Báo cáo khoa học: "Growth, carbon dioxide assimilation capacity and water-use efficiency of Pinus pinea L"

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Original article Growth, carbon dioxide assimilation capacity and water-use efficiency of Pinus pinea L seedlings inoculated with different ectomycorrhizal fungi JM Guehl D Mousain G Falconnet J Gruez 1 INRA, Centre de Recherches de Nancy, Laboratoire de Bioclimatologie-Ecophysiologie Forestière, Champenoux, 54280 Seichamps ; 2 Centre de Recherches de Montpellier, Laboratoire de Recherches sur les INRA, Sym- biotes des Racines, 34060 Montpellier ; 3 Groupement d’Aix-en-Provence, Division des Techniques Forestières Méditer- CEMAGREF, ranéennes, Le Tholonet, 13610 Aix-en-Provence, France (Received 28 June 1989; accepted 8 January 1990) Summary - Three months after sowing, seedlings of Pinus pinea L grown in a nursery on a perlite-Sphagnum peat mixture were inoculated with different ectomycorrhizal fungi: Rhizo- pogon roseolus and Suillus collinitus (2 strains: 1 and 2). The growth medium was maintained well-watered and was fertilized with a dilute Coïc-Lesaint (N, P, K; 3, 2, 7.5 g l solution. ) -1 Fertilization was stopped at the end of the first growing season (October) and growth and gas exchange parameters of the seedlings were assessed prior to the beginning of their second growth season. Inoculation with the 2 S collinitus strains led to the greatest plant elongation, but biomass growth was greatest with R roseolus. Whole plant CO assimilation 2 capacity in the R roseolus treatment was 1.83 times that in the control treatment and 1.38 times that in the S collinitus 2 treatment. The plants infected by R roseolus and S collinitus 1 had similar whole plant CO assimilation capacities, but root and total 2 plant biomass were significantly higher in the R roseolus treatment. This difference could be due partly to greater carbon diversion by the fungal associate in the case of S collinitus 1. Mean water-use effi- ciency (WUE CO assimilation rate/transpiration rate) of the inoculated seedlings (pooled 2 = mean value 7.29 mol kmol was ) -1 significantly (P < 0.05) higher than that of the controls (5.06 mol kmolThis is linked to the double tendency, neither being statistically significant, ). -1 of the infected plants to exhibit higher CO assimilation rates and lower 2 transpiration rates than the controls. Pinus pinea / ectomycorrhiza / growth / CO assimilation / water-use 2 efficiency Résumé - Croissance, capacité d’assimilation de CO et efficience de l’eau de 2 plants de Pinus pinea L inoculés par différents champignons ectomycorhiziens. Des plants de Pinus pinea L âgés de 3 mois et cultivés en pépinière sur un subtrat à base de perlite et de Correspondence and reprints
tourbe blonde de Sphaigne, ont été inoculés avec différents champignons ectomycorhi- maintenu ziens : Rhizopogon roseolus et Suillus collinitus (2 souches, 1 et 2). Le substrat était non limitant et était fertilisé à l’aide d’une solution en permanence à un niveau hydrique à la diluée de type Coïc-Lesaint (N, P, K ; 3, 2, 7.5 g l La fertilisation a été interrompue ). -1 fin de la première saison de végétation des plants (octobre). On a mesuré les caractéristiques 2 et H avant O 2 de taille et de biomasse des plants ainsi que les échanges gazeux de CO était la plus le début de la seconde saison de végétation (février). La hauteur des plants forte pour les plants inoculés avec les 2 souches de S collinitus, mais la croissance pondérale La capacité totale état la plus élevée dans le cas des plants inoculés avec R roseolus. à d’assimilation de CO des plants inoculés par R roseolus représentait 183 % par rapport 2 S collinitus 2. la capacité des plants non mycorhizés et 138 % par rapport au traitement des capacités Les plants inoculés par R roseolus et S collinitus 1 étaient caractérisés par la biomasse totales d’assimilation de CO similaires, mais la biomasse racinaire ainsi que 2 Cette différence totale des plants étaient plus élevées dans le cas du traitement R roseolus. plus importante du carbone assi- pourrait être liée, du moins partiellement, à une utilisation L’efficience de l’eau taux (WUE dans le cas de S collinitus 1. = milé, par l’associé fongique, d’assimilation de CO de transpiration) moyenne des plants mycorhizés (valeur moyenne /taux 2 P < 0.05) à celle des plants générale 7.29 mol kmol était significativement supérieure (tendance, non statistiquement ) -1 non mycorhizés (5.06 mol kmol ). -1 Cela est à relier à la double des plants myco- significative pour chacune des 2 composantes considérées séparément,(A) plus élevées et rhizés à présenter des valeurs moyennes de taux d’assimilation de CO 2 de taux de transpiration (E) plus faibles que les plants non mycorhizés. / croissance / assimilation de CO / efficience de l’eau 2 ectomycorhize / pinea Pinus Smith, and (Harley hormones and INTRODUCTION 1983). Some authors have also shown that fungi can directly affect plant water re- generally infection is Ectomycorrhizal lations. Duddrige et al (1980) demon- accompanied by alterations in the host strated that the mycelium of Suillus plant CO assimilation capacity with ef- 2 bovinus could absorb tritiated water fects on both leaf area and assimilation which was then transported through the rate (A) (Ekwebelam and Reid, 1983; mycelial network to the host plant. Harley and Smith, 1983; Paul et al, Brownlee et al (1983) and Boyd et al 1985; Jones and Hutchinson, 1988). (1986) found that physiologically signifi- Part of the C fixed, 4% to 17% as re- cant quantities of water were being ported by Paul et al (1985), is diverted transported through such mycelia, towards the fungal associate to meet its since the cutting of mycelial strands metabolic requirements (Martin et al, connecting plants to moist peat led to 1987). Despite this specific C cost, the a rapid decrease in leaf water potential, increase of CO assimilation provided 2 transpiration and photosynthesis of the by mycorrhizal infection is often suffi- host plant. Jones and Hutchinson cient to achieve enhanced plant growth (1988) observed higher transpiration (Ekwelebam and Reid, 1983; Harley rates in Betula papyrifera seedlings in- and Smith, 1983). The mechanisms Scleroderma flavidum with oculated commonly proposed for explain- most inoculated seedlings. than in non ing enhanced photosynthesis in my- Little attention has been paid to ex- corrhizal plants involve aspects of P amining the effects of mycorrhizas on and N nutrition, source-sink regulation
g) -2-1 10l water-use and trace elements. Uninoculated efficiency (WUE ratio of = and inoculated plants received the same 2 CO assimilation to transpiration) of fertilization (Moussain et al, 1988). host plants, yet WUE constitutes a After inoculation, the plants were grown major aspect of plant growth limitation outside in uniform nursery conditions in in dry conditions and is subject to Southern France (mediterranean climate) physiological regulation involving onto- with 60% of the natural incident radiation at genic adaptation (Wong et al, 1985; shoot level. Five months after inoculation the root colonization by the mycorrhizal fungi Guehl et al, 1988) and to short term was assessed. The proportion of plants changes in response to environmental colonized by the inoculated fungi was 91, factors (Cowan and Farquhar, 1977; 78 and 9% in S collinitus 1 and 2 and R Guehl and Aussenac, 1987). roseolus, respectively. The mycorrhizal index The purpose of the present study (index ranging from 0 to 5 and representing the frequency of mycorrhizal tips versus the was to assess growth, CO assimilation 2 total number of root apices) of the colonized capacity and WUE in different ectomy- plants was 3.0 in the 2 treatments inoculated corrhizal Pinus pinea seedlings under with S collinitus and 2.5 in the R roseolus non-limiting water supply conditions. treatment, control plants were nonmycorrhi- zal. At the end of the growing season, in Oc- MATERIALS AND METHODS tober 1986, fertilization was stopped and the plants were left in full sunlight conditions as is usual in forestry practice. In February 1987, 30 plants (only mycorrhizal plants for Plant inoculation and growing conditions the 3 inoculated treatments and nonmy- corrhizal control plants) were taken at ran- Isolates of the following ectomycorrhizal fun- dom within each of the 4 treatments and were obtained from basidiocarps harves- gi transferred to Nancy (Northeastern France) ted in a Pinus pinea stand established on a where their gas exchange, biomass and size calcareous sandy soil (La Grande Motte, Hé- characteristics were assessed in controlled rault, France): Suillus collinitus (ss. Flury nec standardized conditions. Gas exchange ss. Sr.; 2 strains, 1 and 2) and Rhizopogon measurements made at this time of year pro- roseolus (Corda in Sturn). Mycelial inocula vide an estimation of the physiological status were grown in aseptic conditions for 7 weeks of the plant just prior to planting-out (Guehl on a perlite-peat mixture (4:1, v/v) moistened et al, 1989). All the plants of the different with a Pachlewski (Pachlewski, 1967) solu- treatments were dormant at the period of tion. gas exchange measurements. At the end of the winter 1986, seeds of Pinus pinea L. were germinated in a heated greenhouse on a perlite-Sphagnum peat Gas exchange and growth measurements mixture (1:1, v/v) in 500 cm anti-coiling 3 containers with 2 easily removable and re- placeable sides (Riedacker, 1978). Three Carbon dioxide and H gas exchange were O 2 months after sowing, each seedling was in- measured with an open gas exchange sys- oculated with 50 ml inoculum brought into tem consisting of 3 assimilation chambers (28 x 15 x 33 cm connected in parallel ) 3 contact with the roots by temporarily remov- ing the 2 sides of the containers. The growth and through which air was passed at a flow rate of 150 l h Air temperature in the . -1 medium was maintained in a well watered state (pF < 1.5) during the whole growth pe- chambers was maintained at 22.0 ± 0.5 °C. riod. Before inoculation the containers were Photosynthetic photon flux density (400- 700 nm) at shoot level was 600 μmol·m -1 ·s -2 watered with water at pH 8.3, which ad- justed the growth medium to pH 6.2. After and was provided by high pressure sodium inoculation the containers were fertilized lamps (Sont, Philips). The CO molar fraction 2 every other week with a dilute Coïc-Lesaint of the air entering the chamber was measu- solution containing major (N, P, K; 3, 2, 7.5 red continuously with an ADC-225 MK2 IR-
GA and was adjusted to 350 ± 5 Pa·MPa 1 -. h, and the different dry- at 80 °C for 48 assessed. There were 9 repli- The difference in CO molar fraction be- weights 2 were uninoculated treatment for the cates tween the airs entering and leaving the (controls), 13 for the R roseolus treatment chambers was measured with a differential which had the highest biomass growth, and ADC-225 MK3 IRGA, alternately for periods 6 for each of the 2 S collinitus treatments. of 3 min for the 3 chambers by means of In addition, 5 S collinitus 2 infected plants an automated switching system. The dew- were used only for whole plant gas ex- point of the airs entering the chambers and change measurements. In 5 individuals of of the different airs leaving the chambers each of the controls R roseolus, and S col- was measured concurrently with the CO 2 linitus 1 treatments of the total projected measurements with a dewpoint hygrometer needle area of the plants was also deter- (System 1 100 DP, General Eastern). The air mined with an image analysis system (TAS) entering the chambers was maintained at in order to assess the specific dry-weight of 1 380 ± 40 Pa water vapour pressure, lea- the needles (dry weight/area ratio). For these ding to leaf-to-air vapour molar fraction dif- different types of measurements, samples ferences (ΔW) in the chambers of between were taken randomly within the different 7.0 and 10.0 Pa·kPa depending on the in- , -1 treatments. For all the variables assessed, tensity of plant transpiration. Because tran- differences between treatments were tested spiration, in turn, depends on ΔW, and in by means of Scheffe’s multiple comparison order to permit comparisons between plants, test. corrections were made using appropriate formulae (Caemmerer and Farquhar, 1981) to set each value to a constant ΔW of 8.5 Pa·kPaGas exchange calculations were . -1 RESULTS made on a needle dry-weight basis, giving ·s -1 nmu;mol·g mol·g CO assimilation rates (A) in 2 growth Size and biomass ·s -1 & and transpiration rates (E) in Measurements of gas exchange rates were ta- Maximum height growth of the plants ken as the steady-state values after a period of 1-2 h adjustment by the seedlings to the (table I) occurred with the treatments S assimilation chamber conditions. collinitus 1 and S collinitus 2 with After gas exchange measurements, the values significantly greater than those plants were separated into their different of the control treatment. Growth in components (whole root system, needles, height of the R roseolus plants was not nonphotosynthetic aerial parts), oven dried
the 2 treatments inoculated with the S significantly different from that of the collinitus strains being intermediate. controls. No significant treatment ef- There was no treatment effect on found for root collar diame- fects were needle/shoot ratio (table I). The plants. The highest total dry ter of the needles of the mycorrhizal plants had in the treatment R weight occurred lower specific needle dry weights with a value significantly roseolus, (table II, S collinitus 2 was not greater than those of the S collinitus 1 measured) than the control plants. and the control treatments, but not than that of S collinitus 2. At the individual level, total plant dry weight (TDW, g) Carbon dioxide assimilation capacity was poorly correlated with plant height (H, mm) (r 0.32, n 34, P < 0.05), = = and better correlated with root collar There was no significant treatment ef- diameter (D, mm) (TDW 1.33D-1.96, fect relative to A (table III) though large = 34, P < 0.05) and with 0.78, n differences were measured among r = = HxD 2 6.35 10 HD -4 2 +1.369, (TDW treatments. However, significant treat- = 34, P < 0.05). Signifi- ment effects were noticed relative to 0.83, n r = = cant differences between treatments whole plant CO assimilation capacity, 2 were found for the root/shoot ratio of the capacity of the R roseolus plants ) -1 nmol·s the plants, with S collinitus 1 having the being 1.81 times (50.5 lowest value (0.59). This low value was greater than that of the control plants primarily due to low root dry weight in and 1.38 times greater than that of the the S collinitus 1 treatment, the esti- S collinitus 2 infected plants. There was mated mean value being even less than no close relationship between the mean in the control plants. The plants in- treatment values of total plant dry fected by S collinitus 2 and R roseolus weight (table I) and whole plant CO 2 had ratios not significantly different assimilation capacity measured at the from that of the controls. end of the growing season (table III), since the S collinitus 2 infected plants The R roseolus infected plants had had higher dry weights than the S col- needle dry weights and areas signifi- linitus 1 infected plants, but lower CO2 cantly greater than those of the control plants (tablesI and II), the values for assimilation capacities.
In fig 1a the individual total dry weight values of the plants are plotted against their total CO assimilation 2 capacities; there was only a weak link- age between these 2 variables. No re- lationship was observed between the total dry weight of the plants and their A values (fig 1b), thus indicating that the weak dependence noticed in fig 1a is attributable solely to the correlation between total dry weight and needle dry weight of the plants (fig 1c). Water-use efficiency The mean transpiration rates of the my- corrhizal plants (table III) were not sig- nificantly different from those of the control plants. However, WUE in the ) -1 kmol control plants (5.06 mol was markedly and significantly lower than that of the infected plants kmolpooled ( ). -1 This is 7.29 mol mean value = to be associated with the double ten- dency, neither being statistically signif- icant, of the infected plants to exhibit higher A and lower E values (table III) than the controls. Fig 2a gives an in- teresting insight into the WUE regula- tion at the individual level: the individual variability of the points rela-
assimilation and transpiration 2 CO tive to the infected treatments (all treat- since the Y-axis inter- ments pooled) appears to be ordered (constant WUE), of the regression line (Y along a unique linear relationship ex- cept = signifi- 0.82) not 5.57X+6.50, pressing almost proportionally between was r =
their review paper, Harley and Smith cantly different from the origin. A re- (1983) reported that in most cases gression line forced through the origin ectomycorrhizal infection will reduce (Y= 7.00 X) has also been repre- the root: shoot ratio. These authors sented in fig 2a. The control plants did noted that in the examples where the not exhibit such a control of WUE: 4 root/shoot ratio was found to be slightly individuals out of 9 had WUE values enhanced by infection, the increase identical to those of the inoculated may be accounted for by the fungal plants, but 5 individuals had markedly sheath biomass if this were to comprise lower WUE values, thus providing a 20% of the weight of the roots. Our re- clear discrimination between uninocu- sults (table I) are consistent with these lated and inoculated plants in fig- general findings, the root/shoot ratio of ure 2a. The data in fig 2b show the the infected plants being lower than (S same discrimination in a total plant as- collinitus 1 treatment) or equal to (R similation vs transpiration graph. roseolus and S collinitus 2 treatments) that of the control plants. DISCUSSION Whole plant CO assimilation was 2 highest in the R roseolus infected plants. Relatively high (though not sig- Ectomycorrhizal infection by R roseolus nificantly different from the controls) had a significant positive effect on bi- values were also found in the S collin- omass growth of Pinus pinea seedling itus 1 and 2 treatments, but biomass- raised over 1 growing season in nurs- and especially root biomass-growth ery conditions, whereas there was no was not enhanced in these latter treat- enhancing effect in seedlings infected ments as compared to the controls. by the 2 S collinitus strains. Ekwebelam Whole plant CO assimilation did not 2 and Reid (1983), Harley and Smith exhibit significant differences between (1983), Tyminska et al (1986) have re- the R roseolus and S collinitus 1 treat- ported similar results indicating that the ments, but root and whole plant bi- extent to which growth was affected by omass were lower in the S collinitus 1 the infection will depend on the fungal treatment. Differential seasonal courses species and strain used as mycobiont. of growth and CO assimilation cannot 2 It should be stressed here that my- be eliminated as an explanation for corrhizal infection had differential ef- these discrepancies. These results may fects on shoot height growth and also suggest that in the S collinitus in- biomass growth, since the S collinitus fected plants C allocation to the 1 treatment produced the tallest plants vegetative sinks of the host plant could without increasing the total plant bi- be curtailed because of important C di- omass compared to the control plants. version to the mycobiont metabolic re- This can be somewhat misleading in quirements (Paul et al, 1985; Martin et field experiments in which height al, 1987). Further evidence for such an growth is often taken as an indicator of interpretation is provided by the low plant vigour. specific needle dry weights found in The present study also provides the S collinitus 1 plants (table II), prob- information regarding the bi- some ably reflecting low needle carbohydrate distribution between the differ- omass contents (Ehret and Jolliffe, 1985) and its components and plant ent high C sink activity (Harley and Smith, modulation by mycorrhizal infection. In
Pi led to a loss of stomatal control and 1983). The greater growth efficiency of wide stomatal apertures, while high Pi the R roseolus infected seedlings could induced stomatal closure. In the same be linked to lower fungal C require- species, Herold (1978) observed that ments (Harley and Smith, 1983; Paul et mannose and deoxyglucose induced al 1985; Tyminska et al, 1986; Marshall wilting by metabolically sequestring Pi. and Perry, 1987) R roseolus appears to Further investigations are required to be a very efficient fungus, worth select- test this hypothesis in the case of con- ing for practical applications. iferous species. Enhanced whole plant CO assimi- 2 The results obtained in the present lation capacity at the end of the grow- study might be of relevance to forestry ing season in the inoculated seedlings practice. Guehl et al (1989) have ob- was probably due to higher values of served that whole plant CO assimila- 2 dry-weight and A needle both capacity was an important tion (table III), though the differences in as- physiological determinant of survival similation rate were not statistically sig- after planting-out in Cedrus atlantica nificant. In the absence of foliar nutrient seedlings. Low CO assimilation 2 determinations, it is not possible to capacities, plus lower and more varia- assess here whether these effects and ble WUE in non-inoculated seedlings, the large variability of A and E within may, at least partly, explain the poor the treatments are due to varying N or growth after initial and survival P nutritional status or to other factors. planting-out commonly observed in Regardless of the physiological different plantation systems around the processes responsible for the high var- world in non-inoculated as compared to iability of CO assimilation both at the 2 inoculated seedlings (Marx et al, 1977; treatment (table III) and individual Le Tacon et al, 1987). (fig 2) levels, CO assimilation and 2 transpiration of the infected seedlings, measured under standard conditions, RÉFÉRENCES in nearly constant proportion (fig- were ure 2). Such a coupling, reflecting near Brownlee C, Duddridge JA, Malibari A, Read constancy of WUE, has been reported DJ (1983) The structure and function of for variations due to mineral nutrition mycelial systems of ectomycorrhizal roots (Wong et al, 1985; Guehl et al, 1989). with special reference to their role in A main result of the present study forming inter-plant connections and pro- viding pathways for assimilate and water is the observation of the absence of transport. Plant and Soil 71, 433-443 coupling between CO assimilation and 2 Boyd R, Furbank RT, Read DJ (1986) Ecto- transpiration, as well as lower WUE in mycorrhiza and the water relations of the control plants (fig 2). It might be trees. In: Proc. 1 Euro Symp on My- st suggested that this lack of stomatal corrhizae: Physiology and genetics (Gi- control is linked to a low ortho- aninazzi-Pearson Y, Gianinazzi S, eds) 1-5 July 1985, Dijon INRA, Paris, 689-693 phosphate (Pi) level in the needles of Caemmerer S, Farquhar GD (1981) Some re- the nonmycorrhizal plants. Mousain lationships between the biochemistry of (unpublished results) found very low Pi photosynthesis and the gas exchange of concentrations in the needles of ju- leaves. Planta 153, 376-387 venile nonmycorrhizal Pinus pinaster Duddridge JA, Malibari A, Read DJ (1980) seedlings. Harris et al (1983) found that Structure and function of mycorrhizal rhizomorphs with special reference to in leaf discs of Spinacia oleracea low
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