Combined Effects of CO2 Enrichment and Drought Stress on Growth and Energetic Properties in the Seedlings of a Potential Bioenergy Crop Jatropha curcas

The rapid growth of worldwide energy demands has led to mounting concerns about energy shortages and has promoted the development of biofuels, which are susceptible to climate change. To evaluate the effects of future environmental changes such as CO2 enrichment and water stress on the growth and biodiesel production of bioenergy plants, we exposed Jatropha curcas to two levels of CO2 concentration (ambient and elevated) and three watering regimes (well-watered, moderate drought, and severe drought) to study its biomass accumulation and allocation, energy cost-gain properties, and photosynthetic response. Elevated CO2 enhanced biomass accumulation of J. curcas by 31.5, 25.9, and 14.4 % under well-watered, moderate drought, and severe drought treatments, respectively, indicating that the stimulating effect was greater under optimum water conditions than in water-deficit conditions. Drought stress significantly increased the biomass allocation to roots, especially the fine roots. CO2 enrichment also increased the root mass fraction, though not significantly. CO2 enrichment significantly enhanced the photosynthetic rate measured under growth CO2 concentration (Agrowth) and decreased foliar N content and therefore construction cost irrespective of watering conditions. Under elevated CO2, J. curcas employed a quicker return energy use strategy indicated by the higher photosynthetic energy use efficiency and lower payback time. There was a pronounced downregulation in the light-saturated photosynthetic rate under the common CO2 concentration (Pmax) under long-term CO2 exposure, due to a decrease in the initial and total ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities and partially lower foliar N content. The significant interaction of CO2 enrichment and watering regimes implied that the stimulation of plant growth by CO2 enrichment may be negated by soil drought in the future. Long-term field experiments manipulating multiple factors simultaneously are needed to explore how the ecophysiological traits measured for J. curcas translate into bioenergy production.