Journal
PLANT CELL AND ENVIRONMENT
Volume 25, Issue 7, Pages 893-907Publisher
WILEY
DOI: 10.1046/j.1365-3040.2002.00875.x
Keywords
carbon, nitrogen, and oxygen stable isotope ratios; fruit; leaf; phloem; xylem
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In this paper, we present an integrated account of the diurnal variation in the stable isotopes of water (delta D and delta (18) O) and dry matter (delta (15) N, delta (13) C, and delta (18) O) 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 delta D and delta (18) O of leaf water showed a pronounced diurnal variation, ranging from early morning minima near 0parts per thousand for both delta D and delta (18) O to early afternoon maxima of 62 and 23parts per thousand, respectively. Xylem sap water showed no diurnal variation in isotopic composition and had mean values of -13.2 and -2.3parts per thousand for delta D and delta (18) O. 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 A modified model taking into account non-steady-state effects (Farquhar and Cernusak, unpublished) gave better predictions of observed leaf water enrichments over a full diurnal cycle.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 delta (15) N, delta (13) C, and delta (18) O of dry matter varied appreciably among components. Dry matter delta (15) N was highest in xylem sap and lowest in leaves, whereas dry matter delta (13) C was lowest in leaves and highest in phloem sap and seeds, and dry matter delta (18) O was lowest in leaves and highest in pod walls. Phloem sap, leaf, and fruit dry matter delta (18) O varied diurnally, as did phloem sap dry matter delta (13) C. These results demonstrate the importance of considering the non-steady-state when modelling biological fractionation of stable isotopes in the natural environment.
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