Adaptations of plants to Environmental Factors: light variations, Temperature variations, Thermoperiodism, and Vernalization For Class 10th, 11th, and 12th
Introduction
Any environmental element that prevents a plant from growing due to its excess or deficiency is referred to as a limiting factor. For instance, freezing temperatures limit the growth of plants at higher elevations and the availability of water in deserts. Light, moisture, and temperature are limiting elements for terrestrial plants. According to Liebig’s Law of Minimal, nutrient levels must be present in minimum quantities for plants to thrive.
According to Shelford’s Law of Tolerance, there should be an ecological minimum and maximum, and that range between the two indicates the tolerance range of plants. The term “stenothermal” refers to plants with a restricted range of temperature tolerance, whereas “eurythermal” refers to plants with a large range of tolerance.
A plant doesn’t thrive equally well everywhere it is found. The range of the optimum refers to the set of environmental conditions where it performs best. The plant adapts in response to changes in certain significant aspects of the environment. Although the external environment is changing, these modifications maintain some crucial elements of the internal environment of the plant. Homeostasis is the term used to describe this propensity. Plants attempt to adapt when environmental factors change significantly. Any morphological, anatomical, physiological, or behavioral characteristic that favours an organism’s success in a certain environmental state and increases its ability to withstand environmental change is referred to as adaptation. Ecophenes or ecads are populations that have morphological characteristics that are not inherited as a result of a diverse environmental environment. Natural selection is what causes populations to perform differently or be more fit. For its geographic dispersion, a given population demonstrates varying degrees of tolerance to a given limiting constraint. These regionally adapted populations are known as ecotypes, and they may have evolved as a result of genetic alterations that led to various responses to changing environmental conditions. Phenotypes are the outward manifestations of an individual’s genetic reactions to a certain set of circumstances. Physiological (functional features), morphological (physical characteristics), or phenological (phenotypic) responses can all be phenotypic (timing of growth, flowering, and other life-history changes). Plant population and distribution are influenced by adaptiveness ranges.
Adaptations to light variations
The two most crucial aspects of light are photoperiodism and relative light needs. Light plays several significant ecological roles. According to their relative need for light for general vegetative development, plants are classed ecologically as heliophytes or sciophytes.
Heliophytes
For optimal growth, heliophytes need full sunlight. Heliophytes thrive in open areas and spread out quickly on disturbed ground. They can withstand extremes of dryness and wetness and produce both photosynthesis and respiration at a rapid rate. Plants that can tolerate the shade produce less photosynthesis and, more crucially, less respiration. In comparison to sciophytes, heliophytes have leaves that are smaller, thicker, and more deeply lobed, as well as well-developed support and conduction systems. Shade leaves are thinner, with weaker lobed surfaces, a higher surface area per weight, fewer stomata, and support and conduction tissues. These modifications boost the effectiveness of light use, expand the area available for light absorption, and lessen the reflection.
If forced to develop in the shadow, heliophytes will respond by heightening quickly to escape the shade. Due to their thinner and wider spacing, the leaves will be more vulnerable to fungal infections and dryness. Plants acquire traits like vertically oriented leaf blades, thicker stems, well-developed conducting elements, and mechanical tissues, thick palisade layers, longer roots, shorter internodes, more branching, higher root/shoot ratios, lower chlorophyll contents, higher respiration rates, higher osmotic pressure from high salt and sugar concentrations, and increased resistance to temperature, drought, and pest damage to avoid bright light. Low light did not significantly slow down the growth of the majority of shade-tolerant trees, despite their slow growth rates. Compared to C3 species, C4 plants exhibit higher degrees of light saturation. CAM photosynthesis in arid plants usually limits PAR. When other environmental conditions are favourable for photosynthesis, cacti often develop in a way that increases light inception.
Sciophytes
Lower light intensity is ideal for sciophyte growth. Sciophytes continue to photosynthesize under low light conditions. The term “facultative sciophytes” refers to some heliophytes that can also thrive rather well in shade. On the other hand, some sciophytes are known asfacultative heliophytes since they can thrive in full sunshine as well. Because sciophytes have reduced dark respiration and, consequently, a lower light compensation point, they can sustain a positive carbon balance even at very low gross photosynthetic rates. When shade plants are exposed to direct sunlight, they lose a lot of moisture and their chloroplasts are damaged by the light. Shade tolerance is common among herbaceous plants in forests.
Different absorptive spectra of the photosynthetic pigments in the marine environment can be used to explain the distribution pattern of various algae. Red algae found in the deepest parts of the ocean have phycobilin pigments, which may absorb the green light that is frequently present there. Shallow water is home to green algae with chlorophyll a and b, whereas brown algae with chlorophyll a, c, and the unique carotenoid pigment fucoxanthin are prevalent in intermediate depths.
Photoperiodism
There are direct and indirect impacts of light. Through photosynthesis, growth, and development, it directly influences metabolism, as well as indirectly through the immediate metabolic reactions and its regulation of morphogenesis. The spectral distribution of radiation affects apical dominance, stem extension rates, and germination. Three primary receptor systems mediate responses to light. Flavins absorb at 450 nm for tropisms and high-energy photomorphogenesis, phytochrome absorbs in two interchangeable forms at 660 and 730 nm for numerous photomorphogenetic responses, and chlorophyll for photosynthesis. Latitude affects the temporal fluctuations in irradiance and its relative day/night duration. The foundation of photoperiodism is this. The start of flowers, the germination of seeds, the breaking of buds, the lengthening of stems, the dropping of leaves, and other processes are all photoperiodic reactions in all temperate zone plants. Since the length of the days in the equatorial zone differs little from season to season from that in the temperate region, this is of little relevance. With the aid of the photoperiodic response, the plant may time the development of its vegetative and floral parts to correspond with climatic fluctuations. The seasonal variations in day and night rhythms are taken into account while planning a plant’s activities. Critical day duration is the signal for these responses. Between ten to fourteen hours, depending.
Plants can be categorized according to their photoperiod as
(I) Short-Day Plants, which only develop and reproduce normally when the photoperiod is less than a critical maximum (12–14 hrs), like Cannabis sativa, Andropogon virginicus, and Datura stramonimum,
(II) long-Day Plants, develop and reproduce normally when the photoperiod is greater than a critical minimum (12–14 hrs). Long-day plants, like Brassica rapa and Sorghum vulgar, are those whose growth and reproduction are boosted by day lengths greater than the critical day length.
(III) Day-Neutral plants since they don’t care about the duration of the photoperiod. such as Cucumis, Poa,
Adaptations to temperature variations
The kinetics of biochemical reactions and the efficiency of enzymes are both impacted by temperature, which has an impact on how plants use their energy. There are large regional differences in the temperature environment of plants. The earth often protects roots from temperature extremes, whereas above-ground structures are exposed to a wide range of temperatures. In comparison to the shaded side, the temperature of exposed leaves, twigs, and buds is higher. Reradiation, convection, and transpiration are how plants regulate their internal temperature.
Plants may require a distinct set of ideal temperatures during various phases of their life cycles. The temperature needed to encourage germination may be lower than the one that encourages bloom growth. Temperatures that are ideal for a species, its ecotypes, and an individual member of a population can differ.
Heat stress
When the environment is too hot, net photosynthesis declines, and respiration takes over. Rapid temperature increases cause normal protein synthesis in heat-tolerant organisms to stop, while also causing the onset of a group of heat shock proteins that aid in short-term survival. The plant’s protein structure is disrupted if the heat is kept up. Due to their high quantities of bound water and high cytoplasmic viscosity, many species of cactus can tolerate high temperatures. Many plants just have stems for photosynthesis and have extremely little or no leaves at all. These plants are referred to as phylloclades, including Opuntia and Muehlenbeckia. Short-term tolerance to heat stress entails a behavioural reaction. Plants that are experiencing heat stress hold their leaves parallel to the sun’s beams rather than horizontally. Because growing and developing plants and organs are more vulnerable to heat stress than mature organs, heat tolerance varies with developmental phases. In general, C4 plants can survive warmer temperatures than C3 plants because they can carry out positive photosynthesis at higher temperatures. The main cause of this drawback of C3 plants is the high rate of photorespiration at warmer temperatures. Although C3 plants may absorb more CO2 per unit of light, they will lose that advantage at higher temperatures because of increased photorespiration. Temperature also affects CAM photosynthesis because low temperatures at night are necessary for CO2 absorption. Stomatal resistance multiplies when leaf temperature rises in desert plants.
Cold stress
Cold climate plants become tolerant of the cold, and even when the temperature falls below what is necessary for growth, photosynthesis and respiration may still be able to continue slowly. Tropical and subtropical plants may be killed by temperatures just above freezing, or about 0 0C. The severity and length of the temperature decline determine the amount of damage caused by chilling and frost. Sensitive plants rapidly lose electrolytes and suffer cell membrane damage. Dehydration is brought on by slow freezing because as ice crystals develop outside the cells, water is drawn out of them. When the temperature drops quickly, ice crystals grow inside the cells, harming the structure of the cell. The cellular contents leak out during tissue thawing, giving frozen plants their watery appearance. By increasing the sugar and alcohol content, which helps reduce the freezing point of the cytoplasm, freezing and frost damage are prevented. This may enable supercooling of cell sap temporarily and without harm.
Plants in frigid climates frequently become dormant throughout the winter to withstand the frost. Plants harden themselves by creating organic chemicals that act as antifreeze, including sugars, amino acids, and substances that are not harmful. Until development begins under favorable conditions, this learned tolerance is maintained. The plant sections that are most vulnerable to freezing stress are the roots, bulbs, and rhizomes. Trees’ terminal buds are less tolerant of cold than their lateral buds, which are more tolerant than their basal buds on twigs. Compared to leaves, woody stems are more resilient. Another method to fend off freezing and frost damage is insulation. Arctic, Alpine, and Temperate zone plants of the cushion type and rosette variety may maintain a temperature that is 10–20°C higher than the ambient air.
Thermoperiodism
A temperature differential between the day and the night is essential for the growth of many plants. Thermoperiodism is the expression of a favourable response to a thermoperiod, which is a diurnal difference. Because of the large daily temperature variations in their occurrence locations, a thermoperiod need is adaptive in pine and fir trees. During the dark period, several species respond favourably to temperature drops.
Dormancy: The interaction of short days and cold temperatures causes the dormancy of apical buds in many woody species of temperate climates. Cold temperatures serve as the environmental cue that allows plants to recognize the end of winter. For their seeds to germinate, the majority of plants that grow at higher latitudes need frigid temperatures. When exposed to freezing temperatures, such seeds become dormant.
Vernalization: Exposure to freezing temperatures at the time of germination affects the blossoming of cereal plants. Vernalization is the term for this response to exposure to cold.