1 Physiological Role of Sulfur in Plants
1.1 Basic Properties & Natural Occurrence Forms
Sulfur is an essential element for plants, atomic number 16, symbol S, with an average crustal abundance of approximately 0.048%. It mainly exists as sulfides (ferrous disulfide FeS₂), sulfates (calcium sulfate CaSO₄, magnesium sulfate MgSO₄), and organic sulfur derived from animal and plant residues.
Plants primarily absorb sulfur in the form of sulfate ions (SO₄²⁻); foliage can also absorb sulfite ions (SO₃²⁻) or low-concentration gaseous sulfur dioxide (SO₂).
1.2 Sulfur Absorption & Transport Mechanisms
Absorption form: Roots take up SO₄²⁻ via ATP-consuming active transport; foliage absorbs sulfur through stomata.
Transport pathways:
- Lateral transport: Root epidermis → cortex → vascular cylinder
- Longitudinal transport: Xylem transports sulfur upward with the transpiration stream; phloem enables bidirectional translocation. Sulfur features moderate mobility, higher than calcium and boron yet lower than nitrogen and magnesium. Distribution trait: Sulfur accumulates in metabolically active tissues including young leaves, immature fruits and root tips. Partial sulfur stored in old leaves can migrate to newly growing organs.
1.3 Existing Forms & Core Traits Inside Plants
- Organic sulfur (>90%): Sulfur-containing amino acids (cysteine, methionine), proteins, enzymes, vitamins (biotin, thiamine), glucosinolates, etc.
- Inorganic sulfur (<10%): Free SO₄²⁻ in cell sap and sulfates stored in vacuoles. Core characteristic: Partially reusable. Organic sulfur in mature leaves can be partially decomposed and translocated; therefore sulfur deficiency symptoms usually first appear on young leaves.
2 Core Physiological Functions
2.1 Structural Backbone of Proteins & Enzymes
Sulfur is an indispensable component of cysteine and methionine, the core raw material for protein synthesis, and forms the active center of enzymes.
It ensures normal protein synthesis and stable cell structure, activates amylase and protease, and regulates carbon and nitrogen metabolism.
2.2 Auxiliary Support for Photosynthesis
Sulfur participates in chlorophyll synthesis and the formation of thylakoid membrane lipids to stabilize photosynthetic structures.
It improves photosynthetic efficiency, takes part in ATP synthesis during light reactions, and supplies energy for dark reactions.
2.3 Functional Factor for Stress Resistance & Defense
It stimulates synthesis of defensive compounds such as glucosinolates, glutathione (GSH) and phytochelatins.
It enhances tolerance to drought, cold and saline-alkali conditions, alleviates heavy metal toxicity, and suppresses pests like aphids and powdery mildew.
2.4 Quality Optimizer for Fruits
Internal quality: Boosts accumulation of proteins and amino acids, raises sugar content, synthesizes aromatic thioether substances and improves fruit flavor.
External appearance: Maintains dark green foliage, promotes uniform fruit pigmentation and reduces malformed fruits. It strengthens fruit epidermis toughness to lower fruit cracking and postharvest losses.
2.5 Regulator of Nutrient Balance
Facilitates synergistic uptake and utilization of nitrogen, phosphorus, potassium, magnesium and other nutrients.
Sulfate ions (SO₄²⁻) maintain cellular osmotic pressure, balance cation concentration and stabilize intracellular environment.
3 Rules of Sulfur Demand at Different Crop Growth Stages
表格
| Growth Stage | Sulfur Demand Intensity | Core Functions | Sensitivity to Sulfur Deficiency |
|---|---|---|---|
| Seed Germination Stage | Low | Stimulate radicle and plumule growth, synthesize primary proteins | Low |
| Seedling & New Shoot Stage | Medium to Relatively High | Build complete photosynthetic system | Medium |
| Flower Bud Differentiation Stage | Relatively High | Support protein synthesis for flower bud development | Relatively High |
| Blooming & Fruit Setting Stage | High | Boost pollination and fertilization, cut physiological fruit drop | High |
| Fruit Expansion & Ripening Stage | Extremely High | Support quality formation (accumulation of sugar and protein) | Extremely High |
| Post-Harvest Recovery Stage | Medium | Accumulate nutrients and reserve sulfur for next cycle | Medium |
4 Hazards & Root Causes of Sulfur Deficiency
4.1 Typical Symptoms of Sulfur Deficiency
Whole plant: Stunted growth, thin weak stalks, few branches and feeble vigor.
Leaves: Uniform chlorosis on young leaves (similar to nitrogen deficiency, yet nitrogen deficiency first yellows old leaves — this is the key distinguishing feature). In severe cases, leaves turn pale white, scorched, small and thin.
Flowers & fruits: Sparse blossoms, low fruit set rate, undersized malformed fruits, uneven pigmentation and low sugar content.
4.2 Main Causes of Sulfur Deficiency
Soil factors: Low native sulfur content in sandy or acidic soil; sulfur depletion caused by continuous cropping; inappropriate pH (sulfur leaches in acid soil and gets fixed in alkaline soil).
Environmental factors: Heavy rainfall at high temperature accelerates sulfur leaching; drought blocks root sulfur absorption.
Cultivation management: Long-term application of sulfur-free fertilizers (urea, diammonium phosphate DAP); insufficient organic fertilizer input.
