The term “heat stress” is somewhat vague and can be used to describe different types and degrees of high temperature events, both short and long term. Heat stress is often defined as stress caused by temperatures above the plant’s physiological optimum growing conditions. The tolerance of plants to high temperatures can vary wildly both within and between species: cotton shows optimum stomatal conductance at 28-30℃, whereas alfalfa performs best at 22-25℃. Different stages of plant growth often vary in their thermotolerance as well, with the reproductive stages being the most negatively affected by heat.
Managing Heat Stress
How to understand and prevent this detrimental process.
What is Heat Stress?
Effects on Plant Health
Heat stress induces the production of an overabundance of reactive oxygen species (ROS) such as hydrogen peroxide and superoxide, leading to oxidative stress. Oxidative stress and high temperatures cumulatively damage cellular components such as proteins and DNA, inhibiting vital metabolic processes. The functions of many enzymes are impaired, cell membrane permeability is affected, and ion leakage begins to increase.
One of the most affected processes is photosynthesis. Chlorophyll pigment synthesis is impaired, and high temperatures also degrade chlorophyll, leading to reduced photosynthetic activity. Rubisco, the enzyme which captures CO2 from the air, is inhibited as well, impairing photosynthesis further. In cucumber, photosynthesis is reduced by 60% at 42℃. This reduction in enzymatic and photosynthetic activity leads to an inability to synthesize starches, sugars, oils, and proteins, causing reductions in biomass and yield.
Effects on Reproduction
While heat stress is detrimental to all parts of the plant, the reproductive organs are most affected, reducing grain yield and quality in many crops. Typically the male organs are more affected than females, but there are exceptions. In pearl millet, the pistil (female) is damaged more than the pollen. High temperatures inhibit anther dehiscence, the process of the anther opening and releasing pollen. Fertilization is an energy-intensive process, so pollen must be full of starches and soluble sugars to power itself. During heat stress, ROS accumulate in pollen and inhibit the synthesis of critical metabolites like starches and proteins, which can render pollen sterile, leading to reductions in overall seed set.
How do Plants Cope?
Plants regulate their temperature through evapotranspiration. When they’re too warm, plants release more water vapor through their stomata and draw cooler groundwater through their roots. However, if temperatures rise too high, root conductance is inhibited, limiting water uptake and evapotranspiration, even when water is plentiful. When plants are unable to regulate their temperatures through evapotranspiration, heat stress begins to occur. Also, heat stress is often accompanied by drought.
Fortunately, plants have developed complex physiological responses to cope with heat stress and oxidative stress. These are the heat shock response (HSR) network and the antioxidant defense system. While certain details are still not understood, the overall process is well researched. High temperatures alter root membrane stability and cause calcium to flow into the plant. This influx of calcium binds to calmodulin, which is the first step in the activation of the heat stress response (HSR) network. This complex network involves the signaling of phytohormones, calcium-dependent protein kinases (CDPKs), phosphatases, and transcriptional regulators, which will ultimately upregulate the expression of HSR genes that make the plants more tolerant to high temperatures. Some of these genes encode for molecular chaperone proteins which increase the stability of other proteins and enzymes under high temperatures.
The antioxidant defense system involves enzymes and non-enzymatic components, such as phytohormones. The antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) all play a major role in detoxifying ROS. ROS are a normal occurrence in plant cells, and must be detoxified regularly. During periods of oxidative stress, ROS are present in much greater concentrations than normal, so the antioxidant system must be upregulated.
The heat shock response (HSR) network as well as the antioxidant defense system help plants survive periods of high stress. A healthy plant, supplied with the proper water and nutrients, can respond to the stresses, adapt, and repair damaged cellular components. If oxidative stress can be controlled by the antioxidant system, photosynthesis can continue, allowing the plant to generate the energy needed to grow and respond to abiotic stresses.
Managing Heat Stress as a Grower
Besides providing crops with plenty of water, exogenous applications of certain nutrients, phytohormones, and signaling molecules have been shown to enhance heat tolerance in plants. All major plant hormones such as abscisic acid, auxin, gibberellins, cytokinins, salicylic acid, jasmonic acid, ethylene, and brassinosteroids have been shown to play a role in heat stress response. However, research for the practical application of these hormones to suppress heat stress is lacking in some cases.
Phytohormone Applications
Jasmonic acid applications are said to have a greater effect on abiotic stress tolerance than other phytohormones. In perennial ryegrass, pre-treatment with 100 µM methyl jasmonate before heat stress increased chlorophyll content and enhanced antioxidant activity, improved relative water content, and decreased electrolyte leakage compared to the control group.
During periods of high temperatures when male sterility is a concern, applications of auxin have been shown to induce cell proliferation and can reverse male sterility.
In grains like wheat, high temperatures inhibit the formation of starches, resulting in high concentrations of soluble sugars in grains. Applications of gibberellic acid and abscisic acid promote starch accumulation in wheat under heat stress.
Salicylic acid (0.5-1.5 mM) has been reported to ameliorate the effects of heat stress, and enhance chlorophyll content, soluble protein, sugar accumulation, and net yield in wheat and other cereals. This effect may be due to salicylic acid upregulating the synthesis of antioxidant enzymes and proteins. In a greenhouse experiment of alfalfa, heat stress decreased plant biomass by 34.62%. However, plants pre-treated with 0.25 mM salicylic acid had 33.07% higher biomass than the control group under heat stress. Heat stressed plants saw a 24.14% reduction in chlorophyll A content, whereas pre-treated plants saw a 16.98% relative increase. Heat stress reduced catalase activity by 41.91%, but pretreatment increased catalase activity by 39.52% relative to the control group. Overall, salicylic acid was shown to ameliorate the effects of heat stress and improve plant growth, physiology, and morphology.
Nutrient Applications
In a pot experiment, wheat grown under heat stress conditions was treated with a foliar spray of either 2 mM or 4 mM silica, and the 4 mM treatment was found to be the most effective at ameliorating stress. Compared to the control group, the wheat treated with 4 mM silica had significantly higher chlorophyll a (45.25%), chlorophyll b (75%), and carotenoids (58.22%). The antioxidant activity was increased as well; catalase activity (45.76%), superoxide dismutase (35.12%), peroxidase 31.54%, and ascorbate peroxidase (21.34%) were all significantly higher following a 4 mM silica spray. The grain yield was increased by 28.64% as well.
Thiourea has also been shown to be a beneficial treatment during times of heat stress. Thiourea contains an -SH group that has redox regulatory properties, which help alleviate abiotic stress by improving antioxidant redox potential and thus detoxifying ROS, upregulating the photosynthetic rate, and increasing water availability. In heat stressed camelina and canola, 1000 mg/L thiourea was the most effective treatment, increasing seed yield per pot by 61% for camelina and 90% for canola, compared to no thiourea applications. The thiourea treatment also helped regulate hydration status and leaf gas exchange under heat stress.
Microbial Inoculants
The ability of many bacteria to produce phytohormones has been well documented. Inoculating crops with plant growth-promoting rhizobacteria may enhance thermotolerance due to the advantages provided by the bacteria, primarily nutrient uptake and phytohormone production. In a 10-day pot experiment, young soybean plants inoculated with a PGPR strain showed greater resistance to heat stress. The heat stressed plants inoculated with the bacterium showed a greater increase in biomass and higher chlorophyll content than the non-inoculated group. Also, the inoculated plants had lower levels of abscisic acid and increased levels of salicylic acid, as well as increased ascorbic acid peroxidase, superoxide dismutase, and glutathione contents, indicating a higher capacity to cope with oxidative stress.
Further Reading
Ahammed, G. J., Li, X., Zhou, J., Zhou, Y., and Yu, J. (2016). Role of hormones in plant adaptation to heat stress. In G. J. Ahammed and J. -Q. Yu (Eds.), Plant Hormones Under Challenging Environmental Factors. Springer.
Ahmad, M., Waraich, E. A., Tanveer, A., and Anwar-ul-Haq, M. (2021). Foliar applied thiourea improved physiological traits and yield of camelina and canola under normal and heat stress conditions. Journal of Soil Science and Plant Nutrition.
Akter, N. and Islam M. R. (2017). Heat stress effects and management in wheat. A review. Agronomy for Sustainable Development.
Fahad, S., Bajwa, A. A., Nazir, U., Anjum, S. A., Farooq, A., Zohaib, A., Sadia, S., Nasim, W., Adkins, S., Saud, S., Ihsan, M. Z., Alharby, H., Wu, C., Wang, D., and Huang, J. (2017). Crop production under drought and heat stress: Plant responses and management options. Frontiers in Plant Science.
Jagadish, S. V. K. (2020). Heat stress during flowering in cereals – Effects and adaptation strategies. New Phytologist.
Khan, M. A., Asaf, S., Khan, A. L., Jan, R., Kang, S., Kim, K., and Lee, I. (2020). Thermotolerance effect of plant growth-promoting Bacillus cereus SA1 on soybean during heat stress. BMC Microbiology.
Mustafa, T., Sattar, A., Sher, A., Ul-Allah, S., Ijaz, M., Irfan, M., Butt, M., and Cheema, M. (2021). Exogenous application of silicon improves the performance of wheat under terminal heat stress by triggering physio‑biochemical mechanisms. Nature.
Rai, K. K., Pandey, N., and Rai, S. P. (2020). Salicylic acid and nitric oxide signaling in plant heat stress. Physiologia Plantarum.
Sarwar, M., Saleem, M. F., Ullah, N., Ali, S., Rizwan, M., Shahid, M. R., Alyemeni, M. N., Alamri, S. A., and Ahmad, P. (2019). Role of mineral nutrition in alleviation of heat stress in cotton plants grown in glasshouse and field conditions. Nature.
Su, Y., Huang, Y., Dong, X., Wang, R., Tang, M., Cai, J., Chen, J., Zhang, X., and Nie, G. (2021). Exogenous methyl jasmonate improves heat tolerance of perennial ryegrass through alteration of osmotic adjustment, antioxidant defense, and expression of jasmonic acid-responsive genes. Frontiers in Plant Science.
Wassie, M., Zhang, W., Zhang, Q., Ji, K., Cao, L., and Chen, L. (2020). Exogenous salicylic acid ameliorates heat stress-induced damages and improves growth and photosynthetic efficiency in alfalfa (Medicago sativa L.). Ecotoxicology and Environmental Safety.
