Introduction
In controlled environments such as plant factories, lighting conditions are critical to plant growth and development. Specifically, Photosynthetic Photon Flux Density (PPFD) and photoperiods (the duration of light exposure) must be fine-tuned for optimal plant yield and health. Understanding and setting appropriate PPFD levels and light periods can prevent plant deformities, promote robust growth, and support the economic feasibility of plant factory operations.
What is Photosynthetic Photon Flux Density (PPFD)?
PPFD measures the amount of light (specifically in the photosynthetically active range of 400–700 nm) that hits a square meter per second (μmol m² s⁻¹). This metric is crucial because it directly impacts photosynthesis – the process plants use to convert light energy into chemical energy. In plant factories, PPFD levels can be controlled to optimize plant growth, ensuring that plants receive adequate light for their development while avoiding excess light that may not yield proportional benefits and could increase operational costs.
Optimal PPFD for Leafy Vegetables and Microgreens
For most leafy vegetables grown in plant factories, a PPFD between 100 and 300 μmol m² s⁻¹ is generally recommended. This range provides enough light for healthy growth and proper plant structure without stretching, which can occur with lower PPFD levels. Specifically:
- PPFD < 100 μmol m² s⁻¹: When PPFD falls below 100 μmol m² s⁻¹, plants tend to stretch excessively, resulting in a light color and improper form. However, for crops like microgreens and sprouts, lower PPFD values can be acceptable due to their shorter growth cycles and different light requirements.
- PPFD > 300 μmol m² s⁻¹: A PPFD above 300 μmol m² s⁻¹ generally benefits the growth of many plants. However, in a commercial setting, this increase in PPFD may lead to higher initial costs due to the need for more advanced lighting equipment. For plant factories focused on economic feasibility, maintaining a balance between sufficient PPFD and cost-effectiveness is essential.
Setting the Daily Light Integral (DLI) for Leafy Vegetables
In addition to PPFD, Daily Light Integral (DLI) is an essential metric for controlled environments. DLI is the total amount of light received over a 24-hour period, measured in mol m² d⁻¹. For leafy crops like lettuce, a DLI between 12 and 17 mol m² d⁻¹ is recommended to achieve optimal growth.
To meet these DLI requirements, various combinations of PPFD and photoperiod (hours of light per day) can be adjusted:
- High PPFD with Short Photoperiod: For instance, a PPFD of 300 μmol m² s⁻¹ over an 11-hour photoperiod can achieve a target DLI of approximately 12 mol m² d⁻¹.
- Moderate PPFD with Longer Photoperiod: Alternatively, a PPFD of 210 μmol m² s⁻¹ for a 16-hour photoperiod can reach a similar DLI, making it a flexible choice for plant factories with varying lighting systems and energy budgets.
Special Considerations: Continuous and Intermittent Lighting
In some cases, plant factories may implement a 24-hour photoperiod, particularly for specific crop varieties that tolerate continuous lighting. This can maximize DLI without the need for extremely high PPFD levels, making it efficient for certain crop types. Additionally, intermittent lighting methods can be used strategically to avoid physiological disorders in sensitive plants, offering precise control over growth cycles and plant health.
Conclusion
Setting appropriate PPFD and photoperiods is a nuanced but vital aspect of operating a plant factory. By carefully managing these parameters, growers can achieve optimal growth rates, maintain healthy plant structures, and ensure cost-effective lighting solutions. With controlled lighting tailored to each crop’s needs, plant factories can produce high-quality leafy greens and other vegetables sustainably and efficiently.
