Preventing light damage

Many plants are stressed and maybe even 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 may damage the plant ("burning"). Greenhouse plant growers will then 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 greehouse. Growers can use the PPM to measure exactly the screening light level of their plants.

The new PPM-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 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 increasing heat development. Since PAR light levels in greenhouses on sunny days can easily reach 1500 µmole/m²/s screening is essential to prevent damage to the plant. The moment of screening is usually reached if the photosynthesis yield measured with the PPM decreases below 40%.