Micro-climatic components influencing the plant growth
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Plant Response to Greenhouse Environment
The productivity of a crop is influenced not only by its heredity but also by the microclimate around it. The components of crop microclimate are light, temperature, air compositions and the nature of the root medium. In open fields, only manipulation of nature of the root medium by tillage, irrigation and fertilizer application is possible. Even in this case, the nature of the root medium is modified but not controlled. The closed boundaries in greenhouse permit control of any one or more of the components of the micro climate.
1.1 Light
The visible light of the solar radiation is a source of energy for plants. Light energy, carbon dioxide (CO2) and water all enter in to the process of photosynthesis through which carbohydrates are formed. The production of carbohydrates from carbon dioxide and water in the presence of chlorophyll, using light energy is responsible for plant growth and reproduction. The rate of photosynthesis is governed by available fertilizer elements, water, carbon dioxide, light and temperature. The photosynthesis reaction can be represented as follows:
CO2 + Water + Light energy ------- Carbohydrates + Oxygen
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Transmedia Greenhouse Considerable energy is required to reduce the carbon that is combined with oxygen in CO2 gas to the state in which it exists in the carbohydrate. The light energy thus utilized is trapped in the carbohydrate. If the light intensity is diminished, photosynthesis slows down and hence the growth. If higher than optimal light intensities are provided, growth again slows down because of the injury to the chloroplasts. The light intensity is measured by the international unit known as Lux. It is direct illumination on the surrounding surface that is 1 meter from a uniform point source of 1 international candle. Greenhouse crops are subjected to light intensities varying from 129.6 klux on clear summer days to 3.2 klux on cloudy winter days. For most crops, neither condition is ideal. Many crops become light saturated, in other words, photosynthesis does not increase at light intensities higher than 32.3 klux. Rose and carnation plants will grow well under full summer light intensities. In general, for most other crops foliage is deeper green if the greenhouse is shaded to the extent of about 40% from mid spring (May) to mid fall (August and September). Thus, it is apparent that light intensity requirements of photosynthesis vary considerably from crop to crop. Light is classified according to its wave length in nanometers (nm). Not all light is useful in photosynthesis process. UV light is available in the short wavelength range, i.e. less Chlorophyll Plant nutrients than 400nm. Large of quantities of it is harmful to the plants. Glass screens are opaque to the most UV light and light below the range of 325nm. Visible and white light has wavelength of 400 to 700nm. Infrared light (700 to 750nm) affects plants, besides causing photosynthesis. Infrared rays of longer wavelengths are not involved in the plant process. It is primarily, the visible spectrum of light that is used in photosynthesis. In the blue and red bands, the photosynthesis activity is higher, when the blue light (shorter wavelength) alone is supplied to plants, the growth is retarded, and the plant becomes hard and dark in colour. When the plants are grown under red light (longer wavelength), growth is soft and internodes are long, resulting in tall plants. Visible light of all wavelengths is readily utilized in photosynthesis.
1.2 Temperature
Temperature is a measure of level of the heat present. All crops have temperature range in which they can grow well. Below this range, the plant life processes stops due to ice formation within the tissue tying up water, and cells are possibly punctured by ice crystals. At the upper extreme, enzymes become inactive, and again process essential for life cease. Enzymes are biological reaction catalyst and are heat sensitive. All biochemical reactions in the plant are controlled by the enzymes. The rate of reactions controlled by the enzyme often doubles or triples for each rise of temperature by 100C, until optimum temperature is reached. Further increase in temperature begins to suppress the reaction and finally stop it. As a general rule, greenhouse crops are grown at a day temperature, which are 30C to 60C higher than the night temperature on cloudy days and 80C higher on clear days. The night temperature of greenhouse crops is generally in the range of 70C to 210C. Primula, mathiola incana and calceolaria grow best at 70C, carnation and cineraria at 100C, rose at 160C, chrysanthemum and poinsettia at 170C to 180C and African violet at 210C to 220C.
1.3 Relative humidity
As the greenhouse is a closed space, the relative humidity of the greenhouse air will be more when compared to the ambient air, due to the moisture added by the evapo-transpiration process. Some of this moisture is taken away by the air leaving from the greenhouse due to ventilation. Sensible heat inputs also lower the relative humidity of the air to some extent. In order to maintain the desirable relative humidity levels in the greenhouses, processes like humidification or dehumidification are carried out. For most crops, the acceptable range of relative humidity is between 50 to 80%. However for plant propagation work, relative humidity up to 90% may be desirable.
In summers, due to sensible heat addition in the daytime, and in winters for increasing
the night time temperature of the greenhouse air, more sensible heat is added causing a reduction in the relative humidity of the air. For this purpose, evaporative cooling pads and fogging systems of humidification are employed. When the relative humidity is on the higher side, ventilators, chemical dehumidifiers and cooling coils are used for dehumidification.
1.4 Ventilation
A greenhouse is ventilated for either reducing the temperature of the greenhouse air or for replenishing carbon dioxide supply or for moderating the relative humidity of the air. Air temperatures above 350C are generally not suited to crops in greenhouse. It is quite possible to bring the greenhouse air temperature below this upper limit during spring and autumn seasons simply by providing adequate ventilation to the greenhouse. The ventilation in a greenhouse can either be natural or forced. In case of small greenhouses (less than 6m wide) natural ventilation can be quite effective during spring and autumn seasons. However, fan ventilation is essential to have precise control over the air temperature, humidity and carbon dioxide levels.
Natural Ventilation for Green House
Natural Ventilation for Green House
Natural Ventilation for Green House1.5 Carbon dioxide
Carbon is an essential plant nutrient and is present in the plant in greater quantity than
any other nutrient. About 40% of the dry matter of plant is composed of carbon. Under normal conditions, carbon dioxide (CO2) exits as a gas in the atmosphere slightly above 0.0345% or 345ppm. During the day, when photosynthesis occurs under natural light, the plants in a greenhouse draw down the level of CO2 to below 200ppm. Under these circumstances, infiltration or ventilation increases carbon dioxide levels, when the outside air is brought in, to maintain the CO2 at ambient levels. If the level of CO2 is less than ambient levels, CO2 may retard the plant growth. In cold climates, maintaining ambient levels of CO2 by providing ventilation may be uneconomical, due to the necessity of heating the incoming air in order to maintain proper growing temperatures. In such regions, enrichment of the greenhouse with CO2 is followed. The exact CO2 level needed for a given crop will vary, since it must be correlated with other variables in greenhouse production such as light, temperature, nutrient levels, cultivar and degree of maturity. Most crops will respond favorably to CO2 at 1000 to 1200 ppm.
