First Lady’, ‘Gold Lady’, ‘Orange Lady’, ‘Marvel Gold’, ‘Yellow Galore’, and ‘Discovery Orange’. Treatments consisted of 17.9 $m$mol·L−1 (low) and 360 $m$mol·L−1 (high) Fe-DTPA incorporated into a nutrient solution. Symptoms of Fe toxicity only developed in the high Fe treatment. Foliar Fe levels in the high Fe treatment was 5.6-times greater in ‘Orange Jubilee’ and 1.7-times greater in ‘Discovery Orange’ than in the low Fe treatment. Iron toxicity, as observed visually on leaves, was most severe in ‘Orange Jubilee’ with chlorotic and necrotic speckling and downward leaf cupping and least severe in ‘Discovery Orange’ with few chlorotic spots.; Irradiation of Fe-chelate solutions resulted in the loss of soluble Fe and the formation of a precipitate that was mostly composed of Fe. The rate of Fe-chelate photodegradation in solution increased with temperature or irradiance, but irradiance had the greater effect. Iron-chelates absorb in the ultraviolet and blue regions of the electromagnetic spectrum; removal of these wavelengths with a spectral filter eliminated photodegradation.; Plants grown hydroponically in the irradiated lab-prepared nutrient solution had root ferric reductase activity 2.2-times greater, foliar Fe level 0.77-times less, and foliar Mn level 1.9-times greater than in plants grown in an identical, but non-irradiated solution, indicating that the plants growing in the irradiated solution were responding to Fe deficiency stress with physiological reactions associated with Fe-efficiency. Plants grown in soilless media and exclusively treated with the commercial irradiated fertilizer had a lower foliar Fe concentration and greater root ferric reductase activity than plants exclusively treated with the non-irradiated fertilizer.