Abstract
Hormesis, which describes the stimulatory effect of low doses of toxic substances on growth, is a well-known phenomenon in the plant and animal kingdoms. However, the mechanisms that are involved in this phenomenon are still poorly understood. We performed preliminary studies on corn coleoptile sections, which showed a positive correlation between the stimulation of growth by Cd or Pb and an increase in the auxin and H2O2 content in the coleoptile sections. Subsequently, we grew corn seedlings in hydroponic culture and tested a wide range of Cd or Pb concentrations in order to determine hormetic growth stimulation. In these seedlings the gas exchange and the chlorophyll a fluorescence, as well as the content of chlorophyll, flavonol, auxin and hydrogen peroxide, were measured. We found that during the hormetic stimulation of growth, the response of the photosynthetic apparatus to Cd and Pb differed significantly. While the application of Cd mostly caused a decrease in various photosynthetic parameters, the application of Pb stimulated some of them. Nevertheless, we discovered that the common features of the hormetic stimulation of shoot growth by heavy metals are an increase in the auxin and flavonol content and the maintenance of hydrogen peroxide at the same level as the control plants.
Highlights
Hormesis is a phenomenon that is defined as the stimulatory effect of low doses of toxic substances, e.g., heavy metals, on a single biological parameter of a given organism [1]
A significant stimulation of elongation growth was observed in the coleoptile sections that had been excised from the corn seedlings that had been treated with 1000 μM Cd or Pb and incubated for 24 h in the control medium (APW) without heavy metals
The corn coleoptile sections that had been excised from seedlings that had been treated with Pb and incubated in a medium without Pb grew substantially better than the coleoptile sections that had been excised from the control seedlings [20]
Summary
Hormesis is a phenomenon that is defined as the stimulatory effect of low doses of toxic substances, e.g., heavy metals, on a single biological parameter of a given organism [1]. Hormesis is considered to be a universal phenomenon that is common in nature, which is independent of the type of stressor, the organism in which it occurs or the physiological process [2,3,4]. This notion is supported by numerous studies that have been carried out on a wide range of organisms (from microorganisms through plants to mammals) [5,6]. Many scientists have connected the stimulation of plant growth during hormesis with a low level of oxidative stress [3,7,8,10,12] Their results show that reactive oxygen species (ROS) are undoubtedly involved in the hormetic effect. There is a dearth of information that could explain the mechanisms that underlie the hormesis phenomenon [6]
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