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44 M. Mleczek et al.

biosynthesis of endogenous salicylic acid in pea leaves. Similar results were
obtained by Metwally et al. (2003) in the case of Cd-treated barley seedlings
(Hordeum vulgare). SA-priming treatment (500 μM) of dry caryopses decreased
cadmium toxicity and was beneficial for all growth parameters (excluding shoot dry
weight), although total Cd in root and leaf tissue remained unaltered. Surprisingly,
the addition of salicylic acid to the hydroponic solution for 24 h had a similar
positive effect on barley seedlings. In addition, SA reduced the level of MDA in
roots of Cd-treated seedlings and increased by 20 % total non-protein thiol content
compared to Cd treatment. According to Kova ´c ˇik et al. (2009), salicylic acid
(50 μM) added to the cultivation medium containing cadmium or nickel soluble
salts (60 μM) altered the rate of metal uptake and translocation from roots to leaves
of chamomile plants (Matricaria chamomilla). Cadmium transport to the photosyn-
thetic organs was greatly reduced by SA treatment, but in roots total Cd was found
at a comparable level. However, nickel accumulation in chamomile leaves was
significantly increased by salicylic acid with simultaneous reduction of its content
in roots, indicating distinct modes of salicylic acid action in chamomile response to
both metals. Plant treatment with SA altered the activity of phenolic metabolism-
related enzymes either in chamomile roots or leaf rosettes. SA enhanced the activity
of shikimate dehydrogenase (SKDH) in leaves and cinnamyl alcohol dehydroge-
nase (CAD) in roots in the case of nickel-treated plants and greatly lowered SKDH
activity in roots in the case of cadmium addition. As a consequence, significant
changes in composition of chamomile phenolics including benzoic and cinnamic
acids were noticed. The accumulation of endogenous SA was strongly induced in
plant roots and aerial organs in the case of nickel and salicylic acid simultaneous
treatment. In the case of cadmium, exogenous salicylic acid enhanced endogenous
SA biosynthesis in roots, but lowered it in leaf rosettes due to the SA-mediated
inhibition of cadmium translocation up to leaves.

3.3 Biochemical Responses to Metals

Growth inhibition, water and nutrient imbalance, decrease of photosynthetic activity
and oxidative stress are only a few of the multiple effects observed in plants growing
in the presence of heavy metals. Metal toxicity is a result of the binding of ions to
functional groups in proteins, nucleic acids or lipids, leading to inhibition of their
activity or structure disruption, or also from the exchange of essential metal ions
from the active centres of enzymes resulting in deficiency effects (Van Assche and
Clijsters 1990). In addition, like most stress factors, an excess of heavy metals may
lead to the generation of harmful reactive oxygen species which react with
macromolecules important for cell functioning (Dat et al. 2000; Clemens 2001;
Clemens et al. 2002;Pittman 2005). One of the first effects of metal toxicity is
the very well documented increase of reactive oxygen species (ROS) in plants.
Normally, ROS produced during different metabolic processes such as photosynthesis
are immediately dissolved by antioxidative enzymes and molecules. Increased