Comprehensive evaluation of alfalfa (Medicago sativa L.) seed vitality based on plasma treatment

Alfalfa (Medicago sativa L.) is one of the superior forage crops that is extensively cultivated in China. The surface of alfalfa seed is rich in dense hydrophobic wax-like substances, thus inhibiting water permeation, seed germination, and growth. Plasma treatment can serve as a physical seed treatment to promote seed germination and crop growth while offering environmental and economic advantages. In this study, the optimal conditions of plasma treatment were obtained to promote the growth of alfalfa. An improved model was constructed to clarify the actual effect of plasma treatment on alfalfa seeds. Zhongmu No. 4 was taken as the experimental material. The binary quadratic orthogonal rotation design was used to combine with germination tests. A systematic investigation was also made to explore the effects of plasma treatment power and treatment time on eight indicators of alfalfa seed germination, including seed germination potential, seed germination rate, seed conductivity, water absorption of the seed coat, seedling root length, seedling shoot length, seedling fresh mass, and seedling dry mass. The variance and principal component analysis were conducted to systematically evaluate the vigor of alfalfa seeds before and after plasma treatment. A mathematical model was established to integrate the plasma treatment power and treatment time with the comprehensive germination text index of alfalfa seeds, in order to seek an optimized condition of treatment plan. The results showed that the plasma treatment positively enhanced the vigor of alfalfa seeds; The variance analysis indicated that plasma treatment significantly (P<0.05) increased the water absorption of seed coat (20.93%), seedling shoot length (16.05%), seedling fresh mass (18.57%), and seedling dry mass (18.39%). There was a significant (P<0.05) decrease in the seed conductivity (28.86%), which was beneficial for the quality of alfalfa seeds. Principal component analysis was implemented to extract three main components with a cumulative contribution rate of 93.38%. A comprehensive evaluation model was established to explore the effects of plasma treatment on alfalfa seeds, according to the scores of the three principal components and their corresponding weights. The highest comprehensive score was achieved in the G12 experimental group (treatment power of 120 W, and treatment time of 12 s). Two factors (treatment power and treatment time) shared the threshold effect on the comprehensive germination test index of alfalfa, with the influence of treatment power greater than that of treatment time. There was no significant interactive effect of the two factors on the comprehensive germination test index of alfalfa (P＞0.05), indicating that the two factors were independent of each other, without substitution and counteraction. The processing conditions were optimized for the plasma treatment with a treatment power of 142 W and a treatment time of 16 s. Experimental verification showed that the comprehensive germination test index of alfalfa was 0.973 under the optimized treatment conditions, with a relative error of 2.83%, compared with the theoretical value of 0.955. An acceptable range of experimental error was achieved to further prove the rationality of the model. Therefore, the optimized conditions were obtained by the model for plasma treatment, indicating a quite reliable measurement. In summary, this finding can provide theoretical support to promote and apply the plasma seed treatment technology. A comprehensive evaluation was offered for the seed quality, which was a positive significance to promote the application of plasma in seed pre-sowing treatment.