Guide to Plant and Crop Stress | Updated for 2023

Crop Stressors

Stresses can harm plants, so crops either avoid or tolerate stress. Plants avoid stress through evolution over long periods. While stress tolerance is a short-term response regulated by genes to change plant biochemistry and metabolism.

In agriculture, researchers develop new genotypes to either avoid or tolerate stress.

Crops face two kinds of stresses based on whether the cause is abiotic or biotic. Losses caused by stress can vary depending on the causal factor:

• Abiotic stresses are environmental factors like drought, salinity, temperature, heavy metal, light, nutrient deficiency, and toxicity. These factors and weeds have a general effect on the whole plant.
• Biotic stresses arise from pests, diseases, and weeds and can yield 25-40% losses, depending on the crops. Besides weeds, biotic stresses have a specific pattern of action and effect depending on the causal organism and are ordinarily restricted to just part of the plant.

The following sections discuss the causes of plant and crop stress.

Temperature

Temperature stresses are the most common form of plant stress. High, low, and freezing temperatures can be stressors, as plant growth is optimal only within a specific range. Each crop has a minimum and maximum temperature that it can tolerate, and these cardinal temperatures depend on species and regions.

• Cool-season crops growing in temperate regions like wheat, barley, or oats have a minimum temperature of around 0-5°C, an optimum of 25-31°C, and a maximum of 31-37°C.
• Hot season or tropical crops can function in higher ranges of temperatures. For example, rice, sorghum, mangoes, etc., need a minimum temperature of 10-12°C, an optimum of 30-32°C, and a maximum of 36-38°C.

The cardinal temperature will vary with the growth stage of crops. Moreover, the diurnal range of temperature is also essential for photosynthesis.

Temperature influences crop productivity as it is important for growth and development:

• Optimum temperature is crucial to maximizing dry matter accumulation.
• Night temperature can have significant effects on shoot growth.

Climate extremes result in 18–43% yield failures, with the exact percentage depending on the crop type. Crops can suffer low and high-temperature stress and injury when temperatures are not optimal.

Low-Temperature Stress

Low temperatures can cause chilling, freezing, suffocation, heaving, and frost injury. Chilling injury can occur at 4°C for temperate crops, 8°C for subtropical crops, and 12°C for tropical crops.

Low temperatures cause changes in enzymatic reactions and membrane transport rates, loss of chlorophyll, etc. As a result, crop survival, photosynthesis, cell division, water transport, and yield can be affected.

High-Temperature Stress

Rising temperatures increase tissue temperature and transpiration while lowering plant water potential. High temperatures also suppress germination and plant emergence.

These temperatures can dehydrate leaves, scorch seedlings and stems, impact shoot growth, photosynthesis, and pollen development, stop flowering or seed set and reduce the absorption and assimilation of nutrients. In addition, high soil temperatures can damage shoots and roots alike. Heat effects on crops are irreversible. For example, crop yield in wheat can be reduced by 80-90% due to heat stress, see Figure 1.

Many annual crops die when temperatures rise above 50°C in the tropics and 35°C in the temperate regions.

Crop stress management of temperature requires monitoring photosynthesis, transpiration, and stomatal conductance.

Water

Water stress occurs due to a significant lack or overabundance of water.

Drought Stress

Next to temperature, drought is the most common crop stressor. Crops can suffer drought stress when soil water levels are low or the transpiration rate is high. We irrigate about 21% of cropland worldwide to deal with drought stress.

Drought stress results in a cascade of biochemical changes that can alter plant physiology, photosynthesis, chlorophyll content, water and nutrient relations, and biomass allocation.

Water deficit will slow growth, biomass accumulation, and crop yield, but crop response to drought depends on species, variety, growth stage, and other environmental conditions.

• Decreasing cell division and enlargement, drought reduces germination and seedling growth, shoot and root weight, and stem elongation in cereals and pulses.
• Drought decreases the number and size of leaves as well as stem length. It can also make flowers sterile, shortening anthesis and seed filling time.

When the duration and severity of water stress increase, the effects can be irreversible. The yield loss caused by drought can be high depending on the crop and the environmental conditions they need, as shown in Table 1.

Table 1: “Yield losses in some major crops caused by drought and heat stress,” Fahad et al. 2017. (Credits: https://doi.org/10.3389/fpls.2017.01147)

Waterlogging Stress

Flooding or waterlogging occurs when crops have too much soil moisture due to excessive rainfall, irrigation, and poor drainage. This impedes the movement of air in the soil. Root respiration is crucial for the roots to produce energy. Moreover, the soil microflora essential for nutrient and carbon cycling needs good air circulation.

Even short-term flooding reduces oxygen levels in the soil, which triggers the production of ethylene after 48-96 hours, leading to chlorosis and leaf decay. Plants growing in water-saturated regions will confine their root systems to shallow soils and are disadvantaged as they can’t access enough water and nutrients. Thus, the whole plant’s development and productivity are reduced.

Warmer temperatures compound the problems created by waterlogging. The germination and emergence stages of crops are most susceptible to waterlogging.

Crop stress management: Drought can be detected by the changes it brings about in chlorophyll content, photosynthesis, transpiration, stomatal opening, leaf area, leaf area index, plant height, biomass, number of flowers, and fruits, number and weight of fruits/ seeds/ grains, and yield. Changes in transmittance, absorbance, or reflectance can be used to measure drought. These changes can be measured by multi- or hyperspectral light bands, with measurements taken in the field or via remote sensing.

Salinity Stress

Salinity stress occurs due to high salt (NaCl) content. Na ions in concentrations higher than 100mM impact enzymes, but the toxic concentrations of Cl ions are unknown. Salinity stress effects occur in three stages:

1. Initially, the high salt concentrations decrease osmotic potential causing water stress in crops due to physiological drought.
2. High sodium ion levels in the plant cause toxicity since most plants are sensitive to even low concentrations of salts.
3. Salinity causes nutrient deficiencies and imbalances since sodium and chloride ions compete with nutrients like potassium, calcium, and nitrous oxide for plant absorption.

As a result of these three effects, plants growing in saline soils have reduced growth and physical molecular damage.

Salinity occurring naturally is called primary salinity. Secondary salinity occurs due to anthropogenic causes, as shown in Figure 2, some of which are:

• Clearing forests or natural vegetation with deep roots and replacing them with shallow-rooted crops that use less water. This practice results in water seeping down to raise the groundwater level, bringing up salts from below.
• Irrigating crops with saline groundwater.
• Irrigating inappropriate soils.

Crop stress management: Scientists measure gas exchange in research to detect plant stress caused by salinity.

Nutrient Deficiency and Toxicity

Compared to micronutrients, plants need macronutrients in great quantities. This is because macronutrients comprise integral parts of compounds that form cells and tissue.

A deficiency of the three primary nutrients, nitrogen, potassium, and potash, induces plant stress responses. Shoot and root growth become restricted; there is less chlorophyll and photosynthesis, and therefore less biomass accumulation.

The number of nutrients available to plants will depend on their overall concentrations and soil-related conditions, like temperature, pH, and moisture. For example, root growth is reduced when temperature and pH are low. When root growth is reduced, crops cannot access enough soil area to get the nutrients they need.

Lower moisture causes micronutrients to remain bonded to soil particles. Moreover, the nutrients will interact with each other positively or negatively to increase or decrease availability respectively.

The effect of nutrient deficiency is specific to each element and depends on the crop’s growth stage. Thus, there may be a seedling failure, leaf symptoms, stunting of plants, and internal damage. Physiological processes can also be limited, yield amount and quality can be lowered, and maturation can be disrupted.

Crop stress management: To limit the stress damage by nutrient deficiency, growers learn to recognize the symptoms and provide the required nutrient. For crops, the nutrient need and fertilizer dosages are well-researched. Practical recommendations are made for specific cultivars and even regions on these foundations. Vegetative indices based on multi- and hyperspectral reflectance can identify various symptoms caused by nutrient deficiency at the field level and via remote sensing.

Nutrient Toxicity

Toxicity can occur when more than the necessary nutrients are applied. This is often the case with nitrogen, phosphorus, and potassium when growers over-apply fertilizers and manures.

• Excess nitrogen makes plants dark green and tall, with high transpiration rates, weak stems, succulent new growth, and delayed maturity.
• Phosphorus toxicity reduces growth by affecting the uptake of nutrients like iron, manganese, and zinc.
• Potash toxicity reduces the uptake of magnesium and calcium, causing a deficiency in those nutrients.

Micronutrient toxicity is not particularly common but can also occur due to overapplication or on farms close to mining areas that release micronutrients. Since the sufficiency range (see Figure 3) between deficiency and toxicity is more restricted for micronutrients, they can easily become toxic to the plants. 

Crop stress management: Visual observation, soil testing, and plant analysis can help researchers identify nutrient deficiencies and toxicities.

Heavy Metal Stress

Soil contamination by heavy metals is becoming common as pollution increases and affects fields, especially those close to urban areas. The heavy metals can bioaccumulate and become toxic for many faunae and people, besides affecting plant processes.

Heavy metals also occur naturally, and plants need many non-essential nutrients, such as copper, zinc, manganese, nickel, iron, cobalt, molybdenum, and selenium.

Other heavy metals like chromium, cadmium, lead, silver, mercury, and arsenic are toxic to plants, even at low concentrations, as plants do not use them in any way.

Heavy metals’ effect on plants depends on their concentrations in bioavailable forms. Moreover, each heavy metal has a different mode of action.

They can bind to tissue sites and compete with essential nutrient assimilation by crops. As a result, they decrease growth processes in roots and shoots. Heavy metals will also impact photosynthetic activity, protein, and enzyme synthesis and utilization, and nutrient transport. They also generate free radicals in cells and disrupt anti-oxidant functions.

The overall effect is that heavy metal toxicity impacts various physiological processes like germination, plant growth, seed filling, and productivity.

These heavy metals can also accumulate in plants and enter the human food chain or be consumed by birds, insects, rodents, etc., harming all that eat them. Even outside the food chain, they accumulate and deteriorate soil quality.

Crop stress management: Heavy metal stress can be detected by chlorophyll content, photosynthesis, leaf area measurement, leaf spectroscopy, and monitoring of root and shoot development.

Light Interactions with Plants

Light is one of the most crucial factors for crop growth and development. Plants can suffer stress when there is too little, excess, or unnaturally fluctuating light.

The leading plant process involving light is photosynthesis. When there is less light, crops will conduct less photosynthesis; when there is too much light, the photosynthetic apparatus can be damaged. Fluctuating and excess light cause photoinhibition of photosynthesis and accumulation of reactive oxygen species (ROS) in photosystem I and II.

By determining how much food is produced, light determines crops’ overall health and productivity. ROS accumulation can also disrupt antioxidant actions.

Besides simply the amount of light, prolonging light exposure causes photoperiod stress. Light quality can also impact plants as too much ultraviolet light (UV) can damage DNA.

The quantity and quality of light crops receive will also influence how they react to other stress. Less light due to shade or shorter days helps in plants’ thermotolerance and cold acclimation. Since photosynthesis regulates stomatal conductance, shade helps increase drought tolerance by closing the stomata.

Light intensity and duration can also influence plant response to many diseases. Plants can recognize pathogens and initiate defense activity. One of the genes that produce resistance against pathogens is activated by light, without which the gene cannot act.

UV light is known to help plants attacked by pests through many mechanisms. Plants suffer more from pests and disease stress in the shade. So, aside from its amelioration of light stress, the simple application of UV light can also help to reduce the number of pest eggs and, thus, pest stress.

Crop stress management: The amount of light stress crops experience can be measured by photosynthetic efficiency and directly measuring the light they receive.

Pest Stress

Insects are the main causal organisms among the smaller pests, followed by other arthropods. Birds and mammals can also cause significant stress by damaging whole plants.

Among biotic stressors, the single group that causes the most loss is insect pests, see Figure 4. Global crop loss due to pests is crop-specific:

• 26–29% in soybean, wheat, and cotton
• 31% in corn
• 37% in rice, and
• 40% in potatoes.

Though $30 000 million is spent annually on insecticides, insect pests still cause a global loss of 30-40% of crop yield. Since there is a trade-off in growth and defense, cultivars bred for higher yields have lower resistance to pests. Having a short lifespan also helps insects to adapt quickly to changing farm conditions, even insecticides or climate change.

Direct losses occur due to dry matter consumption by pests in standing crops, but pests can also reduce the quality of seeds by damaging kernels and contaminating processed products. The pests can be monophagous, feeding on only one crop, or polyphages that consume various grains.

Pest stress detection: Different pests are recognized by the damage they cause, by the presence of eggs, larvae, pupae, and adults on the plants, and by the changes in gas exchanges and leaf area, see Figure 5.

Disease Stress

Diseases are significant yield limiters for 25 food and fiber crops. Globally, diseases cause 9-16% loss in wheat, rice, barley, corn, potato, cotton, and soybeans.

The casual agents include fungi, bacteria, viruses, chromista, and other microbes.

• Fungi are the most common pathogen, causing 85% of plant diseases, such as rust, mildews, rots, anthracnose, yellowing of leaves, etc.
• Viruses are obligate parasites and need a vector to be transmitted, such as insects, mites, nematodes, fungi, or any infected material in contact with plants. Viral plant diseases are necrotic spots, chlorosis, leafroll, mosaic, etc.
• Bacteria are primarily single-celled organisms. About 200 phytopathogenic bacteria, mainly aerobic, live in between plant cells. They are transmitted by air, water, soil, and insect vectors. Bacterial diseases cause spots, wilts, cankers, scabs, galls, and soft rots in shoots, roots, storage organs, and fruits.
• Nematodes are multicellular, wormlike animals that usually attack roots but can also damage stems, shoots, and flowers. The root diseases caused by nematodes are knots, galls, lesions, stunted or excessive root growth, and injured root tips. Cutting water and nutrient supply decreases leaf growth, causing wilting, chlorosis, etc. Ultimately, yield can be affected even when there are no visible symptoms.

Warm temperatures, high humidity, preexisting stresses, and physical and chemical injury catalyze disease onset and spread.

At times, diseases spread so far that they turn into pandemics. For example, the Irish potato famine in 1845 was caused by potato crops infested by Phytophthora infestans, leading to 2 million deaths and mass migration.

Disease stress detection: There is a large overlap in the symptoms produced by various causal agents, see Figure 6. Signs of the organisms can also identify fungus and bacteria, but this is not so for viruses and nematodes.

Weed Stress

Of all the biotic stresses, weeds produce the highest potential losses of 34% compared to 18% by pests and 16% by diseases.

Any plant growing out of turn can be considered a weed. The plants that crops will encounter in this way are native species well adapted to survive in a wide range of environments. These are usually early succession plants growing when land is cleared of forests and pastures. Plowing fields imitates these conditions, giving weeds a chance to invade, then fertilizers give them an early advantage over crops. Weeds can remain in the seed bank in the soil and become persistent endemic if not controlled.

Weeds can cause stress when the crop population is young by competing for nutrients, water, and space. Their canopy can cut access to light and severely limit crop growth.

Once crop plants are more mature and outgrow weeds, the stress and effect of weeds will decrease. However, dense weed growth can transmit and encourage diseases by limiting air circulation, thus limiting yield, see Figure 7.

The timing of weed emergence, intensity and type of weeds, and specific crop factors all influence the amount of damage they cause to yield. Weeds can also cause further stress to crops by transmitting pathogens and pests.

Crop stress management: Keeping track of canopy cover, photosynthesis, chlorophyll content, leaf area, and leaf area index are some methods of monitoring weed stress.

Further Complications

Climate change impacts crops’ ability to deal with stresses of all kinds. Rising temperatures, increasing incidences of droughts, and extreme weather all disrupt cultivar health and performance, as these plants developed for climatic conditions that are less and less accurate. Each degree rise in global mean temperature is expected to bring with it a reduction in yields of 3.2% for rice, 6% for wheat, 7.4% for corn, and 3.1% for soybean.

Various causes of plant stress can also act synergistically, as crops are exposed to many elements simultaneously. For example, temperature, light, and water availability will influence both weed and crop growth and affect their interaction. Similarly, a lack of water and resources will amplify competition if crop density is high.

How to Measure Plant Stress

Measuring plant stress is vital to breeding new varieties of crops that can withstand stress and increase yield. Later, growers must be able to detect and measure the stress for decision-making and crop stress management. Therefore, the tools used to measure plant stress precisely and quickly are vital.

Some tools produced by CID Bio-Science Inc. that can help to measure plant stress are discussed below:

Photosynthesis can measure light, pests, diseases, salinity, weeds, temperature, drought, waterlogging, and stress. The CI-340 Handheld Photosynthesis System can be used for this purpose.

Transpiration, measured by CI-340 Handheld Photosynthesis System, can estimate plant stress due to temperature, salinity, drought, waterlogging, pests, and diseases.

Stomatal conductance can measure temperature stress, drought stress, waterlogging stress, light stress, salinity stress, and pest stress. The CI-340 Handheld Photosynthesis System can measure this process.

Chlorophyll content changes, which can measure nutrient deficiency, pests, diseases, and temperature stresses, can be tracked by the CI-710s SpectraVue Leaf Spectrometer.

Leaf spectroscopy can be used for all biotic stress detection – pests, diseases, and weeds- by using vegetative indices like Normalized Difference Red-Edge, and Modified Red-Edge Normalized Difference Vegetation Index, with the help of a tool like The CI-710s Spectravue Leaf Spectrometer. Leaf spectral measurement can also assess drought, heat stress, nutrient deficiency, and toxicity.

Leaf area and leaf area index (LAI) can estimate plant stresses like pests, diseases, weeds, temperature, drought, waterlogging, temperature, and nutrient deficiency. Leaf area meters provide leaf area and LAI at the leaf level, while the Plant Canopy Imager CI-110 can measure LAI values for a canopy.

Canopy cover can measure plant stress due to weeds, pests, and nutrient deficiency. A primary tool for this purpose is the CI-110 Plant Canopy Imager.

Root growth and distribution estimation by minirhizotrons help in plant stress detection of drought, waterlogging, nutrient deficiency, and diseases.

Field Maturity affected by nutrient toxicity, deficiency, and drought can be measured by near-infrared spectroscopy-based quality meters.

Post-harvest quality estimated by near-infrared spectroscopy-based quality meters can predict the presence and severity of pest and disease stress.

Measurement tools from CID Bio-Science | Measurement tools from Felix Instruments – Applied Food Science

Crop Stress Management is Vital

About 38% of the global land area, or five billion hectares, is used for food production. Of this, one-third is used as cropland and the rest for grazing livestock. Due to the growing population, the land available per capita has decreased from 0.45 hectares in 1961 to 0.21 hectares in 2016. Hence loss in yield due to stress must be minimized to produce enough food for all.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture