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Biological Significance
Temperature
Temperature controls the rate of plant growth. Generally, as temperatures increase, chemical processes proceed at faster rates.
Most chemical processes in plants are regulated by enzymes which, in turn, perform at their best within narrow temperature ranges.
Above and below these temperature ranges, enzyme activity starts to deteriorate and as a result chemical processes slow down or are stopped.
At this point, plants are stressed, growth is reduced, and, eventually, the plant may die.
The temperature of the plant environment should be kept at optimum levels for fast and successful maturation.
Relative Humidity
The relative humidity (RH) of the greenhouse air influences the transpiration rate of plants.
High RH of the greenhouse air causes less
water to transpire from the plants, which causes less transport of nutrients from roots
to leaves and less cooling of the leaf surfaces. High humidities can also cause disease problems in some cases.
For example, high relative
humidity encourages the growth of botrytis and mildew.
CO2
The CO2 concentration of the greenhouse air directly influences the amount of
photosynthesis (growth) of plants. Normal outdoor CO2
concentration is around 350 parts per million (ppm). Plants in a closed greenhouse during a bright day can deplete the CO2 concentration to 100
ppm, which severely reduces the rate of photosynthesis. In greenhouses, increasing CO2 concentrations to 1000-1500 ppm speeds growth.
CO2 is supplied
to the greenhouse by adding liquid CO2.
Lights
Light measurements are taken with a quantum sensor, which measures Photosynthetically
Active Radiation (PAR) in the units micromoles/m2s/sec. PAR is the light which is useful to plants for the process of
photosynthesis. Measurements of PAR give an indication of the possible amount of
photosynthesis and growth being performed by the plant. Foot-candle sensors and lux meters are
inappropriate because they do not directly measure light used for
photosynthesis.
Dissolved Oxygen
Dissolved oxygen (DO) measurements indicate the amount of oxygen available
in the pond nutrient solution for the roots to use in respiration.
Lettuce will grow satisfactorily at a DO level of at least 4 ppm. If no oxygen
is added to the pond, DO levels will drop to nearly 0 ppm. The absence of
oxygen in the nutrient solution will stop the process of respiration and seriously
damage and kill the plant. Pure oxygen is added to the recirculation system
in the ponds. Usually the level is maintained at 8 ppm.
pH
The pH of a solution is a measure of the number of hydrogen ions. The pH of a solution can range between 0 and 14. A neutral solution has a pH of 7.
That is, there are an equal number of hydrogen ions (H+) and hydroxide ions (OH-).
Solutions ranging from pH 0-6.9 are considered acidic and have a greater concentration of H+.
Solutions with pH 7.1-14 are basic or alkaline and have a greater concentration of OH-.
The pH of a solution is important because it controls the availability of the fertilizer salts.
A pH of 5.8 is considered optimum for the described lettuce growing system, however a range of 5.6-6.0 is acceptable.
Nutrient deficiencies may occur at ranges above or below the acceptable range.
Electrical Conductivity
Electrical conductivity (EC) is a measure of the dissolved salts in a solution.
As nutrients are taken up by
a plant, the EC level is lowered since there are fewer salts in the solution.
Alternately, the EC of the solution is increased when water is removed from the solution
through the processes of evaporation and transpiration. If the EC of the solution increases, it can be lowered by adding pure water, e.g., reversed osmosis water).
If the EC decreases, it can be increased by adding a small quantity of a concentrated nutrient stock solution.
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