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.
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