Detailed Explanation of Light Requirements for Legal Horticultural Crops (General Plant Physiology Principles)

In indoor cultivation or greenhouse supplemental lighting, the core elements of light management include: photoperiod (light/dark period), light intensity (PPFD), spectral quality (light color), and daily light intake (DLI).

1. Seedling Stage

Goal: To promote root development, cultivate robust seedlings with short internodes, and prevent etiolation.

Photoperiod: Typically requires a relatively long light duration, 14-18 hours/day, to accumulate sufficient photosynthetic products.

Spectral Requirements: Broad-spectrum light or cool white light with a high proportion of blue light (color temperature 5000K-6500K). Blue light helps inhibit internode elongation, resulting in shorter, sturdier plants, and promotes stomatal opening and chlorophyll synthesis.

Light Intensity: Relatively low, approximately 150-250 µmol/m²/s (PPFD). Excessive light may stress young seedlings.

Key Points: 1. **Even Light Distribution:** Uniform light distribution is crucial to ensure all seedlings receive equal light and grow uniformly.

2. **Vegetative Growth Stage:**

Goal: Maximize healthy leaf and stem growth, establishing a robust photosynthetic system.

Photoperiod: Long days are still required; 16-18 hours/day is a common setting.

Spectral Requirements: Full-spectrum or a spectrum rich in red and blue light. Red light (around 660nm) promotes stem elongation and leaf expansion; blue light (around 450nm) continues to control plant shape and promote leaf thickening. A balance between these two is critical at this stage.

Light Intensity: Significantly increased, typically ranging from 400-600 µmol/m²/s or higher, depending on the crop. Sufficient strong light is key to rapid growth.

Key Points: Ensure light intensity is matched with temperature, carbon dioxide concentration, and nutrient supply to achieve optimal growth rates.

3. **Flowering and Fruiting Stage:**

Goal: Induce flowering, promote flower development, pollination, and fruit enlargement.

Photoperiod: For many flowering crops, changes in the photoperiod are a key trigger signal. The transition from long-day to a "short-day" cycle of 12 hours of light/12 hours of darkness is a common method for simulating autumn environments and inducing flowering.

Spectral Requirements: The ratio of red to far-red light (730nm) becomes particularly important. Increasing the proportion of red light promotes flowering and photomorphogenesis. The ratio of far-red to red light affects the plant's "shade avoidance response" and flowering time. At the same time, adequate blue light is still needed to ensure plant health and quality.

Light Intensity: Typically maintained at a high level, 500-800 µmol/m²/s or higher, to meet the enormous energy demands of flower and fruit development.

Key Points: Strict continuity of the dark cycle is required at this stage; any light pollution can interfere with flowering and lead to physiological disorders.