Impact of Heavy Metals on Changes in the Biochemical Profile of Pea Root Exometabolites

Changes in the qualitative and quantitative indices of the biochemical composition of low-molecular-weight pea root exometabolites caused by the exposure to the toxic concentrations of heavy metals (4 μM of cadmium chloride or 40 μM of cobalt chloride) at the early vegetation stage have been evaluated by HPLC in three different pea genotypes (SGE, SGECDt, and Sofya). A contamination of the nutrient substrate with heavy metal salts has caused an increase in the total yield of sugars and amino acids in the SGE and SGECDt genotypes. In the variety Sofya, this effect has been observed for only sugars. No statistically significant differences have been found between the three genotypes in relation to the total yield of organic acids. The cluster analysis and principal component analysis have revealed a unique SGECDt mutant differing from other genotypes in the case of a combined exposure to salts of both heavy metals. A fractal analysis of the structuring level of a root exudation performed for the largest (in terms of the number of components) amino acid fraction has shown that the values of correlation coefficients demonstrated an increase in the total plant biomass with a simultaneous decrease in the biosystem determination indices. In the absence of stress, the lowest plant system consolidation index has been observed for the variety Sofya. In the presence of heavy metals, the value of this index remains unchanged, which may indicate a plant growth stagnation and its transition to an anabiosis-like state. This supposition is also supported by the data on the growth inhibition of this genotype and a decrease in the yield of amino acids. In the case of exposure to individual heavy metals, the determination index of SGE decreases, while it increases in the case of the SGECdt mutant. A combined effect of both metals on SGECDt was multiplicative. Based on these indicators, one can conclude that the plant spends more resources to attract a potential beneficial microflora in order to form an effective symbiosis and successfully resist a metal-induced stress.