Tolerance of switchgrass to extreme soil moisture stress: Ecological implications

Abstract

Switchgrass (Panicum virgatum L.), a native of eastern and central North America, is a leading candidate as a dedicated biofuel feedstock in the US due to its broad adaptability, rapid growth rate, and ability to grow in low production soils. To begin to characterize the important agronomic and ecological traits related to environmental tolerance of switchgrass, we evaluated fitness under stressful growing conditions. We assessed the germination, establishment, performance, and reproductive potential of four common accessions, both upland and lowland ecotypes, at various levels of soil moisture availability (moisture deficit to flooded) in the greenhouse. Seeds emerged and established (55–90% survival) under all soil moisture conditions (−0.3 MPa to flooded). Transplants of lowland ecotypes performed as well in flooded conditions as in field capacity controls, though flooding reduced performance of upland ecotypes. Drought treatments (−4.0 and −11.0 MPa) reduced tiller length and number, leaf area, and biomass production by up to 80%. However, once established, all plants survived at −4.0 MPa and had the same proportion of tillers in flower as at field capacity. The ability of switchgrass to germinate, establish, and flower in low moisture and flooded conditions, particularly lowland ecotypes, may increase the range of environments suitable for biofuel cultivation, and can serve as a baseline for further ecological studies and genetic improvement.

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.