Synthesized Zn(II)-Amino Acid and -Chitosan Chelates to Increase Zn Uptake by Bean (Phaseolus vulgaris) Plants

Plants require optimum amounts of nutrients for suitable growth and yield production. Accordingly, the most efficient methods of fertilization, including the new ones, may be used. It was hypothesized that the use of organic complexing products including amino acids (AA) and polysaccharides such as chitosan (not previously investigated to our knowledge) can increase Zn uptake by bean plants as such products have the potential to form relatively stable complexes with metals. Some Zn(II)-amino acids complexes (ZnAAC) including [Zn(L − L′)2] [where L − L′ = mono anion of amino acids] as Zn-alanine (Zn-Ala), Zn-glycine (Zn-Gly), Zn-phenylalanine (Zn-Phe), Zn-tryptophan (Zn-Trp), and Zn(II)-polysaccharides complexes including Zn-enzymatic chitosan [Zn-Chi(E. hyd)] and Zn-acidic chitosan [Zn-Chi(A. hyd)] were synthesized and characterized by IR spectroscopy and CHN analyzer. The efficacy of Zn(II) organic complexes as a Zn source was evaluated by growing bean plants in a hydroponic system using a completely randomized design, with three replicates, and investigating: (1) bean growth including root (RDW) and shoot dry weight (SDW), and (2) plant pigments and nitrogen (N) contents. A soil experiment was also conducted to determine the effects of ferritization type (Zn-Gly, Zn-Ala and ZnSO4) and method (soil, fertigation and foliar) on Zn distribution in different plant tissues (root, shoot, pod and seed) as well as on seed yield and seed Zn to total Zn ratio. The elemental analysis supported the formation of Zn(II)-organic complexes. Accordingly, organic ligands coordinated to the Zn(II) ion via their nitrogen and oxygen atoms, and supported the coordination mode obtained from IR spectroscopy. The results confirmed the higher efficacy of Zn(II) organic chelates in supplying Zn for bean in comparison with ZnSO4. Among all synthesized complexes, Zn-Trp and Zn-Gly significantly enhanced plant SDW, and the chitosan treatments effectively increased plant RDW. Zn-Gly and Zn-Ala resulted in the highest Zn concentration (in different plant tissues) and seed yield compared with ZnSO4. The highest seed Zn concentration and yield was resulted by the foliar application of Zn-Gly and Zn-Ala. Although the highest plant shoot uptake of Zn was resulted by the AA treatments, the chitosan treatment ([Zn-Chi (A. hyd)]) significantly increased root Zn uptake. The results indicate, such Zn (II)-organic complexes can provide bean plants with their required Zn, which is of environmental and economical significance.