Photosynthesis, Resource Acquisition and Growth Responses of Two Biomass Crops Subjected to Water Stress

Abstract

This study compares photosynthesis, growth, 13C and 15N labelling patterns of two biomass crops (Arundo donaxL. and Panicum virgatum L.) grown under water stress in greenhouse conditions. Plants were exposed to three water stresslevels: control (C, 100% Pot Capacity), mild stress (MS, 50% PC) and severe stress (SS, 25% PC). Photosynthesis,fluorescence parameters and relative water content were measured at the beginning (Ti) and the end of the experiment (Tf).Biomass parameters were measured at Tf. Short-term double labelling with 13C and 15N stable isotopes was performed in bothspecies. Isotopic analyses of total organic matter, total soluble sugars and the CO2 respired were undertaken at T0 (prelabelling),T1 (24h after labelling) and T2 (7 days after labelling). Immediately after the 13C and 15N labelling, stems andrhizomes seemed to be the main sinks for labelled carbon and nitrogen in both species. Moreover, not all of the labelled carbonand nitrogen substrate was used by plant metabolism after seven days. Decreases in photosynthesis parameters were observedas a consequence of the increase in water stress (WS) in both species, with a greater magnitude decline in giant reed than inswitchgrass. A decrease in height, number of green leaves and total dry weight due to WS was observed in both species. Bothspecies were more 13C-enriched and more 15N-depleted during the increases in WS due to lower stomatal conductance andtranspiration. In general, WS accelerated plant phenology and, consequently, the accumulation of storage compounds in therhizome occurred in response to stress. This effect was more clearly visible in switchgrass than in giant reed.