High temperature effects Cells of most plant species get killed when the temperature ranges from 50 to 60°C. This point of temperature is called thermal death point. But it varies with the species, the age of tissue and the time of exposure to high temperature. It is reported that most plant cells are killed at a temperature of 45 to 55°C. Some plants tissues withstand a temperature of up to 105°C.
The aquatic plants and shade loving plants are killed at comparatively, lower temperature (40°C); where as, for xerophytes it is 50°C. High temperature results in desiccation of the plants and disturbs the balance between photosynthesis and respiration. Higher temperature increases the respiration leading to rapid depletion of reserve food in plants resulting in growth stunted due to incipient or starvation.
Heat injury – Very high temperature often stops growth. The plant faces incipient starvation due to high respiration rates. The plant is stunted and if such a condition persists for a long period the plant is killed. Direct temperature effects are noticeable in young seedlings and transplanted crops. High temperature causes sterility in flowers. The general effects of excessive heat are defoliation, premature dropping of fruits and in extreme cases death of plants.
(i) Sun clad: Injury caused by high temperature on the sides of bark is known as sun clad. This is nothing but exposure of barks of the stems to high temperature during daytime and low temperature during nighttime.
(ii) Stem girdle: It is another injury associated with high temperature. High temperature at the soil surface scorches the stems at ground level. This type of injury is very common in young seedlings of cotton in sandy soil where the after noon soil temperature exceeds 60°C to 65°C. The stem girdle injury is first noticed through a discoloured band a new millimeter wide.
This is followed by shrinkage of the tissues, which have been discoloured. The stem girdle causes the death of the plant by destroying the conductive and cambial tissues or by the establishment of pathogens in the injury. As direct effects on crop plants high temperature causes sterility in flowers. The general effects of excessive heat are defoliation, pre-mature dropping of fruits. In extreme cases, death of the plants may also occurs.
A. Effects of Temperature on Crop Production
Plants can grow only within certain limits 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. During photosynthesis there are certain biochemical processes preceding and following the reduction of carbon dioxide, which are affected mainly by temperature.
As long as light is limiting, temperature has little effect on the rate of photosynthesis. When light is not limiting, it has a profound effect on the rate of photosynthesis. In general, high temperature accelerates growth process. Rarely are high temperatures per se the direct cause of death of plants, provided the water supply is adequate. Retardation of growth and difficulties in fertilization are observed even in heat loving crops such as sorghum, at extremes of temperatures.
The harmful effects of excessive temperatures are usually aggravated by lack of available moisture. Hot dry winds will further increase the damage. Increasing temperatures increases evapotranspiration. The daily alternations of high and low temperature may effect grain production.
B. Factors Affecting Air Temperature
Latitude – The time of occurrence of maximum monthly mean temperature and minimum monthly mean temperature also depends on latitude of a place. (e.g.) The coldest month is January in northern regions of India while December in the south. Similarly, the warmest month is May in the south while June in the north across the country.
Altitude – The surface air temperature decreases with increasing altitude from the mean sea level as the density of air decreases. Since the density of air is less at higher altitudes, the absorbing capacity of air is relatively less with reference to earth’s long wave radiation.
Distribution of land and water – Land and water surfaces react differently to the insolation. Because of the great contrasts between land and water surfaces their capacity for heating the atmosphere varies. Variations in air temperature are much greater over the land than over the water. The differential heating process between land and sea surfaces are due to their properties. It is one of the reasons for Indian monsoon.
Ocean currents – The energy received over the ocean surface carried away by the ocean currents from the warm areas to cool areas. This results in temperature contrast between the equator and poles. The occurrence of El Nino is due to change in sea surface temperature between two oceanic regions over the globe.
Prevailing winds – Winds can moderate the surface temperature of the continents and oceans. In the absence of winds, we feel warm in hot climates. At the same time, the weather is pleasant if wind blows.
Cloudiness – The amount of cloudiness affects the temperature of the earth’s surface and the atmosphere. A thick cloud reduces the amount of insolation received at a particular place and thus the daytime temperature is low. At the same time, the lower layers in the atmosphere absorb earth’s radiation. This results in increasing atmospheric temperature during night. That is why, cloudy nights are warmer. This is common in the humid tropical climates.
Mountain barriers – Air at the top of the mountain makes little contact with the ground and is therefore cold while in the valley at the foothills makes a great deal of contact and is therefore warm. That is, the lower region of the earth’s atmosphere is relatively warmer when compared to hillocks.
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