Cells of most plant species get killed when the temperature ranges from 50 to 60o C. This point of temperature is called Thermal death point.
But it varies with
1. The species
2. The age of tissue and
3. The time of exposure to high temperature
It is reported that most plant cells are killed at a temperature of 45 to 55oC. Some plants tissues withstand a temperature of up to 105oC.
The aquatic plants and shade loving plants are killed at comparatively, lower temperature (40oC); whereas, for xerophytes it is 50o 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 Injuries:
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 day time and low temperature during night time.
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 afternoon soil temperature exceeds 60oC to 65oC.
The stem girdle injury is first noticed through a discolored band a few millimetres wide. This is followed by shrinkage of the tissues which have been discolored.
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 occur.
Temperature aberrations
Heat Wave: A region is considered to be in the grip of moderate heat wave when it recorded maximum temperature exceeds the normal by 5o to 8oC. Heat wave is common in UP (54% Probability) in the month of June. Incidences are maximum in Western UP. Persistence is 5-6 days particularly more in June.
Effect of Heat wave: Already dealt in effect of temperature on crop growth. Thermal death point affects photosynthesis and respiration. Increased respiration depletion of reserve food, sun clad, stem girdle.
Soil Temperature: In many cases soil temperature is more important to plant life than air temperature. It influences the germination of seeds and root activities. It influences the soil-borne diseases like seedling blight, root rot etc.
The decomposition of organic matter will be higher in higher soil temperature with necessary moisture.
It controls the nutrient availability. In tropics, high temperature of soil causes regeneration of potato tubers. It also affects nodulation in legumes.
steve williams answered the question on November 16, 2017 at 05:53