Miscanthus × giganteus (Mxg) and Panicum virgatum (PAVI) are 2nd generation biofuel crops (i.e., those grown for cellulose content) that may be planted throughout the US in large acreages within the coming decades. The capacity for a plant to survive summer drought is often associated with accessing available soil moisture deep in the soil profile. One strategy plants employ is to maintain active root growth into relatively moist regions of the soil profile. We evaluated Mxg and PAVI survival and growth in well-watered and rain-fed (dry) soil moisture environments in 0.5 m x 3 m PVC tubes filled with fine sandy loam soil. The drought stress treatments were rain-fed following an initial saturation in the spring. Each treatment was replicated 4 times. In order to observe root growth in the soil, clear plastic tubes measuring 0.7 x 0.05 m were placed every 0.3 m throughout the length of the tube (-0.3 to -2.7 m). A mini-rhizotron was used to photograph roots at 1 cm intervals, and were collected at 2 to 4 week intervals from May through November. Photos were analyzed for root growth rates and root densities. Soil moisture was measured at locations adjacent to the mini-rhizotron tubes. Following senescence (November) aboveground biomass was harvested, dried, and weighed. Root biomass was sampled in 0.3 m increments (e.g., 0-0.3 m, 0.3-0.6 m), wet sieved, dried, and weighed.

Results/Conclusions

By the end of the growing season, 30 weeks after planting, total root length density of well-watered Mxg was 33.7 times the drought treatment. The root length density of well-watered PAVI is 2.2 times the drought treatment. Both Mxg and PAVI roots reached 2.7 m in the well-watered treatment, while the dry PAVI reached 2.4 m deep and Mxg only reached 0.6 m. The total root mass of the Mxg well-watered treatment was 38.7 fold greater than the drought treatment, 1.16 and 0.03 kg respectively. The total root mass of the PAVI well-watered treatment was 1.7 times the drought treatment, 0.90 and 0.52 kg respectively. Our data show that PAVI does notably better under drought compared to Mxg, by being able to send roots deep into the soil profile. In addition, these data demonstrate that Mxg will be less likely to establish well-rooted plants outside of cultivation with out supplemental water.