Temperature affects the vital process of plant life like manufacture of carbohydrates (Photosynthesis) or protoplasm, which is the physical basis of life, transpiration, respiration, nutrient absorption, and translocation.
Sometimes the temperature may be so low that the plants are unable to manufacture carbohydrates or make protoplasm.
At other times, the temperature may be to high resulting in very high rates of respiration or transpiration.
At high temperature stomata may close down to check high rate of transpiration and in the process cause reduction in the manufacture of carbohydrates.
Higher rates of respiration use up the available carbohydrates which otherwise would have been used for growth and production.
Within each group, different crops need some specific temperature ranges for their proper growth and flowering, which is termed optimum temperature for the plant.
An optimum temperature is that with which a particular plant is able to have maximum photosynthesis to manufacture carbohydrates and have normal respiration rate so that the excess carbohydrates are available for obtaining the highest yields.
Temperature below or above the optimum levels may cause several abnormalities.
The optimum night temperature is very important for crop plants as new cells and the protoplasm for these cells are made only during night.
The night temperatures below or above the optimum range result in low yield. This happens because either the rate of respiration increases to a considerable extent at high temperature or the rate of photosynthesis decreases at a faster rate than the corresponding reduction in respiration rate.
Dormacy and rest period
Lower temperature is also responsible for dormant periods in plants in the temperate zones as the plant parts above ground level cease to grow because of unfavorable weather condition.
Some tropical plants also have periods of dormany because of high temperatures and dry seasons. Some plants have a specific rest period which is controlled by some internal conditions of the plant.
The principal plant structure which has a rest period is the vegetative bud, although some seeds also have rest periods.