Root Dynamics of Bouteloua gracilis During Short-Term Recovery from Drought

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1 The ability of drought-stressed Bouteloua gracilis H.B.K. Lag. ex Steud. to respond rapidly to a small rainfall event with increases in leaf water potential and leaf conductance directed our attention to the role of the roots in these responses. Our first objective was to determine whether new root growth was required to restore the water status to that of a non-stressed plant, following a small rainfall event. The results indicated that surviving roots were able to absorb water at approximately 40% of the rate of new roots, which was sufficient to restore non-stressed leaf water potentials within one day of watering. 2 The second objective was to estimate the rate and amount of new root growth following simulated rainfall events. New root growth occurred approximately 40h after watering. New roots never accounted for more than 15% of total root length but they made possible the continued response to increased water availability. 3 The third objective was to evaluate the importance of previous water stress in the response of Bouteloua gracilis to watering. The response of leaf water potential was influenced by previous water stress as well as by the amount of water added. However, the response of root growth was independent of water stress when 15 mm of water was added and dependent upon water stress in the case of the addition of only 5 mm. *On leave from Departamento de Ecologfa, Facultad de Agronomfa, Universidad de Buenos Aires, Avenida San Martin 4453, Buenos Aires 1417, Argentina. Correspondence: W. K. Lauenroth, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523, USA. 4 A simulation analysis of potential carbon gain, as a result of Bouteloua gracilis responding to a 5-mm rainfall event, suggested that the cost of producing new roots was less than the gain. Key-words: Drought, Bouteloua, root growth, steppe region, C4