Moderate Drought Stress Interferes with the Physiological State and Promotes the Accumulation of Isoflavone in Reproductive Iris domestica Rhizomes

Drought stress is one of the main factors affecting the growth and secondary metabolism of plants. Iris domestica, rich in isoflavones, is a common herbal medicine in China. In this study, the effects of drought stress and rehydration on resistance physiological characteristics and the secondary metabolism of two-year-old I. domestica during the vegetative and reproductive growth period were investigated. The results showed that the dry weight and fresh weight of rhizomes and roots under severe drought stress were significantly decreased, while those under moderate drought stress were not significantly changed. Meanwhile, the SOD activities, POD activities, MDA content and Pro content increased to resist drought at D1 and D2. In the vegetative growth period, the changes in isoflavone concentration and the expression levels of genes in isoflavone synthesis were more dramatic. Isoflavone accumulation was promoted, to some extent, in the reproductive growth period under the D1 drought treatment. In the actual production process, different measures, namely short-term stress regulation in the vegetative growth period and moderate drought stress (13.44% < soil water content ≤ 16.8%) in the reproductive growth period, need to be adopted to regulate isoflavone biosynthesis.