Ears Plant Photosynthesis Monitoring B.V. (EARS-P2M)

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Quite some research has been done on the genetic modification of plants. An important part is dedicated to the development of herbicide-resistant crop species. In many cases photosynthesis inhibitors are concerned. These act by chemical binding to essential protein complexes of the photo-systems. By genetic modification of these proteins, it is possible to create plants that are resistant to specific herbicides. Nature has already incorporated very efficient proteins in the photosystems of plants, giving maximum photosynthesis of about 80%. Therefore such genetic modifications usually have a negative effect on the photosynthetic and consequently growth capacity of the plant.

In the field also genetic modifications occur. Under the influence of intensive herbicide treatment, resistant weed mutants may develop. An example is Solanum nigrum (Black night shade). The resistant version is not sensitive to atrazine. Measurements with the PPM have shown that the photosynthesis efficiency of this mutant is notably lower. So far research concentrated on the increase of herbicide resistance in plants, but not much attention has been paid to the effects of the genetic modification on productivity. The PPM may be used to investigate this.

Many plants are stressed and may be damaged by too much light. During day-time, two phases of photosynthesis are discerned. At the 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 a 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 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 the fist, screening is necessary if the photosynthesis yield measured with the PPM decreases below 40%.