Friday, August 7, 2009

PS 91-164: Responses of Salix nigra to spatial heterogeneity in soil flooding

Samuel C. Pierce and S. Reza Pezeshki. University of Memphis

Background/Question/Methods Inundated soils in wetlands and riparian zones often become a reducing environment within a few days of flooding, resulting in decreased root functioning.While flooding itself is expected to occur on a scale that encompasses the entire root system of a plant, the oxidation-reduction potential (Eh) of the soil reflects processes occurring at much finer spatial scales. Plants may utilize this heterogeneity to their benefit, but experimental comparisons of localized physiological and morphological responses and their relations to whole-plant stress responses are lacking. The objective of this research was to determine if localized morphological and physiological responses of Salix nigra will reflect heterogeneous soil conditions, and if these changes will result in improved functioning at the whole-plant level. The experiment described here utilized split-root minirhizotrons with two separate chambers, allowing for half of the root system of each plant to be flooded while the other half was allowed to drain. Plants from these heterogeneous treatments were compared to plants in which the entire root system was flooded or drained. Following treatment initiation, data on Eh, photosynthesis, and root architecture were recorded at seven-day intervals.  The minirhizotron system allowed for repeated measures of root architecture to be recorded nondestructively. 
Results/Conclusions Soil Eh initially declined to anoxic levels in flooded chambers at moderately reduced levels (60 mV<Eh< 350). Net photosynthesis initially decreased in response to flooding, with split-root S. nigra plants demonstrating a response intermediate to drained and flooded treatments. Chlorophyll fluorescence and chlorophyll content index demonstrated no response. Roots demonstrated little treatment response; however fine root growth appeared to decrease when the entire root system was flooded as compared to either drained or heterogeneous treatments. These preliminary results indicate that in S. nigra moderate soil reduction, while having a short-term effect on photosynthesis, did not substantially decrease root growth unless the majority of the root system is inundated for periods of at least two- to four weeks. On the banks of low-order streams, root systems may be simultaneously subjected to both reduced and aerated conditions for much of the year, whereas storm events may result in the entire root system being subjected to soil reduction for a period of days to weeks. The plasticity of S. nigra to develop specialized roots in response to spatial as well as temporal variation in water level and related soil characteristics explains at least some degree of this species ubiquity.