Báo cáo khoa học: "Growth, gas exchange and carbon isotope discrimination in young Prunus avium trees growing with or without individual lateral shelters"

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Original article Growth, gas exchange and carbon isotope discrimination in young Prunus avium trees growing with or without individual lateral shelters H Frochot JM Guehl 3 C Collet A Ferhi 1 INRA, centre de Nancy, laboratoire Lois de Croissance, F-54280 Champenoux; 2Centre de recherches géodynamiques, université Paris VI, 47, avenue de Corzent, F-74203 Thonon-les-Bains; INRA, centre de Nancy, laboratoire de Bioclimatologie et d’Écophysiologie forestières, 3 F-54280 Champenoux, France (Received 3 November 1992; accepted 5 February 1993) Summary — One-yr-old wild cherry (Prunus avium L) plants were grown as follows: 1) in small cylin- drical shelters (diameter 50 cm, treatment S); 2) in large shelters (diameter 100 cm, treatment L); or 3) without shelter (control, treatment C) during 1 growing season. Treatment C was characterized by higher values of photosynthetic photon flux density (I and of leaf-to-air water vapour pressure dif- ) p ference (Δ W) than treatments L and S. The plants were taller in treatments L and S than in treatment in the latter treatment. The plants of treatment C were also C but biomass production was higher characterized by higher values of CO assimilation rate (A) and of leaf mass per unit area (LMA, ra- 2 tio of leaf mass to leaf area). Relative carbon isotope composition (p of the leaves was higher in )δ treatment C than in treatments L and S, which expresses higher time-integrated values of plant in- trinsinc water-use efficiency (A/g ratio) in the former treatment. There was a positive correlation be- tween &p; and LMA. Thus, LMA, a readily measurable parameter, is a relevant parameter for under- delta standing and modelling water-use efficiency of canopies. I isotope discrimination I carbon lateral shelter I microclimate I growth I leaf gas exchange efficiency / leaf mass per unit area water-use Résumé — Croissance, échanges gazeux et discrimination isotopique du carbone de jeunes merisiers (Prunus avium L) placés ou non dans des abris latéraux individuels. Des plants de merisier (Prunus avium L) âgés de 1 an ont été installés durant une saison de végétation dans 1) des petits abris cylindriques (diamètre 50 cm, traitement S); 2) des grands abris cylindriques (diamètre 100 cm, traitement L); ou 3) sans abri en plein découvert (traitement C) (fig 1). Le traite- ment C était caractérisé par des valeurs plus élevées de rayonnement photosynthétiquement actif ( ainsi que de différence de pression partielle de vapeur d’eau entre feuille et atmosphère (ΔW) Ip) (fig 3). La croissance en hauteur était plus élevée pour les plants du traitement C que pour ceux des traitements L et S, alors que la production de biomasse était la plus élevée dans le traitement C
(tableau I). Les plants du traitement C étaient également caractérisés par des valeurs plus élevées de taux d’assimilation de CO (A) (fig 5) ainsi que de masse foliaire spécifique (LMA, rapport de la 2 masse sur la surface foliaire) (fig 8). La composition isotopique relative en carbone (p des feuilles )δ était plus élevée dans le traitement C que dans les traitements L et S (fig 8). Cela traduit des valeurs intégrées dans le temps d’efficience intrinsèque d’utilisation de l’eau (rapport A/g) plus élevées pour le traitement C (tableau I). On a noté une corrélation positive entre &p; et LMA (fig 8). Ainsi, LMA, qui delta est une grandeur facilement mesurable, constitue un paramètre pertinent pour la compréhension et la modélisation de l’efficience d’utilisation de l’eau des couverts végétaux. abri latéral / microclimat / croissance / échanges gazeux foliaires / discrimination isotopique du carbone / efficience d’utilisation de l’eau / masse foliaire spécifique INTRODUCTION which be beneficial for the plant water can status and water-use efficiency (Aussenac and Ducrey, 1978). The neighbourhood relationships between This study examines the effects of artifi- young trees and the surrounding vegeta- cial lateral shelters simulating the aerial ef- tion are the result of various below-ground fects of an accompanying vegetation - (competition for water and nutrients, alle- without any below-ground relationship - on lopathy) and above-ground (competition young Prunus avium trees. Measurements for light, modification of temperature, air of: 1) microclimatic parameters ; 2) humidity and windspeed) interactions growth ; 3) leaf gas exchange ; and 4) leaf (Gjerstad et al, 1984 ; Radosevich and Os- carbon isotope composition which can lead teryoung, 1987). When neighbourhood re- to time-integrated plant water-use efficien- lationships are dominated by competition cy were made. processes, their global effect will be to re- duce survival and growth of the young trees. However, in situations of high poten- MATERIAL AND METHODS tial evapotranspiration, the presence of ac- companying vegetation may be beneficial for the trees due to lowered evaporative Experimental design demand at the tree level. To analyze the neighbourhood relation- Wild cherry (Prunus avium L) seedlings (Côte ships it is necessary to disentangle the ef- d’Or provenance, Eastern France) were grown fects of aerial and soil factors (Nambiar, in an experimental nursery near Limoges (Mas- 1990). The use of artificial lateral shelters sif Central, France) from spring 1989. On Febru- built around growing young trees may be a ary 15 1990, 48 plants (average height 30 cm) relevant way of studying the effects of aeri- were taken from the nursery beds. In order to al microclimate modifications on the minimize transplanting stress, the plants were immediately placed in containers filled with or- growth and function of plants (Collet and ganic soil and transferred to the experimental Frochot, 1992). The general effect of later- plot near Nancy (northeastern France) where al shading will be to reduce photosynthetic they were planted. The trees were randomly dis- CO assimilation due to lowered leaf inci- 2 tributed into 3 treatments comprised of 16 trees dent photosynthetic photon flux density. each: However, this reduction may be accompa- Treatment S (small shelters). These plants were nied by a decrease in stomatal conduc- surrounded by individual cylindrical shelters with tance and in transpirational water losses a diameter of 50 cm.
Treatment L (large shelters). These plants were surrounded by cylindrical shelters with a diame- ter of 100 cm. Treatment C. Controls without shelters. The shelters constituted of wire net- were a ting supporting a green plastic net with a porosi- ty of 50% (fig 1). Initially, the shelters were 60 cm high. As the seedlings grew, the height of all the shelters was increased so that no plant was greater than its shelter. Four successive height increases were made simultaneously for all shel- ters (fig 2). At the end of the growing season the shelters were 2.5 m high. Bare soil conditions were maintained throughout the experiment by chemical weeding around the shelters and manual weeding within the shelters. Rainfall dur- ing the experimental period (April-September) amounted to 262 mm and additional water no supplied to the trees. was In order to assess the microclimatic condi- tions inside the 2 types of shelters, photosyn- thetic photon flux density (I was measured at ) p 12.00 (solar time) on a sunny day with a quan- Water status tum sensor (Li-Cor, Lincoln, NE, USA) at differ- and gas exchange measurements ent heights above ground. These measure- ments were made when the shelters were 1.5 m high. At the top of the shelter Ip was similar to Water status and gas exchange measurements that outside the shelters (100%). Below 115 cm made periodically between July 11 and Au- were (S shelters) and 75 cm (L shelters), p I was gust 16. These measurements were carried out abruptly reduced to 30% of the ouside I in both p on the 6 tallest plants in each treatment in order types of shelters. Thus, the upper parts (= 30 cm to avoid experimental interference due to trans- for the S shelters and 40 cm for the L shelters) planting stress. Predawn leaf water potential of of the elongating stems were exposed to full the seedlings was measured with a Scholander sunlight around midday while the lower parts pressure chamber and was between -0.1 MPa were shaded all day long. (July 11) and -0.45 MPa (August 16), thus indi- cating an absence of severe drought con- straints. Carbon dioxide assimilation rate (A, μmol m-2 rate (E, mmol m s and-2 -1 ), -1 s transpiration ) leaf conductance for water vapor (g, mmol m -2 ) -1 s were measured using a LI-6200 portable photosynthesis system (Li-Cor Inc, Lincoln, Ne- braska, USA) fitted with a 4-I assimilation cham- ber. Leaf temperature (T was monitored by ) 1 means of a thermocouple in contact with the lower leaf surface. The leaf-to-air difference in water vapour partial pressure (ΔW) was calculat- ed from T and air water vapour pressure. Si- 1 multaneously to the gas exchange measure- ments, I was measured with a quantum sensor p (Li-Cor, Lincoln, NE, USA). Preliminary meas- urements were carried out in order to assess the effects of leaf ageing on gas exchange parame-
ters. A and g were highest for leaf order be- (full sunlight) (fig 3). Air temperature (con- tween 4 and 7. All gas exchange data reported trol treatment) increased progressively hereafter correspond to measurements made from 22.0°C on July 11 to 34°C on August within that zone of the trees which, in the shel- 1 and then decreased to 27°C on August ters, was generally at the transition between the 8. Leaf-to-air water vapour concentration shaded and the full sunlight exposed regions. (ΔW) presented similar time changes with Gas exchange measurements were performed extreme values of≈ 14 Pa KPa and 34 -1 between 11.30 and 13.30 (UT) on 2 leaves per tree. Gas exchange parameters were calculated Pa KPa In both L and S treatments I . -1 p leaf area basis. Leaf area was determined on a was approximately half that in C, except on in situ just prior to the gas exchange measure- August 8 when I was identical in all treat- p ments by means of a portable area meter (Licor ments. The frequency distribution of I in p 3000, Li-Cor, Lincoln, NE, USA). the different treatments is given in figure 4. Carbon isotopic composition Carbon isotopic composition was determined on leaf material. Three leaves from each of the 6 trees in the different treatments were harvested on October 12. These leaves included those in which gas exchange had been measured on August 8. After determination of leaf area, the samples were oven-dried at 70°C for 48 h, weighed and finely ground. Fifteen mg of sam- ple material was then weighed out and com- busted in special quartz vessels under a pure O atmosphere. The carbon was thus quantita- 2 tively converted to CO Relative abundances of . 2 C 13 and 12 were determined using a mass C spectrometer (Finigan Mat). The results are ex- pressed in terms of the conventional δ &permil; nota- tion, according to the relation (Farquhar et al, 1989): 1 - b /R s δ=R [1] refer to 13 ratio in the C 12 C/ where and s R b R sample and in the Pee Dee Belemnite standard (PDB), respectively. RESULTS Microclimate, growth and gas exchange Gas exchange measurements were made 5 sunny days from July 11 to August 8 on with a photosynthetic photon flux density (Ip) of≈1 400 μmol m s in treatment C -2 -1
Pa KPa lower in the sheltered treatments -1 For treatment C a monomodal distribution observed with a modal interval 1 500- compared with treatment C. These be- was as 1 700 μmol m s For the L and S treat- -2 -1 . tween-treatment differences were associat- ments bimodale distribution were ob- ed with differences in leaf temperature served, modal intervals being 250-500 (T whereas water vapour concentration ), 1 and 1 250-1 500 μmol m s No signifi- -2 -1 . in the air was identical in all treatments cant differences were noticed between (data not shown). treatments for T whereas ΔW was≈ 3-4 , a At the end of the growing season (be- of October) trees of treatments L ginning and S were taller than those of treatments C (table I), but root collar diameter and production of biomass were higher in the latter treatment. There was no significant treatment effect on root/shoot biomass ra- tio. Carbon dioxide assimilation rate (A) in the C treatment showed a slight decrease from 18 to 13 μmol m s over the meas- -2 -1 urement period (fig 5). Except on August 8, A was=5 μmol m s lower in treat- -2 -1 ments L and S than in C. This difference was not only attributable to higher values p I in treatment C, but was also linked to a higher assimilation capacity in this treat- ment since in saturating light conditions (I 1 000 μmol m s A was≈ 4.2 μmol -2 -1) > p -2 -1 m s higher in treatment C than in the other treatments (fig 6). Leaf conductance for water vapour diffusion (g) decreased progressively during the measurement pe- riod in all treatments (fig 5). With the ex- ception of July 11, the g values were iden- tical in the C and L treatments, while g was 80 mmol m s lower in S than in the -2 -1 = former treatments. Leaf transpiration rates (E) were highest in all treatments on August 1 (fig 5). Between-treatment differences, similar to those for g, arose for E. Intrinsic instantaneous water-use efficiency (A/g ra- tio) increased in the 3 treatments during the measurement period (fig 7). This pa- rameter was highest in C, lowest in L and intermediate values were noticed in S. In- stantaneous water-use efficiency (A/E ra- tio) was markedly lower in L than in the 2 other treatments.
Carbon isotopic composition and leaf mass per unit area No significant difference in relative isotopic composition (p arose between treatments )δ L and S (fig 8). Carbon isotope composi- tion was significantly higher in the control (-26.83&permil;) than in treatments S (-27.75&permil;) and L (-27.49&permil;) (table II). Leaf mass per unit area (LMA) differed significantly be- tween the 3 treatments with 67.89, 72.95 and 101.79 g m in S, L and C, respec- -1
DISCUSSION There was a significant positive tively. correlation between &p; and LMA both at delta the treatment and individual plant level Climatic parameters (mainly I and ΔW) dif- p fered between the control treatment and (fig 8).
The apparent enrichment factor related the 2 shelter treatments, but no significant isotopic fractionation by the photo- to the difference arose between the 2 latter treat- synthesis processes may be expressed by ments (figs 3, 4). For the leaves situated in an isotopic discrimination defined as (Far- the shaded part of the 2 types of shelters quhar et al, 1989): incident I was ≈ 30% of outside I Upper p . p leaves of the sheltered plants could be ex- posed to full sunlight in the middle of the day. The proportion of these leaves and the duration of full sunlight exposition de- pended on the ratio (tree height/shelter where &a and &patled ; refer to the isotopic com- delta; height) and on the diameter of the shelter. positions of air CO and of the photosyn- 2 Thus, in treatments S and L, I presented p thetic products (ie the leaf material here), a bimodal distribution in the first mode respectively. A typical value of &a is cur- delta; (shaded region of the shelters) being ≈ rently -0.008 (Friedli et al, 1986). 30% of the second (sunlit region of the According to Farquhar et al (1989), iso- shelters) (fig 4). topic discrimination is given by: The ratio of CO assimilation rate (A) in 2 treatments S and L to that in treatment C was ≈ 0.70, which is identical to the ratio of total plant biomass at the end of the grow- where a, the discrimination against 13 2 CO ing season (table I). Carbon dioxide assim- during diffusion into the leaf, is 0.0044 ; b, ilation rate was higher in the control treat- the discrimination during carboxylation, is ment, not only because of elevated I (figs p 0.027 ; C and C (mmol mol are inter- ) -1 i a 3, 4) but also because of higher values of cellular and ambient CO concentrations, 2 light saturated assimilation capacity (fig 6). respectively. Within mature Fagus silvatica and Quer- The diffusion of CO through the stoma- 2 cus petraea canopies, Ducrey (1981) also tal pores is described by: reported a positive relationship between light-saturated CO assimilation rate and 2 the proportion of solar radiation reaching the leaves during their ontogeny. Combining equations [2], [3] and [4] and Leaf conductance values were lower in substituting the different coefficients by treatment S than in treatments L and C (fig their numerical values yields: 5) ; however, this difference cannot be clearly ascribed to differences in microcli- mate parameters, for example I and ΔW p (figs 3, 4). This discrepancy between gas exchange and microclimatic variables could be linked to the fact that no time- Relative carbon isotopic composition (p ) δ integrated values of these 2 types of vari- less negative (-26.83&permil;) in the control was ables were assessed in this study. plants than in the plants of treatments L Carbon isotope composition measure- (-27.49&permil;) and S (-27.75&permil;) which corre- ments of plant material can give access to sponds to higher time-integrated values of time-integrated (lifetime of the measured A/g in the former treatment (table II). organ) values of plant intrinsinc water-use Lower &p; values found in lower forest cano- delta py leaves in comparison with upper leaves efficiency (ratio A/g).
have been attributed to low relative carbon ference in time integration scale between isotope composition of source CO in the the 2 approaches (ie a better integrative 2 air (a linked to the recycling of CO (de- value of the isotopic approach). 2 ) δ pleted in 13 relative to atmospheric CO C 2 The close correlation found be- positive above the canopy) originating from soil tween &p; and LMA (fig 8) at the individual delta respiration (Vogel, 1978 ; Medina and Min- leaf level shows that LMA, a readily mea- chin, 1980 ; Francey and Farquhar, 1982 ; surable parameter, is not only a relevant pa- Medina et al, 1986 ; Gebauer and Schulze, rameter for understanding and modelling 1991).In the present study, different light the spatial structure of CO assimilation in 2 regimes and associated small differences in plant canopies (Aussenac and Ducrey, T and ΔW (fig 3) were not accompanied by 1 1977 ; Ducrey, 1981 ; Oren et al, 1986) but differing &a; values (constant soil respiration delta also be used for understanding and can conditions and constant height above efficiency of canopies. modelling water-use ground) or by changes in other microclimat- In conclusion, in this study we have sim- ic factors such as air temperature or air hu- ulated aerial neighbourhood relationships midity. The difference in &p; found between delta between young Prunus avium trees and an treatment C and treatments L and S can accompanying vegetation in the absence therefore be entirely ascribed to differences of water vapour source constituted by the in isotopic discrimination by the leaves (Δ, transpiration of the accompanying vegeta- eq [3]) which are mainly determined by the tion. Under these conditions the height light regime. Zimmermann and Ehleringer growth of young trees was improved which (1990) also found a negative correlation be- may be of interest from a practical point of tween leaf Δ and the daily integrated values view. However, the trees grown without of leaf incident I in a Panamanian C epi- 3 p shelters were characterized by a higher phytic orchid, Casatetum viridiflavum, grow- biomass production, which was associated ing on trees of a forest canopy. with higher A values than in the trees The high &p; (and thus low Δ) values delta grown with shelters. Thus there was no found here in treatment C could be asso- positive effect of lateral shading on bio- ciated with high A values (figs 5, 6) and mass growth. The control trees were also with high LMA values (fig 8) which prob- characterized by higher water-use efficien- ably reflect high nitrogen contents per unit cy than the sheltered trees. leaf area (no measurements of this param- eter were made in this study). ACKNOWLEDGMENTS The between-treatment differences in the A/g ratio found here on a gas ex- change basis (fig 7) were not totally con- The authors wish to thank M Pitsch and L Wehr- len (INRA Nancy) for their technical assistance sistent with the data obtained with the iso- and AM Chiara (Centre de Recherches Géody- topic approach (table II). In particular, gas namiques, Thonon-Les-Bains) for the isotopic exchange data provided higher A/g values measurements. They are grateful to M Dixon (fig 7) - linked to lower g values (fig 5) - in (INRA, Nancy) for reviewing the manuscript. treatment S than in treatment L, whereas isotopic data also provide higher A/g val- ues in treatment S than in treatment L, REFERENCES whereas isotopic data also provide higher A/g values in treatment C but identical Alg Aussenac G, Ducrey M (1977) Étude bioclima- values in treatments L and S (table II). This tique d’une futaie feuillue (Fagus sylvatica L discrepancy might be attributed to the dif- et Quercus sessiliflora Salisb) de l’est de la
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