Light is a fundamental factor for plant growth, and selecting the right plant lights is crucial to provide adequate illumination. Two important concepts to consider when choosing plant lights are the light compensation point and the light saturation point. These terms are vital in determining the effectiveness of artificial lighting for promoting photosynthesis in plants.
What is Light Compensation Point
The light compensation point is the critical threshold at which a plant's photosynthesis rate equals its respiration rate. In other words, for a plant to grow, the rate of photosynthesis must exceed the rate of respiration. This point is measured in terms of the intensity of photosynthetically active radiation (PAR), also known as PPF (Photosynthetic Photon Flux) or PPFD (Photosynthetic Photon Flux Density). PPFD is the unit of measurement for PPF per unit area.
For instance, a 20W T8 LED tube light, placed 30cm above the plant, can provide approximately 24μmol/s/m² of PPFD. If the light compensation point for a specific plant, such as tomatoes, is 51.6μmol/s/m², it means that the effective radiation for photosynthesis must exceed this value for the plant to grow optimally. In practical applications, this may require the use of multiple lights.
However, it's important to note that some plants, like tomatoes, tend to grow vertically, which complicates the calculation of the required light intensity. In greenhouse settings, 630W LED lights suspended at a height of 4 meters may only provide around 90μmol/s/m² of PPFD.
What is Light Saturation Point
The light saturation point represents the level of light intensity at which a plant's photosynthesis rate is maximized. Beyond this point, further increases in light intensity do not lead to increased growth and can even have adverse effects. The light saturation point for tomatoes, for example, is around 1998μmol/s/m².
In practice, the most powerful artificial lights available, such as 1000W double-ended HPS lights with a PPF of 2100μmol/s, might only provide around 200μmol/s/m² of PPFD when hung in a 6-meter high greenhouse. This falls far short of the PPFD levels achieved by natural sunlight, which can exceed 2000μmol/s/m² during peak daylight hours.
In summary, the selection of plant lights should take into account both the light compensation point and the light saturation point of the target plant species. Understanding these concepts is essential for optimizing growth conditions. While artificial lighting can be beneficial in controlled environments, it is clear that nothing can replace the intensity and quality of natural sunlight for plant growth. Balancing the type and quantity of artificial lighting with these two critical points is key to successful indoor and greenhouse cultivation.