Zinc Nutrition and Microbial Allelopathy for Improving Productivity, Grain Biofortification and Resistance against Abiotic Stresses in Wheat

Abstract

Zinc (Zn) is vital for the plants and humans. Wheat (Triticum aestivum L.) is one of the leading cereals and is consumed as staple by billions across the globe. However, wide scale Zn deficiency has been observed in wheat growing regions. This situation necessities the Zn nutrition in wheat and to improve its grain yield and grain Zn contents. This study was conducted (i) to optimize the source and application of Zn through seed treatments, (ii) to characterize the wheat genotypes for genetic diversity, Zn bioavailability and localization in different seed fractions, (iii) to improve the productivity and grain biofortification of wheat by combined application of Zn and Zn solubilizing plant growth promoting rhizobacteria (PGPR), and (iv) to explore the potential of Zn nutrition in improving tolerance against abiotic stresses (drought, salt, heat, chilling) at the University of Agriculture, Faisalabad, Pakistan and Sabanci University, Istanbul, Turkey. For optimizing Zn seed treatments, Zn was applied as seed priming and seed coating using ZnSO4 and ZnCl2 as sources. Seed priming with 0.5 M Zn and seed coating with 1.25 g Zn kg-1 seed, using ZnSO4 as source, were the best treatments to improve the stand establishment, grain yield and grain Zn concentration of wheat. Twenty-eight wheat genotypes, of diverse morphology, were characterized for genetic diversity and grain biofortification potential. There was very less genetic diversity (0.0335- 0.0677) among the tested wheat genotypes of Punjab, Pakistan. However, there was substantial variation for yield and grain mineral concentration. Maximum grain yield was recorded for Chakwal-50 while highest Zn concentration was measured in Blue silver with Zn application. Application of Zn also enhanced the Zn localization in embryo, endosperm and aleurone along with high bioavailability of Zn. In glass house and field experiments, Zn was applied as preoptimized seed priming and seed coating treatment, soil and foliar applications. Zinc solubilizing bacterial strain viz. Pseudomonas strain MN12 was used in combination with different Zn application methods. Zinc application, by either method, improved the yield and the grain Zn accumulation. However, use of PGPR in combination with Zn seed priming was the best in improving the grain yield by 44% followed by soil Zn fertilization (41%). Application of Zn through soil and foliar methods in combination with PGPR substantially increased the Zn concentration in grain (90%), germ (50%), aleuron (98%) and endospermic fraction (80%) of wheat with high bioavailable Zn (70%) compared to control. Zinc application (3ppm Zn kg-1 soil) proved helpful for improving wheat performance under abiotic stresses viz. cold 10/7ºC day/night, drought (35% field capacity) and salt (2500 ppm NaCl) stresses. Adequate Zn supply (1 μM Zn) also mitigated the detrimental effect of heat stress (36/28ºC day/night). Abiotic stresses severely reduced the growth and productivity of wheat and adverse effect of these stresses was escalated under Zn deficient conditions. However, adequate Zn application improved the grain yield and quality of wheat under abiotic stresses by increasing chlorophyll intensity, photosynthesis, enzymatic activities, relative water contents, leaf Zn, nitrogen and potassium concentration and lower lipid peroxidation. Adequate Zn supply increased the Zn and protein concentration and contents and bioavailable Zn in whole grain and seed fractions by about 2-fold compared to low Zn treatment. In conclusion; Zn application improved the productivity of wheat. Use of Zn solubilizing PGPR in combination with foliage and soil Zn application enhanced the Zn uptake and grain Zn bioavailability. However, seed priming in combination with PGPRs produced more yield with maximum net economic return. Wheat genotypes of Punjab are genetically diverse for grain yield and mineral concentration. However, genotypes Blue silver had the maximum grain Zn concentration and Zn bioavailability. Furthermore, adequate Zn supply can help ameliorating the adverse effect of abiotic stresses and improving the yield and grain quality of bread wheat. Keywords: Zinc, PGPRs, bread wheat, abiotic stresses, biofortification, seed fractions