Table 6.

Maximum theoretical 2′-deoxymugineic acid (DMA) concentrations (no microbial degradation or adsorption) in the close vicinity of wheat roots (Triticum aestivum cv Tamaro; 0.5 mm distance from the root) grown in different soils compared with modified calculations published by Römheld (1991) using nutrient solution (zero iron (Fe)) grown barley as the model plant

		Barley*	Wheat (Triticum aestivum cv Tamaro)
Parameter	Units	Zero Fe solution	Sant	Bol	Xer L	Xer T	Lass	Nadec	SL+10%
Root mass	g	0.03*	1.4	1.7	3.4	1.9	2.2	0.8	1.7
Root density	g mm−3	5.8 e−5*	5.8 e−5*	5.8 e−5*	5.8 e−5*	5.8 e−5*	5.8 e−5*	5.8 e−5*	5.8 e−5*
Root diameter	mm	0.5*	0.5	0.5	0.5	0.5	0.5	0.5	0.5
Root + rhizosphere cylinder diameter	mm	1.5*	1.5	1.5	1.5	1.5	1.5	1.5	1.5
Total root length	mm	2325*	124 522	145 631	296 876	168 817	191 943	67 860	144 988
Root volume	mm³	465*	24 450	28 595	58 291	33 147	37 688	13 324	28 468
DMA release rate	μmol per plant per 4 h	0.090†	0.044	0.007	0.012	0.010	0.010	0.454	0.103
Volume of the rhizosphere cylinder	mm³	725*	18 710	20 198	45 430	20 657	23 128	7789	38 339
Volume of water in rhizosphere cylinder (WC 25%)	mm³	181*	4678	5050	11357	5164	5782	1947	9585
DMA concentration in the rhizosphere cylinder:
Active root zones (20% of total) – root tips	μM	493†	4.5	0.6	0.5	0.7	0.7	85	9.1
Total root system (TRS) average	μM	99†	0.9	0.1	0.1	0.1	0.1	17	1.8

No microbial degradation or adsorption was considered. Italic numbers mark the assumption made for root and rhizosphere dimensions.

^*Values from Römheld (1991).

^†In contrast to extrapolated daily rates used by Römheld (1991), the release period was reduced to 4 h during which DMA samples were taken and DMA peak release was observed. Extrapolation of release rates on a daily basis using DMA exudation observed during peak release will lead to an overestimation of DMA concentrations in the rhizosphere.

Sant, Santomera; Bol, Bologna; Xer T, Xeraco topsoil; Xer L, Xeraco subsoil; Lass, Lassee; Nad, Nadec; SL + 10%, Siebenlinden + 10% CaCO₃.