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We talked about the effects of light intensity and carbon dioxide on plant growth before. For plants, not more light or carbon dioxide is better, and not less is better. Let's take a look at what the light saturation point and light compensation point are.

Light saturation point

Within a certain light intensity range, the photosynthetic rate of plants increases with the increase of light intensity, and when the light intensity rises to a certain level, the photosynthetic rate does not continue to increase.

Light compensation point

Below the light saturation point, when the light intensity decreases, photosynthesis also decreases, and the light intensity when the organic matter produced by the plant through photosynthesis and the material consumed by respiration is in balance is called the light compensation point.

To make it easier to understand, we can briefly outline the difference between photosynthesis and respiration: photosynthesis is the production of organic matter, and respiration is the decomposition of organic matter.

From the above graph, we can see that when the light intensity is 0, the plant only performs respiration. When the light intensity of photosynthesis and respiration are the same, the formation and consumption of organic matter are equal, and dry matter cannot be accumulated, and the light intensity at this time is the light compensation point. When the light continues to increase, photosynthesis rate is greater than respiration rate. At this time, the carbon dioxide produced by respiration is not enough to satisfy photosynthesis, and plants need to absorb more carbon dioxide. That is to say, the light intensity must exceed the light compensation point before plants can accumulate organic matter and grow. If it is below the light compensation point for a long time, the respiration will continue to consume organic matter, and the plant will gradually turn yellow and even die. Therefore, when you need to supplement additional light for your indoor grow, make sure that the intensity of the light source hitting the plant leaves is higher than the light compensation point.

When the light reaches a certain intensity, the photosynthesis rate does not increase with the increase of light intensity, and this intensity is the light saturation point. When the light saturation point is reached, the rate of photosynthesis of plants is the highest, which means the fastest growth rate.

The light saturation point and light compensation point vary by plant species.

According to the degree of adaptation of plants to light, plants are generally divided into sun-loving plants, shade-loving plants and shade-tolerant plants.

Sun-loving plants need strong light, so the light compensation point is high. Shade-loving plants can grow with low intensity light, the light compensation point is low. Shade-tolerant plants have a light requirement between sun-loving and shade-loving plants. The light compensation point and light saturation point of the plant indicate the degree of shade tolerance of the plants. The lower the light compensation point and the light saturation point, the stronger the shade tolerance of the plant.

In 1883,  the German botanist J. Reinke first discovered the phenomenon of light saturation in plants and pointed out that the light saturation point depends on the object under study. Shade-loving plants (eg.deep-water algae) reach light saturation at 10% or less of full sunlight at sea level; sun-loving plants, especially desert or alpine plants, have not yet reached light saturation under direct sunlight at noon.

figure 2: Data is for reference only.