Objectives We analyzed the key indicators for melatonin to promote the resistance of soybean seedlings to low phosphorus stress, from the perspective of regulating root morphology and physiological characteristics.
Methods Soybean seedlings at 1 compound leaf stage (V1 stage) were used as materials to carry out a sand culture experiment. The P concentration 0.5 and 0.05 mmol/L were set up as normal phosphorus control (CK) and low P stress (P0.05), and applying 100 μmol/L of melatonin under low P stress was taken as melatonin treatment (P0.05+M). At the 0, 5, 10, 15, and 20 days after the melatonin treatment, soybean plants and roots were sampled for investigation of plant growth, root morphological indicators, and the determination of physiological indicators.
Results Compared with CK, P0.05 treatment significantly inhibited the growth of soybean seedlings. Compared with the P0.05 treatment, at days 5, 10, 15, and 20, the P0.05+M treatment increased the activities of glutamine synthetase (GS), glutamate synthetase (GOGAT), glutamate dehydrogenase (GDH), glutamate oxaloacetate transferase (GOT), glutamate pyruvate transaminase (GPT), and the contents of soluble protein and free amino acids in roots by an average of 10.63%, 4.3%, 4.8%, 13.73%, 6.18%, 7.05%, 5.23%, but decreased ammonium and nitrate nitrogen content by 9.7% and 5.41%, respectively. On the 10th and 20th days, the P0.05+M treatment increased soybean root length, root volume, number of root nodules, root surface area, average diameter, number of root tip, and root vitality by an average of 4.59%, 27.51%, 12.67%, 7.27%, 3.57%, 22.23%, and 11% compared to the P0.05 treatment; the average nitrogen and phosphorus accumulation of leaves, stems, roots, and the whole plant increased by 108.37%, 201.82%, 58.93%, 87.38%, and 62.69%, 63.24%, 55.84%, and 59.96%, respectively; the content of photosynthetic pigments Chla, Chlb, Chl (a+b), Car and gas exchange parameters Pn, Gs, Ci, Tr increased by an average of 13.02%, 23.02%, 15.66%, 29.59% and 16.64%, 10.4%, 3.2%, 13.04%, respectively; the average increase in plant height, stem thickness, leaf area, number of nodes, leaf dry weight, stem dry weight, root dry weight, and total plant dry matter accumulation was 22.88%, 17.09%, 62.07%, 20.97%, 52.43%, 25.6%, 33.93%, and 38.58%, respectively. The Mantel test results showed that there was a positive correlation between most root morphological and physiological indicators, as well as between them and the aboveground dry matter, nitrogen, and phosphorus accumulation. Random forest analysis further clarified that root nitrate nitrogen content, glutamate pyruvate transaminase activity, root surface area, and ammonium nitrogen content were the four most critical indicators determining the growth rate of soybean seedlings under low phosphorus stress.
Conclusions The most important indicators directly influencing the growth rate of soybean seedlings are root nitrate nitrogen content, glutamate pyruvate transaminase activity, root surface area, and ammonium nitrogen content. Melatonin application significantly improve root development and the activities of glutamate pyruvate transaminase, stimulate the up-translocation of nitrogen and phosphorus in roots, thus alleviate the low-phosphorus stress.
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