Understanding Drought Resistance Strategies: A Guide to Plant Adaptation

• Drought Resistance Strategies include any plant mechanism that responds and adapts to drought, commonly called drought resistance.
• Plants use drought escape mechanisms to prevent encountering climatic established drought conditions by altering their lifespan.
• Drought avoidance strategies avoid the adverse impacts of mild and acute drought on cells by reducing plant water loss.
• Stress tolerance mechanisms cope with the ongoing stress due to severe and long-lasting droughts in plant cells by producing protective compounds and activating ROS scavenging systems.

Terms like drought avoidance, escape, drought tolerance, or drought resistance can be confusing and sometimes interchanged. However, each term has a specific meaning. Since drought is increasing, and we must find ways to limit its impact on agriculture, it is crucial to know what these terms mean. Continue reading to discover the definitions and differences between these often-used phrases.

Drought Stress Effects

Figure 1: ‘‘Stress adjustment developed in plants,’’ Bandurska 2022. (Image credits: https://doi.org/10.3390/plants11070922)

Drought is the most common stress affecting plants and is the number one cause of reducing crop yields.

Drought is a period of little or no precipitation that reduces soil water content. An extended period of dry weather causes injury to plants. Drought stress is felt in plant cells as a water deficit or dehydration, which reduces growth, physiological activity, reproduction, and productivity.

Being sessile, plants have developed molecular and physiological regulatory mechanisms to cope with water scarcity. The plant mechanisms against drought can be adaption or acclimation.

• Adaptation is the evolution of morphological, developmental, and physiological traits that help plants during drought stress. These traits are genetically determined.
• Acclimation involves plant plasticity, where the plant hardens and adjusts to the new stressor. Biochemical, physiological, and structural changes, like alteration to hormones and gene expression, can be responsible for the adjustment to new environmental conditions.

Plant responses to drought can be either adaptation, acclimation, or a combination of strategies. While these mechanisms can ensure biological resistance to drought and allow plants to survive, they can still reduce productivity and yield. So, agricultural resistance is missing. Therefore, it is crucial to understand drought response mechanisms to breed crops that survive drought and maintain productivity and yield.

Drought Resistance

Drought resistance includes all plant responses and adaptations to survive, recover, and reproduce during and after water deficiency, embodying key Drought Resistance Strategies.

Crop breeders define drought resistance as the ability of crops to maintain productivity with minimum loss in a water deficit environment.

One factor that determines the mechanisms used by plants in response to drought is stress severity:

• When plants experience less stress or eustress, the mechanisms aim to cope with the adverse conditions through drought escape or drought avoidance.
• When the drought is severe and leads to distress because of physiological problems, drought tolerance is used to protect the plant from the stress.

Drought avoidance and drought tolerance are the main plant strategies against drought; see Figure 2. Drought recovery mechanisms come into play after the drought.

Drought Escape

Drought escape is the natural or artificial plant adjustment to prevent encounters with drought. The life cycle, growth period, or planting time ensure that plants do not experience any climatic and expected drought. Some examples are as follows:

• Seed dormancy under severe drought is also considered an escape strategy regulated by ABA and gibberellins.
• Farmers use drought escape techniques when choosing crops with short cycles that complete production before the onset of drought.

Drought-sensitive plants in arid and semi-arid areas that experience regular drought use this strategy. Drought escape involves long-term adaptation and is controlled by genetics.

Drought Avoidance

Drought avoidance is the ability of plants to continue fundamental physiological processes during mild and moderate drought. Plants adjust growth rates or morphology to avoid the adverse impacts of drought.

Drought avoidance mainly involves maintaining plant water potentials during water deficit, using three acclimation strategies:

Reduce water loss: Plants reduce water loss by swift responses such as stomatal closure, leaf rolling, and increased leaf surface wax accumulation. Plants using these strategies are alfalfa, rice, and tobacco.

Increase water uptake capacity: Plants enhance water uptake by allocating more resources to develop the root system by increasing the root-to-shoot ratio. Changes made can include more rooting depth and density. Plants also improve water storage ability in individual organs like tubers, cacti stems, candlenut trunks, etc.

Changing growth rates: Plants change their life cycle by speeding up or slowing down the change from vegetative to reproductive stage to avoid complete loss of seed production due to severe drought.

These strategies aim to avoid stress in plant cells by weakening the effects of drought. For example, increasing stomatal conductance limits water loss and osmotic adjustments for growing in water-deficient conditions.

Dehydration avoidance is effective only if the drought is mild and short-lived. However, continuing dehydration avoidance under prolonged drought will result in carbon starvation, as stomatal conductance will reduce the photosynthetic rate. The excess light not used by photosynthesis triggers the creation of reactive oxygen species (ROS) and damages the PSII apparatus, further affecting carbon fixation.

Cell dehydration is unavoidable under prolonged water deficiency conditions, and plants will activate drought tolerance mechanisms.

Figure 2.: Plant responses to abiotic stress factors, coping strategy, and resistance, Bandurska 2022. (Image credits: https://doi.org/10.3390/plants11070922)

Drought Tolerance

Plants use drought tolerance to repair the damage caused by severe and long-term stress. Drought tolerance is the ability of plants to carry on physiological activities, at least to a certain extent, under severe drought.

Cell dehydration due to drought damages cell membranes, disrupting biochemical and physiological processes. The tolerance mechanism involves regulating thousands of genes and metabolic processes to reduce damage that have two main strategies:

Maintain cell membrane integrity: Dehydration tolerance mechanisms maintain membrane integrity and cell homeostasis by increasing cell osmoregulatory molecules to maintain cell turgor pressure to allow normal physiological processes. Metabolic pathways, some of which are controlled by abscisic acid (ABA), produce protective proteins and compounds needed for membrane and enzyme protection.

Reduce toxic substances accumulation: Cell dehydration disrupts the cell respiration process and reduces photosynthesis, generating ROS that creates oxidative stress. Plants activate antioxidant enzymatic and non-enzymatic ROS-scavenging systems to reduce toxic oxidative stress. The enzymes activated are catalase (CAT), superoxide dismutase (SOD), and peroxidases (POX). The non-enzymes are ascorbic acid, flavonoids, carotenoids, glutathione, proline, phenolic compounds, etc., that reduce ROS activity.

Cell membrane stability or injury index is used to indicate drought tolerance.

Drought Recovery

Drought recovery is the plant’s ability to resume growth and biomass accumulation/yield after drought, which has caused leaf dehydration, leaf area loss, and turgor pressure loss.

Recovery processes, initiated in agriculture through rewatering or rains, involve reversing the plant responses to drought. The strategies and extent of recovery depend on drought intensity, duration, and species. In case of severe drought and mortality of plant parts, recovery can be limited.

Some of the recovery processes involved are:

• Reopening the stomata to restore transpiration
• Growing new plant parts to replace dead ones
• Increasing photosynthesis rate
• Reducing peroxidation or ROS scavenging activities

Identifying the plant recovery mechanisms is essential in suggesting vegetation management practices.

Measuring the Various Mechanisms

To study drought resistance, it is crucial to understand the various Drought Resistance Strategies applicable to droughts of varying severity and length.

Researchers use individual or several traits as indicators, broadly classified into drought avoidance-related, drought tolerance-related, and integrated indicators.

Drought avoidance traits are associated with moisture maintenance, water uptake, and use efficiency. Researchers need minirhizotron systems like the CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager to measure root systems and the CI-340 Handheld Photosynthesis System to measure stomatal conductance.

Drought tolerance indicators are mainly physiological parameters like osmotic adjustment and reducing drought damage like changes in chlorophyll content and enzymes. The CI-710s SpectraVue Leaf Spectrometer is ideal for measuring ongoing stress by recording changes in chlorophyll content.

In addition, drought indicators of interest for crops can be survival rate, greenness, fresh or dry weight, seed-setting rate, spikelet number and fertility, grain weight, etc.

Drought resistance in plants is complex and can combine more than one mechanism at different stages of plants and crop cycles. Moreover, drought is unpredictable and dynamic, making it more challenging to evaluate resistance to drought than other stressors.

Sources

Bandurska, H. (2022). Drought stress responses: coping strategy and resistance. Plants, 11(7), 922.

 

Fang, Y., & Xiong, L. (2015). General mechanisms of drought response and their application in drought resistance improvement in plants. Cellular and molecular life sciences, 72, 673-689.

 

Farooq, M., Wahid, A., Kobayashi, N. S. M. A., et al. (2009). Plant drought stress: effects, mechanisms and management. Sustainable agriculture, 153-188. DOI: 10.1051/agro:2008021

 

Ilyas, M., Nisar, M., Khan, N., et al. (2021). Drought tolerance strategies in plants: a mechanistic approach. Journal of Plant Growth Regulation, 40, 926-944.

 

Xu, Z., Zhou, G., & Shimizu, H. (2010). Plant responses to drought and rewatering. Plant Signaling & Behavior, 5(6), 649–654. https://doi.org/10.4161/psb.5.6.11398