Soybean genotypic difference in growth, nutrient accumulation and ultrastructure in response to manganese and iron supply in solution culture

Abstract

Background and aims: The objective of this research was to characterize the physiology and cell ultrastructure of two soybean genotypes subjected to nutrient solutions with increasing concentrations of manganese (Mn) at two contrasting iron (Fe) concentrations. Genotypes 'PI227557' and 'Biloxi' were selected based on their distinctly different capacities to accumulate Mn and Fe. *

Methods: Bradyrhizobium-inoculated plants were grown in hydroponic cultures in a greenhouse. Nutrient solutions were supplied with Mn concentrations ranging from 0.3 to 90 microm, at either 5 or 150 microm Fe as FeEDTA. *

Key results: For both genotypes and at both Fe concentrations, Mn concentrations from 6.6 to 50 microm did not affect shoot, root and nodule mass, or leaf and nodule ureide concentration. Mn concentrations of 70 and 90 microm did not result in visible toxicity symptoms, but hindered growth and nodulation of 'Biloxi'. An Mn concentration of 0.3 microm was, however, deleterious to growth and nodulation for both genotypes, and caused an accumulation of ureides in leaves and major alterations in the ultrastructure of chloroplasts, nuclei and mitochondria, regardless of the Fe concentration. In 'PI227557', there was also a proliferation of Golgi apparatus and endoplasmic reticulum in the cytoplasm of leaf cells, and nodules showed disrupted symbiosomes lacking poly-beta-hydroxybutirate grains concomitantly with a proliferation of endoplasmic reticulum as well as arrested bacterial division. At 15 microm Fe, ferritin-like crystals were formed in the lumen of chloroplasts of 'PI227557' plants. For both genotypes, there was an antagonism between the Fe and Mn concentrations in leaves, the higher values of both microelements being detected in 'PI227557'. The absence of any detectable relationship between Fe or Mn and zinc, phosphorus and copper concentrations in leaves ruled out those micronutrients as relevant for Mn and Fe nutrition in soybeans. *

Conclusions: The results confirmed the greater capacity of 'PI227557' for Mn and Fe accumulation than 'Biloxi' for most nutrient treatments. Hence, 'PI227557' may be a very useful genetic resource both in developing soybean cultivars for growth on low nutrient soils and in physiological studies to understand differing approaches to nutrient accumulation in plants.

Fig. 1.

Concentrations of (A) manganese, (B) iron and (C) phosphorus in leaves of soybean genotypes ‘Biloxi’ (open symbols) and ‘PI227557’ (closed symbols) plants growing at increasing solution Mn concentrations and either low (5 μm, inverted triangles) or high (150 μm, upright triangles) solution Fe concentrations. The standard error of the mean is shown for all data when the value is larger than the symbol.

Fig. 2.

Electron micrographs showing leaf cell structures of soybean genotype ‘PI227557’ grown in 150 μm solution Fe concentration at either (A) 6·6 μm or (B, C, D and E) 0·3 μm solution Mn concentrations. Abbreviations for all parts in alphabetical order: c, chloroplast; cr, heterochromatine; cw, cell wall; er, endoplasmic reticulum; f, ferritin-like crystals; ga, Golgi apparatus; gr, grana; m, mitochondria; mn, external membrane of nuclei; n, nuclei; og, osmiophilic globuli; s, starch grain; v, vacuole. Scale bars: A = 320 nm; B and C = 200 nm; D and E = 240 nm.

Fig. 3.

Electron micrographs showing leaf cell structures of soybean genotype ‘Biloxi’ grown in 150 μm solution Fe concentration at either (A) 6·6 μm or (B and C) 0·3 μm solution Mn concentrations. Abbreviations for all parts in alphabetical order: c, chloroplast; gr, grana; m, mitochondria; n, nuclei; nm, nuclear membrane; og, osmiophilic globuli; s, starch grain. Scale bars: A = 380 nm; B = 430 nm; C = 1·100 nm.

Fig. 4.

Electron micrographs showing ultrastructure of red, central infected region of nodules collected from soybean genotype ‘PI227557’ grown in 150 μm solution Fe concentration at either (A) 6·6 μm or (B) 0·3 μm solution Mn concentration. Abbreviations for all parts in alphabetical order: b, bacteroid; er, endoplasmic reticulum; pb, poly-β-hydroxybutirate granules; pm, peribacteroid membrane. Note in (B) the decreased peribacteroidal space, the absence of poly-β-hydroxybutirate granules, and the apparently constrained cell division, as well as the presence of many vesicles and membrane-like structures in cytoplasm of infected cells. Scale bars: A and B = 630 nm.

Fig. 5.

Dry mass of (A) shoots, (B) roots, and (C) nodules of soybean genotypes ‘Biloxi’ and ‘PI227557’ plotted against the solution Mn concentrations containing either low Fe (5 μm) or high Fe (150 μm) solution concentrations. The standard error of the mean is shown for all data when the value is larger than the symbol.

Fig. 6.

Dry mass of (A) shoots, (B) roots and (C) nodules of soybean genotypes ‘Biloxi’ and ‘PI227557’ plotted against leaf Mn concentration for plants subjected to nutrient solutions with differing Mn concentrations (0·3–90 μm), and either low Fe (5 μm) or high Fe (150 μm) solution concentrations. The standard error of the mean is shown for all data when the value is larger than the symbol.

Fig. 7.

Ureide concentration of (A) leaves and (B) nodules of soybean genotypes ‘Biloxi’ and ‘PI227557’ plotted against leaf Mn concentrations for plants subjected to nutrient solutions with differing Mn concentrations (0·3–90 μm), and either low Fe (5 μm) or high Fe (150 μM) solution concentrations. The standard error of the mean is shown for all data when the value is larger than the symbol.