What is Drought Stress in Plants?
Drought stress occurs when plants do not absorb the necessary amount of water in the soil to remain healthy. They are unable to survive, reproduce, and grow because of this lack of water. On the list of the biggest plant health issues, the drought stress is considered to impact more than 40 percent of the plant species in the world every year. This is mainly due to the fact that the soil becomes excessively dry and when this happens, the plants can no longer sustain their hardened cells, gas exchange and carry out photosynthesis to generate food.
Figure 1: A young green seedling struggles to survive in parched, cracked soil, illustrating the impact of drought stress on plants.
Drought stress is environmental and vegetative. The environmental conditions like low rainfall, prolonged periods of extreme weather, and high levels of evapotranspiration are just a few of the stressors that lead to drought.
The nature of the plant itself (such as the depth of its root penetration, how much water it is able to lose through transpiration, how efficiently it uses water, etc.) determines how well it will survive in such conditions. The impacts of drought stress are enormous and threaten agriculture, horticulture, forestry and urban landscapes that all rely on healthy vegetation to be productive, sustainable and able to provide the required ecosystem services.
How Does Drought Stress Affect Plants?
Drought stress is a cause of various physiological processes to reduce the evaporation of water and sustain vital activities. Some of the earliest responses include the elevation of production of abscisic acid (ABA), a hormone that increases 10 to 50-fold during drought.
The production of ABA also plays a role in stomata closing, therefore, reducing transpiration. This protective process however has a direct negative impact on photosynthesis and it also affects the uptake of carbon dioxide.
Figure 2: Corn plants showing leaf yellowing and browning at the base, clear symptoms of drought stress in agricultural crops. (Image credits: farmprogress.com)
The other strategy plants use is osmotic adjustment. Compatible solutes, e.g. proline, glycine betaine, and soluble sugars, allow plants to sustain water balance in cells and retain turgor pressure even in the face of a limited water supply.
Downside is that photosynthesis is severely affected by long-term stress. They involve both stomatal and non-stomatal factors: stomatal closure restricts the gas exchange, and non-stomatal factors, including the degradation of chlorophyll and impeded electron transport, lower the total photosynthetic activity.
Drought stress changes protein activity and triggers antioxidant defense mechanisms metabolically. During dehydration, the accumulation of reactive oxygen species (ROS) occurs at a high rate and damages body cells without neutralization by antioxidant enzymes. These adaptations are quite different among species.
Deep-rooted trees tend to be more resistant at first, but can be more permanently damaged by water stress when the stress is chronic. Herbaceous plants, on the other hand, tend to wither rapidly, but they have a chance of growing if things go better.
Signs of Drought Stress in Trees
Stress symptoms of drought on trees often appear within a time span of one to two weeks of a lack of water. Among the earliest signs are the momentary midday wilting when the leaves are dropped down in the sun, only to be rediscovered at night. When the drought continues, the drying will become irreversible, and then the scorching of leaves will occur, which consists of the margins and ends of the leaves becoming brown, and the veins becoming green. Leaves turn prematurely yellow, or become curled, or unequally discolored.
Figure 3: A young tree with sparse foliage and extensive branch dieback, demonstrating severe drought stress symptoms. (Image credits: aces.edu)
As the condition becomes more severe, trees can lose leaves to save resources, and this causes a thinning of the crown. Extensive branch dieback, cracking of bark tissue as a result of tissue shrinkage, and, in extreme conditions, loss of large areas of the canopy, may be prompted by severe drought stress.
Symptoms typical to the conifers are the browning of the needles and dieback of the tips (especially at the bottom of the branches) which tends to increase with height. The symptoms are sometimes similar to a disease or insect damage, therefore, it is worth considering soil moisture and a history of irrigation before a diagnosis is made.
Drought Stress Symptoms in Different Plant Types
The symptoms of drought stress are different in different classes of plants. Deciduous trees usually react by turning leaf color even in mid-summer, making early autumn colors, and falling leaves sooner than normal.
This shedding is useful to decrease transpiration, but may weaken the tree in case of repeated shedding.
Evergreen trees have slower falls and the needles or leaves turn brown at the ends and the general vigor is reduced more gradually over a period of months.
Figure 4: An evergreen shrub displaying browning needles and tip dieback, a common drought stress symptom in coniferous plants. (Image credits: missouribotanicalgarden.org)
Drooping foliage, slowed growth, and, in a few instances, going into a dormant state are symptoms typical of shrubs and perennials to survive extended periods of stress.
Plants resistant to drought have inherent mechanisms that slow the apparent symptoms of water stress, whereas species that prefer water, like hydrangeas and willows, wilt away very rapidly.
- Time of year also is a significant factor: drought during spring will slow down budding, drought during summer will cause wilting and scorching of the leaves, and drought in autumn will reduce the carbohydrate content that a plant will need to endure the winter.
What Causes Drought Stress?
Although a deficiency in rainfall coupled with extreme temperatures is the primary source of drought stress, it is commonly exacerbated by poor soil and management practices. Low organic matter soils with poor structure have a difficult time holding water, and once compacted, limit root development and slow down water penetration in the soil. Sandy soils drain too fast, and clay soils possibly retain water in non-accessible forms, which creates stress conditions.
Figure 5: Diagram showing how excessive heat and reduced rainfall cause drought stress, and how plant growth-promoting rhizobacteria (PGPRs) can mitigate abiotic stress to support healthy plant growth. (Image Credits: sciencedirect.com)
- One important factor is the human management practices. Research has shown that inappropriate irrigation techniques cause almost 60% of drought stress in urban settings. Deep and sparse watering favors surface rooting, exposing the plants to drought.
High planting and uncontrolled weed growth improve root competition and prevent easy access to the available water. At the broader level, climate change is increasing the frequency and severity of drought events, and increasing long-term risk in cultivated and natural plant communities.
How to Prevent Drought Stress
The most appropriate way to identify drought stress is through prevention through planning and application of sustainable practices.
It must begin with the right location, and the plants must be placed in the right place to get the right amount of sun, the right type of soil, and the microclimate.
- The preparation of the soil is also important. Compost application of two to four inches has been proven to save 30 to 50% of irrigation requirements by enhancing water absorption in the soil and making nutrients available.
Figure 6: Semi-circular planting basins designed for water harvesting, a traditional drought management technique to improve soil moisture and crop survival. (Image credits: lowimpact.org)
It is important to choose the correct plants. Native species and drought-resistant varieties need less supplemental water and are acclimated to local weather extremes.
Mulching is an additional protective measure that helps preserve moisture in the soil, moderate soil temperature, and inhibit competitive weeds that demand water.
Design of irrigation also becomes key. Drip irrigation systems, where water is supplied directly to root zones, are highly efficient in maximizing the efficiency of the irrigation process and minimizing the evaporation loss.
How to Treat Drought Stress
Immediate treatment is required to aid recovery when drought stress has already become observable. It is advisable to water the tree in an emergency, and the advice should indicate how much water to use, which is about 10 gallons of water per week divided by the inch of trunk circumference on the mature tree. The slow and infrequent use of deep watering methods stimulates root systems to extend further into the soil profile and enhance long-term resilience.
Figure 7: Comparison of wheat plants under drought stress, showing severe yellowing in the untreated control versus healthy recovery in OP-treated plants. (Image credits: news.ucr.edu)
The infiltration and retention of water is enhanced through the use of soil management interventions, including core aeration and the use of organic amendments.
Another way to make sure to use available water efficiently is by eliminating weeds and surrounding vegetation that can cause competition. In others, water loss can be reduced temporarily by foliar sprays with anti-transpirants and by temporary shading of sensitive plants during periods of peak drought.
Recovering should be gradual and at a slow rate so that the secondary stress of sudden overwatering is avoided.
Drought-Resistant Plants and Landscape Design
A sustainable landscape design is a long-term solution to minimising the effects of drought stress. Local plants are especially useful because they need no more than 75% of the water that the introduced species need.
Xeriscaping is a water-saving landscape philosophy which employs tactics like hydro zoning, wherein like-minded plants are pooled together by their relative water requirements.
By so doing, the high-demand species are met, and the resources do not go to waste on the drought-tolerant species that do not demand enough.
Figure 8: A drought-tolerant landscape featuring native plants, gravel mulch, and efficient design elements that minimize water use while maintaining visual appeal. (Image credits: gardendesign.com)
Another approach to water conservation is the reduction of lawn space, which is typically a high water consumer.
Hardscape spaces such as stone, gravel or permeable paving provide structure, but do not impose additional water demand. Even more efficiency is added by the fact that rain gardens and water harvest systems collect the runoff and store it to be used in irrigation. All these measures build strong landscapes that can be appealing and useful even during adverse water regimes.
Professional Drought Stress Management
Professional intervention is the best solution in certain situations, especially for valuable trees or a large area of landscape. Both certified arborists and horticultural consultants can present in-depth studies of the soil, root conditions tests and irrigation system studies. They can also propose or install efficient irrigation systems, which will work automatically, depending on the weather and the soil moisture.
Figure 9: A Blooma Tree professional conducting a plant health and irrigation assessment, an essential step in managing drought stress in landscapes.
Professional implementation of plant health care programs will typically include drought recovery fertilizations schemes, integrated pest management schemes to reduce secondary infestation and structural pruning to enhance resilience.
Very serious situations may even need emergency treatment like specific deep watering, mulching or canopy thinning to stabilize the sick trees. Professional services also help in the correct diagnosis to avoid mismanagement, which will reduce the decline of the plant.
Monitoring and Long-term Drought Management
Continued observation is important to the control of seasonal and inter-annual drought stress. Plant science has now made it possible to make early predictions using instruments like chlorophyll fluorescence sensors, which can detect stress within the first 24 hours of development, before symptoms can manifest themselves. The soil moisture meters give useful real-time information on how to irrigate and when.
The long-term resilience can be supported by seasonal activities, including mulch refreshing, irrigation schedule optimization, and pruning to match the ratio of canopy area to root mass. New leaf growth, root regrowth and less wilting in hot weather are the indicators that should be used to develop the recovery assessment. When viewed in the context of broader climate adaptation, the landscape needs to be redesigned to support the inclusion of a wide variety of drought-tolerant species and eco-friendly irrigation methods, and must be made resilient to the drier future.
Frequently Asked Questions:
How long does drought stress recovery take?
Depending on the species, the intensity of the stressful event, and the type of care the animal receives after the event, recovery may take several weeks to several growing seasons.
Can over-watering cause drought stress symptoms?
Yes, waterlogged soils reduce root oxygen and uptake, and result in symptoms that resemble drought stress, including wilting and yellowing of leaves.
Do drought-stressed plants attract pests?
Plants which are weak are more susceptible to pests. The chemical signals caused by stress can be appealing to insects, including aphids or borers.
What soil moisture level causes drought stress?
Most plants tend to get stressed when the soil water content is below half of the field capacity, but the thresholds also depend on species.
Should I fertilize drought-stressed plants?
Fertilizations is not encouraged when plants are still recovering because the nutrients added are likely to increase metabolic requirements and aggravate stress.
When is drought stress irreversible?
In case of massive death of roots, serious crown thinning or branch dieback, recovery is unlikely and may require removal.
