agronomy - Agrometeorology module 3

TEMPERATURE

·         Temperature refers to the degree of hotness or coldness of a substance or a thing. 

·         Temperature provides a measure of the intensity of heat energy.( Scales of temperature and relationship between scales – study practical record).

Effect of Temperature on Plant growth / Crop Productivity

·         The growth of higher plants is restricted to a temperature between 0 to 60 ^oC and the optimum i.e., 10 ^oC to 40 ^oC.  Beyond these limits, plants are damaged severely and even get killed. 

·         The maximum production of dry matter occurs when the temperature ranges from 20 and 30 ^oC. 

·         High night temperature favours growth of shoots and leaves and it also affects plant metabolism. 

·         On the other hand low night temperature injure the plants.  Tender leaves and flowers are very sensitive to low temperature and frost.

S.No	Plant	Cardinal temperature oC
1	Rice	Minimum	Optimum	Maximum
2	Sorghum	10-12	30-32	36-38
3	Maize	8-10	32-35	40
4	Wheat	8-10	32-35	40-44
5	Barley	3-4.5	25	30-32
6	Sugarbeat	3-4.5	20	38-40
7	Tobacco	4-5	25	28-30
8	Carrot	13-14	28	35
9	Pumpkin	4 –5	8	25
10	Peas	12	32-34	40
11	Oats	4-5	25	28-3
12	Lentils	4-5	30	36
In General
	Cool season crops	0-15	25-31	31-37
	Hot season crops	15-18	31-37	44-50

Cold Injury: ( Low Air Temperature and Plant Injury)

1.      Chilling injury:

• Plants which are adapted to hot climate, if exposed to low temperature for sometime are found to be killed or severely injured. 
• Some effects of chilling are development of chlorotic condition ( Yellowing)

2.      Freezing Injury:

• Plant parts or entire plant may be killed or damaged beyond repair as a result of actual freezing of tissues. 
• Ice crystals are formed first in the intercellular spaces and then within the cells.  Ice, within the cells, causes more injury by mechanical damage on the  structure of the protoplasm and plasma membrane.
• Freezing of water in intercellular spaces results in withdrawal of water from the cell  sap due to dehydration and causes death of cells.Eg., Frost damage in potato, tea, etc.,.

3.      Suffocation:

·         In temperate regions, usually during the winter season, the ice or snow forms a thick cover on the soil surface. 

·         As a result the entry of O₂ is prevented and plants suffer for want of O₂. Ice  coming in contact with the roots prevents the diffusion of CO₂ outside the root zone.  This prevents the respiratory activities of roots leading to accumulation of harmful substances.

4.      Heaving:

• This is a kind of injury caused  by lifting up of the plants alongwith soil from its normal position. 
• The presence of ice crystals increases the volume of soil. 
• This causes mechanical lifting of the soil.

Heat Injuries:

i.                    Sun clad:

·         Injury caused by high temperature on the sides of bark is known as sun clad

ii.                  Stem girdle:

·         High temperature at the soil surface scorches the stems at ground level.

Storm:

A marked atmosphere disturbance characterised by a strong wind, usually accompanied by rain, snow, sleet (rain that freezes as it falls-mixture of rain with snow or hail) or hail and often thunder and lighting.

Thunder Storm:

A storm invariably produced by a cumulonimbus cloud and always accompanied by thunder; usually attended by strong wind, gusts, heavy rain and sometimes hail.  It is usually of short duration, seldom over 2 hour.

Hail: Precipitation in the form of balls or irregular lumps of lice.

Hail Storm: Small round pieces of ice hail) that sometimes fall during thunder storms (frozen rain drops, hail storms). Hails may be sometimes greater in size than a large marble. It falls from cumulonimbus clouds. Hails are destructive to crops - mechanical damage, structures etc.

Hurricane:  A violent tropical cyclone with wind speed of 73 or more miles per hour or 134 and more km/h usually accompanied by torrential (very heavy fall) rain, originating usually in West Indian regions.

Tornado: Tornado - Spanish word - Tornas means "to turn".The smallest vortex (whirlpool, whirl or powerful eddy of air, whirl wind - a whirling mass of water forming a vacuum at its centre, into which anything caught in the motion are drawn). But its wind speed can reach even 300 km/h.

Cyclone means closed circulation about a low pressure centre which is anti clockwise in the Northern Hemisphere. Cyclonic whirls are the "Storms" of middle latitude.

Anticyclone:

Circulation clockwise in northern hemisphere and anti clockwise in Southern hemisphere. This circulation subside whirling @ 10-15 cm/sec. And fair weather generally prevail. The air masses are homogenous with respect to temperature and moisture.

Typhoon:

Any violent tropical cyclone orginating in the western pacific especially in the south China sea.

Dew Point: The temperature to which air has to be cooled in order to reach saturation.

Degree days:

At a given location, the period between planting and harvesting is not a specific number of calender days but rather a summation of energy units, which may be represented as degree days. 

            A degree day for a given crop is defined as a day on which the mean daily temp. is one degree above the zero temp. (that is the minimum temp. for growth) of the plant.

Hmidity:

The terminology related to humidity and concerned with gaseous form of water i.e., water Vapour, several expression  of the amount of water vapour in the air are used.

(i)                 Absolute humidity: It denotes the actual mass of water vapour in given volume of air.  It may be expressed as the number of grams of water vapour in a cubic meter of moist air or mass of water vapour per unit volume of air.

(ii)              Sepcific humidity:  It is defined as the moisture content of moist air as determined by the ratio of the mass of water vapour to the mass of moist air in which the mass of water vapour is contained.

(iii)            Relative humidity: Relative humidity is a common parameter for expressing water vapour content of the air.  It is the percentage of water vapur present in the air in comparison with saturated condition at a given temperature and pressure.  The R.H. can be expressed as

100r

RH      =         -------

                         rw

            Where "r" is the mixing ration of moist air at pressure (p) and temperature and "rw" is the saturation mixing ratio at same temperature and pressure.