Page 54 - Plant-Based Remediation Processes

42 M. Mleczek et al.

Table 3.1 Salicylic acid (free and glucosidal) contents in leaves of Salix viminalis L. cv.
“Cannabina” cultivated hydroponically in Knop’s medium containing Cu 2þ and Ni 2þ soluble
˛
salts (Drzewiecka et al. 2012;Gasecka et al. 2012)
Metal concentration in medium [mM]
0 1 2 3
Free SA
Cu 0.33 0.01 1.16 0.07 4.57 0.09 7.94 0.06
Ni 0.36 0.03 12.23 0.11 5.72 0.08 16.89 0.04
CuþNi 0.92 0.15 2.69 0.29 6.35 0.13 10.73 0.47
Sum of free and glucosidal SA
Cu 2.23 0.07 4.33 0.32 7.94 0.10 21.07 0.38
Ni 2.44 0.05 38.26 0.11 21.62 0.18 61.31 0.31
CuþNi 2.59 0.05 10.41 0.12 7.78 0.79 35.69 0.81

3.2 Impact of Endo- and Exogenous Salicylic Acid on Plant
Tolerance to Metallic Ions

Salicylic acid (SA) is one of the phenolic metabolites widely distributed in plants. It
possesses a complex function in regular plant growth and development, as well as in
tolerance mechanisms against numerous environmental factors of both biotic and
abiotic nature. Biotic stressors cause the enhanced biosynthesis of salicylic acid to
develop the hypersensitive response (HR) (suicidal auto-oxidation at infection site),
and further an intra- and interplant systemic acquired resistance (SAR) with the
induction of pathogenesis-related proteins (PRs) (Raskin 1992). Salicylic acid
function in plant response to abiotic factors directly causing oxidative damage
(mainly of anthropogenic origin, e.g. tropospheric ozone or metallic ions), remains
the subject of on-going debate. Pa ´l et al. (2005) proved the elevated biosynthesis of
salicylic acid upon cadmium stress in young maize (Zea mays L.) seedlings. After
7 days of cultivation in medium containing Cd(NO 3 ) 2 (10, 25 and 50 μM), increased
concentrations of free and bound benzoic (BA), o-coumaric (o-HCA) and salicylic
acid were observed in leaves, without changes in SA content in roots, where only
50 μM Cd treatment enhanced the accumulation of free o-HCA and bound BA. In
our studies, the exposure of basket willow (Salix viminalis L.) to copper and/or
nickel (0.5–3 mM as nitrate salts) in hydroponic solution resulted in a substantial
increase of free and glucosidal SA contents in leaves. However, induction of
salicylic acid biosynthesis in photosynthetic tissue (and/or upstream transport
from roots) differed significantly for both metals analysed. Furthermore, it can be
assumed that synergistic and antagonistic interactions between metal toxicity
occurs taking into consideration the elevation of SA content by these two metals
applied simultaneously (Table 3.1).
Freeman et al. (2005) proved the enhanced accumulation of salicylic acid and its
up- and downstream metabolites (phenylalanine, cinnamic acid and salicyloyl-Glc,
catechol, respectively) across different species of Thlaspi showing Ni/Zn
hyperaccumulation. Furthermore, elevation of free SA levels in Arabidopsis, both