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United States Department of Agriculture

Agricultural Research Service

Research Project: IMPROVED PLANT GENETIC RESOURCES FOR PASTURES AND RANGELANDS IN THE TEMPERATE SEMIARID REGIONS OF THE WESTERN U.S. Title: Root Responses to Short-Lived Pulses of Soil Nutrients and Shoot Defoliation in Seedlings of Three Rangeland Grasses

Authors
item Arredondo, Jose -
item Johnson, Douglas

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 29, 2009
Publication Date: September 11, 2009
Citation: Arredondo, J.T., Johnson, D.A. 2009. Root Responses to Short-Lived Pulses of Soil Nutrients and Shoot Defoliation in Seedlings of Three Rangeland Grasses. Rangeland Ecology and Management. 62:470-479.

Interpretive Summary: The production of new roots is important in determining the capability of rangeland plants to explore the soil for nutrients. The production of new roots in relation to their development patterns and biomass allocation was examined at the critical seeding stage in two important perennial rangeland grasses that occur widely in the great Basin Region of the western U.S. (Whitmar bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Love] and Hycrest crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult. x A. cristatum L. Gaert.]). We compared the responses of Whitmar and Hycrest to the widespread invasive annual grass, cheatgrass (Bromus tectorum L.). Greenhouse-grown seedlings were exposed to four nutrient regimes: uniform-low, uniform-high, soil-nutrient pulse, soil-nutrient depletion, and to either no clipping or clipping. Hycrest was the only species that exhibited root proliferation responses to a nutrient pulse, and this response occurred through root elongation rather than initiation of lateral root branches. Clipped plants of Hycrest interrupted root development following shoot defoliation compared to unclipped plants. In contrast, root developmental patterns were the same for clipped and unclipped plants of Whitmar, whereas cheatgrass exhibited an intermediate response between Whitmar and Hycrest. Our results suggest that a plant's inherent biomass allocation to roots contributes to enhanced root development responses. Gaining a clearer understanding of root responses in rangeland grasses in response to soil nutrients and clipping can help identify critical factors for selecting superior genotypes in plant improvement programs. Such information also may help in identifying possible management tools that could be beneficial in reducing seedling mortality and improving seedling establishment of rangeland grasses. This is especially important in re-establishing perennial plant cover and diversity on cheatgrass-infested semiarid rangelands.

Technical Abstract: Root proliferation is important in determining root foraging capability of rangeland grasses to unpredictable soil-nutrient pulses. However, root proliferation responses are often confounded by the inherent relative growth rate of the particular species being compared. Additionally, proportional biomass allocation to roots (R:S ratio) can be associated with root foraging responses. The influence of relative growth rate and biomass allocation patterns on root foraging responses at the critical seeding stage was examined in two important perennial rangeland grasses that occur widely in the Great Basin Region, U.S.A. (Whitmar bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Love] and Hycrest crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult. x A. cristatum L. Gaert.]) as well as in the widespread exotic invasive annual grass, cheatgrass (Bromus tectorum L.). Greenhouse-grown seedlings were exposed to four nutrient regimes: uniform-low, uniform-high, soil-nutrient pulse, soil-nutrient depletion, and to either no clipping or clipping (80% removal of standing shoot biomass). Hycrest was the only species that exhibited root proliferation responses to the short-lived nutrient pulse, and this response occurred through root elongation rather than initiation of lateral root branches. Overall, defoliation inhibited proliferation-based root responses to a larger extent than topological-based root responses. Defoliated plants of Hycrest interrupted root development (topological index did not change) following shoot defoliation compared to undefoliated plants. In contrast, root topological developmental patterns were the same for defoliated and undefoliated plants of Whitmar, whereas cheatgrass exhibited an intermediate response between Whitmar and Hycrest. Our results suggest that inherent biomass allocation to roots contributes to enhanced capabilities of proliferation-based root responses.

Last Modified: 10/24/2014