This document discusses greenhouse environment factors that influence plant growth, including air and soil temperature, light intensity and quality, and carbon dioxide levels. It provides information on: - The optimal temperature ranges for growth of various crop species. Temperatures outside these ranges can cause chilling, heat stress, or inhibit development. - Factors that control air and soil temperatures in greenhouses, such as passive/active heating and cooling methods. - The importance of light intensity, wavelength, day length, and other light qualities on photosynthesis and plant growth. Optimal levels vary by crop but generally fall within the PAR range of 400-700nm. - Other environmental factors like relative humidity, carbon dioxide, and their effects.

1. BIRSA AGRICULTURAL UNIVERSITY
Protected Cultivation and Secondary
Agriculture
LECTURE 6: GREEN HOUSE ENVIRONMENT
BY
DR. PRAMOD RAI
DEPARTMENT OF AGRICULTURAL ENGINEERING

2. GH Environment
Air environment
Air temperature
Light intensity, quality & day length
RH
Carbon dioxide
Air velocity
Soil environment
Soil temperature
Soil aeration/moisture

3. Temperature
Each plant has certain environmental requirements. To attain the
highest potential yields a crop must be grown in an environment that
meets these requirements.
Most plants function in a relatively narrow range of temperatures.
The extremes of this range may be considered killing frosts at about
0ºC and death by heat & desiccation at about 40ºC.
Air temperature
Chilling injury
Heat Stress
Vernalization
Soil temperature

4. Air temperature
Each kind of crop grows & develops most rapidly at a favorable
range, called the optimum air temperature range. For most crops
the optimum functional efficiency occurs mostly between 12 and
24ºC.
Most crops (and especially vegetables) can be classified according
to the temperature requirements of their optimum air temperature
range.
Temperature requirements are usually based on night temperature.
Those that grow & develop below 18ºC are the cool season crops,
and those above 18ºC are the warm season crops.
Chilling Injury
Most crops are injured at temperatures at or slightly below
freezing.
Susceptibility to cold damage varies with different species and
there may be differences among varieties of the same species.
The susceptibility to cold damage varies with stage of plant
development.

5. Heat stress
When temperatures rise too high, heat destruction of the protoplast
results in cell death. This occurs in the range of 45-55ºC.
In tomatoes, fruits exposed to high temperatures & solar radiation can
reach 49-52ºC. If green fruits are exposed for an hour or more, they
become sunburned; and ripe fruits become scalded.
Transpiration from the leaf stomata helps cool leaves. It has been
calculated that transpiration can reduce heating by about 15 to 25%.
Symptoms of heat injury are the appearance of dead areas in leaves of
hypocotyls and young leaves of many plants.
Vernalization
Vernalization is the exposure of plants to low temperatures for
extended periods of time, which then induces or accelerates flowering
(or bolting). Bolting is unwanted flowering.
The required length of low temperature exposure varies with species.
Certain tubers, corms, and bulbs require low temperatures following
moderately high temperatures before growth occurs.

6. Air temperature for tomato
The tomato grows well in warm weather, air temperatures of
10ºC or below will delay seed germination, inhibit vegetative
development, reduce fruit set and impair fruit ripening.
The tomato plant cannot tolerate frost.
High air temperatures, above 35ºC, reduce fruit set and
inhibit development of normal fruit color.
The optimum air temperature range for normal plant growth,
development and fruit set is between 18.5 and 26.5ºC, with
day & night time temperature ranges being 21 to 29.5ºC and
18.5 to 21ºC, respectively.
The growing day base temperature is 10.5ºC, a temperature
below which growth is negligible.
Although air temperature is a critical factor affecting normal
vigorous plant growth, the canopy (leaf) temperature may be
far more important.

7. Soil temperature
It is generally lower than that of the air but seasonal
fluctuations can occur with depth depending on soil and above
ground factors.
Above ground plant development as well as changes in soil
water status can also influence seasonal changes in soil
temperature at various depths.
It has direct dramatic effects on seed germination, root
development, water and nutrient absorption by roots,
microbial growth and development, organic matter decay. In
general, the higher the temperature the faster these processes
occur.
The size, quality, and shape of storage organs are greatly
affected by soil temperature.
Dark-colored soils absorb more solar energy than light colored
soils. The capacity of water to move heat from one area to
another (conduction) is greater than that of air. Heat is therefore
released to the surface faster in clay soils than in dry sandy
soils.

8. Soil temperature for tomato
The optimum root temperature observed to be between 20 and 30ºC
and at temperatures less than 20ºC, plant growth will be significantly
reduced.
Water uptake by tomato plants is also significantly reduced as the
root temperature decreases. Water use drops sharply as the rooting
temperature is less than 20ºC and when it is greater than 30ºC.
It is observed that as the root temperature ranged from 10 and 40ºC,
uptake of the major elements and micronutrients by tomato plant
was significantly affected.
Fig. 1: The influence of root zone temperature
on major nutrient element uptake by the tomato
plant. (Source: Tindall et.al. 1990.)
Fig. 2: The influence of root zone temperature
on micronutrient uptake by the tomato plant.
(Source: Tindall et.al. 1990.)

9. Species
Min.
Lethal
Temp.
Min.
Biologic
al Temp.
Nightly
Optimal
Temp.
Daily
Optimal
Temp.
Max.
Biologic
al Temp.
Min.
Germinat
ion
Temp.
Opt.
Germina
tion
Temp.
Soil
Opt.
Temp.
oC
CO2
(ppm)
R.H. %
Vegetables
Cucumber 0 to 2 10 to 13 18 to 20 24 to 28 28 to 32 14 to 16 20 to 30 20 to 21
1000 to
3000
70 to 90
Melon 0 to 4 12 to 14 18 to 21 24 to 30 30 to 34 14 to 16 20 to 30 20 to 22 - 60 to 80
Pumpkin 0 to 4 10 to 12 15 to 18 24 to 30 30 to 34 14 to 16 20 to 30 15 to 20 - -
Beans 0 to 2 10 to 14 16 to 18 21 to 28 28 to 35 12 to 14 20 to 30 15 to 20 - -
Pepper 0 to 4 10 to 12 16 to 18 22 to 28 28 to 32 12 to 15 20 to 30 15 to 20 - 65 to 70
Eggplant 0 to 2 9 to 10 15 to 18 22 to 26 30 to 32 12 to 15 20 to 30 15 to 20 - 65 to 70
Lettuce -2 to 0 4 to 6 10 to 15 15 to 20 25 to 30 4 to 6 20 10 to 12
1000 to
2000
50 to 80
Tomato 0 to 2 6 13 to 16 22 to 26 26 to 30 9 to 10 20 to 30 15 to 20
1000 to
2000
55 to 60
Flowers
Carnayion -4 to 0 4 to 6 10 to 12 18 to 21 26 to 32 26 to 32 - 15 to 18
500 to
1000
70 to 80
Rose -6 to 0 8 to 12 14 to 16 20 to 25 26 to 32 30 to 32 - 15 to 18
1000 to
2000
70 to 75
Gerbera 0 to 2 8 to 10 13 to 15 20 to 24 - - 20 to 22 18 to 20 - 60 to 70
Chrysanthe
mum
- 6 to 8 13 to 16 20 to 25 25 to 30 25 to 30 - 18
400 to
1200
60 to 70
Hortensia - - 10 to 18 16 to 20 25 to 27 25 to 27 - 18 to 20 - 70 to 80
Tulip - 4 to 6 12 to 18 22 to 25 - - - 8 to 12 - 70 to 80
Lily - 3 to 5 8 to 15 15 to 20 - - - 10 to 13 - 60 to 70
Daffodil - 3 to 5 8 to 15 15 to 20 - - - 10 to 13 - 60 to 70
Ciclamen - 2 to 4 12 to 18 20 to 22 - - 18 to 20 14 to 16 - 60 to 70
Pelargoniu
m
- 6 to 10 14 to 16 14 to 16 26 to 30 26 to 30 20 to 25 -
1000 to
2000
60 to 70
Table 1: CLIMATICAL CONDITIONS FOR THE CULTIVATION OF DIFFERENT SPECIES
Minimal and optimal thermal level of the ambient, optimal level of CO2, Relative Humidity and soil temperature for vegetable
and flower cultivation
Tables obtained from the book “MODERNE TECNICHE DI PROTECIONE IN OTRICULTURA, FLORICULTURA E FRUTUCULTURA” by Romano Tesi.

10. Growing Degree Days (GDD)
It is a weather based indicator for assessing crop
development.
It can be used to access the suitability of a region for
production of a particular crop, estimate the growth stages of
crops, weeds or even life stages of insects, predict maturity
& cutting dates of forage crops, predict best timing of
fertilizer or pesticide application, estimate the heat stress on
crops, plan spacing of planting dates to produce separate
harvest dates.
It is calculated by taking the average of the daily maximum
and minimum temperatures compared to base temperature.
GDD = (Tmax+Tmin)/2- Tbase
It is accumulated by adding each day‟s growing degrees
contribution as the season progresses.
If the average temperature is below the base
temperature, the GDD value for that day is zero.

11. Control of air/soil temperature
Air temperature
Passive/active cooling
Passive/active heating
Soil temperature
Passive/active cooling of air (indirect effect)
Passive/active heating of air (indirect effect)
Heating of soil (direct)
Plastic mulching (direct)

12. Light
Light intensity/irradiance (radiant flux density
or energy level) is the degree of brightness that a
plant receives & is a major factor governing the rate
of photosynthesis. The quantity or amount of light
received by plants in a particular region is affected
by the intensity of the incident (incoming) light and
the length of the day.
Light quality/spectral quality (the wavelength or
color) refers to the specific wavelengths of light. A
color of light would be the relative distribution of
wavelengths from a radiation or reflective source.
Day length (length of the daily lighting period) is
the duration of the day with respect to the night
period.

13. Solar radiant energy/Sunlight/natural light/Solar Radiation
Light is a form of electromagnetic radiation that is visible to the
human eye.
The radiation that we perceive as sunlight, or the visible spectrum, is a
small fraction of the total electromagnetic spectrum that includes
gamma rays, x-rays, and radio waves.
Measured as Energy [W m-2] Watts per sq. meter, or as Number of
Photons [µMol m -2s-1] micro Mol per sq. meter per sec, within a
waveband
Waveband
Ultra-Violet or
UV
100-400 nm
Visible or
White “light”
380-770 nm
PAR 400-700 nm
Infrared or IR 750-1,000,000 nm
Fig. 3: Electromagnetic Spectrum
Table 2: Waveband of Solar Radiation

14. Solar Radiation
Radiation from the sun can be described by its wavelength or
its frequency.
Light, or visible radiation for humans, can be considered as a
wave (with an associated wavelength), or as a particle of
energy photon (with an associated energy value). When
considered as a photon it may be expressed in energy terms,
Watts per square meter [W m-2], or as the number of photons
[moles of photons] μmol m-2 s-1.
Wavelength has units of meters, typically nanometers (nm)
[one billionth of a meter] or micrometers (um) [one millionth
of a meter]. Frequency has units of cycle per second.
Energy = h * ν =
For radiation (light), as its wavelength increases, its energy
decreases, and as the wavelength decreases, the energy
increases. Thus short wave blue light has more energy than
longer wave red light.

15. Important terms
Transmittance: ratio of transmitted radiant energy to that incident
Absorbtance: ratio of the absorbed radiant energy to that incident
Reflectance: ratio of the reflected radiant energy to that incident
Direct radiation: Straight beam radiation directly arriving from the sun
to the sensor or plant
Diffuse radiation: Radiation has been reflected by the atmosphere or
glazing
Total radiation = direct + diffuse
PAR, Photosynthetically Active Radiation (400-700 nm): waveband of
the electromagnetic spectrum used by the plant for photosynthesis
PPF, Photosynthetic Photon Flux density: number of photons in the
PAR waveband that are incident on a surface in a given time period. The
units are µMol m -2s-1
PI, Photosynthetic Irradiance: radiant energy stream in the PAR
waveband, incident on a surface in a given time period. The units are
Wm-2
Spectral Irradiance (SI): energy value or distribution for each
wavelength within a waveband is measured as µMol m -2s-1 nm-1

16. The spectrum and various wavebands
Of the total solar radiation reaching the earth‟s surface, 97%
of the spectral distribution is within the 280-2,800 nm. Of
this, about 44% is PAR or visible, about 4% is UV, and the
remainder 52% is IR.
The leaf typically absorb nearly 95% of wavelengths between
400-700 nm, while only 5% of the 700-800 nm waveband is
absorbed.
Fig. 4: Radiation spectrum with various wavebands (from Hanan, 1998)

17. Ultra-Violet or UV: It is the wavelengths less than 400 nm. The UV
is divided into UV-A waveband (320-400 nm), UV-B waveband
(280-320 nm), and UV-C (100-280 nm). The shorter wavelengths
have higher energy, thus UV-B and UV-C can be dangerous. Natural
sunlight has a large amount of UV-A, a small amount of UV-B, and
no UV-C.
Visible light: It is based on the sensitivity of the human eye and is
within the 380-770 nm waveband.
The relative absorption of the wavelengths of PAR for the
chlorophyll molecules (type a & b) show that there is a strong
absorption between 400 to 480 nm (blue), and also between 630 to
680 nm (red). Note that there is some absorption at almost all the
remaining PAR wavelengths, but at a significantly reduce relative
value.
Fig. 5: Absorption spectra (from Levine, 1969)

18. Table 3: The “colors” of the radiation visible to humans can be
divided into the following wavebands:
Waveband Color Function in Plant
380-436 nm Violet Uncertain, but may support effect of blue light.
436-495 nm Blue A minimal quantity is necessary to prevent tall, weak plants
495-566 nm Green Unnecessary, but contributes to photosynthesis
566-589 nm Yellow Unnecessary, but contributes to photosynthesis
589-627 nm Orange Optimize for maximum photosynthesis
627-770 nm Red Optimize for maximum photosynthesis; enhances
flowering, stem elongation; Red/Far-red ratio is important
Infrared or IR is the wavelengths greater than 770 nm and up to
1,000,000 nm, and it includes the Near Infrared or NIR which is
within the 770-850 nm waveband, and the IR-A or the short wave
infrared (770-1,400 nm). The IR-A waveband has the greatest
heating effect. The NIR does include wavelengths that influence the
growth of the plant.
Red: Far-red (R:FR) ratio consists of two narrow wavebands which
influence plant growth responses.

19. Fig. 6 shows a sloping curve
representing the increasing rate of
plant growth with increasing radiation
energy applied.
The sloped curve clearly shows that
the growth rate is increasing, but at a
decreasing rate. Ultimately the curve
turns horizontal, meaning that any
additional light energy will not provide
any increase in plant growth rate.
Fig. 7, which includes a comparison of
what the human eye “sees” (line 1),
relative to the wavelengths of light
which have been determined important
to a plant leaf (line 2). Clearly, the
human eye cannot even begin to
respond to many of the wavelengths
before 500 nm and beyond 600 nm.
The plant leaf response, however,
extends beyond the PAR waveband of
400-700nm.
Fig. 6: Rate of plant growth with applied radiation
energy (from Poot Lichtenergie B.V., 1984)
Fig. 7: Relating sensitivity of human eye
(line 1) and cucumber leaves (line 2) for
wavelength of light (nm) (from Poot
Lichtenergie B.V., 1984)

20. Photomorphogenesis
Photomorphogenesis is defined as the ability of light to regulate
plant growth and development, independent of photosynthesis.
Plant processes that appear to be photomorphogenic include
internode elongation, chlorophyll development, flowering,
abscission, lateral bud outgrowth, and root and shoot growth.
Photomorphogenesis differs from photosynthesis in several
major ways. The plant pigment responsible for light-regulated
growth responses is phytochrome. Phytochrome is a colorless
pigment that is in plants in very small amounts.
Only the red (600 to 660 nm) and far red (700 to 740 nm)
wavelengths of the electromagnetic spectrum appear to be
important in the light-regulated growth of plants. The
wavelengths involved in generating photosynthesis are generally
broader (400 to 700 nm) and less specific.
Photomorphogenesis is considered a low energy response-
meaning that it requires very little light energy to get a growth-
regulating response. Plants generally require greater amount of
energy for photosynthesis to occur.

21. Light within GH
The amount of light entering a GH is
influenced by
Type of structure
Shape and pitch of the roof
Orientation to the sun
Location of the equipment within GH
Types of cladding materials & its condition

22. Sensors
Pyranometer sensor: measures solar radiation from 280-2800
nm, 97% of the sun‟s spectral distribution “total solar”
radiation. Units are W m-2
Quantum sensor: is PAR waveband (400-700 nm) measured
as Mol µm-2s-1 or W m-2
Net Radiometer: determines the difference of the radiation
measured above to that being reflected from below a surface
Spectroradiometer:
•splits incoming radiation into individual wavelengths or
prescribed wavebands, then measures the irradiance (energy)
of the photons.
•measures spectral irradiance as µMolm-2s-1 nm-1or Wm-2nm-1

23. Conversion of irradiance and luminosity to PAR or vice versa
Quantum PAR sensors measure light only in the
photosynthetically active part of the spectrum, while
irradiance and luminosity sensors measure light over a
broader range.
For these reasons it is not possible to determine the PAR
value from the readings on a luminosity or irradiance
meter. Like wise a PAR sensor cannot be used to calculate
the total Irradiance (w/m-2).
While it is possible to convert engineering units within a
given measurement parameter such as luminosity,
converting between parameters such as irradiance and
Quantum PAR is not as easy.
This is because most light sources emit a spectrum of
wavelengths and each wavelength carries a different
amount of energy. Therefore the conversion factor is
different for every light source.

24. Artificial lighting
In commercial production, artificial light sources are used in a variety
of ways:
Replacement lighting: complete replacement of solar radiation for
indoor growth rooms and growth chambers
Supplemental or production lighting: used in GH to supplement
periods of low natural light.
Photoperiod lighting: used to stimulate or influence photoperiod
dependant plant responses such as flowering or vegetative growth .
The need for and quality of artificial illumination required is
determined by a number of factors including:
The light requirements of the species being grown
The natural day length
The average hours of sunlight
The sun angle and intensity (latitude and weather)
The amount of structure-induced shading

25. Types of Lamps
In commercial production, artificial light sources are used in a
variety of ways:
Incandescent
Fluorescent
Discharge
In lighting terminology, the word „lamp‟ refers to the light bulb or
tube, and the word luminaire refers to the entire light fixture:
lamp + reflector + ballast + housing.
A perfect artificial light source would provide 100% conversion
of electrical energy into light, in a spectrum optimally balanced
for plant growth. In reality, no such light source exists, not even
the sun.
Lamp efficiency, life span, intensity, spectral quality, cost, and
electrical requirements must be weighed against the crop
demands and the intended application before choosing any
supplemental or solar replacement source.

26. Incandescent
It typically emit light as a result of the heating of a tungsten
filament to about 25000C.
Only about 15% of the energy (watts) applied to an incandescent
lamp is radiated in the PAR (photosynthetically active radiation)
range of 400-700 nm, 75% is emitted as infrared (850-2700) nm,
and the remaining 10% is emitted as thermal energy (> 2700 nm).
They are not very light efficient and a relatively short lamp life, it
is usually not the most effective radiation sources for providing
supplementary light.
They are, however, useful for phytochrome-dependent
photoperiod control since they are relatively inexpensive to install
and operate, they can be cycled on and off frequently, and they
produce large amounts of red and infrared radiation.
Fig. 8: Spectrum of Incandescent lamp (Argus control system ltd, 2010)

27. Fig. 9: Incandescent lamps
(Source: www.brightubengineering.com)

28. Fluorescent
It produce light from the excitation of low pressure mercury
vapor in a mixture of inert gases.
They are more light efficient than incandescent lamps and they
have a much longer life span.
They are available in three load types: normal output 400 mA
(normal output), 800 mA (high output), and 1500 mA (very high
output).
One disadvantage of fluorescent lamps is their relative bulk in
relation to output.
They are available in a range of spectral qualities. Relatively
inexpensive cool white lamps are fine for supplementary lighting,
and „full spectrum‟ lamps are available for replacement lighting
applications.
Fig. 10: Fluorescent of (cool white) lamp (Argus control system ltd, 2010)

29. Fig. 11: Fluorescent lamps
(Source: www.msue.anr.msu.edu )

30. Discharge
It converts electrical energy into PAR very efficiently.
The light intensities and efficiencies obtained by high intensity
discharge are higher than either incandescent or fluorescent lamps.
Metal halide: They provide the best overall spectral distribution of
all horticultural lamps, but are not quite as efficient in energy
conversion as high-pressure sodium lamps in the PAR range,
particularly in the yellow-red spectra.
High pressure sodium: It has become the most popular lamp type
for commercial supplemental lighting in horticulture. They are the
most efficient in the PAR range with the exception of low pressure
sodium lamps .
Fig. 12: Spectrum of Metal Halide
(Argus control system ltd, 2010)
Fig. 13: Spectrum of High Pressure Sodium
(Argus control system ltd, 2010)

31. Fig. 14: High Intensity Discharge lamps
(Source: http://archiandesigns.files.wordpress.com)
Fig. 15: Metal Halide lamps
(Source: http://archiandesigns.files.wordpress.com)
Fig. 16: High Pressure sodium lamps
(Source: https://www.sylvania.com)

32. RADIATION SOURCE EFFICIENCY LUMENS
PER WATT
AVERAGE LIFE
(HOURS)
INCANDESCENT 12 – 26 1000 – 3000
METAL HALIDE 80 – 90 8000 – 20000
HIGH PRESSURE SODIUM 117 15000 – 24000
FLUORESCENT 52 – 84 12000
MERCURY VAPOR 50 – 60 24000+
SPECIES DAILY DURATION
(HOURS)
IRRADIANCE
(W m-2)
TOMATOES
(propagation)
12 – 24 8 – 15
CUCUMBERS
(propagation)
12 – 24 8 – 15
PEPPERS
(propagation)
12 – 24 8 – 15
FOLIAGE PLANTS* 12 – 24 3 – 6
BEDDING PLANTS* 12 – 24 5 – 15
CHRYSANTHEMUMS 12 – 24 (long days)
< 12 (short days)
5 – 15
ROSES 18 – 24 5 – 8
Table 4: Radiant Efficiency for Supplementary Irradiation
(adapted from Norton, 1972)
Table 5: Typical Supplementary Illumination Treatments
for Commercial Greenhouse Crops (various sources)

33. Table 6: Climatic requirement for flowers & vegetables (NCPAH, 2011)
Source: Internet researchable
Sl. No. Name of crop
Temperature (oC)
RH (%)
Light Intensity
(lux)
Day Night
Flowers
1. Camation 16 – 20 10 – 12 60 – 65 40000 – 50000
2.
Chrysanthemum
a) Cut flower
b) Pot
22 – 24
23 – 26
15 – 16
16 – 19
60 – 65
60 – 65
35000 – 40000
35000 – 40000
3. Gerbera 20 – 24 18 – 21 60 – 65 40000 – 50000
4. Orchid 22 – 24 18 – 20 70 – 80 25000 – 30000
5. Anthurium 22 – 25 18 – 20 70 – 80 25000 – 30000
6. Rose 24 – 28 18 – 20 65 – 70 60000 – 70000
Vegetables
7. Tomato 22 – 27 15 – 19 50 – 65 50000 – 60000
8. Cucumber 24 – 27 18 – 19 60 – 65 50000 – 60000
9. Capsicum 21 – 24 18 – 20 50 – 65 50000 – 60000
Nursery
10. Nursery 22 – 27 16 – 19 50 – 65 50000 – 60000

34. Table 7: Optimum temperature, RH and PAR, measured as photosynthetic photon
flux density (PPFD), for selected greenhouse crops grown in warm climatic regions
(NCPAH, 2011)
Crop
Optimum
Temperature
(oC)
Optimum RH (%) PPFD (µmol m-2s-1)
Eggplant
25-28 day
14-16 night
65-75 504
Cucumber 25-30 80-90 400
Tomato
23-27 day
13-16 night
50-60 400
Peppers
22-30 day
14-16 night
60-65 504
Lettuce
24-28 day
13-16 night
65-80 260-290
Strawberry
20-26 day
13-16 night
50-65 200-400
Beans
22-26 day
16-18 night
70-80 336-420
Peas
25-30 day
16-18 night
70-80 672

35. Control of light intensity in GH using shading
Shading should be done very carefully, especially permanent
shading.
It reduces both the temperature & the light intensity.
Therefore it is important to consider both factors when decide
which shading technique to be utilized.
Application of external shading compounds: It will diffuse
light rays & reflect heat, come either in white or green and
can be thinned using paint solvent, difficult to apply & to
remove the uniform coating, semi permanent and during
cloudy & dark days of summer; the plants in the GH may
receive insufficient light.
Installing a shading screen or net over/under the GH
frame: reduce light level is to block out light with some
shading screens made of cloth, polypropylene, polyester or
aluminum-coated polyester, inside screen serve two purposes;
reduce light in the summer and act as a thermal blanket in the
winter.

36. Humidity
The absolute humidty, H is defined as kilogrammes of water
vapour present in one kilogramme of dry air under a given set of
conditions.
H depends upon partial pressure of water vapour, pw in air and
total pressure, P.
Therefore, H can be expressed mathematically as follows :
H =
Percentage humidity
It is the ratio of the weight of water present in 1 kg of dry air at
any temperature and pressure and the weight of water present in 1
kg of dry air which is saturated with water vapour at the same
temperature and pressure.
Percentage humidity = (H/Hs) × 100

37. Relative humidity
It is defined as the ratio of the partial pressure of water
vapour in the air to the partial pressure of water vapour
in saturated air at the same temperature:
RH = (pw/ps) × 100
The relation between percentage humidity and RH
Percentage humidity =
The amount of water vapor that the air can hold
depends on its temperature.
The relative humidity affects the opening and closing
of the stomata which regulates loss of water from the
plant through transpiration as well as photosynthesis.

38. Vapour Pressure Deficit
Plants respond to the difference between humidity levels at the
leaf stomata and the humidity levels of the surrounding air.
Another measurement called the vapour pressure deficit (VPD) is
often used to measure plant: air moisture relationships.
The VPD equals the difference in the amount of water vapour in
the leaf (always assumed to be 100% RH) and the outside air.
Fig. 17: Vapour Pressure Deficit (VPD) increases with temperature, even
when the relative humidity remains constant (BC, 2015)

39. Role of humidity
The main plant mechanism to cope with humidity is
the adjustment of the leaf stomata. Stomata are pores
in the underside of the leaf that open and close in
response to vapour pressure deficit.
Transpiration: Plants can control the rate of water
loss by opening and closing the leaf pores or stomata.
Photosynthesis: Humidity levels indirectly affect the
rate of photosynthesis because C02 is absorbed
through the stomatal openings.
Growth and Quality: Most GH plants tend to grow
better at higher RH. However, mineral deficiencies,
disease outbreaks, smaller root systems and softer
growth are possible consequences of excess
humidity.

40. Control of RH in GH
Humidity in a GH is a result of the balance between transpiration of
the crop and soil ET, condensation on the GH cover and vapour loss
during ventilation.
The humidity requirements for selected crops grown in GH ranges
between RH levels of 50% to 80%.
Humidity is the most difficult environmental factor to control in
terms of GH heating, cooling and related equipment.
The problem is exacerbated by the introduction of energy-saving
mechanisms such as shading, insulation and reduced air exchange,
which are all associated with an increase in humidity.
Evapotranspiration (ET) from plants plays an important role in
determining the air moisture content inside a GH. It is dependent on
plant type, total leaf area, and irradiance levels.
ET rates are higher during the daytime in GH that use solar
irradiance. Plants continually release moisture, which needs to be
removed to maintain RH levels.
Most research activities have focused on removing moisture from
GH.

41. Fig. 18: Active and passive humidity control methods for GH
(Rabbi et. al., 2019)
Greenhouse Humidity
Control
Active Passive
Humidification Dehumidification
Water addition
using
misting/fogging
Water addition
using evaporative
pads
Desiccant
dehumidification
Heat pump
dehumidification
Condensation in
heat exchangers
Solid
Liquid
Shading temperature
control
Ventilation

42. CO2 in open field
The photosynthetic process, a plant leaf combine
molecules of CO2 and water in the presence of sunlight to
form carbohydrates and oxygen.
3CO2 + 6H2O ►C6H12O6 + 3O2
The atmospheric air contains about 0.03 per cent (300
ppm) CO2 , the process continues without any concern
about the availability of CO2.
This level of CO2 in the atmospheric air is sufficient to
meet the photosynthesis requirement of open field crops.
The higher level of CO2 have been observed to
significantly contribute high crop yield.
To create such situation in open field condition is just
impractical.

43. CO2 in GH
In the GH, the enclosed air may have a CO2
concentration of 1000 ppm because of respired CO2
remained trapped overnight.
As the sunlight becomes available, photosynthesis
process begins and CO2 from the greenhouse air gets
depleted.
Owing to this, the CO2 level in greenhouses goes below
even 300 ppm much before noon. Obviously, if the
greenhouse air does not receive additional CO2 from
some other sources, the plant would become CO2
deficient.
The level of CO2 in air at which rate of photosynthesis
equals the rate of respiration, is called compensation
point.
It becomes inevitable to supply CO2 from external source
to maintain favorable levels for optimum production.

44. Factors affecting CO2 uptake
The amount of CO2 uptake by leaves depends
upon several factors:
Plant species and variety
Radiation intensity
Wind velocity
Water stress
CO2 concentration in air
Resistance to CO2 diffusion through the
stomates
Previous history of plant, and
Leaf area

45. Fig. 19: Dinural variation of CO2 concentration
in GH
Fig. 20: Photosynthesis in tolerant and
intolerant species as a function of light intensity
Fig. 21: Relationship between CO2 concentration,
light intensity and photosynthesis in wheat
Fig. 22: Effect of internal water stress on
photosynthesis

46. CO2 enrichment
The CO2 concentration drops below the atmospheric level
whenever the CO2 consumption rate by photosynthesis is
greater than the supply rate through the greenhouse vents.
The solution is to increase the ventilation rate through
forced air, to improve design and management of the
ventilation system, or to provide CO2 enrichment.
The level to which the CO2 concentration should be
raised depends on the crop, light intensity, temperature,
ventilation, stage of the crop growth and the economics of
the crop.
Enrichment reportedly increases crop yield and quality
under a CO2 concentration of 700– 900 μmol mol-1.
Optimal CO2 enrichment depends on the margin between
the increase in crop value and the cost of providing the
CO2 gas.

47. Advantages
Increase in photosynthesis results in increased
growth rates and biomass production.
Plants have earlier maturity and more crops can
be harvested annually. The decrease in time to
maturity can help in saving heat and
fertilization costs.
Supplemental CO2 provides additional heat
(depending upon the method of
supplementation) through burners, which will
reduce heating cost in winter.
It helps to reduce transpiration and increases
water use efficiency, resulting in reduced water
use during crop production.

48. Disadvantages
Higher production cost with a CO2 generation system.
Plants may not show a positive response to supplemental
CO2 because of other limiting factors such as nutrients,
water and light. All factors need to be at optimum levels.
Incomplete combustion generates harmful gases like
sulphur dioxide, ethylene, carbon monoxide and nitrous
oxides. These gases are responsible for necrosis, flower
malformation and senescence if left unchecked, resulting
in a lower quality products.
Additional costs required for GH modification. GH need to
be properly sealed to maintain a desirable level of CO2.
Excess CO2 level can be toxic to plants as well as humans.
On warmer days, it is difficult to maintain desirable higher
CO2 levels because of venting to cool the GH.

49. CO2 enrichment methods
Pure (liquid) carbon dioxide gas
Refillable steel bottles or from a bulk storage vessel. Liquid C02 is normally
free of impurities and is therefore the safest source.
It is easily transportable and distributable (low volume, as it is 100% C02).
Its availability is not related to production of heat. Hence pure C02 is
considered as the ideal method for enrichment.
The price of the commercially available pure C02 cannot compete usually
with that of C02 obtained by combustion of natural gas.
Combustion of an appropriate fossil fuel (Either with small burners
inside GH or central burner & heat storage facility
Small burner inside the GH is the oldest and still most common method for
C02 enrichment. Such burners release heat and flue gases together directly in
the GH (although there are some types with a chimney). A major
disadvantage is that they are used primarily for heating, whereas CO2is
considered of minor importance.
The alternative method for flue gas C02 supply is by using a large burner
connected to a central hot-water pipe heating system. The flue gases of such a
burner can be made available for C02 enrichment.

50. Quantity of CO2 for enrichment
The amount of CO2 required for enrichment is the sum of
amount of CO2 used by plants and loss of CO2 through
infiltration. The amount used by plants varies with the
micro climatic parameters, type of crop, level of nutrition.
The infiltration loss can be determined using the
expression given below:
IL = Vg × N × 10-6 × (DL – 300)
where,
IL = Infiltration loss (m3/h)
Vg = Volume of greenhouse (m3)
N = Number of air charge/h
DL = Designed CO2 level
The loss of CO2 is more in glass houses than plastic film
greenhouse owing to differences in infiltration rate.

51. Selecting the source of CO2
Purity of carbon dioxide: Sulfur dioxide and ethylene are
contaminants that can injure crops when present in very
low concentrations. Is the carbon dioxide gas or the fuel
used to produce carbon dioxide relatively low or free of
these toxic compounds.
Cost of carbon dioxide
Installation and investment costs
Operating costs
Heat of combustion: The heat produced by combustion
units located inside of the GH may present problems in
maintaining optimum day temperatures.
Water vapor: Combustion units produce water vapour
simultaneously with carbon dioxide. Units located within
the GH may increase humidity levels to a point where
diseases can be problem.

52. Table 8: Important Climatic factors influencing plant growth
Sl.
No.
Climatic Factors Important for Desirable level in
GH
1. Radiation / Light Photosynthesis, Photo-
morphogenesis, Photoperiodism
50,000 lux*
2. Temperature Cell division & elongation,
respiration, photosynthesis water
uptake, transpiration etc.
100C – 250C
3. Relative humidity Quality of plant 60% – 80%
4. Carbon dioxide
(CO2)
Photosynthesis 350 ppm – 1000
ppm
5. Air movement /
wind movement
Influences temperature, relative
humidity & CO2 concentration in
the GH, structural stability
Inflow: Outflow
ation should be 1:1
per hour
6. Rainfall /
precipitation
Influences RH, structural stability –
Source: Internet researchable * 1 lux = 1 lumen/square meter

53. Air velocity
Air velocity in the GH is an important factor since
the distribution of temperature and RH follows the
air flow pattern.
Non-uniform distribution of air velocity leads to
non-uniform temperature and RH and consequently
non-uniform crop growth, development and
maturity.
The ideal GH airflow rates ranges between 0.5 and
0.7 m/s and air velocity values above 1 m/s result
in high transpiration rate and water stress.
The role of air velocity in the GH is therefore
indispensable in attempts at microclimate
management.

54. Soil aeration/moisture
Under waterlogged conditions, all pores in the soil
or soilless mixture are filled with water; so the
oxygen supply is almost completely deprived.
As a result, plant roots cannot obtain oxygen for
respiration to maintain their activities for nutrient
and water uptake.
Plants weakened by lack of oxygen are much more
susceptible to diseases caused by soil-borne
pathogens.
Waterlogging due to lack of oxygen in the soil
causes death of root hairs, reduces absorption of
nutrients and water, increases formation of
compounds toxic to plant growth, and finally
retards growth of the plant.

55. If you have any question/suggestion
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