পৃষ্ঠাসমূহ

EFFECT OF TEMPERATURE ON CROP PRODUCTION

EFFECT OF TEMPERATURE ON CROP PRODUCTION

A. Distribution of Plants :

The most influential factors in the climate are temperature and moisture. Plants can grow only within certain limit of temperature. For each species and variety there are not only optimal temperature limits, but also optimal temperatures for different growth stages and functions, as well as lower and upper lethal limits. Temperature determines which species can survive in a particular region.

Fig 1. Effect of temperature on the metabolic rate. Based on the temperature requirement vegetation can be classified as follows-

Table 2. Classification of vegetation based on temperature

Class

Region

Temperature

Type of vegetation

Common crops cultivated

Megatherms

Equatorial and tropical

High temperature throughout the year

Tropical rain forests

Tropical crops like rubber, Cassava, Varieties of rice, etc.

Megatherms

Tropical and sub-tropical

High temperature alternating with low temperature of winter

Tropical deciduous forests

Sub-tropical crops like maize, sorghum, etc.

Microtherms

Temperate and high altitude plants (upto 12,000 ft of tropical and sub-tropical)

Low temperature

Mixed coniferous forests

Temperate crops like wheat, oats, potato etc.

Microtherms

Aretic and Alpine regions (above 16,000 ft in tropics and 12,000 ft in temperature regions)

Very low temperature

Alpine vegetation

Pines, spruce, etc.

2. Germination :

Temperature greatly influences germination of seeds. The inbibition phase of germination is primarily physical and shows low sensitivity to temperature. The subsequent phases of germination (hydration and enzyme activation, enzymatic degradation of storage material, synthesis and growth of embryo) are temperature dependent due to biochemical processes involved.


Cardinal temperature of different crop’s seed germination :

Crop’s Seed

Temperature ( OC )

Minimum

Optimum

Maximum

Rice

Maize

Wheat

Soybean

Barley

11

9

4

9

4

32

33

25

30

22

41

42

32

41

36

Note :

Cardinal Temperature :

Cardinal temperature are those at which physiological process continue at such a rate so that it becomes critical for survival, growth and reproduction of plant.

Or

For survival each plant has a minimum, optimum and maximum temperature which is called cardinal temperature.

3. Photosynthesis :

The effect of temperature on the rate of photosynthesis is little than on other processes. Very high and very low temperatures effect the photosynthetic rate adversely. According to mathaei (1904) the rate of photosynthesis increases with the rise in temperature from 50 to 370C beyond which there is a rapid fall. Between 50 to 370C, the rate of photosynthesis is doubled for every 100C increase in temperature. In desert and C4 plants photosynthesis may run relatively at high temperature.

4. Respiration :

The rate of respiration increases with the rise of temperature up to a certain level, but beyond the optimum limit the respiration rate shows marked decrease. The rate of respiration becomes doubled at the increase of 100C above the optimum temperature provided other factors are favorable. In fact, respiration continues to increase after gross photosynthesis has reached its limitation by CO2 diffusion and enzyme kinetics. Thus the net photosynthesis is reduced (Fig. 3 )

5. Flower Initiation :

In many crops (such as sugar beet, wheat) low temperature is required for flower initiation. The phenomenon of acquisition or acceleration of the ability to flower by chilling / low temperature treatment is termed as vernalization.

6. Induction of sterility :

The occurrence of various cytological abnormalities in meiosis during the formation of the generative cell has been thought to constitute the primary cause of spikelets sterility. These abnormalities due to cold injury at the boot stage of rice plant. Sterility can also occur due to loss of stickiness of stigma and consequently failure of fertilization.

7. Growth Duration :

Crop growth is influenced by temperature as every plant has its specific degree-day. A degree-day or heat unit is the departure from the mean daily temperature above the minimum threshold temperature. The minimum threshold is the temperature below which no growth take place. The threshold varies with different species of plants and for majority ranges from 4.50 to 12.50C. The growing degree-days are calculated with an equation written as :


Growing degree-days (GDD) =

Where,

T max = Maximum daily temperature

T min = Minimum daily temperature

T t = Minimum threshold temperature

It is important for the forecast of crop harvest dates, yield and quality.

8. Pest and Diseases :

Usefully high temperature promotes the growth of weeds, insects and pathogens.

9. Temperature injury to plants :

a. Low temperature injury :

i. Chilling : Plants of tropical or subtropical, if exposed to low temperature above freezing point for some time, are found to killed or injured severely.

ii. Heaving : Injury to plants is caused by a lifting upword of the plant along with the soil from its normal position.

iii. Glaze : The weight of the ice coating on tree limbs cause them broken.

iv. Suffocation : During winter the ice or the snow form a thick cover over the ground and the crop suffers from want of 02

v. Freezing : Water is frozen in the intercellular space of cell.

b. High temperature injury :

i. Starvation injury: Shortage of food reserve for respiration.

ii. Biochemical lesion : Break down of essential compounds like vitamins.

iii. Toxin injury: Anaerobic respiration produces different compounds causing toxicity to plants.

ATMOSPHERE:

The atmosphere is a deep blanket of gases and suspended liquids and solids that entirely envelops the earth.

Structure of Atmosphere :

There are five concentric layers within the atmosphere which can be distinguished on the basis of temperature.

These are as follows-

1. Troposphere

2. Stratosphere

3. Mesosphere

4. Mesosphere

5. Thermosphere, and

6. Exosphere

1. Troposphere :

The lowest layer of atmosphere in which man and other living organisms live is called troposphere. It is about 20 KM in the equator and to KM in the polar region. It is a mixture of gases whose proportion is fairly constant. The composition of troposphere excepting water vapor and dust particles is as follows-

Constituent Percentage by volume

a.

Nitrogen

78.0841

b.

Oxygen

20.9476

c.

Carbon dioxide

0.0318

d.

Argon

0.2340

e.

Neon

0.001818

f.

Helium

0.000524

g.

Methane

0.0002

h.

Nitrogen Oxide

0.00005

i.

Krypton

0.000114

j.

Hydrogen

0.00005

k.

Xenon

0.0000087

l.

Ozone

0.000001

Troposphere is characterized by steady decrease in temperature and it may decrease up to – 600C in the upper layer. It is the region of clouds, storms, and convective motion. The upper layer is called tropopause. [ The temperature of the air decrease by about 10C for every rise in 164.4 meters in altitude.]

2. Stratosphere :

The second layer of air mass extending about 30 KM above tropopause is called stratopause. In this zone the temperature shows an increase from a minimum of about – 600C to a maximum of about 50C. The increase in temperature is due to ozone formation under the influence of ultraviolet rays of solar radiation

O2 20

O2+ 0 O3

Ozonosphere acts as a blanket that reduces the cooling rate of earth as well as prevents the lethal effect of ultraviolet rays.

3. Mesosphere :

It is the third layer of atmosphere next to stratopauss. It is about 40 KM in height. It is characterized by low atomospheric pressure and temperature. The temperature begins to drop from stratopause goes on decreasing with the increase in height and reaches a minimum of about-95oC at a level some 80 to 90 KM above the earth surface. The upper layer of mesosphere is termed as mesopause.

4. Thermosphere :

Next to mesosphere is thermosphere which extends about 500KM above the earth surface and is characterized by steady temperature increase with the height from meso-pause. In this region ultraviolet rays and losmic rays cause iovization of molecules like oxygen, and nitric oxide. In ionosphere, molecules of grass are so coidely spaced that high frequency cudible sound are not carried out by their atmosphere.

5. Exosphere :

The rest region of atomosphere above the thermosphere is called exosphere or outer space which lalks atoms except thase of hydrogen and helium. This extends up to 3290 KM from the earth. Exosphere has a very high temperature due to solar radiation. The earths magnetic field becomes more important than gravity in distribution of atomic particles in the xosphere.