Do trees use reserve or newly assimilated carbon for their defense reactions? A C-13 labeling approach with young Scots pines inoculated with a bark-beetle-associated fungus (Ophiostoma brunneo ciliatum)

Three-year-old saplings of Pinus sylvestris L. were labeled with (CO2)-C-13 prior to inoculating the trunk with Ophiostoma brunneo ciliatum, a blue-staining fungus usually associated to Ips sexdentatus. During incubation, half the trees were submitted to a severe drought that decreased photosynthesis and natural C-13 content in non-labeled saplings. A large C-13-excess was obtained in wood and phloem, especially in the fractions of soluble proteins, starch and soluble sugars of labeled saplings. Drought increased C-13-excess, due to reduced photosynthesis and smaller dilution of C-13 by the addition of newly assimilated C-12. The induced-reaction zones in inoculated saplings displayed large total C (58 g 100 g(-1)) because of the accumulation of secondary metabolites. They also showed much larger 13C-excess than any other compartment: the contribution of stored C to the reaction zones was much higher than that of currently assimilated C. Moreover, drought lowered the contribution of the latter, as shown by the increase of C-13 in the reaction zones. We conclude that stored C was readily mobilized for the construction of reaction tissues, and that the contribution of currently assimilated C was only minor.