Iron Nutrition in Marigold (Tagetes erecta L.)
Joseph P. Albano and William B. Miller Department of Horticulture, Clemson University

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Some cultivars of the improved African type marigolds (Tagetes erecta ) are prone to develop a specific physiological disorder of the leaves characterized by speckled pattern of chlorosis and/or necrosis and downward curling of the leaves. Marigolds grown commercially that exhibited the disorder had excessively high concentrations of iron and sometimes manganese in symptomatic tissue. In several trade and extension publications, the disorder has been identified as an "iron toxicity" of floriculture crops including, but not exclusive to, New Guinea impatiens, Sultana impatiens, cutting geraniums, vinca and some species of Brassica. The disorder, termed 'Bronze Speckle' (authors nomenclature), results in economic devaluation of the crop due to extensive aesthetic damage at a critical marketing period.

The occurrence of 'Bronze Speckle' as a physiological disorder of marigolds and other bedding plants seems to coincide with the use of soilless media and chelated micronutrients, particularly iron. Varying and sometimes inadequate levels of micronutrients for optimum plant growth in soilless media prompted the formulation and wide spread use of liquid fertilizers which incorporate micronutrients. Chelates like diethylenetriaminepentaacetic acid (DTPA), or ethylenediaminepentaacetic acid (EDTA) serve to keep the metal in an available form for plant uptake across a wide pH range. The fate of chelated metals in soilless media is, however, poorly understood. Peats are known to vary widely in the amount of iron which is extractable by chelates. Thus the use of chelated micronutrient fertilizers may lead to excessively high levels of effects of iron or manganese for plant uptake. Primary objectives of this research were (1) to determine the effects of iron-DTPA on the orrurence of the disorder and to document and characterize 'Bronze Speckle' as it occurs in marigolds grown in a commercial soilless media, (2) to determine the effects of iron source and fertilizer solution pH on soilless media, and (3) to determine the effects of Fe toxicity and deficiency stress on plant physiology associated with Fe uptake.

Materials and Methods

'Bronze Speckle '. Experiments were designed to induce and document the disorder 'Bronze Speckle' under controlled conditions. Treatments consisted of base nutrient solution varying in iron concentration (1 to 20 ppm). The 1 ppm iron DTPA treatment (control) represented the commercial application rate of chelated iron. Marigold varieties 'First Lady' and 'Voyager' were chosen for these experiments because of reported susceptibility to the disorder. Plants were grown in 6-cell grow packs in a commercial soilless medium within a controlled environment growth chamber programmed to simulate greenhouse growing conditions in light and temperature. Plants were fertilized as needed and were leached weekly with distilled-deionized water. Leachates were collected for monitoring iron, manganese, and pH of the medium. Plants were harvested when at least one flower per plant was in bloom. Leaf tissue was visually separated into symptomatic and asymptomatic (no symptoms) groups for mineral analysis.

Fe Source and pH of Fertilizer solution . Experiments were designed to determine the effects of iron source and fertilizer solution pH on soilless media. Conditions for these experiments were as described above except that only 'First Lady' was studied and that treatments consisted of a 1 ppm Fe base nutrient varying in combination of Fe source (iron-DTPA or iron sulfate), and pH (4.0, 5.25, or 6.5).

Fe Uptake Physiology . Experiments were designed to study the effects of iron toxicity and deficiency stress on plant physiology associated with iron uptake. 'First Lady' marigold was grown hydroponically in a growth chamber. Treatments consisted of a base nutrient solution incorporated with 0, 1, or 5 ppm iron-DTPA. The ability of roots to cause rhizophere (root zone) acidification was determined throughout the course of the experiment. The ability of roots to reduce ferric iron to ferrous iron was determined at harvest when plants were in full-bloom.

Results and Discussion

'Bronze Speckle '. In these studies, 'Bronze Speckle' was characterized by a speckled pattern of chlorosis and/or necrosis and downward curling of nearly or recently expanded mature leaves. Symptoms of 'Bronze Speckle' increased in severity with increasing iron-DTPA treatment. Plants treated with the highest iron-DTPA concentration (20 ppm) expressed the disorder first with symptoms developing later in plants treated with 15, 5, and 1 ppm iron-DTPA. In the lowest iron-DTPA treatment (1 ppm), symptoms of 'Bronze Speckle" were not present in all plants. This suggests that at rates consistent with those applied in the industry, other factors internal and/or external to the plant may play an antagonistic role in the occurrence of the disorder. The percentage of total leaf dry weight that was affected with symptoms generally increased with increasing iron-DTPA treatments, and symptomatic leaf tissue had a greater iron concentration than corresponding asymptomatic leaf tissue (Table 1). Leaf manganese was similar for both varieties at all treatments.

Iron Source . The source of iron [iron-DTPA or iron sulfate (1ppm)] incorporated into a fertilizer solution affected leachate iron. The iron concentration in leachate for an iron-DTPA fertilizer was 3-times greater than for an iron sulfate fertilizer (Table 2). This is most likely due to the ability of the chelating agent (DTPA) to extract iron from the media. This demonstrates the importance of regularly leaching the medium, especially when chelates are supplied as part of a soluble fertilizer.

Iron Uptake Physiology . At harvest, excised roots of the 0 iron treatrment had ferric iron-DTPA reductase activity 14-fold greater, and an enhanced ability to acidify the rhizosphere than plants grown in the 1 ppm iron-DTPA treatment. Reductase activity and rhizosphere acdification of plants grown in the 1 ppm and 5 ppm iron-DTPA treatments were similar. The ability to cause rhizosphere acidification and an enhanced ability to reduce ferric iron indicates that 'First Lady' marigold is an iron-efficient plant (i.e., 'First Lady' is capable of chemically modifying the root zone and root physiology to enhance iron uptake when avaialble iron is low). This study also indicates that reactions of iron-efficiency are not expressed when plants are supplied with sufficient iron, ruling out the possibility that iron-efficiency reactions are directly responsible for 'Bronze Speckle'.

Conclusions

There does not appear to be a critical concentration of iron associated with symptom occurrence of 'Bronze Speckle' suggesting that tolerance to Fe concentrations in tissue varies depending on available iron levels. Concentrations of iron-DTPA (e.g., 1 ppm) which are typically used in commercial production were sufficient to cause symptoms; increased concentrations were associated with increased severity and higher leaf iron concentrations. Based on these data, we suggest that occurrence of the disorder and its severity in commercial settings may depend on the iron-chelate in the liquid fertilizer program. Iron-chelate concentrations vary with nitrogen concentration of the liquid fertilizer and since nitrogen (N) concentrations may vary significantly with each application, either deliberately or due to poor injector calibration, producers may be unaware of the actual iron-chelate concentrations being applied to the crop. Leachate iron concentrations increased over time, thus the cumulative effects of repeated iron-chelate applications are important factors in the occurrence of the disorder. Attempts to control this disorder by managing nutrient solution or medium pH are unlikely to be fully effective as iron-DTPA remains fairly stable between pH 4-7 and fairly water-soluble (greater than 50%) in a high pH (7.25) peat-based medium several days after application.

Acknowledgments

We thank the Clemson University Ornamental Horticulture Competitive Grants Program, The Fred C. Gloeckner Foundation, Inc., The Scotts Co., and Fafard Inc. for supporting this research.

Last Updated 7/16/98