Báo cáo khoa học: " Vulnerability of young oak seedlings (Quercus robur L) to embolism: responses to drought and to an inoculation with Ophiostoma querci (Georgevitch) Nannf"

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Nội dung Text: Báo cáo khoa học: " Vulnerability of young oak seedlings (Quercus robur L) to embolism: responses to drought and to an inoculation with Ophiostoma querci (Georgevitch) Nannf"

Original article Vulnerability of young oak seedlings (Quercus robur L) to embolism: responses to drought and to an inoculation with Ophiostoma querci (Georgevitch) Nannf A Granier E H Cochard C Delatour Dreyer G Simonin 54280 1 Champenoux; Nancy, Laboratoire de Pathologie Forestière, INRA Écophysiologie Forestière, Équipe Bioclimatologie et Écophysiologie, 2 Unité France 54280 INRA Champenoux, Nancy, (Received 7 June 1993; accepted 27 October 1993) Summary— Possible interactions between an infection with Ophiostoma querci and water stress on pedunculate oak (Quercus robur) were tested with potted saplings. O querci was inoculated into the stems of 3-year-old saplings, and a severe drought was imposed for about 40 d. Drought promoted an irre- versible decline in total leaf specific conductance of all saplings; direct measurement of losses of hydraulic conductivity in twigs and petioles revealed that a strong embolization occurred in the vessels as soon as minimal leaf water potential decreased below -2.5 MPa. This vulnerability to cavitation on rooted seedlings was in agreement with earlier data obtained on cut branches from the same species left to freely dehydrate; a slight artifact was probably due to the onset of occlusions of embolised vessels in the rooted plants. The presence of fungal spores in the stems did not induce any modifica- tion in these water relations on well-watered or stressed seedlings. The role of O querci in the oak decline symptoms as occurring in Europe may therefore be questioned. / water relation- Ophiostoma querci/ hydraulic conductivity water stress I embolism / oak I ships/ oak decline Résumé — Vulnérabilité de jeunes semis de chêne pédonculé (Quercus robur) à l’embolie : réponses à la sécheresse et à une inoculation avec Ophiostoma querci. Les effets potentiels d’une infection par Ophiostoma querci sur la réponse à la sécheresse de jeunes plants de chêne pédonculé ont été testés. O querci a été injecté dans le tronc de plants âgés de 3 ans, et une séche- resse intense a été imposée pendant une quarantaine de jours. La sécheresse a provoqué une dimi- nution irréversible de la conductance hydraulique spécifique de tous les plants. Des mesures directes de perte de conductivité hydraulique dans les rameaux et les pétioles ont montré qu’une forte embo- lie se produisait dès que le potentiel hydrique foliaire était abaissé en dessous de -2.5 M Ce degré Pa. * Correspondence and reprints Symbols and abbreviations:ψ predawn leaf water potential (MPa); ψ midday leaf water potential : wd : wm (MPa); g leaf specific hydraulic conductance (mmol m s MPa E total transpiration (mmol s ). -1 -2 -1 -1 t ); : : L
de vulnérabilité à l’embolie était très voisin de celui détecté en laissant des branches d’arbres adultes dessécher rapidement au laboratoire. Les légères différences observées pour les potentiels se hydriques les plus faibles ont pu être dues à des occlusions de vaisseaux se produisant lors de séche- resses de longue durée. La présence de spores dO querci dans le xylème n’a modifié ni la conductance totale des plants, ni la vulnérabilité des rameaux et des pétioles à la cavitation. Le rôle souvent attribué à ce champignon dans l’induction des dépérissements de chênes en Europe doit être remis en question. sécheresse / embolie / chênes / Ophiostoma querci / conductivité hydraulique / dépérissement INTRODUCTION saplings of Q robur with suspension of a conidia, but were unable to detect any foliar symptoms after this inoculation. They never- Oak stands in Western and Central Europe theless observed the occurrence of local- frequently reported to present severe are ized bark necroses and conspicuous nar- dieback symptoms. In France, pedunculate row strips of browning induced in the xylem oak (Quercus robur L) is often declining, tissue which were sometimes several 10s while sessile oak (Q petraea (Matt) Liebl) of cm long. Moreover, the fungus could be seems to exhibit a better resistance; in Cen- reisolated from these zones even 1 year tral Europe, both species suffer from severe later. Similar results have been described decline. The precise chain of events leading by Balder (1993) with O querci, O steno- to the onset of these decline processes is ceras (Robak) Melin and Nannf, and O pro- still poorly understood. Environmental con- liferum (Kowalski and Butin) de Rulamort. straints, and among them repeated periods The length of these discolorations was highly of water shortage, probably play a major variable among individual trees. These role (Landmann et al, 1993). However an results suggested an important interaction involvement of various pathogens has fre- between xylem structure in oaks and the quently been suspected (Delatour, 1983; ability of Ophiostoma spp to spread in the Kowalski, 1991).Among the numerous fungi conducting tissues following an infection, isolated from declining oak trees, those as has been reported for other vascular belonging to the group of the Ophiostom- pathogens (Beckmann, 1987). atales (Ascomycotina) deserve special attention (Delatour, 1986). Indeed, this fun- However, even if the Ophiostoma spp gal group comprises a number of strong already studied only promoted the occur- pathogens like those inducing oak wilt in of very limited symptoms of tra- rence north-east America (Ceratocystis cheomycosis on oaks under normal water fagacearum (Bretz) Hunt; Gibbs, 1981),or supply, the presence of spores or hyphae the Dutch-elm disease (O novo-ulmi; inside the xylem could possibly affect tree Brasier, Sinclair and Campana, 1978). water relations during drought. Among the These vascular pathogens severely disor- mechanisms which could lead to long-term ganize the water transport in infected trees damage, induction of embolism in vessels (Hall and MacHardy, 1981; Beckmann, and the subsequent dysfunctions in water 1987). transport could be of major importance. Information concerning vulnerability of oaks Ophiostoma querci (Georgevitch) Nannf to cavitation is increasing. Cochard et al has been frequently isolated from declining (1992) showed that significant embolism oak trees(Kowalski, 1991) and is therefore appeared as soon as the leaf water poten- suspected to be involved in the induction of the dieback. To test for this hypothesis, tial dropped below -2.5 MPa on branches of Delatour et al (1993) inoculated young Q robur left to dehydrate freely under labo-
Champenoux. They were watered every second ratory conditions, and that almost all ves- day. During 1991, bud break and flushing sels were embolised around -3.3 MPa. occurred during early March. Seedlings were Measurements made on adult trees in a for- 170-250 cm high and stem diameter ranged from est near Nancy during a gradually increas- 0.5 to 1 cm at the inoculation point. ing drought yielded similar results (Bréda et al, 1993), and confirmed the good agree- ment observed by Tyree et al (1992a) Fungus between embolism induction during drought in situ and during rapid dehydration of cut The strain of O querci (Georgevitch) Nannf was branches. In the present work, we intended isolated from cambial necroses on Q petraea to evidence the cavitation induction patterns (Matt) Liebl during 1985, at Cerrilly, near Chatil- obtained with rooted saplings during slowly lon-sur-Seine (north-eastern France; Morelet, increasing drought. In addition, we tested 1992), and stored on wood pieces at 4°C (Dela- tour, 1991).The inoculum was prepared from cul- for potential interactions between the pres- tures grown during about 1 month on petri dishes ence of spores and hyphae of O querci in (Difco malt agar 3%, 25°C), which produced large the xylem and the sensitivity to water stress. amounts of conidia (Hyalodendron and Pesotum In particular, we tested the hypothesis that stages). Washing each culture with 15 ml steril- the presence of spores and hyphae in the ized water yielded a high density of spores (about xylem vessels could reduce the hydraulic 10 ml adjusted to 106 m The diameter of . -3 8 -1 ) conidia was investigated using microfiltration; no conductivity of our trees, or that they might conidia were smaller than 0.45 μm, but many produce compounds reducing significantly passed 0.8 μm filters. the surface tension of the xylem sap, as reported by Kuroda (1989) who observed that volatile terpenes emitted during the Inoculation infection of Pinus thunbergii by a nematode increased the susceptibility to cavitation. A micro-perfuse connected to teflon tubing con- We therefore inoculated O querci directly taining the conidia suspension was used to inject into the xylem of young oaks, and investi- the suspension directly into the xylem of the gated the patterns of dissemination of the annual growth ring. The absorption was entirely fungus in the xylem, comparing it with that passive, with no additional pressure. simultaneously injected of Indian ink. We then submitted the saplings to water stress by withholding irrigation and followed the 1 Experiment total hydraulic conductance from soil to leaves, and the onset of embolism in twigs Patterns of dissemination of the fungus in the and petioles. tissue following injection were analysed xylem on 48 trees using suspensions of conidia mixed with sterile Indian Ink (5% dilution, Steadler, Mars- matic 745R; sterilisation: 20 min at 120°C). Prior MATERIAL AND METHODS to the use of this mixed suspension, we tested for potential effects of Indian ink and latex paint, another dye frequently used in water relation stud- Plant material ies, on conidial viability (24 h incubation at 25°C). The ink/conidia mixture (0.1 ml) was injected dur- Three-year-old seedlings of Q robur L were grown ing April 1991 into 48 trees at 50 cm below the upper limit of the 1990 growth flush. Spread of in 10 L pots in a peat/sand mixture (50:50 v/v), fer- the fungus inside the xylem was observed through tilized with a slow release fertilizer (Nutricote 100, reisolation from cut segments of stems. Stems N/P/K 13:13:13, Fertil, Paris), and grown in a were disinfected with alcohol, debarked, and glasshouse at the Forestry Research Center of
sliced into 1 segments. Each segment the lost water (water stress) was added during cm was malt/agar medium containing the first drought cycle. During the second, pots placed on a 50 mg -1 of both penicillin and streptomycin. L were left to dry out freely. Predawn (ψ and ) wd midday (ψ leaf water potentials were mea- Different injection procedures were tested: (1) ) wm half of the injections (24) were made under water sured on one leaf of every tree during 6 sunny days with a pressure chamber, before dawn, and to avoid wounding induced cavitation, and half between 12 and 1 pm UT, respectively. Losses in air, and (2) in each group 18 trees were injected of weight were recorded for each plant between 11 at dawn and 6 at midday with about -1.5 MPa AM till 1:30 pm UT (Sartorius IB31000P balance, water potentia. Reisolation was made after 2-3 h, ± 0.1g). Due to the large leaf area of the saplings, and delayed by 24 h on half of the trees. soil evaporation was considered to be negligible and the loss of weight was recorded as the diurnal maximal rate of transpiration (E Total leaf area ). t Assessment of vessel length (LA) of each tree was estimated at the end of the experiment with planimeter (ΔTDevices, UK). a These measurements allowed the computation Vessel lengths were measured in 8 seedlings of a specific soil to leaf hydraulic conductance as using the technique described by Zimmermann reported by Cohen et al (1983), Granier and Colin and Jeje (1981 ) adapted to oaks by Cochard and (1990) and Reich and Hinckley (1989) as: Tyree (1990). A solution of blue pigment (latex paint) was diluted 100/1 in water and passed through a 5 μm filter. The eluate was perfused through stem segments from the distal end, at specific soil-to-leaf hydraulic conductance : L g an over-pressure of 0.015 MPa during 24 h. Per- (mmol m s MPa E maximal transpiration -2 -1 -1 t ); : fusions were applied at 4 different locations: 5 cm (mmol sLA: leaf area (m and ψ and ψ ); -1 ); 2 : wm wd above, and 5, 20, 50 cm below the contact zone predawn and minimal leaf water potential (MPa); between 2 successive growth cycles; 2 saplings in this equation ψ was used estimate of as an wd were used for each of these treatments. The num- the soil water potential. ber of vessels filled with pigments was counted under a dissecting microscope every 2.5 cm. Only vessels included in the current year’s (1991) wood Experiment 3 with a diameter above 20 um were taken into account. The statistical procedure of Zimmer- mann and Jeje (1981) was used to estimate ves- Loss of hydraulic conductivity of twigs and petioles sel length distribution. during drought was examined on 80 seedlings (same substrate, same pots, same height, but 2-3 seedlings grown in each pot) were used for the same treatments as in Experiment 2. Drought Experiment 2 was imposed as in Experiment 2, and ψ mea- wd sured every second day on one of the individuals Total hydraulic conductance during drought was in each pot. Watering was controlled to maintain measured on 16 seedlings grown in individual midday leaf water potential (ψ above -3.3 ) wm 10 L pots. They were inoculated during May with MPa during the first cycle, and no watering was repeated injections at about 10 points all along supplied during the second period of drought. of the stem to the upper 70 satis- The technique developed by Sperry et al (1988), cm ensure a factory dispersal of conidia all over the xylem and described in detail by Cochard et al (1992) for (inoculated trees), or injected in the same way oak trees was used to monitor loss of hydraulic with sterile water (control trees). After 2 months of conductivity. ψ was measured between 11 am wn incubation, 4 treatments were defined: (1) water- and 1 pm UT and the pot rewatered to stop any stressed and inoculated with O querci; (2) water- further induction of embolism. During the follow- stressed and non-inoculated; (3) well-watered ing morning, 5 twigs and 10 petioles were cut off and inoculated; and (4) well-watered and non- under water from the upper crown of the same inoculated (control). Two successive cycles of seedling. Twigs were recut into 2 cm long seg- drought were imposed, each lasting about ments under water. Petioles were prepared in 10-15 d. Pots were weighed every second day the same way, and a segment of the leaf mid-rib and either the total amount (controls) or half of included whenever the petiole was less than 2 cm
long. This procedure was repeated during the experiment on 8 well-watered and 15 water- stressed for both the inoculated and control treat- ments. Embolism was computed as the loss of conductivity, ie as: where k = F// P where k is the actual conductivity (mg s MPa -1 -1 ), , i measured immediately on the sample with a 65 cm head of degassed distilled water containing 0.1% HCl (pH 2); this step was performed as quickly as possible to avoid passive resaturation of the xylem; k is the maximal conductivity, mea- m sured after resaturation of the samples by repeated flushes of a perfusion solution at 0.1 MPa; a single flushing of 15-20 min was usually enough to fully restore maximal conductivity; for strongly embolized samples, 2 periods of 15 min each were used; F is the actual flow of degassed water through the sample (kg s ), monitored -1 with a balance (Mettler, ± 0.01 mg); I is the length of the sample (m), usually 2 cm; and P is the pressure applied to the water (MPa). Maximal conductivity (k was used to calculate ) m the leaf specific conductivity of individual petioles (= k mg s MPa m with LA: leaf area). -1 -1 -1 , /LA, m RESULTS sequences in the 11 remaining from uous injection point till a maximal distance the Vessel lengths varying between 7 and 46 cm (mean 28.7 ± 10.7 cm, very regular distribution with a kur- Distributions of vessel lengths showed fol- tosis of -1.114 and a skewness of -0.022). lowing features (fig 1):(1) vessel lengths The maximal spread was identical whether measured from the top of these 2.5 m high the injection had been made under water saplings sometimes reached values as high or not (respectively 29.9 ± 10.9 and as 80 or even 110 cm; (2) mean vessel 27.4 ± 10.6 cm, Fisher PLSD non-signifi- length increased from top to bottom of the cant at 5%). No significant effect of any of stem; (3) no discontinuity appeared between the other injection procedures could be both growth flushes (1990 and 1991); and observed: injections at predawn, while water (4) more than half of the vessels 5 cm above potential was high, were followed by the the connection were over 5 cm long. same pattern of distribution of the conidia in the stems than injections made at mid- Fungus dispersal in the stem day; furthermore, reisolation after 2 h yielded the same maximal distance of spread than those made after a 24 h delay. The only dif- O querci could be reisolated in continuous ference was related to the speed of absorp- sequences from 37 trees and in discontin-
seedlings. As already observed by Delatour tion of the inoculum: the delay for the com- et al (1992), the fungus remained present plete absorption of the 0.1 ml suspension in the xylem tissue in which it was injected varied between a few seconds (injection at but did not spread further. It nevertheless midday) and a few minutes (injection at induced browning symptoms in the xylem, dawn). which were already detected at the end of In fact, when analysing in parallel spread the drought experiments, that is 2 months of spores and of Indian ink, we observed after inoculation, and again 1 year later (data very similar values of maximal extent for shown). not both the fungus (28 ± 10.7 cm) and the dye (27.4 ± 9.1 cm). Moreover, a very good cor- relation between both values was detected Total soil-to-leaf hydraulic conductance (fig 2). Inafew cases, the extent of dye was (g response to drought ) in L slightly lower, which was probably related and inoculation to the difficulty of detecting pigmentation when only very few vessels were stained. We concluded that the main factor control- Reduced watering imposed a rapid decline ling the extent of the fungus was probably of ψ which reached low values of about , wd the dimension of vessels into which coni- -2 MPa after 18 d. Complete rewatering dia were injected, and that the mechanism allowed a recovery to high values around governing the spread of these conidia was -0.5 MPa in less than 2 d. The second a passive diffusion similar to that of partic- drought cycle yielded even stronger reduc- ulate ink. tions to -3.8 MPa (fig 3c). ψ decreased wm approximately from -2 to -3 MPa during the The survival of the fungus in the inocu- first drought cycle; rehydration yielded only lated trees was shown through reisolations a slight recovery, and finally the second during following year made on 3 inoculated drought cycle resulted in a decline to -3.8 MPa. The difference between ψ and ψ wm wd remained high during periods of adequate water supply, but decreased strongly dur- ing the stress. Control trees showed almost constant values of g with no significant difference L between inoculated and non-inoculated trees (fig 3a). Mean values were 1.03 and 1.02 mmol m s MPa for inoculated -2 -1 -1 and controls, respectively, which were not statistically different (Fisher PLSD, 5%). The value of g declined rapidly to very L low values during the first drought cycle (fig 3b); it recovered only partially after re- watering, and decreased again during the second cycle. The lowest values reached during drought were around 0.25 mmol m -2 -1 -1 s MPa No significant difference between . inoculated and control saplings could be detected during this stress evolution (Fisher PLSD, 5%).
increased after the 2 periods of water stress and yielded 60% loss of conductivity, but again no difference was detected in rela- tion to inoculation (table I). We plotted all measured values of embolism against the lowest values of ψ wm experienced by the saplings prior to the measurement (fig 5). Two major observa- tions could be drawn from these vulnerabil- ity curves: (1) curves from inoculated and non-inoculated saplings overlapped com- pletely showing that no effect of the inocu- lation with O querci was detected on peti- oles or twigs; and (2) twigs and petioles displayed approximately the same vulner- A direct plot of g against ψ (fig 4) ability to embolization. L wd showed that the observed declines appeared Finally, we compared these vulnerabil- at rather high values of ψ (around -1 MPa). wd curves obtained on petioles from pot- ity Stronger stress intensities only induced lim- ted saplings dehydrated at a rather slow ited additional depression of g . L rate (40 d) with those from branches rapidly dehydrated (a few hours, data obtained with adult Q robur, Cochard et al, 1992, fig Embolism in twigs and petioles 6b). Both curves showed strong similari- ties, with cavitation beginning around -2.5 During the whole experiment, loss of MPa and increasing steeply around -3 hydraulic conductivity remained very low in MPa. The second half of the curves well-watered saplings, and no significant diverged: on potted saplings, losses of con- ductivity remained significantly lower than inoculation-related difference appeared on on cut branches at low water potentials. twigs or petioles (table I). Embolism strongly
This discrepancy could be explained by the fact that, on slowly dehydrating potted saplings, embolized vessel could be pro- gressivély plugged and therefore unable to refill under pressure during our mea- surements. This would lead to underesti- mates of maximal conductivity (k and is ) m a consequence of drought-induced loss of conductivity. Calculated values of leaf spe- cific conductivities (k decreased sig- /LA) m nificantly with the lowest values of ψ (fig wm 6a), which can only be the consequence of decreases in k with increasing dura- m tion of drought. We corrected our data for this artefact, using the regression coeffi- cient between minimal potential and k , m and obtained the new vulnerability curve displayed in figure 6b, which is similar to that obtained with cut branches.
drought was very similar to that measured DISCUSSION with different oak species (Cochard et al, 1992): cavitation began at leaf water poten- tials around -2.5 MPa. Oaks display an Water relations of oak saplings intermediate response between vulnerable submitted to drought species like Juglans regia (Tyree et al, 1992b), or Populus deltoides (Tyree et al, Vessel length distribution in current year’s 1992), and more resistant ones like Junipe- wood of Q robur saplings was in agreement rus virginiana or Cedrus atlantica (Tyree with the results obtained by Cochard and and Ewers, 1991; Cochard, 1992). Tyree (1990) with mature Q rubra and Q We clearly showed that twigs and peti- alba. oles displayed very similar vulnerabilities to The maximal length of about 1 m same embolism, as had already been shown on observed, and the distribution along was several different oak species (Cochard et the growing axis was similar, with the short- al, 1992). Oaks present no hydraulic seg- est vessels located near the distal end of mentation based on differential susceptibil- the current year’s shoot, and the longer ones ity to cavitation, while some species like in the earlier growth segments. common walnut (Juglans regia) do; in the specific hydraulic conductance of Total latter petioles are much more vulnerable is a good parameter describing trees ) L (g than twigs (Tyree et al, 1992b). the overall efficiency of water extraction from The good agreement between the results soils and transport to the shoots (Reich and obtained with our saplings dehydrating over Hinckley, 1989; Granier and Colin, 1990; a relatively long period and earlier data Bréda et al, 1993). The values obtained here obtained by Cochard et al (1992) on excised were in close agreement with those reported branches, showed that the cavitation-induc- for oaks by Reich and Hinckley (1989). The ing processes are probably of the same g decreased in response to drought, simi- L nature in situ and on cut branches. This larly to what had been reported in many observation confirms the suitability of the species (Granier et al, 1989; Bréda et al latter method, as already shown by Tyree 1993). Such decreases may be due either to et al (1992a). Apart from this rather good changes in the hydraulic properties at agreement between both methods con- soil-root interface or to xylem embolism with cerning the water potential inducing onset of stresses. Under natural conditions strong cavitation (around -2.5 MPa), a significant mainly ascribed to reversible they were discrepancy appeared for stronger deficits: reductions of hydraulic conductivity at the the loss of conductivity was more progres- soil-root interface (Bréda et al, 1993). In sive in potted saplings, and 50% loss was the present study, the decline in g could , L reached at about -3.2 MPa, while for sev- only be partially reversed by rewatering. ered branches it had already been reached This poor recovery may be partly ascribed to -2.6 MPa. A very likely explanation for this the fact that vessel embolization participated lies in the fact that during gradually increas- in the decrease in g under such low leaf , L ing drought, embolization may rapidly water potentials, as has been demonstrated become irreversible, due to vessel plugging. by direct measurements of losses of con- This leads to artefacts in the estimate of ductivity in twigs and petioles. loss of conductivity with Sperry’s resatura- tion technique. The decrease of the leaf- The vulnerability to cavitation evidenced specific conductivity of petioles with increas- by our measurements of loss of hydraulic ing drought duration and intensity we conductivity on twigs and petioles during a
evidenced in this work is a good argument (Delatour et al, 1993), thus demonstrating for this hypothesis. Nevertheless, a cor- that the initial dispersal was not followed by rection of our data based on the asumption any further spread into adjacent vessels. that leaf specific conductance should be This point clearly distinguished O querci constant in the absence of drought, did not from O novo-ulmi which spreads readily all completely overcome the differences. They over the xylem of young elms after a few could also be partly due to differences days (Delatour et al, 1993). among juvenile and adult trees, but no Despite this poor ability to colonize xylem information is yet available on age-related tissues, O querci survived in the stems of changes of vulnerability in current year young oaks and induced browning symp- wood of trees. toms which were detected just 2 months after inoculation. This observation confirms many earlier observations of brown spots Effects of an inoculation in the xylem of inoculated trees and with O querci on water relations seedlings (Przybyl and Delatour, personal communication). The presence of living coni- dia in the xylem tissue and vessels could Our results brought some insight into the induce potential synergistic effects with mechanisms leading to the initial dispersal drought. Two complementary observations of spores of O querci following a direct inoc- following massive injection at several heights ulation. A rapid dispersal of conidia at a rate in the stem showed that no such effects and a distance very similar to that observed occurred: (1) the total leaf-specific hydraulic for ink particles was observed in the xylem conductance (g was not modified on well- ) L of our potted saplings. Underwater injec- watered or stressed plants by the presence tion, aimed at avoiding wounding-induced of the fungus; and (2) cavitation induction cavitation, did not modify it as compared to occurred at exactly the same rate in both direct injection. Furthermore, neither the rate of transpiration nor the leaf water poten- cases. tial prevailing during injection had any effect Vascular pathogens have been described on this dispersal. In fact, the initial dispersal to induce losses of hydraulic conductivity in of the conidia appeared to be a passive pro- young seedlings of Q rubra inoculated with cess very similar to that occurring with par- Ceratocystis fagacearum; in this case, ves- ticles of Indian ink, and was probably mainly sels became non-conductive and tyloses, controlled by the length of the vessels into gum and material depositions were which both were injected. Pit membranes observed (Jutte, 1977). In seedlings of impeded their transport into adjacent ves- Ulmus americana inoculated with O ulmi, sels, as their pores probably do not exceed Newbanks et al (1983) detected rapid 0.17 μm diameter (Ewers and Fisher, 1989). embolization along a 10 cm segment of stem The few discrepancies that we observed above the wound. No such direct occlusion between the dispersal of ink particles and of vessels was observed in our trees, as fungus reisolation were probably due to the leaf-specific hydraulic conductance was not higher sensitivity of the latter method. Sim- significantly modified. Another potential ilar observations have already been reported action of vascular pathogens has been by Mace et al (1971) for Fusarium oxyspo- hypothesized: the emission of compounds rum. decreasing the surface tension of xylem sap Reisolations made after 1 year showed and consequence increasing the sus- as a that the fungus remained viable, but still to cavitation. Such hypothetical ceptibility sequestered in the same stem segments effects would only act under high tensions in
the xylem, ie during periods of water stress. Cochard H (1992) Vulnerability of several conifers to air embolism. Tree Physiol 11, 73-84 This hypothesis was considered and dis- Cochard H, Tyree MT (1990) Xylem dysfunction in Quer- cussed by Kuroda (1989) who suspected cus: vessel sizes, tyloses, cavitation and seasonal that volatile terpenes induce cavitation in changes in embolism. Tree Physiol 6, 393-407 the case of Pinus thunbergii infected with Cochard H, Granier A, Bréda N, Aussenac G (1992) the pine wood nematode (Bursaphelenchus Vulnerability to air embolism and hydraulic archi- tecture of three European oak species (Quercus xylophilus). For O querci, we observed no petraea (Matt) Liebl, Q pubescens Willd, Q robur L). direct vessel occlusion as the total soil-to- Ann Sci For 49, 229-233 leaf hydraulic conductance remained con- Cohen Y, Fuchs M, Cohen S (1983) Resistance to water stant. Nor did we detect increased suscep- uptake in a mature citrus tree. J Exp Bot 34, 451- tibility to cavitation in uninfected twigs. As 460 a result, water stress did not lead to Delatour C Les dépérissements de chênes (1983) en Rev For Fr 35, 265-282 Europe. enhanced disorders in inoculated vs non- Delatour C (1986) Le problème des Ceratocystis inoculated saplings. We must therefore con- européens des chênes. EPPO Bull 16, 521-525 clude that O querci is unable to induce any Delatour C (1991) A very simple method for long-term hydraulic dysfunction in young oaks even storage of fungal cultures. Eur J For Path 21, 444-445 during a strong drought. This observation, Delatour C, Ménard J, Vautrot A, Simonin G (1993) added to the increasing number of reports of Assessment of the ophiostomatales pathogenicity: the low pathogenicity of Ophiostoma species Ophiostoma querci on oak compared to O novo-ulmi elm. "Recent advances in studies on oak decline on on adults trees in situ in Europe (Balder, (N Luisi, P Lerario, A Vannini, eds), Bari, 59-67 1992) reinforces the hypothesis of the Ewers FW, Fisher JB (1989) Techniques for measuring absence of implication of this fungus in the vessel lengths and diameters in stems of woody oak decline processes reported. plants. Amer J Bot 76, 645-656 Gibbs JN (1981) European forestry and Ceratocystis species. Bulletin OEPP/EPPO 11, 193-197 ACKNOWLEDGMENTS Granier A, Colin F (1990) Effets d’une sécheresse édaphique sur le fonctionnement hydrique d’Abies bornmulleriana en conditions naturelles. Ann Sci For The authors very grateful to JE Ménard, who are 47, 189-200 was a great help in making the measurements. Granier A, Bréda N, Claustres JP, Colin F (1989) Vari- This research was partly supported by the Com- ation of hydraulic conductance of some adult conifers mission of the European Community (DG XII) under natural conditions. Ann Sci For 46s, 357-360 under the project STEP CT 900050C Water Hall R, MacHardy WE (1981) Water relations. In: Fungal stress, xylem dysfunctions and dieback mecha- Wilt Diseases of Plants (ME Mace, AA Bell, CH Beck- nisms in European oak trees. man, eds). Academic Press 255-298 Jutte SM(1977) A wood anatomical contribution to the understanding of vascular wilt disease. IAWA Bull 4, 77-81 REFERENCES Kowalski T (1991) Oak decline. I. fungi associated with various disease symptoms on overground portions of Balder H (1993) Pathogenicity of Ceratocystis spp in middle-aged and old oak (Quercus robur L). Eur J For oaks under stress. Recent advances in studies on Path 21, 136-151 Oak decline" (N Luisi, P Lerario, A Vannini, eds), Kuroda K (1989) Terpenoïds causing tracheid cavita- Bari, 31-39 tion in Pinus thunbergii (Bursaphelenchus xylophilus). Beckman CH (1987) The Nature of Wilt Diseases of Ann Phytopathol Soc Jpn 55, 170-178 Plants. The American Phytopathological Society, St Landmann G, Becker M, Delatour C, Dreyer E, Dupouey Paul, Minnesota, 175 p JL (1993) Oak dieback in France: historical and Bréda N, Cochard H, Dreyer E, Granier A (1993) Water recent records, possible causes, current investiga- tions. In: Rundgespräche der Kommission für Ökolo- transfer in a mature oak stand (Quercus petraea): seasonal evolution and effects of a severe gie Bd5 "Zustand und Gefährdung der Laubwälder, drought. Can J For Res 13, 1136-1143 Munich, Germany, 97-114
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