The Role of Micronutrients in Crops

Micronutrients are nutrients found in very low concentrations in the soil but are essential for normal crop growth. They primarily include elements such as zinc, manganese, boron, iron, and molybdenum. Within plants, micronutrients serve as components of various enzymes or coenzymes, playing a crucial role in promoting and regulating the synthesis of chlorophyll and proteins. Although these trace elements are present in only small amounts within plants, a deficiency in any single trace element can inhibit growth and development, leading to reduced yield and quality. Trace elements must not be applied in excessive amounts; overapplication is not only economically wasteful but can also cause crop toxicity. To effectively harness the yield-enhancing benefits of trace element fertilizers, fertilization must be tailored to specific soil types and crop varieties. 　　
(1) Zinc: Zinc is an essential element for the synthesis of tryptophan, a precursor to auxin. Since zinc is a necessary component of tryptophan synthase and a component of carbonic anhydrase—an enzyme that catalyzes the reaction CO₂ + H₂O → H₂CO₃—its role is critical. Since plants typically absorb and release CO₂ in a water-soluble form, zinc deficiency affects both respiration and photosynthesis. Zinc is also a component of glutamate dehydrogenase and carboxypeptidase, and thus plays a role in nitrogen metabolism. Zinc deficiency prevents the synthesis of tryptophan from indole and serine, thereby inhibiting auxin production. This leads to stunted plant growth and the condition commonly known as “dwarf leaf disease.” For example, when fruit trees such as apple, peach, and pear suffer from zinc deficiency, their leaves become small and brittle, grow in clusters, and develop yellow spots. This condition is common in northern orchards during the spring. The zinc content in crops generally ranges from a few parts per hundred thousand to a few parts per million of dry matter. Among plant organs, the apical bud contains the highest zinc content, followed by the leaves, with the stem containing the least. Zinc is a component of various enzymes in plants and is involved in chlorophyll formation; for example, zinc deficiency in crops can cause leaf chlorosis. Zinc also plays a beneficial role in enhancing crops’ resistance to cold, heat, drought, and salinity. Applying zinc fertilizer promotes crop growth and development, alters the seed-to-stem ratio, and increases economic yield. Zinc is mobile within the plant and is most concentrated in young, tender organs. Symptoms of zinc deficiency in crops typically include stunted plants, chlorosis, gray-green or yellow-white spots on leaves, and underdeveloped root systems. Crops that show significant yield increases with zinc fertilization include corn, rice, cotton, wheat, sugar beets, and grapes. Experimental demonstrations have shown that zinc fertilizers not only increase crop yields but also significantly improve crop quality. Available forms include zinc sulfate, zinc oxide, and zinc chloride. Zinc sulfate is the most commonly used zinc fertilizer; it is applied at a rate of 1–2 kg per mu when incorporated into the soil, at a concentration of 0.01%–0.05% for foliar spraying, at a concentration of 0.02%–0.05% for seed soaking, and at a rate of 1–3 g per kilogram of seed when mixed with seeds. 
(2) Manganese Manganese is a key component of the photosynthetic oxygen-releasing complex; manganese deficiency inhibits photosynthetic oxygen release. Manganese is an essential element for the formation of chlorophyll and the maintenance of its normal structure. Manganese also acts as an activator for many enzymes, such as certain phosphotransferases and, in the citric acid cycle, citrate dehydrogenase, oxaloacetate dehydrogenase, α-ketoglutarate dehydrogenase, malate dehydrogenase, and citrate synthase—all of which require manganese for activation. Therefore, manganese is involved in both photosynthesis and respiration. Manganese also acts as a cofactor in nitrate reduction; in the absence of manganese, nitrate cannot be reduced to ammonia, and plants are unable to synthesize amino acids and proteins. In the absence of manganese, plants cannot form chlorophyll; the interveinal areas of the leaves lose their green color, while the veins remain green. This is the primary distinction between manganese deficiency and iron deficiency. In crop organs, manganese is typically most abundant in leaves, followed by stems, and then seeds; it is more concentrated in green tissues than in non-green tissues. Manganese is a structural component of chloroplasts and an essential nutrient for maintaining chloroplast structure; it promotes photosynthesis in crops. Applying manganese fertilizer to cotton not only reduces bud and boll drop but also significantly increases the yield of early-harvested Grade 1 lint. Manganese cannot be recycled within the plant; symptoms of manganese deficiency first appear on young leaves: chlorophyll in the leaves decreases, resulting in chlorosis between the veins, while the veins and areas near them remain green. Types of manganese fertilizers include manganese sulfate, manganese carbonate, manganese chloride, and manganese oxide. Manganese sulfate is a commonly used manganese fertilizer. It is applied to the soil at a rate of 1–2 kg per mu, used for foliar spraying at a concentration of 0.05%–0.2%, for seed soaking at a concentration of 0.05%–0.1%, and for seed treatment at a rate of 4–8 g per kilogram of seed. 
(3) Boron Boron is closely related to pollen formation, pollen tube germination, and fertilization. Boron participates in sugar transport and metabolism. It enhances the activity of uridine diphosphate glucose pyrophosphorylase, thereby promoting sucrose synthesis. Uridine diphosphate glucose not only participates in the biosynthesis of sucrose but also plays a crucial role in the synthesis of various sugars, such as pectin. Boron also promotes plant root development and has a particularly significant effect on the formation of root nodules in legumes, as it influences the transport of carbohydrates, thereby affecting the root’s uptake of nitrogen from rhizobia.