Inside and outside rhizosphere parameters of barley and dose-dependent stress alleviation at some chronic metal exposures

A pot experiment was designed to study the variability of some inside and outside mycorrhizosphere characteristics of barley (<i>Hordeum vulgare</i> L.) and the potential transfer of Cd, Ni and Pb in a metal-contaminated calcareous chernozem soil. Substrates of the pots were taken from a long-term field experiment site at Nagyhörcsök, Hungary, where the cadmium (Cd), nickel (Ni) and lead (Pb) were spiked as single salt application at four levels (0, 30, 90 and 270 mg kg⁻¹ dry soil) 12 years prior to this study. Beside the biomass production and element content of plants, the total catabolic enzyme activity measured by fluorescein diacetate analysis (FDA) and the colonization parameters of arbuscular mycorrhizal fungi (AMF); the infection intensity (M%) and the arbusculum richness (A%) were determined. After 12 years, the indigenous mycorrhiza fungi in the soils might be adapted to the contaminated environments, as a function of metals and their applied doses. Stress-defense strategies of the fungal-plant symbiosis, such as the better functioning of the AMF by enhanced arbusculum richness or by the improved phosphore-mobilization capacity was found mainly at the middle (90 mg kg⁻¹) doses of metals. Increasing quantity of Cd above the maximum permitted concentration in the soil could enhance the biomass production of barley roots and reduce the Cd translocation towards the shoots. Outside rhizosphere parameters as the FDA enzymatic activity were stronger influenced by the long-term metal stress, than the inside mycorrhiza colonization, showing the protecting effect of the symbiosis both for the macro- and microsymbionts. Mycorrhizosphere conditions are part of the common plant-microbe strategies and plant-defending mechanisms that can result in a better stress-alleviation at chronic metal-exposures.