Influence of nutrient supply and clipping on root architecture of three range grasses

Root architecture was evaluated for 3 important range grasses from the Intermountain West, including 'Whitmar,' a cultivar of bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) A. Love); 'Hycrest,' a cultivar of hybrid crested wheatgrass (Agropyron desertorum (Fisch. ex Link) Schult. x A. cristatum (L.) Gaert.); and cheatgrass (Bromus tectorum L.). Three greenhouse experiments were conducted.; In the first experiment the influence of clipping frequency and intensity on root architecture were evaluated. Defoliation (intensity and frequency) did not produce significant changes in root topology (Pe-slope index). The 3 grass species, however, did differ in root system topology across harvests. The Pe-slope index did vary with advancing seedling age, and Hycrest and cheatgrass exhibited reduced branching complexity, whereas Whitmar showed increased branching complexity. Specific root length (SRL) decreased through time for Hycrest and Whitmar but remained constant for cheatgrass. In Hycrest and cheatgrass, increases in branching complexity were associated with increases in SRL.; In the second experiment the effect of spatial nutrient availability and clipping on root architecture was studied. In a split-root system technique, treatments involving high and low nutrient levels were supplied in homogeneous or heterogeneous fashion. In addition, half the plants had 80% of their foliage removed. Plants exposed to a low-nutrient supply in the heterogeneous treatment increased root branching complexity (lower topological index) more than roots exposed to a high-nutrient supply, except for undefoliated cheatgrass, which showed the opposite response. In Hycrest and Whitmar, differences in the topological index observed between the high- and low-nutrient treatments were increased with defoliation. In cheatgrass the lowest a-slope index was observed in the high-nutrient level; however, with defoliation the same treatment exhibited the highest index. Overall, greater root biomass and root length were observed in the high-nutrient treatment than in the low-nutrient treatment. Apparently, architectural plasticity and inherently low relative growth rates allowed Whitmar to achieve similar biomass in either low- or high-nutrient conditions.; In the third experiment, the effect of transient and continual nutrient conditions as well as defoliation (0 and 80%) was evaluated. Initial nutrient supply did not consistently affect root architecture and root morphology. Defoliation also did not influence root topology, but decreased external-external (EE) and external-internal (EI) link lengths in Hycrest and EI link length in Whitmar. Some of the responses in link length may reflect trade-offs between carbon cost and root function. In summary, my studies indicated that root architecture was influenced by both defoliation and nutrient supply.