Does root pruning increase yield and water-use efficiency of winter wheat?

A pot and a field experiment were conducted to assess whether seeding density in winter wheat affects grain yield and water-use efficiency when combined with root pruning. Both experiments compared four treatments, namely (i) low (normal) plant density with no root pruning; (ii) low plant density with root pruning; (iii) high plant density (25% higher than low density) with no root pruning; and (iv) high plant density with root pruning. Roots to a depth of 25 cm were cut back to keep their length to 13 cm and to limit their lateral spread to 13 cm from the plant. In the pot experiment, two water regimes were employed from stem elongation: (i) plants maintained at 85% field capacity, and (ii) plants maintained at 55% field capacity by regular watering. Low rainfall in spring at the terminal stage of plant growth served as natural water stress in the field experiment. In the field, the higher plant density induced higher root biomass at all sample depths at anthesis. Root pruning significantly decreased the root biomass in the upper soil layer (0-40 cm) and increased the root biomass in the deep soil layer (80-120 cm). When water was limited, increasing the seeding density by 25% reduced the grain yield, but with adequate water increasing the seeding density increased the grain yield. Root pruning increased the grain yield, but there was no interaction between seeding density and root pruning on grain yield in either the pots or field. Root pruning reduced water use between stem elongation and anthesis which led to more available soil water in the field between anthesis and maturity, and increased the rate of flag leaf photosynthesis at anthesis, whereas the reverse was true of increasing seeding density. Measurements of chlorophyll fluorescence suggested that initially root pruning induced a stress in the plants, but that the pruned plants rapidly recovered so that by anthesis and during grain filling the level of stress was reduced. Increasing the seeding density increased the number of spikes (fertile tillers), but decreased the grain yield per spike and had no effect on the thousand-kernel weight. In contrast, root pruning reduced the number of spikes in both the field and pots, but increased the grain yield per spike and thousand-kernel weight. Under our experimental conditions, restricting the root size by root pruning increased the yield and water-use efficiency of winter wheat in water-limited environments.