Temperature as an Ecological Factor
Temperature is one of the important ecological factors affecting the growth and reproduction of plants. All organisms depend upon a certain temperature or range of temperature. There is relatively little biologic activity below O°C due to immobilization of water or above 500°C because very high temperature denatures the enzymes. The plants have both minimal and maximal temperature for survival but they can grow and reproduce only within a range of optimal temperature. The plants regulate their temperature by losing part of energy they absorb. They maintain their heat balance by reradiation, convection and transpiration.
The temperature varies in space and time due to altitude, latitude, proximity to water body, wind, vegetation cover and other environmental factors which modify its effects.
Heat and Temperature
All organisms live in a thermal environment characterized by heat and temperature. Heat is a form of kinetic energy possessed by all substances that result from the random motion of molecules within the substance.
Temperature is direct measure of the average kinetic energy possessed by individual molecule of a substance. Temperature expresses the substance’s intensity of hotness and measures its tendency to give up heat.
Plants and Thermal Environment
Plants are fixed at a place, therefore cannot move to a more favorable situation, thus they experience a wide range of temperatures. Their roots, crown and parts experience different temperatures during the day. The internal temperature of plant parts is influenced by heat gained and lost to the environment. Plants regulate their temperature by losing part of the energy they absorb. They maintain their heat balance by reradiation, convection and transpiration.
Pants tolerate extremes of cold by frost hardening, i. e., formation of or addition to protective antifreeze compounds in the cells of shoots, buds, roots and seeds. Plants may also have insulation or lower the freezing point of their cells through supercooling. Plants are limited in their response to excessive heat, but cells can make short-term adaptive changes.
Importance of Temperature to Plants
Temperature and Metabolism
Temperature affects metabolic processes of plants by influencing the kinetic of the chemical reactions and effectiveness of enzymes. The body temperature of plants varies with the environmental conditions vital metabolic processes such as transpiration, photosynthesis, respiration, seed germination, etc. are affected by temperature for every 10°C temperature, the metabolic rate doubles.
In general, there is little metabolic activity at temperature below 0°C or above 40°C. however, some arctic and alpine plants carry on photosynthesis at temperature below freezing point and a number of plants are active at temperature above 40°C.
Temperature, Growth and Reproduction
Plants can grow and reproduce only with in a range of optimal temperatures. Each part of life cycle, a plant may have a different set of optimal temperatures. For example, the temperature necessary to stimulate germination may be lower than the temperature that favors flower development. The optimal temperatures vary among species and among individuals in a population.
In many plant species low temperature induces quiescence (temporary inactivity) and in others it helps in breaking dormancy. Similarly, low temperatures are often necessary to stimulate the formation of flower buds (Vernalization). Also, the seeds of many plants require chilling under most condition (stratification).
Temperature and Physiological Processes
Heat Stress: Heat affects the physiological processes of plants. This is called heat stress. On a hot summer day, a leaf may become too warm and net photosynthesis declines because of inactivation of photosynthetic enzymes. Also, rise in temperature shut down normal protein synthesis. If heat persists -it disrupts the protein structure of the plant. Most plants die at temperatures above 50°C.
High temperature can injure and protoplasm. When temperature above maximum for growth, a plant enters quiescent state.
Cold Stress: When temperature drops below the minimum for growth of plant, a plant becomes dormant, even though respiration and sometimes continue slowly, Chilling results in chlorosis (breakdown of chlorophyll), precipitation of proteins, freezing of water in intercellular spaces resulting in outward movement of water from protoplast (dehydration) crystals, and formation of ice within protoplast causing injury to protoplasm. This is known as cold stress.
With a drop-in temperature the viscosity of water is doubted. Therefore, the low temperature reduces the ability of the soil to supply water to roots.
Low air temperature causes frost cracks in shoots. The stem open along one radius. These lesions are important because these permit entries of parasitic fungi into the living tissue.
Temperature and Distribution of Plants
In terrestrial habitats the temperature limits the ranges of plant species directly or indirectly. It sets pole ward and equatorial limits. Temperature in combination with moisture determines the general distribution of vegetation. It has long been known that when equal areas are considered, the number of plant species increases in an equatorial direction from both poles. Temperature values are maximum at the equator and decrease gradually towards the poles. The division of earth’s vegetation into different zones such as equatorial tropical, coniferous, alpine vegetations rests on the marked variations in temperature at different latitudes. Similarly, altitude (height above the sea level) affects temperature values on mountains there is a decrease in temperature with increasing altitude, thus vegetation shows distinct zonation at different altitudes. In addition to temperature, rainfall and topography, soil and geology are also important in distribution of plants at attitudes. The successive zones of vegetation from base upwards are tropical, sub-tropical, temperate and alpine.
In aquatic habitats diurnal variations are not much pronounced in large water bodies because of its high latent heat the water does not become so cool at night and moreover the effects of wind and water currents lessen the effects of local cooling and heating. However, in small ponds the diurnal variations are much pronounced. The submerged vegetation experience lesser fluctuations of temperature.
Three different zones of vegetation can be found in aquatic habitats:
Epilimnion, the zone experiencing vertical gradient of decreasing temperature.
Thermocline or metalimnion, a short zone of rapidly falling temperature.
Hypolimnion, the bottom cold -zone where no temperature gradient is evident.
Temperature and Plant Diseases
The ability of parasitic fungus to enter and develop within a host organism depends upon temperature. For example, at temperature below 13°C the seedlings of most strains of maize become susceptible to disease.