Importance of micronutrients: zinc and copper in the production of spring barley ( Hordeum vulgare L.): a review

Barley (Hordeum vulgare L.) is a major cereal crop in Europe and an important source of food, feed, and malting products, with grain quality increasingly valued for its nutritional components, including β-glucan. Zinc (Zn) and copper (Cu) are essential micronutrients that regulate diverse physiological and biochemical processes underpinning barley growth, yield formation, grain nutritional quality, stress tolerance, and disease resistance. However, their agronomic effectiveness is highly context-dependent and influenced by soil properties, genotype, environmental conditions, fertilization practices, and application method and timing. This review critically examines the physiological and agronomic roles of Zn and Cu in spring barley, focusing on their effects on growth, yield components, grain quality, and plant defense. Evidence indicates that adequate Zn and Cu supply can improve yield formation, including spike density and thousand-kernel weight, while supporting antioxidant defense, reactive oxygen species homeostasis, cell-wall integrity, and metal-mediated plant–pathogen interactions. However, these benefits occur within narrow concentration ranges; deficiency restricts physiological processes and productivity, whereas excessive accumulation can cause phytotoxicity, oxidative stress, and nutrient imbalance. The review further evaluates emerging management strategies, including precision and variable-rate fertilization, chelated and nano-formulations, seed priming, controlled-release fertilizers, rhizosphere engineering, microbial approaches, and agronomic biofortification. Despite advances, significant gaps remain in understanding Zn–Cu interactions, genotype-specific responses, micronutrient-mediated immunity, and combined Zn–Cu management under different NPK regimes. Future research should integrate site-specific and genotype-informed nutrient management with precision technologies and advanced fertilizer systems to enhance nutrient-use efficiency, grain nutritional quality, disease resilience, and sustainable spring barley productivity.