Báo cáo khoa học: "Effects of drought on the induced defence reaction of Scots pine to bark beetle-associated fungi"

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Note Effects of drought on the induced defence reaction of Scots pine to bark beetle-associated fungi L Croisé F Lieutier INRA, Station de Zoologie Forestière, Ardon, 45160 Olivet, France (Received 3 April 1992; accepted 14 September 1992) Summary — Water stress was imposed on Scots pine saplings grown in a greenhouse. Predawn leaf water potential (ψ was monitored regularly while carbohydrate reserves were quantified in ) wD the phloem, xylem and shoots. Trees were inoculated in the bole with a bark beetle-associated fun- gus at 4 different periods of the year; the induced defence reaction was quantified 2-3 wk later by its length and the total amount of resin. A decrease in the induced reaction (length and resin quantity) was observed in the most severely stressed trees. However, no clear relationship could be estab- lished between the concentration of carbohydrates (soluble or hydrolysable) and the intensity of the defence reaction. pinus sylvestris / phytopathogenic fungus / induced defence reaction / water stress / pre- dawn leaf water potential / carbohydrate reserve Résumé — Action d’un stress hydrique sur la réaction de défense induite du pin sylvestre contre des champignons issus d’insectes Scolytides. Un stress hydrique a été appliqué en serre sur des pins sylvestres de 6 ans. Des champignons phytopathogènes préalablement isolés d’in- sectes Scolytides ont été directement inoculés dans le liber de ces arbres afin de préciser l’effet de la contrainte hydrique sur les caractéristiques de la réaction locale de défense (longueur et quantité totale de résine). L’état hydrique des arbres a été suivi régulièrement par des mesures du potentiel hydrique de base au niveau des aiguilles (ψ Les réserves glucidiques de l’arbre ont été dosées ). wD dans le liber, l’aubier et les pousses au moment des inoculations. Quatre séries d’expérience ont ainsi été réalisées sur les mêmes arbres, de mai à octobre 1989. Pour des potentiels de base allant jusqu’à -2 MPa, ψ est systématiquement et négativement corrélé avec les caractéristiques de la wD réaction de défense dans chaque série d’expériences (tableau I). Aucune différence concernant la teneur en glucides n’apparaît entre les arbres stressés et les témoins dans les différents comparti- ments considérés (tableau II). Pinus sylvestris / champignon phytopathogène / réaction de défense induite / stress hydrique / potentiel hydrique foliaire/ réserve glucidique
INTRODUCTION MATERIALS AND METHODS In coniferous trees attacked by bark Water stress treatments beetles, an induced reaction visible as a resinous impregnation of the tissues sur- Fourteen 6-yr-old (2.20 rri high) Scots pines rounding the point of aggression plays a grown in pots were divided into 2 groups and determining role in the resistance of the watered with a drop-by-drop system. Group A tree. In the majority of cases, the reac- (control) received 20 I of water per wk per tree, = tion appears to be induced, or at least while group B was subjected to water stress be- considerably amplified by the presence ginning on May 12, Julian d 133. After this date, of fungi introduced by the insect (Berry- the group B trees received 3 I of water per wk per tree until June 16, Julian d 168, 9 I from man, 1972; Christiansen and Horntvedt, June 16-July 3, Julian d 185, 6 I from July 3- 1983; Raffa and Berryman, 1983; Cook August 7, Julian d 220, and again 3I per wk per and Hain, 1986; Christiansen et al, 1987; tree thereafter. Lieutier et al, 1988; Lieutier, 1992). The success or failure of each attack de- pends on the outcome of the struggle be- Tree water status tween the bark beetle and its associated fungi, and the intensity of the defence re- Predawn leaf water potential (ψ was meas- ) wD action at the site of attack. The defen- ured with the pressure chamber technique sive ab,lity of a tree can be represented (Scholander et al, 1965). The measurements by the attack density threshold above were carried out on 2 pairs of needles per tree which the tree is no longer able to resist. from the current year shoots. Needles were col- Since the production of induced resin is lected between 4:30 and 5:30 am GMT every 10-20 d. Stress intensity at a given date of inoc- costly in terms of energy (Croteau and ulation was evaluated as the average (ψ ) of wD Loomis, 1975) this threshold should be measurements made on the same tree between higher the lower the energy mobilized in 6 d prior to and 16 d after inoculation. each reaction. The attack density thresh- old appears to depend on the vigor of the tree at the time of attack (Berrymn, Tree defence reactions 1978; Waring and Pitman, 1983; Mulock and Christiansen, 1986). Tree vigor is The reactions were induced with artificial inocu- conditioned by several factors among lations of Ophiostoma brunneo-ciliatum (Math- which climatic factors are probably deter- K) or Leptographium wingfieldii (Morelet), 2 As- mining. A number of authors have report- comycete fungi previously isolated from the pine bark beetle Ips sexdentatus Boern and Tomicus ed that the worst bark beetle damage in piniperda L respectively (Lieutier et al, 1989). conifer forests are often preceded by a These fungi had been cultivated and purified as major period of drought (Christiansen et monospore cultures on a malt-agar medium. al, 1987; Mattson and Haack, 1987; and Five-mm diameter agar implants from 3-wk-old references therein). Thus, there is prob- cultures were introduced into the trees at the ably a relationship between the water cambium level accoring to a technique derived from Wright (1933) and which has been previ- status of a tree, the attack density ously described (Lieutier et al, 1989). One inocu- threshold, and the characteristics of the lation was performed per tree on May 22 (Julian induced reaction. The present paper in- d 143), July 11 (Julian d 193), 1989 with O brun- vestigates the relationships between tree neo-ciliatum, August 16 (Julian d 229), and Sep- water status and the characteristics of tember 21 1989 (Julian d 265) with L wingfieldii. the induced reaction. Two or 3 wk after inoculation, the bark around
the inoculation points was pulled away to ex- RESULTS pose the reaction zone of the phloem. This reac- tion zone was then measured (length) and cut Water status of trees into samples which were immediately placed into dry ice under a nitrogen atmosphere. These samples were kept at -35 °C until analysis for Watering of control trees kept their pre- total resin content. Samples of non-inoculated dawn leaf water potential at relative stable phloem, as well as sapwood and shoot axes (phloem and xylem) of new growth were taken levels between -0.3 and -0.6 MPa during on December 21, 1988, March 20, 1989, and on the whole experiment (fig 1). In stressed the days of inoculation; they were frozen, and trees, 2 phases can be distinguished. The stored in the laboratory at -35 °C before analy- first coincides with the first inoculations sis for their carbohydrate content. Non- and was characterized by a strong water inoculated phloem and sapwood were sampled deficit (ψ down to -2.1 MPa). The sec- wD near the inoculation point, and the shoots at the ond, where ψ was between -0.55 and end of lateral branches directly above the inocu- wD lation point. -1.2 MPa, began at Julian d 182 and con- tinued until the end of the expriment. Analyses Defence reactions The resin content in the induced reaction zones of the stressed The defence reaction zone was measured according to a previously de- significantly shorter scribed method (Lieutier et al, 1989). The re- only trees was once sults were expressed as the total quantity of res- than that of the control trees (May; 15 ± 4.7 in present in fresh whole reaction zones. mm for the control and 5.6 ± 3.0 mm for Carbohydrates, separated into a soluble and hy- the stressed trees). Total quantity of resin drolysable fraction, were measured and ex- in the defence reaction zone never differed pressed in terms of glucose equivalents by the significantly between the 2 categories of anthrone colorimetric method used by Mokrash trees. (1954) and modified by Sauvard (1988).
the correlations between or hydrolysable) content of stressed trees However, and the characteristics of the de- and that of control trees. Otherwise, the | wD |ψ fence reactions (length and resin quantity) carbohydrate content of each compartment were constantly negative (table I). Three of did not vary very much over time. them (2 for the length and 1 for resin quan- tity) differed significantly (P ≤ 0.05) from zero and another (concerning resin quanti- DISCUSSION ty) was almost significant (P = 0.058). Since the trees recovered from all inocula- tions it may be hypothesized that the Carbohydrate reserves (table II) length of the reaction zone was proportion- al to the duration of the struggle between After statistical analyses, phloem, xylem the tree and its aggressor, and that total and shoots showed no significant differ- quantity of induced resin included in the re- ences between the carbohydrate (soluble action zone was proportional to the quanti- ty of energy invested by the tree in its de- fence. Under these conditions, our results suggest that the aggressors were arrested more rapidly and that the quantity of ener- gy invested for that purpose was lower in the stressed trees. We cannot a priori discard a direct ef- fect of stress on the fungus. However, phy- topathogenic fungi are generally more tol- erant than plants to water deficits (Pinon, 1986; and references therein). According to Cook and Papendick (1972; in Pinon, 1986), fungal growth is still possible for wa- ter potentials as low as -3 to -5 MPa. Sometimes, water stress can even pro- voke stimulation of that growth, as ob- served by Bagga and Smally (1967; in Pin- on, 1986) for the aspen canker in in vitro
cultures. Under these conditions it seems in response to water stress Pos- (table II). unlikely that the decrease in the induced intense water sibly longer a or a more reaction in our experiment was due to a stress would have been necessary to in- decrease in fungal vitality. duce such modifications. Indeed, Grieu et al (1988) reported an increase of the solu- Some data on response regarding the ble carbohydrates in Douglas fir needles between water stress and the relationships for strong water stress (ψ -1.6 MPa). wD = induced defence reaction have been ob- However, these authors observed only tained. A constant negative correlation was weak variations of these compounds in the observed between the characteristics of needles and in the roots of the same tree the defence reaction and the predawn leaf and in those of Pseudotsuga macrocarpa water potential. Thus, it seemed that when and Cedrus atlantica, with a leaf predawn water stress increased, both reaction water potential below -2 MPa. It is there- length and quantity of induced resin in that fore difficult to consider the observed varia- reaction decreased. The decrease in the tions in the tree defence reactions to be a amount of induced resin is in agreement consequence of variation of stored carbo- with the decrease observed by Lorio hydrates. (1986) for constitutive resin in Pinus taeda during a period of severe drought. It is also These conclusions agree with the re- in agreement with the findings of Paine sults of Christiansen and Ericsson (1986) and Stephen (1987) who noticed for the who reported that the level of stored starch same species a less important induced re- was not correlated with Picea abies resis- action in the dominated trees than in the tance to infection by Ophiostoma poloni- dominant trees. Stephen et al (1983) cum. Owing to the fact that resin synthesis claimed that an important defence reaction is costly for the tree (Croteau and Loomis, would correspond to trees resistant to bark 1975), Christiansen and Ericsson (1986) beetle attacks. In Norway spruce, Chris- have suggested that the flow of assimilates tiansen et al (1987) observed that marked might be the main source of energy for the reactions could develop in weak trees development of the defence reactions. close to death. This observation seems to Stored starch might thus be a complemen- be in opposition to our present results; tary source of energy when the current however the situation described by these flow is not sufficient. Otherwise, the study authors was obtained with an inoculation of resin biosynthesis in Pinus pinaster density above the lethal threshold; in these (Bernard-Dagan, 1988) suggested that the conditions, the fungus extended to the ascending flux of soluble carbohydrates whole phloem. It was very different from from roots might also be an important en- our situation with isolated inoculations ergy source for the induced reactions. Ac- which were always contained by the tree cording to the same authors, the possible response. Nevertheless, Lorio (personal catabolism of the heartwood resin might lo- communication) observed an increase in cally participate in the defence mecha- constitutive resins during a limited period nisms at the beginning of the annual of drought, and Lieutier and Ferrell (1988) growth activity. reported an increase of induced reaction in Scots pine when tree growth efficiency de- creased. ACKNOWLEDGMENTS Our results did not demonstrate any modification in the amount of stored carbo- grateful to E Christiansen The authors are and in the shoots, phloem xylem hydrates and E Dreyer (INRA, France) (NISK, Norway)
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