Comparative study of the toxic effect of Zn in Lactuca sativa and Brassica oleracea plants: I. Growth, distribution, and accumulation of Zn, and metabolism of carboxylates

• Brassica oleracea is much more tolerance than Lactuca sativa, since in this latter species the root as well as foliar biomass was negatively affected.
• MDH and CS activities appear to be key in organic anions synthesis under Zn-toxicity conditions.
• The organic anion citrate could be determinant in the tolerance and greater accumulation of this element in leaves of B. oleracea.

Zinc (Zn) is an essential micronutrient for plants, animals, and microorganisms. However, in environmental situations of heavy-metal soil pollution, Zn constitutes a major problem for worldwide agricultural production. Organic anions are compounds that have a special structure which proves indispensable to tolerate excess Zn. The aim of the present work was to determine whether carboxylate metabolism is a key physiological process to select and/or generate plants tolerance to Zn toxicity (0.5 mM). For this, we make a comparative analysis of the toxic effect of Zn between two horticultural plants of great agricultural interest, i.e. Lactuca sativa cv. Phillipus and Brassica oleracea cv. Bronco. The tolerance under Zn-toxicity conditions was greater in B. oleracea than in L. sativa despite to show a higher Zn concentration in shoot. Regarding organic anions metabolism, in leaves of L. sativa the enzyme malate dehydrogenase (MDH) notably increased its activity under Zn-toxicity conditions and both under control conditions as well as in Zn toxicity the main organic anion was malate. With respect to B. oleracea under Zn-toxicity conditions the MDH activity declined with respect to control increasing malate levels in leaves, also under control as well as toxicity conditions the citrate synthase (CS) activity was high and the predominant organic anion was citrate. These results suggest that both in programmes of phytoextraction as well as in biofortification with Zn that are based on the fertilization enriched with this element, B. oleracea is more effective than L. sativa and that the organic anion citrate could be determinant in the tolerance and greater concentration of this element in leaves.