Temperature is one of the most critical environmental factors influencing seed germination and subsequent plant growth. It directly affects enzymatic activity, cellular metabolism, and physiological processes necessary for plant development. However, temperature fluctuations—sudden variations in temperature within short periods—can have both positive and negative effects on seed germination and growth, depending on the species and environmental conditions.
This article delves into how temperature fluctuations influence the various stages of seed germination and plant growth, explores the underlying mechanisms, and discusses strategies to mitigate adverse impacts, particularly in the context of climate change.
Seed Germination: A Delicate Balance
Seed germination is the process through which a dormant seed transitions into a seedling, involving imbibition (water uptake), activation of metabolic processes, and embryonic growth. Temperature is a key regulator at every stage.
Seeds of different plant species have specific temperature ranges within which germination occurs efficiently. This range includes minimum temperature, the lowest at which seeds can germinate, optimum temperature, which supports maximum germination, and maximum temperature, the highest at which germination can occur. Fluctuations beyond these thresholds can either inhibit or promote germination.
For certain plant species, such as those native to temperate regions, alternating day and night temperatures can enhance germination. This phenomenon, known as thermoperiodism, mimics natural environmental cues that seeds require to break dormancy. For example, fluctuations can help break dormancy in seeds like lettuce (Lactuca sativa) and celery (Apium graveolens). They also stimulate enzymatic activities, such as amylase production, aiding in the mobilization of seed reserves.
However, extreme or unpredictable temperature variations can have detrimental effects on seed germination. Non-optimal temperatures slow down water absorption and metabolic activity, delaying germination. High-temperature fluctuations can denature proteins and disrupt membrane integrity, reducing germination rates. Furthermore, such conditions may weaken seed defenses, making them prone to fungal and bacterial attacks.
Impact on Seedling Growth
Seedling growth, the crucial phase that determines a plant’s establishment and survival, is particularly sensitive to temperature fluctuations. These variations can have far-reaching effects on multiple physiological processes, including root and shoot development, photosynthesis efficiency, cellular stability, and stress responses.
Root and Shoot Development
Temperature plays a critical role in cell division and elongation, processes that underpin the growth of roots and shoots. At optimal temperatures, root systems develop robustly, allowing effective anchoring and efficient absorption of water and nutrients. However, exposure to suboptimal or fluctuating temperatures can cause severe disruptions:
- Roots: Prolonged low or high temperatures reduce the rate of mitosis and cell elongation in root tissues. This leads to stunted or poorly developed root systems, limiting the seedling’s ability to absorb essential nutrients and water from the soil. The reduced uptake directly impacts the seedling’s overall growth and vigor.
- Shoots: Temperature stress affects the production and distribution of plant hormones like auxins and gibberellins. Auxins regulate cell elongation, while gibberellins promote shoot growth and leaf expansion. Alterations in these hormonal balances due to temperature stress can cause abnormalities such as shortened stems, smaller leaves, or etiolation (excessive elongation under low temperatures).
Photosynthesis Efficiency
Temperature fluctuations critically influence photosynthesis, the primary source of energy for seedling growth. Photosynthesis depends on the stability and activity of key enzymes and pigments, including chlorophyll and RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase), which drives carbon fixation in the Calvin cycle.
- Chlorophyll stability: Sudden temperature drops or rises can degrade chlorophyll, reducing the plant’s ability to absorb light efficiently. This limits the energy available for photosynthesis.
- Enzyme activity: Enzymes involved in photosynthesis operate within specific temperature ranges. Fluctuations outside these ranges slow down enzymatic reactions, reducing carbon assimilation and, consequently, growth.
- Reduced stomatal activity: Temperature variations may also lead to erratic stomatal opening and closing. Reduced stomatal conductance limits CO₂ uptake, further suppressing photosynthesis.
Cellular Stability and Oxidative Stress
Abrupt temperature changes can destabilize cellular functions, leading to oxidative stress. This stress arises when the production of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂) and superoxide radicals, exceeds the plant’s capacity to neutralize them.
- Membrane damage: ROS can cause lipid peroxidation, compromising the integrity of cellular membranes. This disrupts the transport of nutrients and ions, further weakening the seedling.
- DNA and protein damage: ROS can also harm genetic material and denature essential proteins, impairing vital cellular processes like DNA replication and protein synthesis.
Stress Responses and Growth Trade-Offs
To cope with temperature fluctuations, plants activate stress responses that help them survive but often come at a cost to growth.
- Osmolyte accumulation: Seedlings accumulate osmolytes like proline, which stabilize proteins and cellular membranes during stress. However, the energy and resources diverted to produce osmolytes reduce energy availability for growth.
- Activation of stress-related genes: Plants upregulate genes involved in heat shock proteins, antioxidants, and other stress-related pathways. While these mechanisms enhance resilience, they consume energy that would otherwise support biomass production.
Impact of Climate Change
Global climate change has amplified the frequency and intensity of temperature fluctuations, posing new challenges for seed germination and plant growth. Erratic germination patterns may result as seeds either germinate prematurely or fail to germinate altogether due to unpredictable environmental cues. These fluctuations can also alter phenology, disrupting the timing of flowering, fruiting, and seed setting. For crops like wheat, rice, and maize, the combination of unpredictable germination and altered phenology often results in reduced productivity, making agricultural systems particularly vulnerable.
Mitigation Strategies
Given the growing threat of temperature fluctuations, several strategies have been developed to mitigate their impact on seed germination and growth.
Modern breeding programs focus on developing temperature-tolerant crop varieties with traits such as enhanced dormancy-breaking mechanisms, robust root systems to withstand stress, and improved photosynthetic efficiency under variable conditions. Seed priming techniques, such as hydropriming (soaking seeds in water), osmopriming (using osmotic agents like polyethylene glycol), and hormonal priming (treating seeds with growth regulators like gibberellic acid or abscisic acid), improve seed resilience to temperature stress.
Creating favorable microclimates through practices like mulching, shading, and using protective coverings can buffer seeds and seedlings from temperature extremes. Controlled environment agriculture (CEA), which includes technologies like greenhouses and growth chambers, allows precise regulation of temperature, ensuring optimal conditions for germination and growth. Additionally, the application of biostimulants, such as plant hormones, microbial inoculants, and natural extracts, can enhance stress tolerance in plants by boosting antioxidant activity and stabilizing cellular functions.
Conclusion
Temperature fluctuations have a profound impact on seed germination and plant growth, influencing critical physiological and metabolic processes. While certain species benefit from mild fluctuations, extreme and unpredictable changes often lead to reduced germination rates, stunted growth, and lower crop productivity.
With climate change exacerbating these fluctuations, it is imperative to adopt innovative strategies, including breeding, seed priming, and controlled agriculture, to ensure sustainable plant growth. By understanding the complex interactions between temperature and plant biology, researchers and agriculturalists can better prepare for future challenges, safeguarding food security and ecological stability.
References:
Bewley, J. D., & Black, M. (1994). Seeds: Physiology of Development and Germination. Springer Science & Business Media.
Bradford, K. J. (2002). Applications of hydrothermal time to quantifying and modeling seed germination and dormancy. Weed Science, 50(2), 248–260.
Baskin, C. C., & Baskin, J. M. (2014). Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination. Academic Press.
Wahid, A., Gelani, S., Ashraf, M., & Foolad, M. R. (2007). Heat tolerance in plants: An overview. Environmental and Experimental Botany, 61(3), 199–223.
Prasad, P. V. V., Boote, K. J., Allen, L. H., & Thomas, J. M. G. (2002). Effects of elevated temperature and carbon dioxide on seed-set and yield of kidney bean (Phaseolus vulgaris L.). Global Change Biology, 8(8), 710–721.
Farooq, M., Basra, S. M. A., Wahid, A., & Siddique, K. H. M. (2010). Improving the drought tolerance in plants through exogenous application of osmoprotectants. Journal of Agronomy and Crop Science, 196(4), 253–266.
Finch‐Savage, W. E., & Bassel, G. W. (2016). Seed vigour and crop establishment: Extending performance beyond adaptation. Journal of Experimental Botany, 67(3), 567–591.
IPCC. (2021). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press