Temperature is one of the most important ecological factors. It regulates many physiological processes of the plant. The metabolic processes are low at a certain minimum temperature. It increases at particular temperature called optimum temperature. The plant grows best at optimum temperature. Metabolism again decreases at maximum temperature. The plants cannot survive above this temperature. Both extremely low and high temperatures have adverse effect on plant growth.
- Low temperature: Low temperature causes cold injuries. Water is frozen into ice crystals in the intercellular spaces. It causes injury to cells.
- High temperature: Extremely high temperature cause adverse effects on a number of vital physiological processes like respiration, transpiration, protein metabolism etc. These effects cause stunting and finally death of plants. This is called as Heat Injury.Different kinds of plants have various ranges of minimum-optimum-and maximum temperatures. Plants differ considerably in temperature tolerance. Generally, there is little metabolic activity at temperature below 0°C or higher than 40°C. However some arctic and alpine plants can carry on photosynthesis at temperature below freezing point. Numbers of plants are active at temperature above 40°C. Some arctic and alpine plants die even at moderately warm temperatures. But almost all seed plant become inactive or killed after 55°C. On the other hand a few kinds of blue-green algae can grow in hot springs between 55-70°C. Certain species of yeast can tolerate temperature of 114°C in dormant conditions. Bacteria can tolerate temperature between 120-130°C. A few fungi can withstand temperature up to 89°C.
Temperature classification of plants
The plants are classified into the following categories on the basis of temperature requirements.
1. Megatherm: The plants live in high temperature throughout the year are called megatherms. These plants are found in equatorial and tropical rain- forests.
Mesotherm: The plants living at high temperature of summer, alternating with low temperature of winter are called mesotherms. They are found in deciduous forest of tropical and subtropical regions.
- Microtherm: The plants which live in extremely low temperature are called microtherms. It includes plants of temperate and high altitudes (upto 12000 feet of tropical and subtropical region). These regions are dominated by mixed Coniferous forest.
- Hekiskotherm: It includes plants of Arctic and Alpine regions • (above 16000 teat in tropics and 12000 feet in temperate) with very low temperature. Alpine vegetation prevails in such locality.
Ecophyiological responses of temperature
Temperature is an ecological factor. It affects the rate of many physiological processes in plants.
- Transpiration: Rise in temperature increases transpiration rate. High temperature influences the saturation deficit of the atmosphere. It decreases the humidity of air. Thus more water is transpired. High rate of transpiration also increases the rate for absorption of water from the soil.
- Photosynthesis: Photosynthesis occurs over a wide range of temperature. In some desert plants photosynthesis continues even at 80°C. Most of the algae require lower temperature range for photosynthesis than the higher plants. The optimum temperature for photosynthesis for most of the plants is 25 to 35°C. Photosynthesis stops at 40°C in temperate plants and at 50°C in tropical plants.
- Respiration: The rate of respiration increases with the rise of temperature. It is maximum at optimum temperature. But it decreases rapidly above optimal temperature.
- General distribution of plants: Temperature and moisture determine the general distribution of vegetation. Different belts of vegetation occur between the equator and the poles. Vegetation is primarily determined by heat. Thus plants which grow in a hot climate cannot grow in a cold climate and vice versa. Therefore, same crop are not cultivated in all regions of the world. Different crops are cultivated in different region i of the world.
- Germination of seed: Temperature also affects the germination of seed. Every seed has optimal temperature. Seed cannot grow below or above this temperature.
- Spreading of diseases: Temperature and humidity affects the spreading of plant diseases. Low temperature along with high humidity favours the attack of rust. damping off, seedling blight, foot rot and root rot.