Effects of soil water restriction on root growth and root morphology of perennial ryegrass and pasture brome

To study the adaptability of herb roots to drought environments, perennial ryegrass (Lolium perenne) and pasture brome (Bromus valdivianus) were supplied 80%–85% plant available water (PAW) and 20%–25% PAW in a pot experiment. By analyzing of root appearance, biomass accumulation, and root morphology of perennial ryegrass and pasture brome, an effective production strategy for the two forage grasses under extreme drought stress was explored. The results showed that the appearance of the root tips differed slightly between the two forage grasses. Pasture brome had long and dense root hair and a long elongation area in the root tips, whereas perennial ryegrass had sparse root hair and a short elongation area. Extreme drought stress (20%–25% PAW) resulted in uneven root thickness and malformation in perennial ryegrass. No obvious damage was observed in pasture brome roots. There was no significant difference between species and soil water content in terms of biomass accumulation and distribution in the shoots and roots of the two forage grasses. However, there were significant differences in root length, root area, root diameter, root tips, and root forks between the perennial ryegrass and pasture brome. The root quantity of perennial ryegrass was significantly higher than that of brome (P≤0.01); the root length and root area were approximately 1.5 times those of pasture brome; and the root tips and forks were more than twice those of pasture brome. Pasture brome roots were significantly thicker than perennial ryegrass roots, with root diameters of 0.315 mm and 0.259 mm, respectively. The lateral root branches of the pasture brome root were short, thick, and dense, whereas the lateral roots of the perennial ryegrass were long and thin. Drought stress significantly reduced the total root length of the two forage species and promoted the radial growth of roots in the pasture brome. In conclusion, pasture brome had a more developed root-hair-area, and perennial ryegrass had more root quantity. Therefore, perennial ryegrass is distinguished mainly by its root quantity and adopts an adaptive strategy of extensive water absorption, whereas pasture brome is distinguished by well-developed root hair, high lateral root branch density (quality), and adaptive strategies to ensure effective water absorption.