Greenhouse Illumination Control
Conference contribution, 2010
Illumination of European greenhouses consumes electricity equivalent to 1.5 times the electricity production in Sweden. This amount could be reduced by approximately 75% by improving the illumination. Today HPS lamps (High pressure sodium lamps) are used. The spectrum of the HPS lamp fits poorly with the action spectrum of the photosynthesis, why most of the illumination cannot be used. By combining LEDs of different wavelengths, a spectrum closer to the action spectrum of the photosynthesis can be achieved. Experiments carried out with Heliospectra HB LED-lamp on the crop basil showed 40% improved efficiency compared to HPS lamps.
Further improvements are possible, since the LED-lamp allows the intensity of each wavelength-band to be adjusted to the varying needs of plants. A plant’s need, and its efficiency in using light, can be measured through plant fluorescence and reflectance. The aim of this project is to derive mathematical models that correlate plant growth with reflectance and illumination, and fluorescence with physiological state of the plant and plant stress. These models will then be used for a control system for the LED-lamp with which the plants are able to modulate their own light environment. Expected results are decreased energy use, predictable harvest and higher crop quality.
Reflectance and growth rate
Reflectance from plants has showed a general relationship to chlorophyll content in leafs, leaf area index and plant hight. Estimates of green biomass has been done from remote sensing of reflectance from satellites as well as on ground level through the use of reflectance indices. These indices, for example the NDVI, are based on relations between the reflectance of the wavelengths that are the least and the most sensitive to chlorophyll content. Since plant reflectance is sensitive to chlorophyll content it is reasonable to assume that it can be correlated also to growth rate. By remote (at about one meter distance) and on leaf measurements of plant reflectance and measurements of dry matter content in plants we hope to find a correlation between growth rate and plant reflectance. The relationship between growth rate and reflectance is then to be used for non destructive determination of biomass content and prediction of time for harvest.
Fluorescence and plant stress
Fluorescence measurements contains information about the physiological state of plants. The physiological state of a plant is affected by access to specific wavelengths of light (especially 680 and 700 nm), CO2, water and nutrients. Unbalanced access to these growth factors blocks the transfer of absorbed light energy and causes deexcitation of chlorophyll molecules, which gives rise to fluorescence. Hence fluorescence intensity can be used to measure if the plants are stressed due to lack of any of these parameters.
From fluorescence measurements it can also be determined whether the plants are exposed to damaging light intensities. Damaging light intensities causes break down of chlorophyll.