The influence of temperature and relative humidity on δ 13 C values of C3 plants in growth chamber experiments

Experimental setting and methods Three different C3 plant species (Vicia faba var. minor cv. Fribo, Eucalyptus globulus and Brassica oleracea var. medullosa cv. Gruner Ring) were cultivated under controlled conditions in four growth chambers at the phytotron facility at the GSF (Research Centre for Environment and Health) in Neuherberg (for description of the chambers see Payer et al. 1993). The experimental setting consisted of seven climatic conditions that differed in relative humidity and/ or temperature. The experiments were carried out in two series including the repetition of one climatic condition. Other climatic parameters than relative humidity and temperature (e.g. irrigation and radiation) were kept identical in all chambers. CO2 concentrations in each chamber were regularly controlled and air samples were taken for δC-analyses. Both CO2 concentration and δC values of the air did not differ significantly between any of the chambers. The plants were set to the growth chambers immediately after germination and further cultivated there for 56 days. After that period the plant material was harvested for isotopic analyses of dry organic matter (δC), cellulose nitrate (δC, δH) and plant water (δO, δH). Here only results from δC analyses of cellulose nitrate are discussed. Cellulose nitrate was prepared from dried stem material using standard methods (Epstein et al. 1976). Cellulose nitrate was combusted in vacuo in glass tubes in the presence of copper and copper oxide and the resulting gases were separated cryogenically in a preparation line (Stichler et al. 1982). The purified CO2 was trapped in an ampoule and isotope ratios were determined with an IR-mass spectrometer (Delta-S, Finnigan MAT, Germany). The analytical error including the preparation of cellulose nitrate was 0.1‰ (one standard deviation).