Diurnal variation in the stable isotope composition of water and dry matter in fruiting Lupinus angustifolius under field conditions

In this paper, we present an integrated account of the diurnal variation in the stable isotopes of water (δD and δ18O) and dry matter (δ15N, δ13C, and δ18O) in the long‐distance transport fluids (xylem sap and phloem sap), leaves, pod walls, and seeds of Lupinus angustifolius under field conditions in Western Australia. The δD and δ18O of leaf water showed a pronounced diurnal variation, ranging from early morning minima near 0‰ for both δD and δ18O to early afternoon maxima of 62 and 23‰, respectively. Xylem sap water showed no diurnal variation in isotopic composition and had mean values of −13·2 and −2·3‰ for δD and δ18O. Phloem sap water collected from pod tips was intermediate in isotopic composition between xylem sap and leaf water and exhibited only a moderate diurnal fluctuation. Isotopic compositions of pod wall and seed water were intermediate between those of phloem and xylem sap water. A model of average leaf water enrichment in the steady state (Craig & Gordon, pp. 9–130 in Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Palaeotemperatures, Lischi and Figli, Pisa, Italy, 1965; Dongmann et al., Radiation and Environmental Biophysics 11, 41–52, 1974; Farquhar & Lloyd, pp. 47–70 in Stable Isotopes and Plant Carbon–Water Relations, Academic Press, San Diego, CA, USA, 1993) agreed closely with observed leaf water enrichment in the morning and early afternoon, but poorly during the night. A modified model taking into account non‐steady‐state effects (Farquhar and Cernusak, unpublished) gave better predictions of observed leaf water enrichments over a full diurnal cycle. The δ15N, δ13C, and δ18O of dry matter varied appreciably among components. Dry matter δ15N was highest in xylem sap and lowest in leaves, whereas dry matter δ13C was lowest in leaves and highest in phloem sap and seeds, and dry matter δ18O was lowest in leaves and highest in pod walls. Phloem sap, leaf, and fruit dry matter δ18O varied diurnally, as did phloem sap dry matter δ13C. These results demonstrate the importance of considering the non‐steady‐state when modelling biological fractionation of stable isotopes in the natural environment.