Effects of water and nitrogen on root/shoot ratio and water use efficiency of winter wheat

Water and nitrogen (N) fertilizer have been identified as the two key factors that influence wheat root and shoot development. Root/shoot ratio and water use efficiency (WUE) of winter wheat were studied in the field and tub experiments at the Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences in the North China Plain. The field experiment involved with 5 irrigation treatments (with zero to 4 irrigations) during winter wheat growth period. The tub experiment involved 15 treatments of five levels of N (0 g·tub^-1, 2 g·tub^-1, 4 g·tub^-1, 6 g·tub^-1, 8 g·tub^-1) and three levels of irrigation (with 140 mm, 210 mm, 280 mm irrigation), with 6 replicas of each treatment. The PVC tub used in this experiment was 1 m in depth and 20 cm in diameter. The bottom of each tub was sealed with plastic film and buried in the field. Field data showed that soil water content significantly influenced root and shoot biomass accumulation. The root/shoot ratio was not affected by irrigation when water content of the top 60 cm soil layer was above 60% of field capacity. Below this soil water level, root/shoot ratio increased with decreasing soil water content. The tub experiment showed that the root/shoot ratio was significantly driven by N dose and not by the combined effect of water and N. Root dry weight decreased and grain yield increased with increasing N dose under deficit water supply. Concurrently, WUE increased with increasing N dose. Under sufficient water supply, winter wheat yield initially increased with increasing N dose to a certain level, after which it no longer changed with increasing N dose. A threshold value for N dose was noted under sufficient water supply. While under deficit water supply, more N meant higher WUE by reduced root/shoot ratio. This study suggested that N and water regulated biomass allocation to the parts of the above-ground and below-ground systems of the plant that influenced grain production and WUE. Both experiments showed a negative correlation between root/shoot ratio and WUE. That implied that higher root/shoot ratio increased biomass allocation to root but at the same time reduced above-ground biomass and WUE.