Understanding the Impact of High Temperatures on Crop Roots

• Due to climate change-related high temperatures on crop roots, root architecture is altered.
• Roots change the carbohydrate/amino acid ratio and lipid metabolism to limit growth disruption and activate heat pathways.
• Temperature-induced hormone changes trigger signals to activate root responses to stress.
• Roots must also deal with climate change effects like drought, nutrient deficiency, salinity, and pathogen increase to survive and grow.

Rising soil temperatures affect the root’s ability to uptake water and nutrients, affecting crop yield. So, crops with a root system that adapts to changing conditions are necessary for food security. Morphological, physiological, metabolical, and hormonal root traits of improved root adaptation are being researched to develop climate change-proof cultivars. A review by Calleja-Cabrera et al. (2020). summarizes what we know so far about these root traits.

Warmer Soils And Air

Climate change will increase air temperature, resulting in more incidences of heat waves, less rain, and more extreme weather events.

Global air temperatures are expected to rise by 1.5-2°C by 2050. Recent research shows that soils tend to get warmer than the air, and extreme heat is more likely in soils. For example, when high temperatures occur in 10% of the month, high soil temperature will occur in 20% of the days.

Differences between soil and air temperatures are higher in summer than winter, and during the day than night. Soil temperatures have a gradient, are higher at the surface, and decrease with increasing depth. Moreover, the increases in soil temperature are not uniform across the globe.

Both air and soil temperature rises will affect crop growth and yield since soil temperatures affect soil moisture levels more than air temperatures.

Besides temperature, other effects caused by high temperatures must also be considered. These include increased drought, salinity, nutrient deficiency, and disease incidence.

Figure 1.: Root response to increased temperature. Calleja-Cabrera et al. (2020). (Image credits: https://doi.org/10.3389/fpls.2020.00544)

Temperature Sensing

Roots sense high temperatures directly and indirectly. Shoot demand for water or increased carbon allocation to roots are the indirect cues. Directly, higher temperatures alter the stability of membranes, proteins, nucleic acids, and cytoskeleton components.

The root responds to the changing growing conditions by altering its morphology, physiology, metabolism, and hormonal system, see Figure 1.

Root Morphological Changes Due to High Temperatures on Crop Roots

Higher temperatures change several root architecture traits, affecting its ability to function properly, see Figure 2..

Nutrient and water uptake by the roots is reduced due to the following changes caused by high temperatures:

• Primary roots get shorter
• Lateral roots’ number, growth, and emergence angle are reduced
• Shoot-borne adventitious and nodal root number and elongation are inhibited
• The root-to-shoot ratio is reduced as root carbon allocation is less

Figure 2. Response of major root traits to increasing temperatures in crops. Calleja-Cabrera et al. (2020). (Image credits: https://doi.org/10.3389/fpls.2020.00544)\

Most of the changes adversely affect root, but some changes are positive and help in water and nutrient uptake:

• A boost in root: soil surface because of an increase in the root hair length and number, and amount of second and third-order roots
• Increase in root diameter
• Deeper roots because lateral roots angle is reduced

Deeper soils are cooler due to the soil temperature gradient. As soil warmth reduces and growing conditions become more favorable, there is more growth in the lower than upper root parts.

These root traits are found in heat-tolerant cultivars and can be helpful in further breeding programs.

Root Physiology Changes

Changes in root functions of water and nutrient uptake are species-specific.

Water Uptake

Water movement occurs in root cells through the membrane via channels called aquaporins or by diffusion. Aquaporin activity and water uptake increase due to higher temperatures in wheat and pepper. However, warmer conditions in maize and broccoli reduce aquaporin activity and increase membrane fluidity. However, under extreme temperatures, the membrane becomes rigid and less fluid, adversely affecting water uptake.

Nutrient Uptake

In crops that experience reduced root growth due to rising temperatures, nutrient uptake of macro and micronutrients is affected, as in tomatoes. In maize, higher temperatures reduce only phosphorus and potassium uptake. However, in Agrostis stolonifera, a grass fodder, though root numbers fall, there is a higher uptake of nitrogen, phosphorous, and potassium.

Reduction in both water and nutrients can reduce crop growth and yield. Root response and adaptation become crucial in these conditions, primarily through communication between plant organs.

Aerial and Belowground Communication

Root changes that help aboveground growth are indicators of communication between below and aboveground plant organs. This communication can have various effects on roots.

Plants that manage to have an extensive root architecture or more root activity help provide water for more evaporation, leading to plants’ cooling and increased photosynthesis.

However, carbon allocation is reduced due to high soil temperatures, which reduce root system size and functioning. In these cases, plants prioritize carbon allocation to flowering and seed development.

Hormonal Changes Due to Rising Temperature

When exposed to high temperatures, the plant hormones involved in root growth change concentrations.

• Salicylic acid, abscisic acid, and ethylene levels increase due to heat stress.
• Auxin, cytokinin, and gibberellin levels fall due to heat stress in roots.

The changes in hormone levels help roots respond to heat stress by triggering signal transduction pathways, which pass a signal from the stimulus (heat stress) by converting it to another form. So, the physical heat message is sent as molecules, such as proteins, calcium ions, etc, to activate a morphological or physiological response.

Metabolic Response To High Temperatures

Roots undergo significant metabolic changes to maintain homeostasis and ensure plant survival.

Crops and fodder species show a common pattern in roots’ metabolism changes due to heat stress:

• A reduction in carbohydrates like glucose, fructose, sucrose, galactose, or xylose, and glycolytic cycle enzymes.
• Accumulation of amino acids such as proline, which is an osmoprotective compound that ameliorates cellular damage due to stress.
• Reduction in lipids like phospholipids, fatty acids, and glycerolipids to limit membrane fluidity.
• Reactive oxygen species (ROS) increase due to heat stress, which can harm cells and roots.

Effects of Temperature-Associated Abiotic and Biotic Stress

The root system integrates the effects of drought and temperature to produce responses to adapt to the stresses for plant survival, see Figure 3.

Figure 3. Effect of increasing temperature and associated abiotic stresses on root growth. Calleja-Cabrera et al. (2020). (Image credits: https://doi.org/10.3389/fpls.2020.00544)

Drought

Drought will become frequent due to less rain and higher temperatures. Root growth slows due to drought initially, but drought avoidance mechanisms are triggered as water deficiency increases, which increases primary and secondary root growth. When roots have to respond to drought and high temperatures, the response depends on the species and development stage. C3 plants are more affected by drought cum heat stress than C4 plants. C3 plants allocate more resources to roots to deal with the stress, as in sunflowers, whereas C4 maize reduces root growth.

Salinity

Drought is often accompanied by more salinity. The ion concentrations and toxicity increase due to less soil moisture and can severely reduce plant growth.

Plants absorb excessive amounts of sodium, whose accumulation can lead to cytotoxicity. Salinity also causes oxidative stress and ROS production, which damages cells. Though roots are more resistant to salinity stress than leaves, it inhibits root growth and architecture. Higher temperatures intensify the salinity effects on roots. The changes are species-specific. Salinity reduces root growth in wheat, whereas drought and heat elongate roots. Salinity has little impact in barley, but ROS levels increase, so the plants produce more proline.

Nutrient Availability

Soil temperature changes the nutrient levels by encouraging the decomposition of organic matter and making more nutrients available. However, nitrogen and phosphorus mineralization and availability remain unchanged. Roots growing in nitrogen-deficient soil grow longer and deeper; there are more lateral roots in phosphorus-deficient soils. Plants could have reduced nitrogen in combination with high temperatures, which inhibit root growth.

Biotic Stress

Global warming has been shown to increase pest and pathogen numbers and reduce crops’ coping capacity. New pathogen strains adapted to the new environment have emerged. Soil-borne diseases have become severe due to higher temperatures; for example, Phytophthora spp. are increasing root rot in forest trees. Enriching soil microflora can help control diseases, but high temperatures have also reduced the diversity and abundance of beneficial microbes, making additional amendments necessary as part of management practices.

Tools for Studying Roots

The morphological, physiological, metabolic, and hormonal responses are controlled by genetics, whose mechanisms are not entirely understood yet. Several other gaps exist in our knowledge of crop root response to rising temperatures. Phenotyping roots while studying root adaptation has been challenging. Minirhizotron systems using transparent root tubes installed in the field and scanning root traits by imagers like CI-602 Narrow Gauge Root Imager and CI-600 In-Situ Root Imager produced by CID Bio-Science Inc. is one of the standard techniques that can be used to get more information on underground root dynamics. These tools are crucial for understanding the impact of high temperatures on crop roots.

Sources

Calleja-Cabrera, J., Boter, M., Oñate-Sánchez, L., & Pernas, M. (2020). Root growth adaptation to climate change in crops. Frontiers in Plant Science, 11, 523645.

 

García-García, A., Cuesta-Valero, F.J., Miralles, D.G. et al. Soil heat extremes can outpace air temperature extremes. Nat. Clim. Chang. 13, 1237–1241 (2023). https://doi.org/10.1038/s41558-023-01812-3

 

Mulligan, R. M., Chory, J., & Ecker, J. R. (1997). Signaling in plants. Proceedings of the National Academy of Sciences, 94(7), 2793-2795.

 

Zhang, H., Liu, B., Zhou, D., Wu, Z., & Wang, T. (2019). Asymmetric Soil Warming under Global Climate Change. Int J Environ Res Public Health.16(9):1504. doi: 10.3390/ijerph16091504.