Biomass Accumulation and Partitioning of Eastern Gamagrass Grown Under Different Temperature and CO2 Levels

Eastern gamagrass has been reported to have one of the highest photosynthetic rates of any C4 species but data on temperature x CO2 interactions are lacking. This study was conducted to determine the potential effects of future increases of atmospheric carbon dioxide on growth, biomass accumulation and root/shoot carbon allocation under three day/night temperatures and two CO2 levels. Eastern gamagrass (cv. Pete) plants were grown in 1 m soil bins containing sand:vermiculite (1:1), fertilized weekly with a complete nutrient solution in closed, transparent SPAR (Soil, Plant, Atmospheric Research) chambers maintained at 370 or 740 :mol mol CO2 and 20/14°, 27.5/21.5° or 35/29°C day/night temperatures, and allowed to develop from mid-May to mid-October. Three harvests were taken during this period. Leaves were collected during the first two harvests. During the final harvest, leaves, crowns, and roots were collected from each individual plant. The optimum day/night temperature under our conditions for biomass accumulation in the leaves (35/29°C) was higher than that for the crowns and roots (27.5/21.5°C). Biomass accumulation in leaves increased two-fold over the entire temperature range. Temperature had a greater effect on vegetative growth than CO2. CO2 enhanced biomass accumulation was modest, restricted to leaves, and observed only at higher temperatures and later in development. Under optimum soil moisture conditions in the SPAR chambers, high amounts of carbon were captured in the above ground biomass for later incorporation into soil. This study demonstrates the potential of eastern gamagrass to capture carbon for sequestration under projected global climate change scenarios. INTRODUCTION Eastern gamagrass [Tripsacum dactyloides (L.) L.] is a robust, warm season, perennial bunch grass which is native to North America, Central America, and upper South America. It produces high yields of palatable and digestible forage with protein content comparable to alfalfa (Horner et al., 1985; Coblentz et al., 1998; Bidlack et al., 1999). It exhibits tolerance to a wide range of environmental stresses and soil conditions including drought, flooding, aluminum toxicity and acid soils (Foy et al., 1999; Gilker et al., 2002; Krizek et al. 2003). It is an attractive species for use in sustainable agriculture because of its ability to penetrate acid, compact soils (Clark et al. 1998; Krizek et al. 2003) and to reduce the runoff of nutrients and sediment to nearby streams, when planted as a buffer strip (Ritchie et al., 2000). Increases in the Earth’s atmospheric carbon dioxide (CO2) concentration and associated changes in global climate have gained world-wide attention. There is keen interest among scientists as to how projected increases in CO2 and the associated increase in atmospheric temperature will affect crop production of C3 and C4 species (Reddy et al., Proc. XXVI IHC – Sustainability of Horticultural Systems Eds. L. Bertschinger and J.D. Anderson Acta Hort. 638, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 294 1994; Kimball et al., 2002). Studies are needed to determine the response of eastern gamagrass to elevated CO2 if this warm season grass is to be considered as a potential candidate for carbon sequestration. Eastern gamagrass has been reported to have one of the highest leaf photosynthetic rates of any C4 species (Coyne and Bradford, 1985) but data on temperature x CO2 interactions are lacking. This study was conducted to determine the potential effects of future increases of atmospheric carbon dioxide on growth, biomass accumulation and root/shoot carbon allocation at three temperatures and two levels of CO2. MATERIALS AND METHODS Eastern gamagrass plants were grown in six naturally lit Soil-Plant-AtmosphereResearch (SPAR) plant growth chambers. These chambers are useful for studying canopy and ecosystem or small-plot responses to combinations of variables in controlled fieldlike environments (Tingey et al., 1996; Reddy et al., 2001). Each SPAR unit consists of a soil bin containing the rooting medium and a 1.27 cm thick acrylic (Plexiglas G) chamber that accommodates the aerial plant parts. Each chamber measures 2.5 m high x 2.0 m long x 0.5 m wide. A door in the bottom of each chamber is hinged for easy access to the plants. There are ducts on the northern face that connect to the cooling system. Conditioned air is introduced at the top of the chamber, flows down through the plant canopy, and is returned to the ducts just above the soil level. The soil bins containing the rooting medium measure 1.0 m high x 2.0 m long x 0.5 m wide and are not temperature controlled. The south face consists of tempered glass to allow root observations. Variabledensity shade cloths, positioned around the edges of the canopy, are adjusted to simulate the presence of neighboring plants. Temperature, CO2, and relative humidity are controlled and monitored by a computerized data acquisition and control system. Germtec II treated seed of eastern gamagrass (cv. Pete) were obtained from Gamagrass Seed Company (Falls City, NE) and sown in the greenhouse on a hot pad at 30C. On May 16, 2001, after 3 weeks in the greenhouse, seedlings were selected for uniformity and transplanted into six SPAR chambers. Seedlings were selected that had not developed lateral shoots. Two rows of eight plants each were transplanted into the 1 m soil bins containing sand:vermiculite (1:1) mixture and fertilized weekly with a complete nutrient solution. Plants were grown at three day/night temperatures (20/14°, 27.5/21.5°, or 35/29°C) and two CO2 levels (370 or 740 :mol mol). Three SPAR chambers were maintained at each of these temperatures at 370 :mol mol CO2 (ambient) and three were maintained at 740 :mol mol CO2 (elevated). The thermoperiod was adjusted weekly. Water was supplied 2-3 times a day by a drip irrigation system. Analysis of variance was used to determine statistical significance at the 0.05 level of probability.