ENGLISH VERSION

ZESZYT 340

Jolanta Stanisława Molas
The uptake of nickel by cabbage plants (Brassica oleracea L.) 
and its phytotoxicity in relation to the chemical form applied 
to the substrate

The subject of the paper concentrated on the relation between the chemical form of nickel applied to the substrate (water medium/soil) and the process of its assimilation, content in plants and the degree and symptoms of its phytotoxicity. Cabbage plants (Brassica oleracea L.) cultivar Slava from Enkhouizen were grown in water and pot cultures with different reaction of the medium and the soil and the content of float particles in the soil. The object of the study was a sulphate form of nickel and its three chelate forms, i.e. Ni(II)-Gly, Ni(II)-citrate (abbr.: Ni(II)-Cit and Ni(II)-EDTA). The chelates differed with respect to their chemical nature and the origin of ligand, stability constant and ionic speciation, including molecular mass and ionic charges. Many research methods were used to analyse the soil and the plants, including physical, chemical, physiological, biochemical methods as well as the methods of light and electron microscopy (TEM, SEM) and histochemical and RTG microanalysis techniques. 
The results of experiments showed that assimilation of nickel by experimental plants and risk of its phytotoxic effects are a function of chemical form and content of this element in the substrate as well as substrate properties, i.e. the reaction of water medium and the soil  and the content of float particles in granulometric composition of the soils. Interactions between these variables were statistically significant. Along with the growth of substrate pH from 5.2 to 7.6 and the content of float particles in the soil the amount nickel absorbed by experimental plants from all its four chemical forms decreased. However, the degree of this reduction was varied and looked as follows:  NiSO4·7H2O > Ni(II)-Gly > Ni(II)-Cit >> Ni(II)-EDTA. As a result, the difference between assimilation of nickel from the used chemical forms of this metal grew smaller and the arrangement of these forms with respect to the content of nickel in plants changed. Generally, when pH ranged from 5.2 to 7.2 the plants assimilated more nickel from the sulphate form than from chelate form whereas with pH = 7.6 the plants assimilated more nickel from chelate forms than from the inorganic form.  
Chemical properties of the examined forms of nickel determined both immobilisation  of this metal in the substrate and its phytoavailability and its assimilation at the stage rhizosphere → apoplast → symplast of the root. Transport of nickel ions at the stage rhizosphere → apoplast → symplast of the root and, finally the content of this metal in cabbage decreased together with a decrease in the content of cations as compared to neutral ions and anions in the ionic speciation of these forms and with the growth of molecular mass of the ions and the stability constant of nickel chelates. It needs to be emphasized that there was no positive correlation between the content of nickel in plants and the content of nickel forms soluble in 0.1 mol · dm-3 HCl in soils with addition of examined forms of this metal.
It was found that regardless of the chemical form of nickel in the substrate and apoplast of the root some amounts of this element moved from apoplast to symplast by endocytosis and direct transport and the transport of nickel within symplast was based on membrane flow. Partly this metal was re-transported from symplast to apoplast by exocytosis. Malonic acid played a dominant role in chelation of this metal in vivo; free amino acids were less significant, playing some part only at the initial stage of nickel assimilation and phytochelatins (PC2) were the least significant.
Distribution of nickel assimilated from all the four applied chemical forms was typical of excluding species, i.e. roots assimilated more nickel than over ground organs with external tissues of the root and pericycle as well as vascular tissue assimilating much nickel. Distribution of nickel in the leaf was similar to its distribution in the leaves of hyper-accumulators of this metal in which nickel accumulates in the apical zone and edge zone of the leaf mainly in epidermis cells. Amelioration of nickel consisted in its segmentation in vacuole and cytoplasmic vacuole as well as in cell walls. Also edge cells in the basal zone of the root (except for the root exposed to Ni(II)-EDTA),  stoma cells,  hydathodes and even leaf epidermis with cuticle participated in amelioration of nickel.
There was a positive correlation between the degree of nickel phytotoxicity determined on the basis of the crop of dry mass of the plants  and its concentration in plants.  Intensity of damage at all levels of plant organisation was closely connected with the assimilation and distribution of this metal in plants, organs and tissues. The symptoms of phytotoxicity of the examined forms of nickel included chlorosis and necrosis of leaves, changes in the architecture of the root system, morphological deformation of plant organs, disintegration of tissues and damage of cells and their organelle. Cell walls were more sensitive to the toxicity of Ni(II)-EDTA chelate than the protoplasts of root cells; their structure underwent significant disintegration  resulting in a decomposition of external cells of root tissue.  Protoplasts of root cells were more sensitive to the toxicity of the remaining three forms of nickel with cell membranes being the most sensitive.  
The mechanism of toxicity of the examined forms of nickel consisted in lipid peroxidation, among other things. The content of one of the products of  lipid peroxidation, i.e. malonic dialdehyde (MDA) was closely connected with the concentration of nickel in a given organ.  Concentration of MDA was lower than the content of nickel in the roots of plants exposed to Ni(II)-EDTA chelate.