IRON TOXICITY STRESS CAUSES BRONZE SPECKLE, A SPECIFIC PHYSIOLOGICAL DISORDER OF MARIGOLD (TAGETES-ERECTA L)

A specific physiological disorder, bronze speckle (J.P.A.'s nomenclatu re), was consistently induced in 'First Lady' and 'Voyager' marigold w ith Fe-DTPA concentrations greater than 0.018 mM Fe-DTPA (1 ppm) appli ed to a soilless medium. The disorder was characterized by specific sy mptomology distinguished visually by speckled patterns of chlorosis an d necrosis, and downward curling and cupping of leaves. The percentage of total leaf dry weight affected with symptoms generally increased w ith increasing Fe-DTPA treatments, Symptomatic leaf tissue had a great er Fe concentration than corresponding asymptomatic leaf tissue. Leaf Ran concentrations in symptomatic and asymptomatic tissue were similar . In 'First Lady', older leaf tissue accumulated more total Fe and was associated with more severe symptoms than younger tissue. Media leach ate Fe concentrations increased over 6 weeks and were larger at greate r Fe-DTPA treatments. Adjustment of nutrient solution pH to 4.0, 5.25, or 6.5 did not alter media pH, nor did it prevent disorder symptoms. Application of Fe-DTPA containing nutrient solution to a soilless medi um resulted in leachate Fe levels 3 times greater than for FeSO4 treat ments. Chemical names used: ferric diethylenetriaminepentaacetic acid, monosodium salt (Fe-DTPA).