Temperature affects morphology, physiology, and biochemistry of plug seedlings of Astragalus membranaceus

Astragalus membranaceus is widely used to improve human immune system. However, the standardization of growth conditions for the commercial production of A. membranaceus is not yet available. Therefore, the effect of temperature on A . membranaceus seedlings growth was investigated in this study. Seedlings were grown for 4 weeks under different daytime/nighttime temperatures, 25 °C/15 °C, 20 °C/20 °C, or 20 °C/15 °C, in controlled growth chambers with a 16 h photoperiod and 150 µmol m −2 s −1 photosynthetic photon flux density provided by white light-emitting diodes and 70% relative humidity. Results showed that 25 °C/15 °C significantly increased root and shoot mass, stem diameter, and the ratio of total dry weight to whole plant length. Besides, photosynthetic related reactions including chlorophyll level, internal carbon dioxide, photosynthetic assimilation rate, and stomatal conductance were also enhanced in seedlings grown under 25 °C/15 °C. In addition, 25 °C/15 °C accelerated the accumulation of primary and secondary metabolites containing carbohydrates, protein, phenol, and flavonoids. A higher reactive oxygen species level and increased activities of antioxidant enzymes were observed in seedlings grown under 20 °C/20 °C and 20 °C/15 °C than 25 °C/15 °C. Therefore, the results suggested that 25 °C/15 °C promoted the growth of A. membranaceus seedlings, and higher average daily temperature and the difference between daytime and nighttime temperatures may induce compact shoots and strong roots with improved nutrient quality and medical care value.