This document discusses the effects of soil and air temperature on plant growth. It covers several topics: 1. Soil temperature affects plant growth both directly and indirectly, and different crops thrive at different soil temperature ranges. Solar radiation, organic matter decomposition, and microbial activity contribute to soil heating. 2. Canopy architecture and leaf orientation impact photosynthesis by influencing light interception. Temperature, along with other factors, also affects canopy temperature depression. 3. Both high and low air and soil temperatures can inhibit plant growth. Optimal temperatures exist for photosynthesis, root development, and germination for different plant species. Temperature impacts plant metabolic processes and disease susceptibility.

1. Temperature variation in soil and plant
canopy architecture
P. Dhamodharan
2019502205
Department of agronomy

2. Soil temperature
• Soil temperature is an important plant growth factor like air, water and nutrients.
• Soil temperature affects plant growth directly and indirectly.
• Specific crops are adapted to specific soil temperatures.
• It also influences the structure, moisture, aeration, microbial and enzyme activity
of the soils.
• All soil heat comes from two sources, viz, radiation from the sun and sky and
conduction from the interior of the earth.
Eg: Apple grows well when the soil temperature is about 18°C, maize 25°C, potato
16 to 21°C, and so on.

3. Sources of soil heat
• Solar radiation
• Microbial decomposition of organic matter
• Respiration by soil organisms
• Internal source of heat

4. Transmission of heat
The transmission of heat within the soil is dependent on
• the physical properties of soil particles,
• their degree of compaction and
• the moisture content of the soil.
• Heat transfer through the soil also depend on the specific heat of the
soil. Solid materials are responsible for conduction while pores are
responsible for convection and radiation.

5. Solar constant
• It is given by the ratio L /4πr 2= 1.94cal/min/sq.cm
where L is the luminosity of the Sun and r the distance
• varies by +/-3%
• The Sun-Earth distance is smaller when the Earth is at
perihelion (first week in January) and larger when the
Earth is at aphelion (first week in July).
• Solar flux, or concentrated sunlight, is a measure of how much light energy is being radiated in a given
area. Solar flux can be characterized by the familiar W/m² or kW/m².

6. Factors Affecting Soil Temperature
1. Aspect and slope:
These factors are of great importance in determining soil temperature outside the tropics.
• In the Northern Hemisphere, a south-facing slope is always warmer than a north-facing slope or a
level plain.
• The reverse is the case in the Southern Hemisphere.
• The difference in soil surface temperature exceeds the difference in air temperature.
2. Soil texture:
Because of lower heat capacity, sandy soils warm up and cool down more rapidly than clay
soils; hence, they are at a higher temperature during the day and a lower temperature at night.

7. 3. Tillage:
• By loosening topsoil and creating mulch, tillage reduces the heat flow between the surface and
subsoil.
• Because a mulched surface has a greater exposed area and the capillary connection with moist
layers below is broken, cultivated soil has greater temperature amplitude than uncultivated soil.
• At noon, the air temperature 2.5 cm above the soil surface can be 5 to 10 C higher in cultivated
soil as compared to uncultivated soil.
4. Organic matter:
• Organic matter reduces the heat capacity and thermal conductivity of soil, increases its water-
holding capacity, and has a dark color which increases its solar radiation absorptivity.
• In humid climates, because of a large water content, peat and marsh are much cooler than
mineral soils in spring and warmer in winter.
• However, when organic soils are dry, they become warmer than mineral soils in summer and
cooler in winter.

8. Temperature regime of soil
• Energy absorbed by the soil surface may be re-radiated to the atmosphere as
long wave radiation.
• This may heat the air above the soil by convection, raise the surface temperature
or conducted downward and raise the temperature of lower layers (Wynne and
Marlowe 1979).
• The temperature regime of soil surface responds closely to the radiant energy
budget.
• The temperature regime of the soil surface comprise two cyclical periods, the
diurnal and annual. Day time heating and night time cooling are responsible for
diurnal period and annual period result from variation in short wave radiation
throughout the year (Marshall and Holmes 1988)

9. Effects of soil temperature on some soil properties
and plant growth
• Effects on biological properties of the soil
• Effects on chemical properties of the soil
• Effects on physical properties of the soil

11. Soil heat bank
• The soil heat bank refers to the amount of heat absorbed and retained by the soil during the day.
• This heat is then radiated back into the crop canopy overnight to warm flowering heads,
minimising frost damage.
• The amount of heat stored depends on a number of factors such as row spacing which affects
canopy closure, soil colour, stubble loads and soil moisture.
• A moist soil profile will store more heat than a dry soil.

12. Soil temperature versus depths for (a) day (b) year (Oke 1978)

13. Effect of soil temperature on plant growth
Soil temperature requirements of plants:
• The soil temperature requirements of plants vary with the
species.
• The temperature at which a plant thrives and produces best
growth is called optimum range (temperature).
• The entire range of temperature under which a plant can grow
including the optimum range is called growth range.
• The maximum and minimum temperatures beyond which the
plant will die are called survival limits.

15. Availability of soil water and plant nutrients
• Low temperatures reduce the nutrient availability, microbial activities and root
growth and branching.
• The ability to absorb nutrients and water by plants reduces at low temperatures.

16. Soil Temperature Management
• Use of organic and synthetic mulches: Mulches
keep soil cooler in hot summer and warm in cool
winter.
• Soil water management:
High moisture content in humid temperate
region lowers soil temperature.
• Tillage management: Tilling soil to break the
natural structure reduces the heat conductance
and heat loss. A highly compact soil looses heat
faster than loose friable soil.

17. Methods Of Measuring Soil Temperature
• Mercury soil thermometers
• Thermo couple and thermister based devices
are also available.
• Infra-red thermo meters measure the surface
soil temperature.
• Automatic continuous soil thermographs
record the soil temperatures on a time scale.
• The International Meteorological
Organization recommend standard depths to
measure soil temperatures at 10, 20, 50 and
100 cm.

18. Effect of soil temperature on
root development
Potato

20. Seasonal foliage temperatures of wheat (cv. Kanking) and cotton (cv. Paymaster 145) - Lubbock, Texas.
• Foliage temperatures were measured with a 50° field-of-view Teletemp Model 50 infrared
thermometer (Teletemp Corp., Fullerton, CA) positioned at 1.5 m above the crop.
• Instruments were scanned at 1 min intervals with a 15 min average computed and stored.
• The infrared thermometer viewed an area of 0.75 m2, with the same area continuously sampled (from
Burke et al., 1988)
The vertical lines represent the temperature range that comprises the species-specific thermal kinetic window as determined from the
changes in the apparent Km of purified enzymes with temperature.

21. Mean weekly photosynthetic rate (fmol CO2/m2/s) and duration of photosynthetic activity (weeks, in
parentheses), and grain biomass of two wheat genotypes grown at two temperature regimes
Few studies have successfully partitioned the effects of elevated CO2 on growth and maintenance
respiration. Both components appear to decline, probably because of decrease in leaf protein
levels. (Wullschleger et al., 1994).

22. Plant canopy
• A plant canopy consists of an assemblage of plants with leaves that possess a
particular spatial distribution and assortment of angle orientations (de Wit,
1965; Monsi et al., 1973).
• The major factors affecting canopy photosynthesis, through light interception,
include the angular relationship between leaves and Earth-sun geometry and the
leaves' vertical and horizontal positions.

23. Factors affecting canopy temperature depression (CTD) in plants

26. Leaf Architecture
• Theoretical calculations predict that photosynthetic rates of canopies with
erect leaves, and high leaf area indices, are less inclined to light-saturate.
• Consequently, canopies with erect leaves can achieve photosynthetic rates
that are 70-100% greater than those whose leaves are arrayed horizontally.
• The spatial pattern of plant stands and leaves also affects canopy
photosynthesis.
• Crowns that shade 100% of the ground attain canopy photosynthetic rates
that are almost double those that shade only 25% of the ground (Wang et
al., 1992).
• Clumping of leaves within a crown enhances the probability of beam
penetration through canopies and increases rates of canopy
photosynthesis as compared to a canopy with leaves that have a random
spatial distribution and spherical angle distribution (Gutschick, 1991;
Baldocchi and Harley, 1995)

28. Temperature
• The response of canopy CO2 exchange rates to temp. is parabolic. The temp. optimum of canopy
CO2 exchange rates of many crops and forests growing in temperate continental climates, under
full sunlight, is on the order of 20-30ºC.
• The temperature optimum, however is very plastic, it can vary with species, ecotype, site, and
time of year (Stenberg et al., 1995).
• In general, leaf photosynthesis decreases markedly at leaf temperatures exceeding 37ºC This
diminution occurs from a decrease in membrane stability.
• The zero crossing for canopy CO2 exchange occurs in the range between 30 & 35~ (Jeffers and
Shibles, 1969; Baldocchi, 1997).
• Freezing reduces subsequent photosynthetic capacity appreciably.
• Low soil temperatures can also reduce photosynthesis through effects on water balance and
stomatal conductance (Stenberg et al., 1995).

29. Cardinal Temperatures
1. A minimum temperature below which no growth occurs: For typical cool-season
crops, it ranges between 0 and 5 C, and for hot-season crops between 15 and 18 C.
2. An optimum temperature at which maximum plant growth occurs: For cool-
season crops, it ranges between 25 and 31 C, and for hot-season crops between 31
and 37 C.
3. A maximum temperature above which the plant growth stops: For cool-season
crops, it ranges between 31 and 37 C, and for hot-season crops between 44 and
50C.

31. Soil Temperature and Crop Germination
Soil temperatures influence
• The germination of seeds,
• the functional activity of the root system,
• the incidence of plant diseases, and
• the rate of plant growth

32. • Excessively high soil temperatures are also harmful to roots and cause lesions on the stem.
• Low temperatures impede the intake of nutrients.
• Soil moisture intake by plants stops when they are at a temperature of 1 C.
• Root growth is generally more sensitive to temperature than that of above ground plant parts
• Germination of warm-season grasses is very poor during the winter season.
• Slower germination rates during cooler seasons require long periods of soil water availability at the
surface to enable germination
• Germination of sunflower, maize, and soybean is very poor when day/night soil temperature is above
21/12°C

33. In the tropics, high soil temperature
causes degeneration of the tuber in
potato. Optimum soil temperature
for this crop is 17 C. Tuber formation
is practically absent above a soil
temperature of 29 C.

34. Impact of Soil Temperature on Plant Growth
• The daytime soil temperature is more important than the nighttime temperature
• Favorable soil temperature is needed for ion and water uptake
• Soil temperature controls the rate of maize development while the meristem is
underground
• Increased soil temperature accelerates the rates of leaf tip appearance and full
leaf expansion, enabling the crop to more rapidly attain maximum green leaf-area
index
• This enables a better synchrony between the time of peak radiation interception
and peak radiation incidence.
• The extent to which soil temperature affects yield will therefore vary with sowing
time and the latitude of the crop’s location(Stone, Sorensen, and Jamieson,
1999).

35. • Optimal soil temperature for growth of wheat plant roots during the vegetative
stage is below 20°C and is lower than that for the shoots.
• Temperatures higher than 35°C have been shown to reduce terminal root growth
and accelerate its senescence.
• Root growth may cease altogether if soil temperatures drop below 2°C.
• Studies have shown (Porter and Gawith, 2000) that an air temperature of –20°C is
lethal for root survival, although this must be translated into a soil surface
temperature, which would, in most cases, be higher.

36. Air temperature on plant canopy
• The midday high temperature increases the saturation deficit of
plants.
• It accelerates photosynthesis and ripening of fruits.
• The maximum production of dry matter occurs when the temperature
ranges between 20 and 30 C
• When high temperature occurs in combination with high humidity, it
favors the development of many plant diseases.
• High temperature also affects plant metabolism.

37. • High night temperature increases respiration.
• It favors the growth of the shoot and leaves at the cost of roots, stolons,
cambium, and fruits.
• It governs the distribution of photosynthates among the different organs of the
plants, favoring those which are generally not useful for human consumption.
• High night temperature also affects plant metabolism.
• It accelerates the development of noncryophytes
• Most crop plants are injured and many are killed when the night temperature is
very low.
• Tender leaves and flowers are very sensitive to low temperature and frost. Plants
that are rapidly growing and flowering are easily killed.
• Low temperature interferes with the respiration of plants. If low temperature
coincides with wet soil, it results in the accumulation of harmful products in the
plant cells.

39. Canopy temperature effect on rice
• Grain yield of rice is highly correlated with minimum temperature.
• A prediction model in the Philippines (Pamplona et al., 1995) showed that the
high yield observed especially during the dry season is due to lower minimum
temperature.
• Higher grain yield corresponds with a seasonal minimum temperature of 22.5°C,
compared to an average seasonal minimum temperature of 24.2°C.

40. Canopy temperature effect on cotton
• Yield and fiber characteristics respond to variations of daily mean and amplitude
of temperature (Liakatas, Roussopoulos, and Whittington, 1998).
• Mean temperature reduction improves yield components, but high temperatures,
particularly high day temperatures, increase fiber length, uniformity, and
strength.
• Large daily temperature amplitude produces an intermediate number of flowers
and the lowest retention percentage.
• Fruiting and yield are increased by a reduction in temperature down to the
threshold mean temperature of 22°C.

41. Temperature and Photosynthesis
• The rate of photosynthesis
and respiration increases
with an increase in
temperature, until a
maximum value of
photosynthesis is reached.

42. • The range of temperature in which
photosynthesis is more than 90 percent of
the maximum obtainable can be regarded as
optimum.
• This range is narrower for net photosynthesis
than for gross photosynthesis, because while
gross photosynthesis is still operating at top
speed in the optimum range of temperatures,
the rate of respiration increases, diminishing
the net photosynthetic yield.

43. Effect of Spacing on temperature variation

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