The Impact of Magnetic Field and Gibberellin Treatment on the Release of Dormancy and Internal Nutrient Transformation in Tilia miqueliana Maxim. Seeds

Tilia miqueliana Maxim. is a deciduous tree species that belongs to the Tilia genus in the Tiliaceae family. Tilia is a unique tree species found in China, known for its strong adaptability and drought resistance [1]. It is majestic, making it suitable for planting as a street and courtyard tree, widely distributed in Jiangsu, Anhui, Zhejiang, and Jiangxi Provinces [2]. Tilia is a plant with both medicinal and edible properties. Its flowers have medicinal uses and serve as an important source of honey. Additionally, Tilia is also considered to be timber [3]. Currently, the natural habitat of Tilia is facing threats, resulting in a sharp decline in population and individual numbers, indicating its endangered status [4]. The main factors contributing to the decline of the Tilia population are human activities and the difficulties of reproduction under natural conditions in the wild [5]. The seeds of Tilia exhibit low plumpness, deep dormancy characteristics, low germination rates, and irregular germination, leading to a limited natural regeneration capacity. Chinese researchers have extensively investigated and developed various methods for the reproduction of Tilia [6,7,8]. Previous studies have confirmed that the dormancy of Tilia seeds is classified as combinational dormancy (PY + PD) [9], including both physical and physiological dormancy [10]. Treatment involving H₂SO₄ soaking, GA₃ solution soaking, and 0–5 °C cold stratification has been found to effectively break seed dormancy and promote germination [11].

Gibberellins are essential during seed development [12] and promote the release of dormancy and the completion of germination [13]. In addition, GAs can enhance the ability of seeds to resist external environmental stress during germination [14,15]. Among them, GA₃ can release seed dormancy of some woody plants, such as Eucommia ulmoides, Prunus yedoensis, and Ulmus rubra Muhl [16,17,18]. Magnetic field treatment is a cost-effective, convenient, and safe method for treating seeds. Magnetic field treatment can enhance seed germination rates through increasing enzymes activities associated with seed metabolism, thereby improving overall germination potential [19]. Helianthus annuus L. seeds exhibit low production under soil moisture stress. The application of a 200 MT magnetic field to Helianthus seeds could effectively improve crop growth and yield [20]. This suggested that magnetic field treatment not only impacted the internal physiological metabolism of seeds but also mitigated the inhibition of seed germination caused by adverse conditions. Currently, it has been found to have a positive impact on seed germination in tree species such as Pinus pinea, Elaeis guineensis, and Fagus orientalis Lipsky [21,22,23]. Although magnetic field treatment is widely used in agricultural and forestry seed researches [24], it is rarely applied to releasing physiological dormancy of Tilia seeds. There is limited research on the combined effects of magnetic field treatment, GA₃ solution soaking, and cold stratification on seeds with combinational dormancy. This experiment aimed to investigate the effects of a comprehensive treatment involving magnetic field, GA₃, and cold stratification on releasing seed physiological dormancy and internal substance transformation in Tilia seeds. Finally, we expected to find a new method to release the physiological dormancy of Tilia seeds.

Previous studies have found that the effects of magnetic treatment on seed germination are influenced by factors such as plant seed types and treatment conditions [32]. The effects of different magnetic field intensities on Bupleurum chinense and Linum usitatissimum L. seeds were investigated. It was found that treating Bupleurum seeds with a magnetic field intensity of 100 MT for 55 min yielded the best effect [33]. On the other hand, treating Linum seeds with a magnetic field intensity of 350 MT resulted in the highest germination rate [34]. In this study, we compared magnetic field intensities of 150 MT and 250 MT to determine the optimal treatment scheme. Regardless of whether the seeds were treated with a separate magnetic field or with magnetic field + GA₃ soaking, the optimal treatment scheme was found to be the 150 MT magnetic field treatment. Among the different treatments, the most effective one was the use of a 150 MT magnetic field treatment for 65 min. After 75 days of cold stratification treatment, the germination rate of the M₁₅₀T₆₅G₀ seeds reached 75% and showed a significant increase of 19% compared to CK₁. It was found that the application of a separate magnetic field treatment could effectively promote the release of dormancy and enhance the germination rates of Tilia seeds to varying degrees. It is important to adjust the magnetic field intensity and processing time accordingly to cope with different types of seeds.

GAs play an important role in embryo growth and seed development and germination [35]. The application of exogenous GA₃ can modify the hormone balance in plant seeds and reduce the production of inhibitory substances. This promoted a shift in the internal environment of seeds from inhibiting germination to promoting embryonic development [36]. After 60 days of cold stratification, the Tilia seeds treated with CK₂ successfully broke dormancy. The effect of CK₂ on releasing seed dormancy was significantly superior to that of separate magnetic field treatments. M₁₅₀T₈₅G₁₄₄₃ was the most effective treatment for Tilia seeds among the comprehensive treatments. The germination rate of M₁₅₀T₈₅G₁₄₄₃ seeds reached 89% after 75 days of cold stratification. However, the highest germination rate among separate magnetic field treatments was only 75%. The M₁₅₀T₈₅G₁₄₄₃ reduced the time for seed dormancy release by 15 days compared to CK₂ and demonstrated significant superiority over CK₁. It was evident that the M₁₅₀T₈₅G₁₄₄₃ not only reduced the time for seed dormancy release but also enhanced seed germination rates. Therefore, the M₁₅₀T₈₅G₁₄₄₃ was significantly superior to both CK₂ and separate magnetic field treatments. There was an interaction effect observed between the magnetic field treatment and GA₃ soaking treatment. In this study, it was necessary to treat all seeds with cold stratification to effectively alleviate the dormancy of Tilia seeds. Cold stratification treatment is an effective method for woody plants to alleviate seed dormancy [37], so it can be used to solve the physiological dormancy of Tilia seeds [38]. And GA₃ treatment can release seed dormancy early and shorten the duration of cold stratification [39]. Combined with magnetic field treatment, the germination time of most seeds was effectively advanced by 15 days. At the same time, it also improved the germination potential, making the germination more regular. The physiological effects of magnetic field treatment on plant seeds are evident in both seed germination and internal physiological metabolism. Taking into account both aspects of the test results can help determine the optimal seed treatment plan.

There are various perspectives on selecting physiological indices to assess the impact of magnetic field treatment on seed germination. Enzyme activities of POD, PPO, and SOD were measured in Glycine max var. seeds [40]. Echinacea purpura leaves, picked from seedlings grown from electromagnetically treated seeds, were used to assess changes in secondary metabolites, including vitamin C and phenolic acids [41]. A large amount of nutrient transformation indicates seed dormancy release. Most stored substances in dormant plant seeds are insoluble. However, during seed dormancy release, these substances gradually decompose into smaller molecules or dissolve. Simultaneously, the levels of soluble substances within the seeds gradually increase. After seed dormancy releasing, germination necessitates nutrient consumption, while metabolic activities involve the decomposition of sugars, proteins, and crude fats to provide energy. For example, as the seeds mature, soluble proteins are converted into storage proteins [42]. After the release of seed dormancy, the improvement of related protease activity will promote the increase of soluble protein content [43]. Both the M₁₅₀T₈₅G₁₄₄₃ and the CK₂ were effective in releasing the dormancy of Tilia seeds in this study. The soluble sugar contents of both treatments initially increased during the process of seed dormancy release, reaching its peak at 30 and 45 days of cold stratification treatment, respectively. Upon entering the germination stage, the soluble sugar contents of the seeds decreased sharply. The soluble sugar content of the M₁₅₀T₈₅G₁₄₄₃ seeds peaked earlier than CK₂, with the largest decrease observed at 75 days. Therefore, the M₁₅₀T₈₅G₁₄₄₃ comprehensive treatment was more effective in increasing the soluble sugar content in seeds compared to the CK₂. Additionally, it consumed more soluble sugars during seed germination and enhanced the internal physiological metabolism of the seeds.

This study evaluated the release of seed dormancy by measuring the nutrient transformation in seeds. α–Amylases play a crucial role in converting nutrients [44]. GA₃ influences the production of a–Amylases to control the amount of some nutrients [45]. β–Amylases are exohydrolases. They further fully convert starch into glucose and sucrose [46,47]. Proteases participate in various physiological activities such as storage protein hydrolysis [48]. Following the CK₂ and M₁₅₀T₈₅G₁₄₄₃ treatments, the levels of internal starches and soluble proteins in Tilia seeds significantly decreased. This phenomenon may be attributed to the enhanced activities of starch hydrolases and proteases. During the dormancy release and germination process of Tilia seeds, the M₁₅₀T₈₅G₁₄₄₃ potentially increased the presence of various hydrolases in the seeds. This facilitated the hydrolysis of storage materials and provided essential resources and energy for the growth and development of seed embryos.