Heat units, also known as growing degree days (GDD), are a crucial concept in plant physiology and agricultural science, providing a measure of heat accumulation used to predict plant development rates and stages. This measure is particularly useful in understanding and forecasting the growth phases of plants, such as flowering, fruiting, and maturity, which are temperature-dependent. Key points on the importance of heat units in plant physiology include: Predicting Phenological Events: Heat units help predict significant events in a plant’s life cycle, such as germination, flowering, and harvest times. This is vital for farmers and gardeners to optimize planting schedules and manage crop cycles efficiently. Agricultural Planning: By calculating GDDs, agriculturists can decide the best times for planting, irrigating, applying fertilizers, and controlling pests. This can lead to better crop yields and improved management of resources. Varietal Selection: Different plant varieties have specific heat unit requirements. Understanding these requirements helps in selecting the right varieties for a particular climatic zone, thus maximizing productivity and sustainability. Climate Change Adaptation: Monitoring heat units over time can provide insights into shifting climate patterns and help in developing strategies to adapt agricultural practices to changing environmental conditions. Research and Breeding: In plant breeding, heat unit data can help in developing varieties with desired traits such as drought tolerance or shortened growing periods, which are particularly valuable in regions facing climatic stresses.

1. Welcome
Brahmesh Reddy B R
PAMB 2088

2. Heat units and GDD
IN REGULATING CROP GROWTH

3. Concept of heat unit
Many physiological processes such as respiration, photosynthesis, germination,
flowering, protein denaturation and the physical processes like, transpiration,
evaporation etc are temperature dependent.
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

4. Concept of heat unit
The heat unit concept assumes that there is a
direct and linear relationship between plant growth and temperature.
It starts with the assumption that the growth of plants is dependent on the
`total amount of heat`
to which it is subjected during its life period.
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

5. Concept of heat unit
A degree-day, or a heat unit, is the
departure from the mean daily temperature above the base temperature.
This minimum threshold is the temperature below which no growth takes place.
The threshold varies with different plants, and for the majority it ranges from 4. 5
to 12.5°C, with higher values for tropical plants and lower values for temperate
plants.
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

6. The portions of the curve that are
nearly horizontal or flat (A & C)
represent thermal environments
that are either too cold or hot for
the organism
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

7. Between the lower and upper
temperature thresholds resides a
range of temperatures (B) wherein
development increases with
increasing temperature
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

8. Mean temperature methods are most widely used in humid regions where
diurnal temperature fluctuations are relatively small during the growing season.
The single sine curve approach is more common in ‘semi-arid and arid regions’
that experience large diurnal fluctuations in temperature
Computation of heat units
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

9. The single sine curve procedure reconstructs the daily temperature cycle by
forcing a sine curve through the maximum and minimum temperatures for the
day
Computation of heat units:
1. SINE CURVE HEAT UNITS
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

10. The area bounded by the sine curve
b/w upper and lower temperature
thresholds represents the
temperature or heat that
contributes to growth and
development (grey area)
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

11. Note that temperatures above the
upper threshold and below the
lower threshold do not contribute to
growth and development and are
excluded
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

12. A heat unit value of 15 DDF simply
means that:
thermal conditions on that date
support a development rate
equivalent to 15°F above the lower
temperature threshold for the
organism
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

13. In this case with a heat unit value of
15 DDF one simply extends a
vertical line from the x-axis at 15
degrees above the lower
temperature threshold until the line
intersects the S-shaped curve to
obtain the relative rate of growth
and development
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

14. Figure represents an early spring
day where nighttime temperatures
(35ºF) are well below the lower
threshold and daytime
temperatures (75ºF) reside
between the two thresholds.
Heat unit accumulation on this cool
day totals just 6 DDF.
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

15. Figure represents a typical
midsummer day where the
minimum temperature (68ºF) falls
between the two temperature
thresholds and the maximum
temperature (108ºF) is well above
the upper temperature threshold.
Heat unit accumulation in this case
totals 26 DDF
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

16. (Tmax + Tmin)/2 is the average daily temperature and Tbase is the minimum
threshold temperature for a crop
Computation of heat units:
2. STANDARD DEGREE-DAY METHOD
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

17. 1. The use of degree-days for calculating the temperature-dependent
development of insects, birds, and plants is widely accepted as a basis for
building phenology and population dynamics models.
Uses / Applications
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

18. Uses / Applications
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
2. Most applications of the growing degree-day concept are for the forecast of
crop harvest dates, yield, and quality.

19. Uses / Applications
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
3. A potential area of application lies in estimating the likelihood of the
successful growth of a crop in an area in which it has not been grown
before.
The growing degree-day concept can also be applied to the selection of one
variety from several varieties of plants to be grown in a new area.

20. Uses / Applications
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
4. Another application of the concept can be to change or modify the
microclimate in such a way as to produce nearly optimum conditions at
each point in the developmental cycle of an organism.
The concept is also applied to plants other than crop plants and to the
issues of growth and development of insects, plant pathogens, birds, and
other animals.

21. Drawbacks / Weaknesses
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
1. Though the degree-day concept is simple and useful, it lacks theoretical
soundness

22. Drawbacks / Weaknesses
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
2. the conditions that impact the physiological state of an organism (such as
nutrition and behavior-based thermoregulation) are not considered

23. Drawbacks / Weaknesses
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
3. error associated with the assumptions and approximation processes used
in estimating developmental rates and thresholds, and the limitations of
available weather data.

24. Drawbacks / Weaknesses
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
4. a lot of weightage is given to high temperature
- early maturing
- high biomass

25. Drawbacks / Weaknesses
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
5. The effects of topography, altitude, and latitude on crop growth cannot be
taken into account.

26. Drawbacks / Weaknesses
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
6. Wind, hail, insects, and diseases may influence the plant growth and
development, but these cannot be accounted for in this concept.

27. Drawbacks / Weaknesses
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
7. Soil fertility may affect crop maturity. This cannot be explained in this
concept.
Drawbacks outweigh uses

28. Importance of Heat Units in Crop Production
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Temperature controls the developmental rate of many organisms. Plants require
a certain amount of heat to develop from one point in their life cycles to another.
This measure of accumulated heat is known as physiological time.

29. Importance of Heat Units in Crop Production
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Theoretically, physiological time provides a common reference for the
development of organisms.
The amount of heat required to complete a given organism’s development does
not vary; the combination of temperature (between thresholds) and time will
always be the same.

30. Importance of Heat Units in Crop Production
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Physiological time is often expressed and approximated on hourly or daily time
scales using units of degree-hour (°hr) or degree-day (°D).

31. Importance of Heat Units in Crop Production
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Each developmental stage of an organism has its own total heat requirement.
The fifth-leaf stage of wheat occurs either at an average of 21 calendar days
after germination or 350 degree days after germination.

32. GDD: Growing degree days
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
“Growing degree days” (abbreviated GDD or DD) is a way of assigning a heat
value to each day. Growing degree days are calculated in each day using the
Tmax, Tmin and Tbase.
Generally, the threshold temperatures are higher values for tropical crops and
lower values for temperate crops.
GDD =
Tmax - Tmin
2
- Tbase

33. GDD: Growing degree days
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Crops Temperature (°C)
Wheat, barley 4.4
Potato, oats, sugarbeet 5.0
Sorghum, maize, groundnut 8.0-10.0
Rice 10.0-12.0
Tobacco 13.0-14.0
Threshold Temperature of Agricultural Crops

34. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Effect of heat units on
PHYSIOLOGICAL PROCESS
The specific amount of heat units required for the plant at each stage from its
germination to harvest of the crop would vary and the important processes are
listed below:
● Growth and development
● Growth parameters
● Nitrification
● Biomass Physiological maturity
● Yield

35. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Effect of heat units on
PHYSIOLOGICAL PROCESS
1.1 Phenology
The plants in higher temperature environment during floral induction would
produce higher primodia with same initiation rate and duration of induction on an
average with one leaf every 4 °C temperature range of 15 to 32 °C.
: 1. Growth

36. Phenology
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Effect of heat units on
PHYSIOLOGICAL PROCESS : 1. Growth

37. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Effect of heat units on
PHYSIOLOGICAL PROCESS : 1. Growth
CROP PHYLLOCHRON (DAY ℃)
Sorghum 54
Cotton 20
soybean 18.9
Pearl millet 75-98
maize 38-45.5
Leaf appearance : The rate of leaf emergence is affected by temperature. An
increase in temperature speeds up leaf emergence.

38. End of Leaf Growth: The end of leaf growth is influenced by temperature,
vernalization and photoperiod. The development of leaves stop in this stage and
takes duration of thermal time of 10-15 phyllochrons.
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Effect of heat units on
PHYSIOLOGICAL PROCESS : 1. Growth

39. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Grain Filling: It is an important phenological stage to set grain size. The thermal
time varied with different genotypes.
The thermal time requirement for genotypes is 500 °D after flowering or
20 °D after anthesis to maturity at which a rapid increase in grain weight takes
place.
Effect of heat units on
PHYSIOLOGICAL PROCESS : 1. Growth

40. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Comparison of growth
chamber (dark) and field
(light). Thermal time
requirements of various
developmental stages for
the Pisum sativum using
growth chamber determined
cardinal temperatures
Effect of heat units on
PHYSIOLOGICAL PROCESS : 2. Development

41. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Effect of heat units on
PHYSIOLOGICAL PROCESS : 2. Development

42. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
: 3. NITRIFICATION
Effect of heat units on
PHYSIOLOGICAL PROCESS
Nitrification process will be initiated according to the degree-day accumulation.
It is assumed that the moisture is correct.
By the time 1000 growing degree-days have accumulated, 90% of the applied
nitrogen has been converted to the nitrate form.

43. : 4. PHOTOSENSITIVITY
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Effect of heat units on
PHYSIOLOGICAL PROCESS
Regulation of growth in photosensitive crops (varieties) regulated by
temperature is based on:
● A rise in temperature increases the rate at which leaves emerge.
● The number of developed leaves on main culm before heading is fairly
constant for a given variety.
● Because of the above two process, the number of days from sowing to
heading is fairly constant under a given temperature regime.
● A rise in temperature increases the rate of grain filling after flowering.

44. Impact of Excess or Deficit of Heat Units on Crop
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Flowering Performance:
The GDDs are must for the development of crop growth. The excess or deficit
GDD accumulation causes delay in flowering, or early flowering, in some crops
failure of flowering, yield loss, reduction in biomass occur.

45. Flowering
Accumulation of GDD is used to estimate the expected time of flowering.
The deficit growing degree days or heat unit accumulation at the time of
flowering leads to delayed flowering.
The excess GDD accumulation at flowering causes early flowering.
Impact of Excess or Deficit of Heat Units on Crop
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

46. Rice Heading Stage:
In rice where the temperature drops from 24 °C to 21 °C a sharp increase in days
to heading occur. A temperature drop by 1 °C leads to 13-day delay in heading.
When the temperature increases above 24 °C, days to heading decreases to 91
days at 27 °C and to 86 days at 30 °C. A temperature raise of 1 °C above 24 °C
shortens the number of days to heading by less than 2 days.
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Impact of Excess or Deficit of Heat Units on Crop
Specific examples

47. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Impact of Excess or Deficit of Heat Units on Crop
Specific examples
Yield Reduction in Soybean
The study of HUE versus seed and stover yield
Early sown crop produced more dry matter and also resulted in higher yield of
both seed and stover than the late sown crop as they had accumulated more
growing degree days.

48. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Impact of Excess or Deficit of Heat Units on Crop
Specific examples
Yield Reduction in Soybean
The study of HUE versus seed and stover yield

49. Sunflower Yield Variation Depends on GDD
The accumulation of GDD is very less in spring than in the autumn season. Low
to high accumulation of GDD is compared with the yield and yield components.
The reduced achene yield was due to low GDD accumulation.
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Impact of Excess or Deficit of Heat Units on Crop
Specific examples
Spring r2 = .928
Autumn r2 = .969

50. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Heat unit requirements

51. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Heat unit requirements

52. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

53. Phenological shift by climate warming
Cereal crop phenology is very sensitive to climate change as compared to other
agronomic crops
Numerous methods have been utilized to elucidate the intensity of the effect of
climate change on agronomic cereal crops. The methods include analysis of
satellite images on vegetation greenness, measurement of net primary
production with Normalized Difference Vegetation Index (NDVI)
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

54. Wheat phenology trend
sowing (S), emergence (E), anthesis (A) and maturity (M) for wheat crop were
delayed by 9.5, 1.3, 5.3 and 5.4 days decade−1
phenological-phases S-A, A-M along with S-M were reduced by 5.5, 4.6 and 5.7
days decade−1
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

55. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

56. Rice phenology trend
sowing, transplanting (T), anthesis and maturity were earlier by 7.9, 6.6, 5.0 and
5.0 days decade−1
anthesis and maturity were earlier by 5.4, 3.2, 6.2 and 4.8 days decade−1 in
Madagascar. The phenological phases of sowing to transplanting, transplanting
to anthesis and A-M were reduced by 2.9, 1.6 and 5.2 days decade−1, respectively
in China during 1981 to 2009 as well as in other parts of world
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

57. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

58. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

59. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

60. Case studies
WHEAT
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

61. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

62. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

63. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
The crop sown on 15th November took maximum days to attain different
phenological stages and required maximum heat units which got reduced
with subsequent delay in sowing
The higher HUE in 15th November sown crop could be attributed to the
proportionate increase in dry matter per each heat unit absorbed.

64. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

65. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Among different irrigation levels, the maximum values of heat units were
noticed under full irrigation treatment and with the increase in moisture stress
from tillering to dough stage a significant reduction in heat
The less heat use efficiency for water stress conditions (I1, I2 and I3)may be
ascribed by less yield as well as growing degree days as compared to full
irrigation

66. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

67. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

68. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R
Crop growth and seed yield were relatively higher in the first sown wheat crop
varieties because of more congenial weather conditions during the entire crop
growth period.
crop micro environment changed due to different sowing dates which
strongly influenced different phenological stages and crop yield.

69. Case studies
RICE
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

70. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

71. – Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R

72. THAT’S IT!!!
– Heat units and GDD
In regulating crop growth
PPH 605 (0+2)
Crop Physiology
Brahmesh
Reddy B R