Life is full of adversity and challenges that test our abilities to survive and overcome. If the hardships become too great, we can succumb to their ill effects or find a way to resist and become stronger, ready to move on to the next challenge. All living organisms go through this cycle, but each species and individual has its own troubles and specific mechanisms to cope with those troubles. Plants have been inhabiting our planet for at least 700 million years, so they witnessed and successfully survived severe climate changes and quite a few cataclysms. Such experiences equipped plants with an array of strategies that help them deal with a diverse set of adverse conditions. Scientists like to classify these conditions into two groups – abiotic and biotic stress.
Abiotic stress is a result of the non-living negative factors in a plant’s environment, while biotic stress includes the negative effects of living organisms. We shouldn’t fall into the trap of viewing these two types of stress separately, as they are closely intertwined. However, for the sake of clarity, we will explain the effect of each type individually and how plants react to them.
Environmental conditions are the primary limiting factors when it comes to plant growth. Temperature, humidity, light, and nutrient availability, as well as the chemistry and structure of the soil, play a pivotal role in successful plant development. Tolerance ranges for these conditions can vary between species and individuals of the same species. For example, an avocado plant won’t be very pleased if it’s colder than 50°F, while wheat needs low temperatures to produce the best grains. Roses thrive in loamy soils abundant in organic matter, while lavender produces the most fragrant flowers in sandy, dry soils. Plant breeders produce cultivars of the same plant species to optimize and accentuate certain qualities, like frost and heat tolerance, drought tolerance, resistance to certain diseases, and similar.
Any deviation from the physio-chemical tolerance range of the specific plant is viewed as abiotic stress. The most common abiotic stressors include drought, flooding, cold or frost, heat, nutrient deficiency, salty soils, heavy metals, air pollutants, and chemical agents (pesticides). Let’s look at the effects of each and how plants react to them.
Different plant species and different organs and tissues of the same species have specific temperature requirements. These requirements change during different phases of a plant’s growth. Plants rely on the environmental temperature to perform their basic functions normally, and any extreme can be lethal. Depending on the side of the thermometer we’re looking at, we can divide temperature stress into two basic types – cold and heat stress.
Effects of cold stress
Low (<60°F) and freezing (<32°F) temperatures primarily affect plants by changing the content and the structure of their cell membranes. The membranes normally have a viscose structure, but they become gelly as the temperature decreases. As such, they are not able to perform their function properly. Additionally, cold and freezing temperatures negatively affect photosynthesis and inactivate enzymes that play an important role in metabolic processes. The secondary effect of cold stress is the deficit of water since the root isn’t able to effectively absorb moisture from the cool soil.
The symptoms of cold stress usually include:
- leaf chlorosis (yellowing)
- dark, necrotic spots on leaves
- plant rapidly loses water (becomes dry)
- slowed or stomped growth of the plant
Certain plants can adapt to low-temperature conditions if the shift is gradual. They do this by activating enzymes that increase the concentration of unsaturated fatty acids in cell membranes and accumulating carbohydrates, sugar alcohols, and other compounds that make the plants more resistant to the negative effects of low temperatures.
Effects of heat stress
Heat stress occurs when plants are exposed to temperatures that are above their optimal range by 20 or more degrees. Such high temperatures can cause irreversible damage to plants, interrupting their growth and development. Similar to cold stress, heat stress primarily causes changes in the structure of the cell membranes. It also inhibits photosynthesis and disrupts the activity of proteins and enzymes. Above a certain temperature, proteins and enzymes cannot function properly or even go through denaturation, while cell membranes become more fluid so the transport of essential compounds is interrupted. Heat stress indirectly affects plants by intensifying the process of transpiration (loss of water through stomata), causing a deficit of water.
The symptoms of heat stress usually include:
- very slow or very intensive growth
- early maturation
- limp leaves
- progressive drying of the plant
- fruit cracking
Some plants, like prickly pear (Opuntia) and houseleeks (Sempervivum), have adapted to very hot and arid conditions and they can tolerate hellish 140-150°F. They have a specific type of photosynthesis and metabolism that enables them to have an incredible water use efficiency – crassulacean acid metabolism or CAM photosynthesis. However, most plants we grow don’t have such adaptations, so they rely on certain compounds like heat shock proteins, hormones, and salicylic acid. Some plants are able to acclimatize to increased temperatures through gradual exposure, but the effect is short-lasting.
Light provides energy for photosynthesis and regulates the growth and development processes of plants, also known as photomorphogenesis. Unlike photosynthesis, where the energy from light is directly collected and transformed into chemical energy, photomorphogenesis uses light as a signaling system that informs the plants about the changes in the environment. The plants use this information to adapt their growth and development optimally according to the given conditions.
Plants respond to light with their photoreceptors – chlorophyll and carotenoids in case of photosynthesis, and phytochromes and cryptochromes in case of photomorphogenesis. Talking about light stress, we need to have in mind that light affects plants with both its quality and quantity. For that reason, scientists usually view light stress through its different wavelengths, so we have:
- infrared light stress
- visible light radiation stress
- ultraviolet (UV) radiation stress
- ionizing radiation stress
We’ll talk a bit more about visible light and UV radiation stress since they have the greatest impact on plant cultivation.
Visible light as a stressor
Plants growing under the Sun are exposed to varying amounts and intensities of visible light. The part of the electromagnetic spectrum that visible light encompasses includes physiologically active radiation. This type of radiation includes photosynthetically active radiation (PAR) which spans from 400-700nm, but also blue light (300-400 nm) and far red light (700-800 nm). Although physiologically active radiation is crucial for plant development, if it’s intensive it can cause stress in plants. High-intensity physiologically AR damages chloroplasts, reducing the photosynthetic activity of plants. The excess energy chlorophyll molecules receive creates a problem in the energy transfer between the molecules, resulting in damaged cell membranes and organelles.
UV light as a stressor
UV light is a part of the Sun’s electromagnetic spectrum that has short wavelengths and can inflict damage to essential macromolecules like proteins, lipids, and DNA. There are three types of UV radiation – UV-A, UV-B, and UV-C. UV-C is completely absorbed by atmospheric gases, UV-B partly, while UV-A reaches the Earth’s surface unfiltered. The latter two types are the ones that cause damage to plants and other living organisms, especially UV-B. While short-term exposure usually doesn’t cause any lasting adverse effects, long-term exposure to UV-A and UV-B can be very destructive.
The short wavelengths of UV-A and UV-B pass through cells effortlessly. On their way through, they can hit macromolecules like proteins, lipids, and DNA. The affected proteins and lipids in the cell membrane deteriorate its structural integrity and increase its permeability. UV-B is known to cause mutations and interrupt the transcription and replication of DNA. As such it can have a devastating effect on plant cells. Aside from these direct effects, UV-A and UV-B can also negatively affect plants indirectly. They do this by making them more susceptible to pathogens and changing their environment (for example, decreasing soil fertility by killing beneficial microbes in the top layer).
The tolerable amount and intensity of light can vastly differ between plant species. Some plants thrive under direct sunlight (like lavender and rosemary), and some love the full shade. But most plant species are somewhere in between. Depending on their proclivity towards light and ecological niche, plants have developed different adaptations to protect themselves from the negative effects of visible and UV light. Their strategies most commonly include forming a thick, waxy layer on their epidermis, and synthesis of various molecules and enzymes that have the ability to absorb the radiation or repair the damaged DNA.
Water availability is another key factor in plants’ growth and development. If the amount of the water released through transpiration exceeds the amount that the plant absorbs, a water deficit occurs. This can be caused by a lack of water in the environment, or because the plant isn’t able to absorb it due to some physiological problem. This means that even if there is enough moisture in the environment plants can become deficient. Adverse conditions like low temperature and a high concentration of salts in the soil can make it very challenging for the plant to take up the available water.
Depending on the causal agent, we can differentiate between two types of water deficit – physical and physiological drought. Physical drought occurs when there is a lack of water in the plant’s environment. This type of drought is usually the first that comes to mind when talking about water deficit stress. On the other hand, physiological drought includes water being present in the environment, but plants not being able to absorb it or use it properly. High concentration of salts in the soil, presence of pathogens, and low temperatures are some of the most common causes of physiological drought.
Water deficit affects almost all physiological processes in plants. To name a few – protein and cell wall synthesis, cell division, the activity of enzymes and hormones, opening and closing of the stomata, photosynthesis, and senescence. Plants react to water deficit by producing abscisic acid (ABA), which changes the expression of their genes and regulates the activity of a very important hormone – ethylene. This hormone functions by inhibiting plant growth and regulating senescence. In case of a water deficit, the concentration of ethylene in the root increases, and the tissues stop growing. To battle this, the plant increases the concentration of ABA, which prevents the ethylene from inhibiting root growth. ABA helps the plant to tolerate unfavorable conditions, but for a limited amount of time.
Plants that naturally grow in arid areas have evolved a variety of adaptations to dry conditions. These adaptations include both structural and biochemical adaptations. If we take a look at a cactus, we’ll see an extreme adaptation to a dry and hot environment. Its leaves are transformed into spines to minimize water loss, and the stem is optimized for water storage. Unfortunately, most of the plants we cultivate have a bad time in dry conditions. Those beautiful flowers and juicy fruits need a considerable amount of water. Aside from establishing and sustaining a good watering regimen, stimulating root growth can also make the plant more resilient to water deficits. A well-developed root can cover a greater volume of soil, absorb every last bit of water, and store some for the black days.
Too much water can cause just as serious problems as a deficit. Water and air compete in the soil, so as the concentration of water increases the concentration of air decreases. Overwatering actually creates oxygen deficiency in the soil and the root is first and most affected. Anaerobic patches have a lower pH, choke out the roots, and attract a wide variety of pathogens that cause rotting. If the anoxic conditions are severe, the plants can start to wither in a matter of hours and die off within 2-4 days. The lack of oxygen triggers the plants to reduce water absorption and close their stomata. As such, overwatering paradoxically causes a water deficit.
How will plants react to water excess depends on how well can they deal with a lack of oxygen. Plants naturally growing in wet environments display a variety of structural and metabolic adaptations. Their roots are often shallow, have a specific structure of tissues, and develop adventive and aerial growths. However, most of them are only able to withstand water excess for a limited amount of time, as they adapted to periodic flooding. Plants like rice, potatoes, wheat, corn, cauliflower, cabbage, and spinach can tolerate anoxic conditions, while tomatoes, soy, and pea are sensitive.
High salt concentration in the soil is one of the most limiting factors in crop cultivation all over the world. According to FAO, more than 400 million hectares of topsoil (0-30 cm) and more than 800 million hectares of sub-soil (30-100 cm) globally have increased salt concentration. Since indoor gardening allows full control of the growing conditions, the high salt concentration in the soil is rarely a problem. However, it can occur with overfertilizing.
Salt excess affects water availability, photosynthesis, transpiration, enzyme activity, metabolism of nitrogen compounds, and availability of certain minerals and nutrients. Such a wide array of effects is due to osmotic and ionic stress in the cells. Plant cells use ions of certain elements, like potassium (K), sodium (Na), and chlorine (Cl), for a variety of physiological and metabolic processes. Salts bind water, so their excess in the substrate can indirectly affect plants by reducing water availability and causing a physiological drought.
Plants display different levels of sensitivity depending on the species and phase of growth. Most plants are tolerant during germination but become sensitive during the early vegetative phase. Depending on their ability to withstand saline soils, plants are generally divided into two groups – glycophytes and halophytes. Glycophytes are plant species that thrive in soils with low salt content, and they include most plants we cultivate. On the other hand, halophytes occupy saline habitats, adapting to conditions very few others can tolerate.
Intensifying industrialization and urbanization produce a mounting amount of harmful materials that often end up polluting the environment. Nitrogen oxides, ozone, heavy metals, microplastics, petrochemicals, and pesticides are the most common pollutants that affect plant growth. They interfere with photosynthesis, damage leaf tissues, cause nutrient deficiencies, and interrupt physiological processes within plants. As a result, plants can display a variety of symptoms. Stunted growth, blotches on leaves, yellowing, necrosis, branch dieback, premature leaf drop, and bud blast are the most common ones. Yield loss caused by pollution costs US crop producers more than a billion dollars annually, having a greater impact than some common pathogens and pests.
To a which degree can the plant tolerate a certain pollutant is genetically predisposed. Some members of the plant kingdom are very capable of handling toxic substances, and some can be even used for remediation. For example, willows, oilseed rape, and Indian mustard have the ability to absorb and accumulate heavy metals in their tissues. As such, they can effectively extract the toxic metals from the soil and restore even heavily polluted sites. However, most plants we grow for food and enjoyment don’t have such majestic abilities. Lucky for indoor growers, pollution is rarely a problem in this type of plant cultivation, as the quality of air, soil, and water are easily controlled.
Aside from the elements, plants also face adversity in the form of various living organisms. The world is full of fungi, bacteria, viruses, nematodes, insects, and other herbivores that are constantly looking to occupy or consume nutritious, juicy plant tissues. Since the enemies are so varied and they cannot run away from them, plants had to develop a wide variety of weapons and defenses. Their armory includes physical, chemical, and physiological mechanisms. Which mechanism or combination of mechanisms will be used, depends on the type of enemy. Sometimes even on the specific species. We can make a general classification of plant adversaries and split them into two groups according to the way they harm plants – pathogens and herbivores.
Plant pathogens include organisms, infective molecules, or particles that can infect plant tissues and cause diseases in them. Such organisms include species of fungi, bacteria, viruses, phytoplasmas, nematodes, protozoans, and parasitic plants, with the first three groups being the most significant. For the infection to occur, there needs to be a certain degree of compatibility between the plant and the pathogen. Compatible pathogens recognize their host plant through specific chemical signals. If the plant doesn’t have the “right” signal, the pathogen won’t be able to infect it, and we consider such plants resistant. Another prerequisite for the infection is that the pathogen is virulent, i.e. able to cause disease. An avirulent pathogen simply doesn’t have the right weaponry to attack the plant.
According to the degree of damage pathogens cause, we can put them into three general categories:
- biotrophs – pathogens that occupy plant tissues but cause minimal damage
- necrotrophs – pathogens that cause the plant to die off and then colonize it
- hemibiotrophs – pathogens that initially cause little damage, but if the disease progresses they have lethal effects
Plants can withstand attacks from compatible, virulent pathogens with different resistance mechanisms that can be divided into two groups – passive and active mechanisms. Passive mechanisms include various constitutive structural and chemical adaptations that enable the plant to prevent infections. It is a type of resistance mechanism that is always in effect, no matter if the plant is attacked by a pathogen or not. For example, certain plant species have a thick, waxy layer over their leaves, stem, and/or roots, which physically make it harder for the pathogen to get in, and it also prevents water retention on the tissues, making conditions less favorable for fungi and bacteria to establish. Plant cells can also contain certain chemicals like tannins, phenols, terpenes, alkaloids, saponins, and similar, which inhibit the growth of pathogens.
Active or induced resistance mechanisms are in effect after the plant is attacked by a pathogen. Such mechanisms can include local and systemic responses. A good example of a local response is when the plant discards an infected leaf to prevent disease spread, or when it isolates the infected tissues with cork. Plants also have a variety of chemical weapons that help them deal with attackers. These include pathogenesis-related (PR) proteins, antimicrobial compounds like phytoalexins and lectins, and hormones like salicylic acid, ethylene, and jasmonic acid.
The recognizing process between the pathogen and the plant can run both ways. Some plants are able to recognize their pathogens, which allows them to react in a timely manner and stop the pathogens from inflicting any greater damage. We consider these plants resistant, as they react to the infection very quickly (in less than 24 hours) and kill off the affected cells, effectively preventing disease spread. The dying cells also send signals to warn their healthy neighbors to produce and accumulate antibiotic compounds, like phytoalexins, pathogenesis-related proteins, and toxins. This rapid cell death followed by stress signaling and accumulation of specific compounds is called a hypersensitive response.
Infected plant cells emit signals that can elicit various reactions in the neighboring cells and tissues. These reactions can be as fast as the hypersensitive response or occur sometime later, during disease development. We can generally put these local responses into three categories:
- Changes in the tissue structure – plants can make their cell wall harder or produce cork to create a physical barrier that will stop the pathogen from spreading further
- Closing of the stomata – bacteria and fungi can get inside plant tissues through stomata, so the plants close them; however, it is important to note that many pathogens developed adaptations that make them able to produce compounds that re-open the stomata
- Metabolic changes – these responses include production of various compounds and metabolites that help the infected tissues fend off the attack; the compounds and metabolites can highly vary between plant species, and some are even species-specific
Systemic acquired resistance (SAR)
As opposed to the previous two types of responses, systemic acquired resistance occurs in plant tissues that are distant from the infection site. SAR usually ensues a day or two after the hypersensitive reaction and can last for months. It is genetically encoded and elicited by systemic signals coming from the infected tissues. The exact type of response always depends on the specific pathogen. Studying and identifying these SAR responses is a hot topic in agricultural biotechnology, with its evergoing quest to minimize yield losses from pathogens and other threats.
Aside from the microscopic threats that lure in the environment, plants also encounter various macroscopic organisms ready to take a bite of their tasty tissues. Insects are the most numerous and varied bunch in that category, but we shouldn’t forget larger animals like birds and mammals. To battle these pests, plants developed a number of tactics and structural and chemical adaptations.
Thorns, hairs, spines, waxy cuticles, and thicker cell walls are the most common structural adaptations plants employ. A very interesting example of this type of adaptation is the North American grapevine species. Many North American grapevines are resistant to a very damaging pest called Phylloxera vastatrix, an insect related to aphids. This is because they have thicker cell walls on their roots and leaves, which make it hard for the insect to penetrate their epidermis. European grapevine varieties haven’t developed this adaptation, so grape producers that cultivate such grapevines often graft them onto a rootstock of a North American variety to prevent the insect from affecting the root.
Blackberries, blueberries, roses, holly, and many other plant species have thorns and spines that discourage deer and other ruminants from eating their leaves. Some herbaceous plants have leaves covered in hairs which make their chewing unpleasant, and in the case of the stinging nettle, very irritating.
When it comes to chemical adaptations, plants are equipped with an incredible variety of genes that encode the production of compounds that serve to repel, poison, or kill the herbivore trying to eat them. Some of these adaptations are pretty straightforward. For example, every part of the deadly nightshade plant contains alkaloids that are highly toxic. These alkaloids are permanently present in the plant’s tissues, only their concentration changes in different growth phases. Some plants like lavender, citronella, and rosemary, try to prevent the herbivores from ever touching them by producing essential oils that repel them.
On the other hand, some plants employ more complex chemical tactics to get rid of their pests. When black poplars are attacked by gypsy moth caterpillars, their leaves emit a specific compound that attracts parasitic wasps. The wasps lay their eggs inside the fleshy caterpillars, providing shelter and food to the future offspring. As the wasp larva develops inside the caterpillar, it eats it from the inside, effectively killing it so it never gets a chance to reach adulthood and reproduce. This way the poplars prevent the gypsy moths from increasing their numbers too much and inflicting greater damage.
Interaction of abiotic and biotic stress
Plants are rarely exposed to the negative effects of a single abiotic or biotic factor; they rather experience combinations of some of these stresses in different intensities. Depending on their effects, these combinations can be synergistic or antagonistic.
What we mean when we say that two factors display synergistic effects is that their combined negative effect on the plant is far worse than the sum of the individual negative effects. For example, mild nitrogen deficiency and excess light individually do not cause significant damage in some plants. However, when combined, they can greatly affect their metabolism. Another example is temperature extremes. High temperatures can increase the sensitivity to certain pathogens, and, as mentioned earlier, low temperatures can cause physiological drought. Plant stresses can also have an antagonistic relationship, which means that one factor prevents other/s from having an effect. An example of an antagonistic relationship can be found in tomatoes – drought makes them more resistant to grey mold (Botrytis cinerea).
One very important thing to note is that different plant species can react differently to the same combination of stresses. Every species has its specific genetic background that is a result of millions of years of adaptation. Plants carry the memory of their ancestors who survived numerous adversities, retaining knowledge of how to defend themselves again them.
How can we make plants more stress-resilient?
Making plants more stress-resilient has been one of the main priorities for plant breeders for a very long time. Now, it is an especially burning topic due to the worsening effects of climate change. Luckily, modern technologies and advancing knowledge allow us to employ and develop an array of approaches to creating more resistant plants. We can generally separate those approaches into two categories – the ones that are focused on producing resistant individuals through genetic improvements, and the ones focused on inducing stress resistance.
Breeding genetically favorable plants
The simplest method, which we commonly used throughout our history, is to take cuttings and seeds from the most resistant plants in a population, propagate them, and repeat the process continuously. This type of selection produced many common crop varieties we know and love today. Another easy way to produce resilient plants is by hybridizing them with their resilient relatives, or by grafting, like in the case of North American and European grapevine.
However, the cutting edge of plant breeding today lies in sophisticated molecular techniques and gene editing technologies. Such methods give us the freedom to introduce resistance genes to our non-resistant plants not just from unrelated, incompatible plants, but also from other forms of life. Commercial transgenic crops are now a big industry, especially in the USA, and land surfaces occupied by them are increasing by the year.
Inducing stress resistance
Aside from selective breeding and gene editing, we can use various chemical and biological activators of the plant’s immune responses to induce resistance in it. Such methods are most commonly used to increase resistance to pathogenic organisms. The activators include synthetic chemicals, plant extracts, beneficial organisms, fragments of the pathogen’s cell wall, and other natural and artificial substances. Although inducing stress resistance often cannot be 100% effective in pathogen control, it can significantly decrease the severity of infection.
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