Plant Stress Physiology: Investigating Responses to Drought, Heat, and Climate Change

By Dr. Priyom Bose, Ph.D.Reviewed by Lily Ramsey, LLM

Plant physiology entails a broad spectrum of processes, such as photosynthesis, respiration, tropism, and synthesis of hormones and metabolites, which are associated with their growth and sustainability.¹

Under different stress conditions, plants undergo a wide range of physiological changes to adapt to them. This article is focused on plant’s response to drought, heat, and climate changes.

​​​​​​​Image Credit: tony4urban/Shutterstock.com

Physiological Responses of Plants to Adapt to Drought, Heat, and Climate Change

Global warming has led to rapid changes in climatic conditions that has significantly contributed to warmer, wetter, or drier weather conditions.² During drought, the water content of the soil is extremely low causing cellular stress characterized by reduced plant water content and cell dehydration.³

Heat stress is caused due to elevated temperature with intensified solar radiation. Draught and heat stresses are mutually connected, for instance, during a drought, the transpiration rate increases which increases leaf surface temperature. On the other hand, during heat stress the plant water consumption increases along with soil moisture evaporation.⁴

The reduced water availability during drought causes loss in turgor pressure and inhibits photosynthesis and long-distance transport. Severe drought leads to cellular hydration which destabilizes membrane structures and protein. However, a prolonged drought leads to irreversible cellular damage and cell death^.5

Plants combat drought stress by shifting life cycles, preventing tissue water loss, and ensuring cellular survival under low tissue water potential. It must be noted that each plant differently responds to drought stresses.³ More generally, when a plant encounters soil dehydration during drought, it closes its stomata to conserve water. The accumulation of abscisic acid (ABA) in guard cells induces stomatal closure.⁶

Drought tolerance is also conferred by the activation of reactive oxygen species (ROS) scavenging pathways, the accumulation of protective molecules, and increased biosynthesis of compatible solutes. However, over-accumulation of ROS can lead to cell death.

Similar to drought stress, heat stress also disrupts membrane systems and metabolic pathways. To combat this condition, plants open their stomata to enhance transpiration which provides a cooling effect to their leaves. The accumulation of heat shock proteins (HSPs), removal of ROS, and photosynthetic adaptations protect the plant from heat stress. HSPs provide thermotolerance to plants under heat stress.⁷

Under drought and heat stress, plants also reduce carbon fixation. Furthermore, a decrease in the function of carbon fixation enzyme ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) has been documented at elevated temperatures or heat stress. Photorespiration also occurs in plants under drought conditions due to low cellular carbon dioxide concentrations.

Methods for Studying Plant Stress Physiology

Several physiological, molecular, and imaging strategies have been designed to study plant stress physiology. Some of the key strategies are discussed below:

Molecular Biology and Gene-Based approaches

Genomics allows large-scale gene function analysis with high throughput technology. Expressed sequence tags (ESTs), microarrays, serial analysis of gene expression (SAGE), and cDNA libraries, provide gene expression profiles. Plant defense genes are transcriptionally activated by biotic and abiotic stresses. For instance, the GF14b gene is activated during drought stress.⁸

Since proteins are directly associated with plant stress, proteomics is a powerful tool to understand plant response to stress. The RNA levels of specific ethylene-responsive-element-binding factors (ERF) genes are regulated by drought stress. Transcription factors, particularly basic-domain leucine-zipper (bZIP) are associated with important plant processes.⁹ TGA/octopine synthase-element-binding factor (OBF) proteins belong to the bZIP protein family that is closely associated with the regulation of the expression of some stress-responsive genes.

Metabolomics, particularly metabolic fingerprinting, and metabolite profiling, have also been used to assess plant stress response. This analytical tool entails measurement of the plant metabolites using either gas chromatography (GC) or liquid chromatography (LC) coupled to nuclear magnetic resonance spectroscopy (NMR) or mass spectrometry (MS).¹⁰

Biochemical Tests and Physiological Assessments

The simplest strategy to determine stress is through assessing plant height, shoot diameter, color of leaves, wilting symptoms, leaf necrosis, senescence, and phenotypical variations. Furthermore, estimation of leaf water potential, absolute and relative transpiration rates, and relative water content in leaves help determine plant stress.³

Stomatal conductance, photosynthetic water use efficiency, and intracellular CO₂ concentration in leaves help evaluate plant stress. Biochemical tests for proline, soluble sugar, and ABA content in plant tissues, are performed to determine plant stress. Enzymatic activities, particularly ATPase, catalase, glutathione reductase, and superoxide dismutase are analyzed to investigate plant stress. Membrane stability and electrolyte leakage are important factors in determining plant status.¹¹

The presence and absence of symbiotic relationships of the plant with microbes may indicate the plant's physiological status due to stress. Scientists also measure electrical capacitance to assess root growth and activity. 

To understand the changes in plant metabolism and regulatory mechanisms under stress conditions, a combination of conventional physiological approaches with genomics, proteomics, and metabolomics analysis is performed.

Imaging Techniques

In hyperspectral imaging, both imaging and spectroscopic methods are used to generate multi-dimensional data. This method has become popular in plant phenotyping and stress detection in agriculture. Hyperspectral imaging has been used to detect drought stress in barley, and maize. Typically, this imaging technique uses a wavelength ranging between 250nm and 2500nm.¹² 

Multispectral imaging has been used to detect stress in tomato plants. A major advantage of this imaging technique is portability and flexibility. Other imaging techniques used to detect plant stress are RGB (visible or red-green-blue) imaging, thermal Imaging/thermography, and fluorescence imaging.

Future Outlook

It is imperative to understand how rapid climate changes impact plant’s stress management systems. This information will help better manage the plant stress condition in the future. The impact of biotic and abiotic stress influences plant productivity, which could also threaten food security.

At present, imaging technology has proved to be a powerful tool to assess and monitor plant stress. This has significantly helped breeders, agronomists, and physiologists to prevent and assess plants' responses to stress conditions. A more efficient platform is required to manage and process the enormous amount of data linked to genotype-stress combinations. 

The existing imaging techniques, for example, hyperspectral imaging must be equipped with cameras in the full range of 350–2,500 nm in a cost-effective manner to assess plant stress more efficiently.

References

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