Literature Review
Maize (Zea mays L.) is an essential cereal crop grown worldwide under diverse environmental conditions. As a C4 grass species, maize is moderately sensitive to abiotic stresses, including salinity, drought, and temperature fluctuations (Biabani et al., 2017). The maize kernel contains high levels of starch (72%), protein (10%), and oil (4%), providing substantial nutritional value (Poucar-Menacho et al., 2017). Maize surpasses other major cereals like wheat and rice in total production. In addition to being a staple food crop, maize is used extensively for industrial products, biofuels, animal feed, and other derivatives like corn syrup (Poucar-Menacho et al., 2017). Consequently, seed germination is fundamental to natural ecosystems and modern agriculture. With global awareness of the delicate balance between food production and population growth, understanding the germination process is vital for supporting crop yields (Poucar-Menacho et al., 2017). General observations reveal that maize can thrive in diverse environments with considerable temperature variations. While this is the case, there is a need to identify the optimal conditions under which the species can thrive, as its production is essential in mitigating world hunger. This means optimizing every step of production, from germination to seed development. To that end, significant research is dedicated to maize germination, growth and development processes. One of the areas that has attracted significant attention is germination and how various abiotic factors, like temperature, affect the germination rate of maize seeds.
Germination and emergence are critical early stages in the plant life cycle that allow efficient use of available nutrients and water. Multiple environmental factors influence seed germination, with temperature being incredibly impactful. Temperature regulates germination timing and plant species’ geographic distributions (De Ron et al., 2016). As reproductive units containing the next generation, seeds sustain the continuity of plant populations (Shu et al., 2015). Cardinal temperatures define the minimum, optimum, and maximum temperature ranges that permit seed germination. The minimum temperature is challenging to establish since germination may be slowly occurring but not yet observable (Shu et al., 2015). The optimum temperature enables the fastest and highest germination percentage. The maximum temperature is defined as the point where proteins essential for germination become denatured (Shu et al., 2015). For most seeds, optimal temperatures are 15-30°C, while maximums are typically 30-40°C (Shu et al., 2015). Furthermore, each stage of germination has distinct cardinal temperatures. Therefore, temperature sensitivity can vary throughout the process due to its complexity. The temperature response depends on species, variety, origin, seed quality, and harvest date (De Ron et al., 2016). Generally, seeds from temperate regions need lower temperatures than tropical seeds and domesticated seeds can germinate across wider ranges than wild types (De Ron et al., 2016). High-quality seeds also tolerate broader temperature ranges.
Seed germination is a complex physiological process that transforms a dormant seed into a growing seedling. It begins when a dry seed imbibes water, causing the seed coat to swell and split (De Ron et al., 2016). Imbibition activates the seed’s inner metabolism, including respiration and enzyme activity. As germination proceeds, the embryo emerges by extending the radicle and plumule, which develop into the roots and shoots (De Ron et al., 2016). Germination involves numerous organized biochemical events, such as mobilization of seed energy reserves, absorption of endosperm nutrients, and shifts in gene expression (De Ron et al., 2016). In addition, critical cellular activities like transcription, translation, and DNA repair are restored to support growth. On the other hand, cell elongation and division facilitate embryonic axis expansion and seedling establishment.
Maize growth and quality are heavily influenced by seed germination. It is regulated by interactions between the seed genotype, its internal physiological state, and external abiotic conditions (De Ron et al., 2016). Maize seeds require a specific range of environmental factors to optimize germination. Key variables include temperature, light exposure, and water availability. The collective abiotic conditions provoke physiological responses in the seed that ultimately determine germination success (Biabani et al., 2017). In turn, successful germination affects maize population size, distribution, and abundance (Poucar-Menacho et al., 2017). No single factor acts in isolation – the demand for one condition depends on others.
Temperature is a critical environmental factor regulating all stages of seed germination. It influences cellular energy levels, enzyme activities, protein synthesis, and gene expression. Increasing temperature typically raises ATP content and the activity of enzymes like lipase and aminotransferases (De Ron et al., 2016). However, high temperatures can reduce the rate of protein synthesis. Temperature stress also induces the transcription and activity of antioxidant enzymes in seeds (De Ron et al., 2016). An optimal temperature range results in the most rapid germination. Since germination involves multiple phases, each stage has specific cardinal temperatures. Moreover, temperature sensitivity can vary throughout germination due to its complexity. The temperature response depends on seed traits like variety, quality, and age. During germination, temperature significantly impacts respiration and sugar metabolism (De Ron et al., 2016). Abnormal respiration disrupts cellular redox homeostasis required for germination (De Ron et al., 2016).
Previous studies show that temperature critically regulates the timing of seed germination. As seeds absorb thermal energy, they accumulate heat units until sufficient metabolic activity initiates germination (Shu et al., 2015). Researchers using a temperature gradient found rapid, comparable germination between 20-35°C in maize seeds (Biabani et al., 2017). However, moderate temperatures around 20-30°C appear optimal for initiating maize germination (Biabani et al., 2017). Temperatures exceeding this range significantly altered enzymatic activities, ATP levels, and protein synthesis (De Ron et al., 2016). Germination at 15°C delayed to 7 days, while 10°C prolonged germination to 34 days (Biabani et al., 2017). Even after 45 days, no germination occurred at 5°C. Literature suggests maize seeds have minimum and maximum germination thresholds around 6.2°C and 45°C respectively (Biabani et al., 2017). Fluctuating temperatures with a 45°C daytime peak also inhibited germination (Biabani et al., 2017). Meanwhile, minor differences occurred in germination timing and seed physiology between 20-30°C (Biabani et al., 2017).
Literature Cited
Biabani, A., Zarei, M., Sanchulli, S., & Romani, A. (2017). Effect of temperature and duration of seed placement at different temperatures on seed germination characteristics of barley. Applied Research of Plant Ecophysiology, 4(1), 173-186.
De Ron, A. M., Rodiño, A. P., Santalla, M., González, A. M., Lema, M. J., Martín, I., & Kigel, J. (2016). Seedling emergence and the phenotypic response of common bean germplasm to different temperatures under controlled conditions and in the open field. Frontiers in Plant Science, 7, 1087. https://doi.org/10.3389/fpls.2016.01087
Paucar-Menacho, L. M., Martinez-Villaluenga, C., Dueñas, M., Frias, J., & Peñas, E. (2017). Optimization of germination time and temperature to maximize the content of bioactive compounds and the antioxidant activity of purple corn (Zea mays L.) by response surface methodology. LWT-Food Science and Technology, 76, 236-244.https://doi.org/10.1016/j.lwt.2016.07.064
Shu, K., Meng, Y. J., Shuai, H. W., Liu, W. G., Du, J. B., Liu, J., & Yang, W. Y. (2015). Dormancy and germination: How does the crop seed decide?. Plant biology, 17(6), 1104-1112. https://doi.org/10.1111/plb.12356
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