Ambient Temperature

Rates of biochemical reactions within an organism are strongly dependent on its temperature. The rates of reactions may be doubled or tripled for each 10 degree increase in temperature. Temperatures above or below critical values may result in denaturation of enzymes and death of the organism.

A living organism is seldom at thermal equilibrium with its microenvironment, so the environmental temperature is only one of the factors determining organism temperature

Other influences are fluxes of radiant and latent heat to and from the organism, heat storage, and resistance to sensible heat transfer between the organism and its surroundings

Ambient Temperature

Air temperature is a measure of how hot or cold the air is. It is the most commonly measured weather parameter. More specifically, temperature describes the kinetic energy, or energy of motion, of the gases that make up air. As gas molecules move more quickly, air temperature increases.

Air temperature affects the growth and reproduction of plants and animals, with warmer temperatures promoting biological growth. Air temperature also affects nearly all other weather parameters

Air temperature affects:

  • the rate of evaporation
  • relative humidity
  • wind speed and direction
  • precipitation patterns and types

The growth of higher plants is restricted to the temperature between 0 – 60 degree C and crop plants are further restricted to 10 – 40 degree C.


  Soil Temperature

Soil temperature is a factor of primary importance for many physical, chemical, and biological processes. It governs:

  • Evaporation and soil aeration
  • All kind of chemical processes and reactions within the soil
  • Biological processes such as seed germination, seedling emergence and growth, root development, microbial activity


Soil temperature varies in response to changes in radiant, thermal and latent energy exchange processes that take place preliminary through the soil surface. The effects of these phenomena are propagated into the soil profile by a complex series of transport processes.

  Soil Cardinal Temperature

Cardinal Soil Temperature are the temperatures that determine the plant growth

  • Minimum Temperature  - below which can’t grow
  • Maximum Temperature above which plant growth stops
  • Optimum Temperaturemaximum plant growth

Example Wheat – Min : 3–4.5 ; Opt : 25 ; Max :30-32

Conceptually, base temperature is the temperature at which development stops through cold. As temperature increases above the base, the process goes progressively faster until it reaches the optimum temperature. The optimum is the temperature at which development is fastest. Higher temperatures than the optimum can slow development and at temperatures well above the optimum development may stop altogether and the plant may die.

Thermoperiodism : Response of plants to regular change in temperature

  Temperature Behaviors

As the surface gets warmer, heat is transferred away from the surface by convection to the air layers above and by conduction to the soil beneath the surface.

Temperature extremes occur at the surface, where temperatures may be 5 to 10° C different from temperatures at 1.5 m, the height of a standard meteorological observation.

microenvironment may differ substantially from the macroenvironment.

The graph shos the variation in temperature with soil depth. After 1.5m the variation becomes constant.

  Temperature Variations

The diurnal temperature wave penetrates much farther in the atmosphere than in the soil because heat transfer in the atmosphere is by eddy motion, or transport of parcels of hot or cold air over relatively long vertical distances, rather than by molecular motion.

The larger the transport distance, the more effective eddies are in transporting heat, so the air becomes increasingly well mixed as one moves away from the surface of the earth. This mixing evens out the temperature differences between layers.

They are steep close to the surface because heat is transported only short distances by the small eddies. Farther from the surface the eddies are larger, so the change of temperature with height (temperature gradient) becomes much smaller                                                  .                                                               

Diurnal Variations

The fact that the temperature maximum occurs after the time of maximum solar energy input is significant. This is known as temperature lag. This type of lag is typical of any system with storage and resistance to flow.

Diurnal temperature variation – e.g., peak daily temperature typically occurs after noon (Maximum solar radiation are generated at 12pm but on earth its 2pm when there is maximum solar heating)

Seasonal lag – e.g., peak annual temperature typically occurs after the summer solstice

Annual Variations

Also note that the time of maximum temperature (around day 200) significantly lags the time of maximum solar input (June 21 ; day 172).

The explanation for this lag is the same as for the diurnal cycle. Random temperature variations can also occur apart from this




  Soil temperature variations

Temperature extremes occur at the surface where radiant energy exchange occurs, and that the diurnal variation decreases rapidly with depth in the soil

Temperatures measured at three depths, the amplitude decreases rapidly with depth in the soil, and that the time of maximum and minimum shifts with depth

The variation or increase in temperature is steep in the layer immediately after surface and this gradually decreases after 1.5 m the heat exchange is almost constant in micro environment. The graph below explains the heat transfer in various layers of soil.

Explaination: the soil surface will show maximum temperature at 14:00 hours because of maximum solar input, however the soil layer at depth of 12cm will show maximum temperature at 18:00 hours and the lowest layer at 24cm will show it at 22:00 hours of the day. This all happens because of heat transfer process in soil.

Remember we are talking about plant's micro environment otherwise there is a Geothermal gradient.

Geothermal Gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior.



  Temperature & Biological Development

Temperature strongly influences the rates of all metabolic processes, thus all aspects of the growth and development of an organism.


Development can be defined as the orderly progress of an organism through defined stages from germination to death


Development differs from growth, which we define as the accumulation of dry matter

Plant stages of deveopment :

  • Germination
  • Emergence
  • Leaf appearance
  • Flowering,
  • Maturity

A simple example of how the development rate is affected by temperature is given below for melon fly

Egg Development in Melon fly

Development time is short at temperatures between 20° and 30" C, but increases markedly at both higher and lower temperatures. Above 37° C and below 15" C, development times are very long

  Thermal Time/GDD

Monteith(1977) uses the term thermal time to describe a time scale in which the development rate of organisms is constant. It has also been referred to as physiological time or p-time

Units of thermal time are day-degrees or hour-degrees

The concept is often called as Growing Degree Days (GDD) which relate plant growth, development and maturity to ambient temperature

Its a measure of the accumulated amount of heat (in degrees Celsius) above a base temperature taken from one point on the landscape over time.

The concept of GDDs assumes that plant growth is directly related to the average daily temperature. It disregards factors such as soil temperature, differences in the pattern of night and day temperatures and other variations caused by the stage of growth of the plant. The GDDs for each day are added together, or accumulated throughout the growing season.

GDD Assumptions

Assumption for calculating GDD

  • The growing region of the plant is at air temperature
  • The hourly air temperature does not go below the base temperature or above the maximum temperature during a day
  • The process being predicted is linear with temperature between the base and maximum temperatures

GDD Calculation

Growing Degree Days (GDD)/ Growing Degree Units (GDUs)/ Heat Unit/ Thermal Unit

Base temperature is the lowest temperature where metabolic processes result in a net substance gain in aboveground biomass (Tbase). 10degree C is the common average Tb

Accumulated GDD (AGDD)

Sum of day by day GDD gives the accumulated temperature by plant or accumulated GDD

Thermal Time

Every phase of development requires a minimum accumulation of temperature before that stage can be complete and the plant can move to the next stage

In effect, the plant senses the temperature every day and adds the average for that day to a running total up to the total required for the stage. This running total is called thermal time or the heat sum for the phase and the thermal units are degree days (°Cd)


Example : Wheat

For wheat, base and optimum temperatures aren’t always 0° C and 25° C respectively. They actually start lower and rise with development. The figure shows that although plants can grow at 0° C during the seedling stages, they make slow progress at the heading stage if temperatures are much below 10° C. Fortunately, varieties differ in their base and optimum by as much as 7° C at any stage. In general, winter wheat can develop at lower temperatures than spring wheat.

 GDD Application 

Phenology is the science of the influence of climate on the occurrence of biological events

Drawbacks in GDD

  • Higher weightage is given to Tmax, although above 25 degree can be detrimental for plants
  • Daily range of temperature is not considered
  • Threshold temperature change at different crop stages, its not considered
  • Soil fertility, wind, disease, insects can affect the growth of plants that are not accounted

Errors from the growing point temperature not being at air temperature can be significant. For example, the growing point in corn is below the soil surface in early developmental stages, and failure to use soil temperature during this time can result in errors of five days or more in prediction

Angus et al. (1981) report that the base temperature fall into two groups, one centered around 2° C, and the other around 11° C. The former are representative of temperate species such as wheat, barley, pea, etc., and the latter of tropical crops such as maize, millet, and sorghum.

Tbase and GDD for emergence of crops

  GDD Fluctuations

Temperature Extremes can deviate during different times

The correct estimate of thermal time would be obtained by shortening to one hour, and summing hour-degrees to determine thermal time for the day. This is often done with insect models, where development times are short and good precision is required

GDD and Environmental Variables

Effective day degree concept includes other  variables in GDD. The development rate concept can be extended to other environmental variables which alter the relationship between development and temperature

Example Photoperiod & vernalization also change rate of development

Minor modifiers include drought, nutrition and solar radiation.

Major modifiers however are photoperiod and vernalization



Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light and dark periods.

A plant which flowers under long-day conditions, but not when days are short. When the daylength is shorter than eight hours, no development occurs. For days longer than 16 hours, development occurs at the temperature determined rate. Any change in the length of photoperiod affects the plant.


Vernalization is the promotion of flowering in plants by cold treatment given to plants or imbibed seeds

Plants must be exposed to certain length of time to become and remain vernalized. Vernalization is required for re- productive growth of winter wheat.