Although trace elements constitute less than 0.1% of the total nutrients required for rice growth, they play an irreplaceable role as “catalysts” and “regulators” in the plant’s life activities. As core components of enzymatic reactions, carriers of electron transport, and key factors in substance synthesis, they profoundly influence every physiological process of rice, from seed germination to grain maturation. With the widespread adoption of high-yielding rice varieties, increased cropping intensity, and decreased use of traditional organic fertilizers, the depletion and imbalance of trace elements in the soil have become increasingly prominent, posing a significant bottleneck to improving rice yield and quality. This paper aims to systematically describe the physiological functions, deficiency and toxicity symptoms, and comprehensive management strategies of six essential trace elements—zinc, iron, manganese, copper, boron, and molybdenum—in rice cultivation.
Core Physiological Functions and Mechanisms of Action of Trace Elements
1. Zinc (Zn): The “Regulatory Switch” of Growth
Zinc is an essential component of over 300 enzymes in rice plants, and its core function lies in regulating the synthesis of auxin (indole-3-acetic acid, IAA). Zinc directly controls the production of tryptophan, the precursor of auxin, by affecting the activity of tryptophan synthase, thus governing cell division and elongation. Zinc deficiency leads to a sharp decrease in IAA content in the roots and above-ground growth points, resulting in a drastic reduction in tillering and severe stunting of the plant, a typical “stunted growth” phenomenon. In addition, zinc is involved in ribosome stability, protein synthesis, carbohydrate metabolism (as a component of carbonic anhydrase), and the reactive oxygen species scavenging system, making it a fundamental element for maintaining plant metabolic homeostasis and stress resistance (such as resistance to low-temperature damage).
2. Iron (Fe): The “Electron Hub” of Energy Conversion
Iron is a cofactor of core proteins such as cytochromes and ferredoxins in the electron transport chains of photosynthesis and respiration. In chloroplasts, iron is an essential component of photosystem I and II and the cytochrome b6/f complex, directly driving the conversion of light energy into chemical energy. Iron deficiency hinders chloroplast development and disrupts thylakoid structure, leading to severe chlorosis between the veins of new leaves. At the same time, iron is also an important component of nitrogenase (in legumes) and nitrate reductase, affecting nitrogen metabolism. Rice absorbs iron through a reduction mechanism; in poorly aerated, acidic, and reducing soils, excessive Fe²⁺ ions can lead to toxicity, causing “bronzing disease.”
3. Manganese (Mn): The “Key” to Photosynthetic Oxygen Evolution
Manganese plays an absolutely crucial role in the oxygen-evolving complex (OEC) of photosystem II (PSII) in photosynthesis, directly catalyzing the splitting of water molecules and the release of oxygen and electrons. Manganese deficiency will lead to a significant decrease in photosynthetic oxygen evolution efficiency and insufficient accumulation of assimilates. Manganese is also the metal center of superoxide dismutase (Mn-SOD), responsible for scavenging superoxide radicals in mitochondria and protecting cell structures. In addition, it participates in activating indole-3-acetic acid oxidase, regulating auxin levels. Manganese exhibits variable oxidation states, and its availability is greatly influenced by soil pH and redox potential.
4. Boron (B): The “Bridge” for Reproduction and Transport
Boron has unique physiological functions, mainly related to the synthesis of cell wall structural substances. Boron forms stable borate ester cross-links with rhamnogalacturonan II (RG-II) in pectin, maintaining the integrity and toughness of the cell wall. This function is crucial for rapidly growing root tips and pollen tube elongation. Boron deficiency leads to abnormal cell wall structure and inhibited pollen tube development, resulting in rice pollen sterility, manifested as “flowering without fruiting” and a sharp increase in empty grain rate. Boron also participates in the long-distance transport of carbohydrates in the phloem; boron deficiency affects the transport of sugars to the grains, reducing grain weight.
5. Copper (Cu): The “Guardian” of Antioxidant and Lignification Processes
Copper is the active center of many oxidases (such as cytochrome oxidase, superoxide dismutase-Cu/Zn SOD, ascorbate oxidase, and polyphenol oxidase). These enzymes are involved in terminal oxidation of the respiratory chain, reactive oxygen species scavenging, and lignin biosynthesis. Therefore, copper plays a key role in enhancing stem mechanical strength, improving lodging resistance, and disease resistance (e.g., participating in phytoalexin synthesis). Copper deficiency leads to weak plants and curled leaves, but copper is a heavy metal element, and the range between the required amount and the toxic amount is very narrow; excess copper can severely poison the roots and inhibit growth.
6. Molybdenum (Mo): The “Converter” of Nitrogen Metabolism
Molybdenum is the only metal cofactor of nitrate reductase and nitrogenase. For rice (a non-nitrogen-fixing plant), its core function lies in nitrate reductase. This enzyme reduces nitrate nitrogen (NO₃⁻) absorbed by the roots to nitrite nitrogen (NO₂⁻), which is the first step in nitrogen assimilation. Molybdenum deficiency will lead to the accumulation of nitrate nitrogen in the plant, preventing its conversion into amino acids and proteins, causing a false “nitrogen starvation” phenomenon; even if the soil nitrogen is sufficient, the plant will show nitrogen deficiency chlorosis. Molybdenum is easily fixed in acidic soils, resulting in very low availability.
Field Diagnostic Atlas of Nutrient Deficiencies and Toxicities
Accurate symptom identification is a prerequisite for effective correction. Each element deficiency has a specific distribution pattern:
- New Tissue Sensitive Type: Symptoms of iron, zinc, manganese, and copper deficiencies first appear in new leaves or growing points because these elements are not easily translocated from old leaves to new leaves.
- Old Tissue Sensitive Type: Symptoms of molybdenum, potassium, and magnesium deficiencies first appear in the lower and middle old leaves because they are reusable within the plant.
- Reproductive Organ Sensitive Type: Boron deficiency mainly affects pollen and seed development; symptoms may not be obvious during the vegetative growth stage.
Typical Complex Symptom Differentiation:
- Chlorosis Differentiation: Iron deficiency shows uniform yellowing or whitening between the veins of newly formed leaves, with the veins remaining green and the boundaries clearly defined; zinc deficiency shows whitening at the base of new leaves, with the midrib losing its green color, often accompanied by reddish-brown spots and severe stunting of the plant; manganese deficiency shows light green stripes between the veins of new leaves, with small brown necrotic spots.
- Growing Point Abnormality Differentiation: Boron deficiency leads to necrosis of the root and stem growing points and pollen malformation; calcium deficiency also leads to growing point necrosis, but is often accompanied by hook-shaped adhesion of the tips of the top leaves.
Toxicity symptoms are equally important: In acidic, strongly reducing paddy fields, iron, manganese, and aluminum toxicities are common. Iron toxicity manifests as reddish-brown rust spots between the veins of old leaves, gradually spreading; manganese toxicity leads to brownish-black spots on old leaves, with black manganese oxide deposits on the underside of the leaves; aluminum toxicity severely inhibits root elongation, resulting in short, thick, brown roots.
Integrated Management Strategy: A Systemic Approach from Soil to Leaves
1. Precise Application Based on Soil Testing
Determining the availability of micronutrients in the soil is the cornerstone of scientific management. Key considerations include:
pH is the master switch: Soil pH directly affects the availability of all micronutrients. Most micronutrients (Fe, Mn, Zn, Cu) are highly available under acidic conditions, but can become toxic under strongly acidic conditions; in alkaline (calcareous) soils, they are easily fixed. Boron and molybdenum are most available under slightly acidic to neutral conditions. Therefore, adjusting acidic soils with lime, or improving alkaline soils with sulfur and organic fertilizers, is a fundamental prerequisite for addressing micronutrient deficiencies.
Antagonistic effects: Antagonism between nutrients is common. For example, high phosphorus soils strongly inhibit zinc absorption (P-Zn antagonism), and heavy application of phosphorus fertilizer in high-yield fields often induces zinc deficiency; potassium antagonizes magnesium and boron; excessive copper and zinc inhibit each other. Balance must be considered when fertilizing.
2. Multi-stage, Multi-pronged Supplementation Techniques
Base fertilizer (long-term solution): For elements that are known to be deficient over large areas (such as zinc deficiency in rice paddies in southern my country and the Yangtze River basin), mixing zinc sulfate (15-30 kg/ha) with NPK fertilizers or fine soil and applying it as a base fertilizer or top dressing provides lasting and economical results.
Seed treatment (economical and efficient): Soaking seeds in a solution containing micronutrients (e.g., 0.1% ZnSO₄, 0.05% ammonium molybdate) or seed coating ensures a healthy start for seedlings, especially suitable for fields prone to nutrient deficiencies during the seedling stage.
Foliar spraying (rapid correction): Foliar spraying is the fastest way to correct deficiencies during critical nutrient-demanding periods such as tillering and heading, or when deficiency symptoms appear. Common concentrations: ZnSO₄ 0.2-0.3%, FeSO₄ 0.2-0.5% (a small amount of citric acid needs to be added to prevent oxidation), borax 0.1-0.2%, ammonium molybdate 0.05-0.1%. Spray 2-3 times, with an interval of 7-10 days. Dipping seedling roots: Dipping the roots in zinc oxide or zinc sulfate slurry during transplanting is a traditional and effective method for zinc-deficient fields.
3. Building a Healthy Soil Micro-ecosystem
Increasing organic fertilizer application is a long-term solution: Organic matter acts as a “reservoir” for trace elements, improving their availability through chelation and buffering pH changes. Long-term application of farmyard manure and straw return to the field can significantly reduce the incidence of trace element deficiencies.
Water management regulates availability: Alternating wetting and drying of paddy fields (drying the field) can improve the soil’s redox state, reduce iron and manganese toxicity under reducing conditions, and promote root vitality, thereby improving nutrient absorption capacity.
Selecting tolerant varieties: Plant breeders have developed some rice varieties that are tolerant to zinc deficiency, iron deficiency, or aluminum toxicity. In areas known to have specific elemental problems, selecting corresponding tolerant varieties is an economical and effective biological solution.
Conclusion
Managing trace elements is not simply a matter of “replacing what’s missing,” but a systematic engineering project involving soil chemistry, plant physiology, and agronomic practices. The core lies in understanding the synergistic and antagonistic relationships between elements, the dynamic relationship between soil conditions and availability, and the needs of rice at different growth stages. Future research directions will focus more on: 1) developing efficient and environmentally friendly nano-trace element fertilizers and novel chelating agents; 2) using molecular markers to assist in breeding rice varieties that efficiently utilize trace elements or are tolerant to toxicity; 3) establishing precise variable-rate fertilization techniques based on remote sensing and leaf spectral diagnosis. Only through scientific and refined management can we fully unlock the physiological potential of trace elements, ensuring food security while achieving high-quality, efficient, and ecologically sustainable rice production.