Mulched drip irrigation is a common water-saving irrigation technology that can improve water resource utilization efficiency in arid areas. The change of irrigation method affects the growth of crop root system and then regulates the growth of aboveground organs. However, there have been limited comparative studies on how mulched drip irrigation affects the cotton root growth and regulates the relationship between cotton root and shoot. The objective of this study was to determine whether mulched drip irrigation could achieve water-saving and high-yield cotton cultivation by regulating the growth redundancy of root and shoot. Under field conditions, mulched drip irrigation (conventional drip irrigation and excessive drip irrigation) and traditional flood irrigation (conventional flood irrigation and reduced flood irrigation) were used in this experiment. Traditional flood irrigation was used as the control group. The results showed that compared with the traditional flood irrigation, the higher soil water content under mulched drip irrigation increased the specific root length (SRL), which promoted the growth of fine root length and biomass at the full flowering stage and prophase full boll stage. The growth of fine roots increased the root-shoot ratio (R/S) of mulched drip irrigation, and then inhibited the excessive vegetative growth of the aboveground parts. When entering the late full boll stage and boll opening stage, the fine root biomass and fine root mass fraction decreased rapidly under the mulched drip irrigation, resulting in a lower R/S. The smaller R/S of mulched drip irrigation facilitated the distribution of photosynthetic products to reproductive organs, which increased the boll number per plant by 7.30 %-25.10 %, thus increasing the boll loading of fine root system and seed cotton yield. However, compared with conventional drip irrigation, over irrigation led to excessive vegetative growth of cotton under excessive drip irrigation, which resulted in the decrease of boll opening rate and seed cotton yield by 8.71 %-17.19 % and 14.14 %-24.27 %, respectively. Moreover, the smaller R/S at the late growth stage enabled conventional drip irrigation to achieve the maximum water use efficiency (WUE). Therefore, mulched drip irrigation promoted the root growth of cotton and inhibited the vigorous vegetative growth of shoot before the prophase full boll stage, and then reduced the growth redundancy of root and increased the productive capacity of the fine root system after the prophase full boll stage, which increased boll number per plant and yield. However, only under the appropriate irrigation amount (390 mm), mulched drip irrigation can increase both yield and WUE, thus achieving the goal of water-saving and high-yield cultivation.
