Floriculture species differ in susceptibility to developing micronutrient disorders, particularly iron and manganese toxicity or deficiency, depending on the efficiency at which micronutrients are taken up by plant roots and the solubility of micronutrients as a function of pH (Albano and Miller, 1998; Argo and Fisher, 2002). The solubility of inorganic Fe3+ decreases 1000-fold for each unit increase in pH (Lindsay, 1979). Decreased solubility results in low levels of water-extractable iron in soilless substrates when pH is above 6 (Peterson, 1981). Appearance of iron deficiency in iron-inefficient species such as calibrachoa (Calibrachoa ×hybrida) develops at high substrate pH levels (pH > 6.4) and often requires supplemental applications of chelated iron fertilizer (Fisher et al., 2003).
Cultivars of iron-efficient floriculture species have been shown to differ in their tendency to accumulate excess iron/manganese at low substrate pH (Albano and Miller, 1998; Harbaugh, 1995). Marigold (Tagetes erecta L.) cultivars developed different degrees of “leaf bronzing” resulting from toxic iron levels in mature leaves after high micronutrient concentrations were applied to the substrate (Albano and Miller, 1998). Susceptible cultivars of pentas (Pentas lanceolata Benth.) developed lower leaf necrosis at substrate pH less than 5.5, which was correlated with high tissue iron levels (Harbaugh, 1995).
Cultivars of agronomic crop species grown at high pH and in calcareous soils are also known to differ in susceptibility to iron deficiency (Fröechlich and Fehr, 1981; Gao and Shi, 2007; Marschner, 1995; Norvell and Adams, 2006). Typical symptoms of iron deficiency include interveinal chlorosis of young shoots and reduced shoot growth during early stages and can progress to severe stunting and shoot tip death in later stages (Marschner, 1995; Römheld, 1987). Symptoms of iron deficiency are well documented for floriculture species, with photos of iron deficiency for a range of floriculture species including calibrachoa published by Argo and Fisher (2002), Gibson et al. (2007), and others.
Strategies for evaluating agronomic crop species for sensitivity to iron deficiency include growing cultivars in noncalcareous and calcareous soils and measuring differences in shoot chlorosis, growth, and yield (Fröechlich and Fehr, 1981; Graham et al., 1992; Hintz et al., 1987; Niebur and Fehr, 1981). Fröechlich and Fehr (1981) used percent reduction in plant height and yield to compare soybean (Glycine max L.) cultivars grown in calcareous vs. noncalcareous soils. Gao and Shi (2007) used hierarchical cluster analysis to group peanut (Arachis hypogaea L.) cultivars by sensitivity to iron chlorosis based on leaf SPAD chlorophyll content, physiologically “active” leaf iron at flowering stage, and pod yield.
Genotypic differences in iron efficiency has not been studied in calibrachoa, which often shows iron deficiency symptoms at high substrate pH or low iron fertilizer level (Wik et al., 2006). The objective of this study was to compare 24 genotypes of calibrachoa for their sensitivity to showing iron deficiency symptoms (reduced shoot growth, chlorophyll content, tissue iron concentration, and flower number as well as chlorosis and necrosis on new shoots) when grown at high vs. low substrate pH. Twenty of the genotypes were commercial cultivars from four breeding companies, in addition to four experimental genotypes. Eleven genotypes were propagated from seed and the remainder from vegetative cuttings. We hypothesized that differences in sensitivity may be related to the tendency for a genotype to increase pH and thereby reduce iron solubility, and/or higher demand for iron (milligrams iron per plant, from either a high required iron concentration per unit dry weight, or high vigor in terms of dry weight gain).
In a greenhouse factorial experiment, seedling plugs and rooted liners of each genotype were transplanted into 11.4-cm-diameter containers and grown for 13 weeks in a soilless peat:perlite substrate at low (initial 5.4) and high (initial 7.1) substrate pH, with analysis of final substrate pH and substrate-electrical conductivity, leaf SPAD chlorophyll content, total shoot dry weight, tissue iron concentrations, and visual indexes of iron chlorosis symptoms and flower number.
Albano, J.P. & Miller, W.B. 1996 Iron deficiency stress influences physiology of iron acquisition in marigold (Tagetes erecta L.) J. Amer. Soc. Hort. Sci. 121 438 441
Argo, W.R. & Fisher, P.R. 2002 Understanding pH Management for Container-Grown Crops. Meister Publishing, Willoughby, OH
Fisher, P.R., Wik, R.M., Smith, B.R., Pasian, C.C., Kmetz-González, M. & Argo, W.R. 2003 Correcting iron deficiency in calibrachoa grown in a container medium at high pH HortTechnology 13 308 313
Fröechlich, D.M. & Fehr, W.R. 1981 Agronomic performance of soybeans with differing levels of iron deficiency chlorosis on calcareous soil Crop Sci. 21 438 441
Gibson, J.L., Pitchay, D.S., Williams-Rhodes, A.L., Whipker, B.E., Nelson, P.V. & Dole, J.M. 2007 Nutrient deficiencies in bedding plants. Ball Publishing, Batavia, IL
Graham, M.J., Stephens, P.A., Widholm, J.M. & Nickell, C.D. 1992 Soybean genotype evaluation for iron deficiency chlorosis using sodium bicarbonate and tissue culture J. Plant Nutr. 15 1215 1225
Hintz, R.W., Fehr, W.R. & Cianzio, S.R. 1987 Population development for the selection of high-yielding soybean cultivars with resistance to iron-deficiency chlorosis Crop Sci. 27 707 710
Karla Y.P. 1998 Handbook of reference methods for plant analysis. Taylor & Francis Group, LLC, Boca Raton, FL
Landsberg, E.C. 1986 Function of rhizodermal transfer cells in the Fe stress response mechanism of Capsicum annuum L Plant Physiol. 82 511 517
Lindsay, W.L. 1979 Chemical equilibria in soils. John Wiley and Sons, Inc., Caldwell, NJ
Manuel, D. & Alcántara, E. 2002 A comparison of ferric-chelate reductase and chlorophyll and growth ratios as indices of selection of quince, pear and olive genotypes under iron deficiency stress Plant Soil 241 49 56
Marschner, H. 1995 Mineral nutrition of higher plants, 2nd ed. Academic Press, San Diego, CA
Marschner, H., Römheld, V. & Kissel, M. 1986 Different strategies in higher plants in mobilization and uptake of iron J. Plant Nutr. 9 695 713
Norvell, W.A. & Adams, M.L. 2006 Screening soybean cultivars for resistance to iron-deficiency chlorosis in culture solutions containing magnesium or sodium bicarbonate J. Plant Nutr. 29 1855 1867
Puustjarvi, V. & Robertson, R.A. 1975 Physical and chemical properties, p. 23–38. In: D.W. Robinson and J.G.D. Lamb (eds.). Peat in horticulture. Academic Press, London, UK
Römheld, V. & Marschner, H. 1983 Mechanism of iron uptake by peanut plants: I. FeIII reduction, chelate splitting, and release of phenolics Plant Physiol. 71 949 954
Vetanovetz, R.P. 1996 Tissue analyses and interpretation, p. 197–220. In: D.W. Reed (ed.). Water, media, and nutrition for greenhouse crops. Ball Publishing, Batavia, IL
Whipker, B.E., Cavins, T.J., Gibson, J.L., Dole, J.M., Nelson, P.V., Fonteno, W. & Bailey, D.A. 2003 Water, media, and nutrition testing, p. 47–70. In: D. Hamrick (ed.). Ball Redbook, Crop Production. 17th ed. Vol. 2. Ball Publishing, Batavia, IL
Wik, R.M., Fisher, P.R., Kopsell, D.A. & Argo, W.R. 2006 Iron form and concentration affect nutrition of container-grown pelargonium and calibrachoa HortScience 41 244 251