Tolerance of switchgrass to extreme soil moisture stress: Ecological implications
Introduction
The United States has set an ambitious goal of integrating biofuels into the nation's energy portfolio, which includes 61 billion liters of non-grain-based liquid fuels by 2022 [1]. It is estimated that 22–61 million hectares of land will be required for cellulosic feedstock cultivation to meet this mandate [2], [3], and by 2050 cellulosic biomass will be cultivated on an estimated 1500 million hectares globally [4]. Much of the area for dedicated biofuel production must occur on less productive marginal land, which will require crops with tolerance to stressful conditions [5]. The leading candidates for biofuel crops are perennial rhizomatous grasses which possess the agronomically desirable traits of broad climatic tolerance, rapid growth rates, high yields, growth on low production soils, and few natural enemies [6].
Despite growing interest in using biomass crops for energy production, little is known about the basic biology and physiological ecology of many of these species [2]. Therefore, there exists the need to characterize the physiological and environmental tolerances of each biofuel crop to identify ecosystems most suitable for agronomic production. Additionally, economic viability of these crops may require that genetic modification play a considerable role [5]—making basic physiological studies important baselines for future crop improvement. Once described, these factors can be integrated into risk analysis and bioclimatic, agronomic, and economic models [7], thus leading to safer and more sustainable use of these potentially important crops [2].
To be competitive with conventional energy sources and curb supplantation of food crops, biofuel cultivation will likely be relegated to less productive soils and will require minimal inputs of water, fertilizer, and pesticides [8]. Water availability will be a major limiting factor to cultivating biofuel crops in the midwestern and western US [9], owing to diminishing availability of surface and ground water, and constricting water rights. Biofuel crops are being bred and genetically modified for enhanced abiotic stress tolerance traits (e.g., drought, heat, cold, metal, salt) that will expand the available cultivatable area [5].
Switchgrass (Panicum virgatum) is a leading dedicated biofuel feedstock candidate in the US due to its broad adaptability, rapid growth rate, and ability to grow in low production soils [10]. Switchgrass is a warm-season rhizomatous perennial formerly common in the North American tallgrass prairie, with a native range spanning from the Atlantic Coast to the Rocky Mountains, and from northern Mexico to southern Canada, though it is not native to California and other western states [11]. Two distinct ecotypes of this C4 grass are recognized: lowland tetraploids, primarily from the southern extent of the native range; and upland octaploids, primarily from the mid to northern extent of the native range [10]. The ecotypes tend to occupy different edaphic conditions: upland ecotypes are associated with mesic to xeric environments, while lowland ecotypes are associated with hydric soils and are common in floodplains [12]. Several dozen cultivated varieties of each ecotype are commercially available, most of which are high-yielding selections from native populations [10]. The species includes tremendous variation in performance relative to environmental variables [13], though lowland ecotypes typically produce larger yields than upland ecotypes [10]. Although no studies have examined this in detail, evidence suggests that upland ecotypes would outperform lowland types under low soil moisture availability, and vice versa under excess soil moisture [12], [14].
A previous study has demonstrated that much of eastern North America is highly suitable for switchgrass production, though the Mediterranean climate of California is unsuitable without irrigation—both of which are related to available soil moisture (Barney and DiTomaso, unpublished data). Therefore, the objective of this study was to quantify the soil moisture stress tolerance of switchgrass. By evaluating currently available switchgrass cultivars, we are establishing the baseline for tolerance to soil moisture environments, which future genotypes—whether genetically modified or not—can be compared against. In this study, we evaluated fitness and reproductive potential of two cultivars each of the upland and lowland switchgrass ecotypes under soil moisture availability ranging from extreme drought to flooded conditions. In a second experiment we evaluated emergence and establishment potential under these extreme conditions.
Section snippets
Materials and methods
To evaluate the soil moisture stress tolerance of currently available switchgrass cultivars we implemented two greenhouse studies. The first experiment was designed to evaluate the tolerance of established plants to soil moisture conditions ranging from extreme drought to flooding. The second experiment was designed to evaluate if seeds introduced to these extreme conditions could germinate and establish.
Transplant stress tolerance
Soil moisture profiles differed only slightly among cultivars, with drought treatments reaching ∼5% moisture (−4.0 MPa), and extreme drought further drying to ∼3% (−11.0 MPa) (data not shown). The stress-free control started at ∼35% moisture and was reduced to between 16 and 22% by the end of the experiment, but with a negligible change in soil water potential (∼0.01 MPa). All cultivars in the flooded treatment required supplemental watering starting 8 weeks after treatment initiation to maintain
Discussion
Under greenhouse conditions, switchgrass displays broad tolerance to soil moisture conditions. To varying degrees, both lowland and upland ecotypes germinated, established, and flowered under low soil moisture (≤−0.3 MPa) and flooded conditions. Lowland ecotypes outperformed upland ecotypes under flooded conditions for the ecological traits of tiller production and tiller length, leaf area, biomass, and photosynthetic water-use-efficiency. Surprisingly, lowland switchgrass accessions performed
Conclusions
Switchgrass demonstrates broad tolerance to soil moisture availability by germinating, establishing, and reproducing under both moisture deficit and flooded conditions. Environmental variability throughout its vast native range has likely led to this adaptive tolerance, which appears greater in current cultivars than in wild-types of a few generations ago [12]. However, there may be a fitness trade-off for broad environmental tolerance (e.g., reduced competitive ability), as switchgrass is
Acknowledgements
We would like to thank Charlie Campbell, Nicholas Eattock, Carlos Figuero, Jacinta Gimeno, and Salil Saxena for help with planting, collecting data, and harvesting. This work was supported by University of California Discovery grant GCP06-10233.
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