Plant Physiol, April 2003, Vol. 131, pp. 1544-1554

Water Relations Link Carbon and Oxygen Isotope Discrimination to Phloem Sap Sugar Concentration in Eucalyptus globulus

Environmental Biology Group and Cooperative Research Center for Greenhouse Accounting, Research School of Biological Sciences, Australian National University, G.P.O. Box 475 Canberra, Australian Capitol Territory 2601, Australia (L.A.C., G.D.F.); and School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Nedlands, Western Australia 6907, Australia (D.J.A., J.S.P.)

A strong correlation was previously observed between carbon isotope discrimination (¹³C) of phloem sap sugars and phloem sap sugar concentration in the phloem-bleeding tree Eucalyptus globulus Labill. (J. Pate, E. Shedley, D. Arthur, M. Adams [1998] Oecologia 117: 312-322). We hypothesized that correspondence between these two parameters results from covarying responses to plant water potential. We expected ¹³C to decrease with decreasing plant water potential and phloem sap sugar concentration to increase, thereby maintaining turgor within sieve tubes. The hypothesis was tested with analyses of E. globulus trees growing on opposite ends of a rainfall gradient in southwestern Australia. The ¹³C of phloem sap sugars was closely related to phloem sap sugar concentration (r = 0.90, P < 0.0001, n = 40). As predicted, daytime shoot water potential was positively related to ¹³C (r = 0.70, P < 0.0001, n = 40) and negatively related to phloem sap sugar concentration (r = 0.86, P < 0.0001, n = 40). Additional measurements showed a strong correspondence between predawn shoot water potential and phloem sap sugar concentration measured at midday (r = 0.87, P < 0.0001, n = 30). The ¹³C of phloem sap sugars collected from the stem agreed well with that predicted from instantaneous measurements of the ratio of intercellular to ambient carbon dioxide concentrations on subtending donor leaves. In accordance, instantaneous ratio of intercellular to ambient carbon dioxide concentrations correlated negatively with phloem sap sugar concentration (r = 0.91, P < 0.0001, n = 27). Oxygen isotope enrichment (¹⁸O) in phloem sap sugars also varied with phloem sap sugar concentration (r = 0.91, P < 0.0001, n = 39), consistent with predictions from a theoretical model of ¹⁸O. We conclude that drought induces correlated variation in the concentration of phloem sap sugars and their isotopic composition in E. globulus.