Effects of Regulated Deficit Irrigation on Yield and Quality of Isatis indigotica in a Cold and Arid Environment

Although regulated deficit irrigation may improve crop yields, little research has been conducted on the effects of water deficits on Isatis indigotica, a popular herbal medicine. Field experiments were conducted in 2016 and 2017 to study the effects of regulated deficit irrigation on the net photosynthetic rate, yield, water use efficiency (WUE), and quality of I. indigotica in northwest China. Plants at the vegetative and fleshy root growth stages were subjected to mild, moderate, and severe water deficits, and their photosynthetic physiological indexes, yield, and WUE were measured. Moderate and severe deficits, but not a mild deficit, significantly decreased the net photosynthetic rate and dry matter accumulation. The yield and WUE under mild deficit were markedly higher, reaching 8239.56 kg center dot ha(-2) and 8390.80 kg center dot ha(-2), respectively, in the vegetative stage and 24.11 kg center dot ha(-2)center dot mm(-1) and 23.62 kg center dot ha(-2)center dot mm(-1), respectively, in the fleshy root growth stage, while severe deficits significantly reduced yield and WUE. Mild and moderate deficits increased the content of (R,S)-goitrin, indirubin, and indigo, improving root quality, but severe deficits decreased these compounds. Therefore, a mild water deficit in the vegetative and fleshy root growth stages is optimal and can reduce water consumption and improve I. indigotica quality and WUE without reducing yield.