Preventing light damage
Many plants are stressed and may be damaged by too much light. During daytime two phases of photosynthesis are discerned. At low light level the photosynthesis rate is limited by the available light. With increasing light level the photosynthesis rate reaches the point where the energy for the reaction and the uptake of CO2 are in equilibrium. At higher light levels the photosynthesis cannot increase anymore and the excess light will inhibit photosynthesis and may damage the plant ("burning"). Greenhouse plant growers will use light screens to reduce the light level. However they do not know the exact light level to start screening the plants. Moreover the screening light level is somewhat variable and will not only depend on species but also on variety, light adaptation after winter, possible water shortage and CO2 levels applied in the greenhouse. Growers can use the PPM to measure exactly the screening light level of their plants.
The miniPPM-300 is most suitable for this purpose. This instrument measures fully automatically the photosynthesis light curve of a plant species. The figure above shows an example of such measurement. The red line presents the photosynthesis rate as a function of the photosynthetic active radiation intensity (PAR). Optimum photosynthesis occurs at 780 micromole/m2s. Photosynthesis at that level is about 190 micromole/m2s, which corresponds to a dry matter production of 1.9 gram/m2hr. At higher light levels photosynthesis decreases again and damage may occur due to increasing increasing heat development. Since PAR light levels in greenhouses on sunny days can easily reach 1700 µmole/m2s screening is essential to prevent damage to the plant. As a rule of fist, screening is necessary if the photosynthesis yield measured with the PPM decreases below 40%.