This presentation deals with various scientific theories that explain the mechanism of plant growth.

2. Various theories/laws regarding crop growth in
relation to growth factor, possibility and scope
of CO2 has been proposed by different
scientists.

3.  Justus Von Liebig proposed law of minimum in
1840 which states that the growth of plants is
limited by the plant nutrient present in smaller
quantity, all other being in adequate amount.
 This has been re-stated as barrel concept. A barrel
with staves of different lengths cannot contain
anything above the height of the shortest stave.
similarly, growth can be no greater than allowed
by the factor lowest in availability.
 The level of plant production can be no greater
than that allowed by the most limiting of the
essential plant growth factors.

5.  Blackman in 1905 proposed law of optima and
limiting factor.
 He stated that when a process is conditioned
to its rapidity by a number of separate factors,
the rate of the process is limited by the pace of
the slowest factor.

6.  Mitscherlich in 1909 developed an equation
relating growth with the supply of plant nutrients.
 When plants are supplied with adequate amount
of all but one limiting element, their growth is
proportional to the amount of this one limiting
element.
 Plant growth increases as more of this element is
applied but not in direct proportion to the amount
of growth factor added.
 An increase in growth with each successive
addition of the limiting elements is progressively
smaller.

7. Mitscherlich expressed this mathematically as :
dy / dx = (A-y)C
Where, dy/dx is growth rate
A is the maximum possible yield with sufficient
level of all growth factors
y is the yield obtained at a given level of growth
factor x and c is the proportionality constant
where x=0 and y=0

9.  Wilcox (1929) proposed inverse yield nitrogen
law which states that the power of growth or
yielding ability of any crop plant is inversely
proportional to the mean nitrogen content in
the dry matter.
 A crop plant with high mean percentage of
nitrogen in dry matter has less dry matter
production potential than a crop plant with
low percentage of nitrogen.

10.  Macy in 1936 proposed that relationship exists
between sufficiency of a nutrient and its
percentage content in the plant.
 According to him, there is a critical percentage
of each nutrient in each kind of plant. Above
that point, there is luxury consumption and
below that point there is poverty adjustment.
 This poverty adjustment is proportional to the
deficiency until a minimum percentage is
reached.

11.  Carbon is a constituent of all organic
compounds, carbohydrate, protein or fatty
acid.
 Its source in the plant is carbon dioxide which
enters in the leaf stomata from the atmosphere
through diffusion and then utilized in
photosynthesis.
 Hence, the concentration of carbon dioxide in
the atmosphere is important in deciding the
crop yields.

12.  Carbohydrate enrichment experiments
conducted in glass and plastic houses
confirmed the above hypothesis.
 Wittwer (1966) reported spectacular yield
increases in a green house from CO2
enrichment. A similar experiment in a plastic
house at IARI, New Delhi on vegetable crops
further substantiated the above hypothesis.
 Similar studies under field conditions have not
been possible as regulating CO2 concentration
in open field is not possible.

13.  yield of crops is higher in valleys due to high
partial pressure of CO2 in valleys. The reason
for higher percentage of CO2 in plant
atmosphere in valleys is low wind velocity
than in plains.
 Increasing CO2 pressure in the plant
atmosphere is beneficial but under controlled
conditions only because CO2 is known to
increase temperature also and higher
temperature leads to more evaporation and
consequently higher chances of crop failure
under rainfed conditions while more irrigation
requirement under irrigated conditions.

14.  Higher temperature also leads to higher
respiration and consequently more loss of
photosynthates and thus probably no gain in
net assimilation rate.
 Besides above, high temperature may deplete
the organic matter in soil much faster. Thus,
CO2 enrichment has to be viewed in totality
before arriving at some conclusions.

15.  Since, piping gas into an open field may be
wasteful as wind will blow it off, we have only two
sources :
(1) Dry ice - The application of dry ice may alter the
CO2 pressure only temporarily because of its high
rate of evaporation.
(2) Organic matter - This is cheaper and long lasting
but a weak source.
 Delivery of CO2 to the field crops is still a basic
problem and probably this is the reason why field
experiments on CO2 enrichment could not be yet
precisely conducted.

16.  Unless an efficient source of carbon dioxide is
available, it is difficult to make use of this
knowledge on a field scale i.e. for field crops.
 As for horticultural crops, plastic structures can
be fabricated and used to regulate CO2
concentration and eventually obtain higher
yields.

17.  Often the agronomists think this as a chance to
boost the crop yields but it has its own
disadvantages because it simultaneously
increases the global temperature.
 Any further increase in CO2 level will further
increase the temperature and that may result in
more agronomic droughts and there will be
crop failures, shifting of crop boundaries
towards polar regions and loss of plant and
animal biodiversity.

18.  It refers to failure of crops due to insufficient water
supply; under very high temperature conditions
the evaporation demand will be higher than the
water supply. Such conditions which results in
failure of crops are considered as agronomic
drought.
 High temperature leads to warming of air. So, air
will become lighter and rise up in the form of
eddies (circular movement) thus, creating a
vacuum. To fill this vacuum, air from surrounding
areas would blow to this place leaving it with
excess CO2.

19. no. C3 plants C4 plants
1. The first product of photosynthesis is
a three carbon compound formed via
the Calvin-Benson pathway. E.g.
Wheat, Rice, Barley, Rye and Oat.
The first product of
photosynthesis is a four
carbon compound formed via
the Hatch- Slack pathway.
E.g. Corn, Sorghum, millets
and Sugarcane.
2. Less respond to light intensities. Respond to higher light
intensities than C3 plant.
(Double than C3)
3. Translocation rate are slow. Translocation is about twice
as fast as in C3 leaves.

20. no. C3 plants C4 plants
4. Less efficient user to carbon
dioxide. CO2 compensation
point is 50-150 ppm of CO2.
More efficient user of carbon
dioxide. CO2 compensation point
is 0-10 ppm of CO2.
5. Lower photosynthetic efficiency
due to high photorespiration.
High photosynthetic efficiency
due to low photorespiration.
6. Lower Net Assimilation Rate. Higher Net Assimilation Rate.
7. Adversely affected by high
temperature
Not adversely affected by high
temperature.

21. no. C3 plants C4 plants
8. Many C3 plants become
unproductive at temperature from 25◦
- 35◦c.
C4 plants increase in productivity
at these temperatures.
9. Lower water use efficiency in this
type. The mean dry matter produced
for each 1000 g of water used was
1.54 g.
Greater water use efficiency in
this type. The mean dry matter
produced for each 1000 g of water
used was 3.29 g.
10. Growth rates are slower than C4
plants. 13 gm/day.
Growth rates are much greater
than C3 plants. 22 gm/day.
11. C3 plants suffer an oxygen stress and
are adversely affected by the oxygen
level of 21% found in nature.
C4 plants do not suffer an oxygen
stress.

22. Wheat Rice

23. Maize
Sugarcane

24.  Crassulacean Acid Metabolism (CAM)
In this type, plants are characterized by their
adaptiveness to arid regions. Xerophyte plants
are of this type. e.g., Pineapple, cactus.
 Photosynthetically Active Radiation (PAR)
Light energy within the spectral wavelength of
0.4-0.7 microns or 400-700 nanometers, which is
useful in photosynthesis process by plants.
 Photosynthesis
The process that transforms carbon dioxide
(CO2) into food; the basis of all crop yields and
the essences of agriculture.

25. Pineapple Cactus

26.  Photorespiration
A form of respiration stimulated by light, found in
C3 plants and having no known essential function.
 Photoperiod
The duration of the light period between sunrise
and sunset, including the light period.
 Photoperiodism
Reproductive response of plants to the relative
length of light (Day) and dark periods (Night) in a
day.

27.  Short Day Plant (SDP)
Flower initiation takes place when the days are
short (less than 10 hrs.) or when dark period is
long. Most of the tropical crops like rice,
sorghum, maize etc. are short day plants.
 Long Day Plants (LDP)
Long day plants require comparatively long days
(usually more than 14 hrs.) for flower initiation.
They put forth more vegetative growth when
days are short. Most of the temperate crops like
wheat, barley and oats are long day plants.
 Day-Neutral Plants (DNP)
Plants in this category do not require either long
or short dark periods. Photoperiod does not have
much influence for basic change for these plants.
E. g. Cotton, sunflower, buckwheat, etc.