Arundo donax water use and photosynthetic responses to drought and elevated CO2

Arundo donax L., commonly known as giant reed or Carrizo grande, has been identified as an excellent biomass feedstock, because of its high yields delivered from low nutrient inputs. Two criticisms of cultivating A. donax are that it has a history of biological invasion, and also that it may require great quantities of water to sustain its rapid growth. Yet, there is little research reported quantifying the water-use requirements; and it is unknown how growth and water-use will be altered by the atmospheric enrichment of carbon dioxide (CO2) in combination with drought, two environmental conditions that have been predicted to occur in regions where A. donax is cultivated or has colonized. An experiment using close-topped CO2 chambers was conducted to study the interactive effects of elevated CO2 and limited water on A. donax growth and leaf physiology. Enrichment of atmospheric CO2 from 400 to 800 μmol mol−1 decreased transpiration rates by 100% (p < 0.05). Reduced transpiration delayed drought responses and extended periods of assimilation, but ultimately could not prevent desiccation and photosynthetic decline during extreme drought. Reduced transpiration also increased water use efficiencies (WUE). A linear model created from whole-plant water use estimates A. donax consumption at 186,500 and 139,500 L H2O Mg−1 (11.65 and 8.72 L H2O MJ−1), at 400 μmol mol−1 or 800 μmol mol−1 CO2 respectively. The improved WUE of plants grown in high CO2 was still less than values reported for Miscanthus, a C4 bioenergy feedstock. Moreover, comparisons between A. donax stable carbon isotope (13C) discrimination and values reported for other C3 species suggest that A. donax has relatively high conductance levels, and will likely transpire more water than most species. These findings present the first reported values for A. donax water-use in response to atmospheric enrichment of CO2.