To reveal photosynthetic characteristics and biomass yield is important for evaluating introduced species adaptation to local environments.  A field experiment was conducted over three consecutive years (2011–2013) to evaluate photosynthetic characteristics, soil water content, aboveground biomass accumulation, and water use efficiency (WUE) in switchgrass (Panicum virgatum L.) populations exposed to three row spacing (20, 40 and 60 cm) treatments in two growth months (June and August) on the semiarid Loess Plateau of China.  Results indicated that net photosynthetic rate (P_n), transpiration rate (T_r), instantaneous water use efficiency (WUE_i) and plant height of switchgrass showed an increased trend, but aboveground biomass production and WUE showed an decreased trend with enlarged row spacings over the three years.  The maximum daily mean P_n values (17.9, 18.4 and 19.7 µmol CO₂ m^–2 s^–1) were observed in 2011, and the highest aboveground biomass production (67 771.8, 6 976.8 and 6 609.2 kg ha^–1) were recorded in 2012 for 20, 40 and 60 cm, respectively.  A close correlation between tiller numbers and aboveground biomass production (r=0.907) was observed.  P_n was positively and significantly correlated with biomass per tiller, but it showed a negative correlation with aboveground biomass production.  Our results confirm that wide row spacing is beneficial for single plant development, while narrow row spacing favors biomass production and water use of switchgrass in the region.  It also implies that single leaf growth and performance could explain the switchgrass community density differences, while fails to account for the aboveground biomass production.

A large portion of the Loess Plateau of China is characterized as “marginal” with serious land degradation and desertification problems.  Consequently, two policies, Grain for Green and Western Development Action were established by the Chinese government in response to the demand for ecological protection and economic development in the Loess Plateau.  These policies are designed to increase forest cover, expand farmlands, and enhance soil and water conservation, while creating sustainable vegetation restoration.  Perennial grasses have gained attention as bioenergy feedstocks due to their high biomass yields, low inputs, and greater ecosystem services compared to annual crops.  Moreover, perennial grasses limit nutrient runoff and reduce greenhouse gas emissions and soil losses while sequestering carbon.  Additionally, perennial grasses can generate economic returns for local farmers through producing bioenergy feedstock or forage on marginal lands.  Here, we suggest a United States model energy crop, switchgrass (Panicum virgatum L.) as a model crop to minimize land degradation and desertification and to generate biomass for energy on the Loess Plateau.

Two pot experiments were conducted to study the effects of root pruning at the stem elongation stage on non-hydraulic root-sourced signals (nHRS), drought tolerance and water use efficiency of winter wheat (Triticum aestivum). The root pruning significantly reduced the root weight of wheat, but had no effect on root/shoot ratio at the two tested stages. At booting stage, specific root respiration of root pruned plants was significantly higher than those with intact roots (1.06 and 0.94 mmol g-1 s-1, respectively). The soil water content (SWC) at which nHRS for root pruned plants appeared was higher and terminated lower than for intact root plants, the threshold range of nHRS was markedly greater for root pruned plants (61.1-44.6% field water capacity) than for intact root plants (57.9-46.1% field water capacity). At flowering stage, while there was no significant difference in specific root respiration. The SWCs at which nHRS appeared and terminated were both higher for root pruned plants than for intact root plants. The values of chlorophyll fluorescence parameters, i.e., the effective photosystem II quantum yield ( PS II), the maximum photochemical efficiency of PS II (Fv/Fm), coefficient of photochemical quenching (qP), and coefficient of non-photochemical quenching (NPQ), in root pruned plants were significantly higher than in intact root plants, 7 d after withholding of water. Root pruned plants had significantly higher water use efficiency (WUE) than intact root plants in well-watered and medium drought soil, but not in severe drought condition. In addition, root pruning had no significant effect on grain yield in well-watered and medium drought soil, but significantly decreased grain yield in severe drought condition. In conclusion, the current study showed that root pruning significantly altered nHRS sensitivity and improved WUE of winter wheat in well-watered and medium drought soil, but lowered drought tolerance of winter wheat in severe drought soil. This suggests a possible direction of droughtresistance breeding and potential agricultural measure to improve WUE of winter wheat under semiarid conditions.