F. Lieutier

Do trees use reserve or newly assimilated carbon for their defense reactions? A 13C 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 13CO2 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 13C content in non-labeled saplings. A large 13C-excess was obtained in wood and phloem, especially in the fractions of soluble proteins, starch and soluble sugars of labeled saplings. Drought increased 13C-excess, due to reduced photosynthesis and smaller dilution of 13C by the addition of newly assimilated 12C. 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 13C 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.RésuméDe jeunes pins sylvestres (Pinus sylvestris L.) âgés de trois ans ont été marqués avec du 13CO2 puis inoculés dans le tronc avec Ophiostoma brunneo ciliatum, un champignon habituellement associé au scolyte Ips sexdentatus. Pendant l’incubation, la moitié des arbres a été soumise à une sécheresse sévère qui a fortement réduit la photosynthèse et l’abondance naturelle en 13C des individus non marqués. Un fort excès en 13C a été détecté dans le bois et le phloème ainsi que dans les protéines solubles, l’amidon et les sucres solubles des individus marqués. La sécheresse a amplifié cet excès, du fait d’une photosynthèse réduite et donc d’une moindre dilution du 13C par du 12C récemment assimilé. Les zones de réaction induite autour des points d’inoculation présentaient de fortes teneurs en C (58 g 100 g−1), du fait de l’accumulation massive de métabolites secondaires. Elles présentaient également un excès de 13C plus marqué que n’importe quel autre tissu : ces zones de réaction étaient donc essentiellement constituées à partir de C provenant des réserves avec une faible contribution de C récemment assimilé. De plus, la sécheresse a augmenté la contribution du C de réserve, comme le montre l’augmentation de l’excès de 13C dans les zones de réaction.

Inter-tree variability in the induced defense reaction of Scots pine to single inoculations by Ophiostoma brunneo-ciliatum, a bark-beetle-associated fungus

Abstract In 1987 and 1988, 100 trees year −1 were characterized by their age, height, diameter, area of sapwood section at breast height and growth efficiency indices. At different periods in the year, they were each inoculated with Ophiostoma brunneo-ciliatum at two diametrically opposite locations on the bole. The characteristics of the induced defense reactions (length of reaction zone and total resin quantity) were read in the phloem. The defense reaction zones of old trees (75–115 years old) always contained a greater total quantity of induced resin than the young trees (27–40 years old). They were also longer than in the young trees in 1987. The length of the reaction zone was smaller in June than later in the year, but the quantity of resin remained the same. Growth efficiency was always negatively correlated with the length of the reaction zone, but its correlation with the total quantity of resin in the young trees varied according to the year. In the young trees, total resin quantity was always positively correlated with tree diameter and area of sapwood section. The results are discussed in relation to the resources involved in the trees induced response and with the levels of attack density thresholds above which the trees defense is overcome. It is suggested that these resources are mainly current photosynthates in the case of young trees, while they could be otherwise in the case of old trees. It is pointed out that whilst there is undoubtedly a relationship between single reactions to individual attacks and growth efficiency, the low correlation coefficients do not allow the conclusion that lesion length or total resin from this reaction can be used as indices of the trees defense capability.

Bioassays on Leptographium wingfieldii, a Bark Beetle Associated Fungus, with Phenolic Compounds of Scots Pine Phloem

The fungicidal activity of five phenolic metabolites occurring in Scots pine phloem (Cat, Tax, PS, PC and PSM) and of phenolic extracts from unwounded (fresh) and wounded phloem of Scots pine were measured against Leptographium wingfieldii, a fungus associated with Tomicus piniperda. Bioassays were performed in micro-wells in a standard medium in the absence and presence of Scots pine phloem. In both media, methanol extracts from the unwounded and wounded phloem, PS, PSM and PC inhibited fungal growth. Tax and Cat had no effect or stimulated fungal growth. The presence of Scots pine phloem in the liquid medium reduced the inhibitory effect of phenolic compounds. At low concentrations (10−4–10−5 M), the fungus seemed to be able to degrade PS, PSM and PC while, whatever the Cat and Tax concentrations, it did not seem to degrade them. These results suggest that three phenolic compounds have a potential fungitoxicity in vitro and that, in vivo, these phenols could play a role in the efficiency of the induced reaction to stop fungal growth.

Defence reactions of Norway spruce (Picea abies Karst.) to artificial insertion of Dendroctonus micans Kug. (Col., Scolytidae)

Swarming Dendroctonus micans were artificially inserted into Norway spruce on several occasions from 1987 to 1989. Resin flow was measured 2 cm below the beetle hole, either immediately below or 10 and 20 mm away on the same side as gallery extension. Four to six weeks later, the bark was removed and the status of the beetle recorded (alive, dead, successful or failed attacks, etc.), as were size and characteristics of the maternal gallery. The induced reaction was also characterized, by size and quantity of induced resin it contained (concentration and total quantity). Beetles and galleries were collected to test for the presence of fungi. In 1990, scarifications were performed on other spruces, and beetles were inserted into various holes located in relation to these slits; the same parameters as before were measured.

Wound reactions of Scots pine (Pinus sylvestris L) to attacks by Tomicus piniperda L. and Ips sexdentatus Boern. (Col., Scolytidae)

Bark beetles Tomicus piniperda and Ips sexdentatus were artificially inserted in Scots pines. Attacks were also provoked in other trees by attractants. Resin flow corresponding to each introduction was measured in the case of artificial insertions. In all cases, gallery length, reaction zone length and the total quantity of induced resin contained in the whole reaction were measured 5–6 weeks after insertions or attacks. The attempt of gallery construction (beginning of gallery boring) and the beginning of oviposition were also noted in each case, as well as the presence of associated fungi.

Do Fungi Influence the Establishment of Bark Beetles in Scots Pine

Most aggressive bark beetle species (Coleoptera: Scolytidae) are associated with fungi which play an important role in the establishment of these beetles on conifers. The success or failure of establishment appears to depend on the capability of the tree to defend itself, especially the so-called hypersensitive reaction directed against the associated fungi (Reid et al. 1967; Berryman 1972; Safranyik et al. 1975; Raffa and Berryman 1982, 1983; Christiansen and Hornvedt 198 3). However, almost nothing is known about the mechanisms of the establishment of the European bark beetles on their usual host trees except for Ips typocrraphus in Norway spruce.