Fruit coloration is the final stage of quality formation; fundamentally, it is the result of a coordinated series of physiological and biochemical processes: chlorophyll degradation, the emergence of carotenoids (yellow/orange), and the massive synthesis and accumulation of anthocyanins (red/purple) in epidermal cells. Fertilizers provide the “raw materials” and “environmental regulation,” whereas plant growth regulators act directly as “signal switches” to control the expression of pigment-synthesis genes. Only by combining the two can one achieve the goal of vibrant, uniform, and safe coloration.
Fertilizers: Providing the Material Basis and a Balanced Environment for Coloration
When it comes to coloration, more fertilizer is not necessarily better; the key lies in the balance between elements and the precise supplementation of critical ones.
1. Key Macronutrient: The “Double-Edged Sword” Effect of Potassium
Potassium is widely recognized as an element that promotes coloration. Its mechanism involves facilitating the transport and conversion of photosynthetic products into the fruit, thereby increasing sugar content—and sugars serve as both precursors and signaling molecules for anthocyanin synthesis. Research shows that treating tomatoes with high concentrations of potassium can double lycopene levels compared to standard treatments and trigger coloration earlier.
However, excessive potassium application can severely inhibit coloration. Studies on “Fuji” apples indicate that as potassium application increases, fruit coloration follows a trend of “rising then falling.” Excess potassium promotes potassium uptake while inhibiting magnesium uptake, leading to reduced anthocyanin content and increased carotenoid content, resulting in yellow rather than red skin. Supplementing with magnesium alongside nitrogen and phosphorus can significantly raise the optimal potassium application range—from 250g to 1000g per tree—effectively mitigating the negative effects of high potassium levels.（1）
Therefore, applying potassium sulfate or potassium dihydrogen phosphate during the coloration stage is a reliable choice. Potassium dihydrogen phosphate supplies both phosphorus and potassium and is highly water-soluble, making it suitable for foliar spraying (typically at a concentration of 0.2%–0.3%).
2. Secondary and Trace Elements: “Supporting Roles” That Cannot Be Ignored
The balance between magnesium and calcium is crucial. The effect of calcium fertilizer on apple coloration is complex; studies indicate that calcium application can increase fruit nitrogen and iron content while simultaneously reducing anthocyanin concentrations and increasing carotenoids—effects that are detrimental to coloration. However, calcium is crucial for maintaining fruit firmness and reducing soft or cracked fruit, so supplementation remains recommended for crops such as citrus. The key lies in avoiding the exacerbation of antagonism between calcium and potassium.
Boron facilitates carbohydrate transport, thereby indirectly supporting coloration. Titanium, a beneficial element, has been proven in dragon fruit coloration programs to enhance the intensity and brightness of the red skin color.
3. Organic and Supplementary Nutrients
Substances such as amino acids, alginic acid, and humic acids do not directly “color” the fruit; however, they stimulate root vitality, enhance photosynthetic efficiency, and improve the tree’s nutritional status, thereby establishing a favorable physiological foundation for coloration. During the citrus color-change period, a common practical strategy is to apply potassium dihydrogen phosphate via fertigation in combination with amino acids or alginic acid.
Plant Growth Regulators: Precisely Regulating Pigment Synthesis Signals
Plant growth regulators have direct and potent effects; consequently, precise timing and concentration are critical during application.
1. Ethephon: A Classic Ripening and Coloring Agent
Ethephon releases ethylene within the plant and serves as a key hormone for promoting fruit ripening and coloration. It accelerates chlorophyll degradation and promotes the accumulation of carotenoids and anthocyanins.
| Crop | Recommended Concentration/Usage | Critical | Period Precautions |
| Grape (Kyoho) | 500–1000 ppm; spray on fruit clusters | Early stage of fruit color change | Excessive concentration (>500 ppm) may cause fruit drop; add 10–20 mg/L NAA (naphthaleneacetic acid) to prevent berry drop |
| Citrus | 200–400 ppm; spray application | Autumn color-change period | Strictly control concentration; excessive levels may cause leaf drop and affect quality |
| Persimmon | 400-fold dilution (approx. 1000 ppm); spray or dip | Onset of coloring | Registered use is for ripening acceleration |
| Tomato | 800–1000 ppm (spray fruit) or 2000–3000 ppm (paint fruit) | Color-change period | Avoid spraying stems and leaves to prevent yellowing |
2. S-Abscisic Acid (ABA analog): A new choice for safe coloring
S-Abscisic acid is a naturally occurring sesquiterpene plant growth regulator. It induces stress resistance in plants and promotes the accumulation of sugars and storage proteins, thereby safely enhancing coloration. Its advantages include relatively minor side effects and a low risk of causing fruit softening or leaf drop.
Registered usage for grapes involves a 170–250-fold dilution, applied as a single, uniform spray on fruit clusters when approximately 20% of the fruit has naturally begun to change color. Official labels note that efficacy is significantly influenced by variety, climate, and vine vigor; users without prior experience should conduct small-scale trials first.
3. Brassinosteroids: A synergistic “booster”
Brassinosteroids do not directly trigger pigment synthesis, but they improve the plant’s physiological status, enhance photosynthesis, and promote nutrient uptake. In citrus color-enhancement programs, they are often sprayed in combination with potassium dihydrogen phosphate, S-abscisic acid, and secondary/micro-nutrients to achieve a synergistic effect.
Core Principles and Risk Warnings
Regardless of whether fertilizers or regulators are used, certain bottom-line principles must be observed:
First, controlling nitrogen is a prerequisite. The application of quick-acting nitrogen fertilizers must be strictly controlled or halted during the coloration stage. Excessive nitrogen can cause fruit to “re-green,” delay coloration, and severely reduce sugar content and flavor quality.
Second, elemental balance is more important than the excess of any single element. More potassium is not necessarily better; excessive potassium can induce magnesium deficiency, leading to poor coloration and yellowing of the peel. When applying potassium, it is essential to ensure the supplementation and balance of magnesium and calcium.
Third, regulators are a “double-edged sword.” Regulators like ethephon are highly effective, but excessive concentrations or improper timing can easily cause leaf drop, fruit drop, and fruit softening. Always consult the registered label or conduct small-scale trials before large-scale application. Adhere to safety intervals before harvest; for instance, it is recommended to stop using ethephon on grapes 10 days prior to harvest.
Fourth, coloration is the result of comprehensive management. Fertilizers and regulators are merely tools; a healthy tree vigor, reasonable crop load, sufficient sunlight, appropriate temperature differentials, and proper water management are the fundamental guarantees for natural fruit coloration.