Tadashi Takakura

A new relative referencing method for crop monitoring using chlorophyll fluorescence

The measurement of plant chlorophyll fluorescence has been used for many years as a method to monitor a plants health status. These types of methods have been mostly relegated to the laboratory. The newly developed Relative Referencing Method allows for the measurement of chlorophyll fluorescence under artificial lighting conditions. The fluorescence signal can be determined by first taking a reference signal measurement, then a second measurement with an additional fluorescence excitation source. The first signal can then be subtracted from the second and the plants chlorophyll fluorescence due to the second lighting source can be determined. With this simple approach, a photosynthesizing plant can be monitored to detect signs of water stress. Using this approach experiments on tomato plants have shown that it was possible to detect water stress, while the plants were continuously illuminated by fluorescent lamps. This method is a promising tool for the remote monitoring of crops grown in a CELSS-type application.

Water and Water Vapor Environment

Water movement in soil is much more complicated than that of CO2 because not only is water present in two phases, one liquid and the other gas, but also water movement is related to heat flow. However, in order to simplify the problem and understand the system clearly, it is again assumed that vapor flow in soil is independent of heat flow, as we assumed in the chapter on heat flow analysis. Furthermore, we will focus only on vapor movement in the present chapter, because in most cases water flow is assumed to occur in parallel with water vapor flow and they are combined. This can be condensed into what is the diffusion coefficient for both.

Plant Response to the Environment

As mentioned in the preceding chapter, it is very important to include a plant growth sub-model in the total model. However, plant growth, particularly under cover, is sophisticated, and modeling it is not simple. This is one of the main reasons why most greenhouse models do not have or have only rather simple sub-models of plant growth. In the open field, the time courses of environmental conditions such as air temperature and carbon dioxide concentration are smoother than those in some greenhouses where the objective is to optimize the environment. Basic response curves, such as photosynthesis and respiration responses to light and temperature conditions, have been studied for many years and are useful for modeling plant growth in open fields. In protected cultivation such as heated greenhouses with CO2 enrichment facilities, some conditions can be changed drastically and positively.

Definition of Covering and Properties of Covering Materials

There are many kinds of covering materials and techniques available and used in practical agriculture. It is rather difficult to distinguish one technique from the other in some cases because the principle involved is the same. The following terms are used for these systems: mulchings, row covers (tunnels), floating mulches (floating row cover), rain shelters, and unheated greenhouses.

Temperature Environment under Cover

Temperature is another important environmental condition for plant growth. The temperature of the plant environment can be modified by cover the area. In the daytime, the main energy source is, of course, the sun, and its energy is stored in the soil. The transmissivity of the film used for covering is an important factor in getting a large amount of energy into the soil surface. At night, the energy source is thus the soil layer. The main heat loss from the surface is due to long wave radiation, and the emissivity of the cover is a key factor in this.

Solar Radiation Environment

Solar radiation is one of the most important environmental factors for plant growth. In general it is known that radiation is generated from a material close to a blackbody. Solar radiation received at the earth’s surface varies with the season because of the planetary relation between the sun and the earth. Therefore, for cultivation in controlled environments, it is very important to calculate how much of the solar radiation we can utilize at a given place on the earth at a given time of the year.

CO 2 Environment

Carbon dioxide concentration in the air is another important factor for plant growth. The CO2 concentration in the open air varies daily. On a sunny day the CO2 level is fairly constant around 300 to 350 µl/l due to photosynthesis by plants, and it rises to around 400 to 450 at night. The CO2 level is more important under cover, especially in plastic houses where air exchange is normally restricted in order to raise temperatures in a cold climate. As a result, the carbon dioxide concentration falls below the outside level during the daytime due to photosynthetic CO2 uptake. Therefore, sometimes CO2 enrichment is conducted in plastic houses.