Abstract
Hormesis is a dose–response phenomenon observed in numerous living organisms, caused by low levels of a large number of stressors, among which metal ions. In cities, metal levels are usually below toxicity limits for most plant species, however, it is of primary importance to understand whether urban metal pollution can threaten plant survival, or, conversely, be beneficial by triggering hormesis. The effects of Cd, Cr and Pb urban concentrations were tested in hydroponics on three annual plants, Cardamine hirsuta L., Poa annua L. and Stellaria media (L.) Vill., commonly growing in cities. Results highlighted for the first time that average urban trace metal concentrations do not hinder plant growth but cause instead hormesis, leading to a considerable increase in plant performance (e.g., two to five-fold higher shoot biomass with Cd and Cr). The present findings, show that city habitats are more suitable for plants than previously assumed, and that what is generally considered to be detrimental to plants, such as trace metals, could instead be exactly the plus factor allowing urban plants to thrive.
Highlights
Hormesis is a dose–response phenomenon observed in numerous living organisms, caused by low levels of a large number of stressors, among which metal ions
The highest Cd accumulation in roots was observed at 2 μM in S. media (67.88 mg/kg DW), while in shoots the highest Cd concentration was found in P. annua (13.75 mg/kg DW)
The presented data demonstrated that low concentrations of the toxic metals Cd, Cr and Pb, comparable to those found in urban soils, can trigger hormesis at various degrees in P. annua, S. media and C. hirsuta
Summary
Hormesis is a dose–response phenomenon observed in numerous living organisms, caused by low levels of a large number of stressors, among which metal ions. It is assumed that hormesis is an adaptive response to stress, possibly triggered by an initial disruption of homeostasis by low levels of biotic or abiotic stressors, often followed by a process of overcompensation aimed at re-establishing the previous s tatus[1] and at protecting the organism through the stimulation of cellular defence m echanisms[4,5]. Behind this mechanistic interpretation there are complex reactions that still need validation[6]. Some main action mechanisms have been identified to be at the base of plant hormetic responses triggered by metal ions[28,29]: (i) ionic interactions between different chemicals present in the soil
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