Báo cáo khoa học: "Photosynthesis, leaf area and productivity of 5 poplar clones during their establishment year"

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article Original Photosynthesis, leaf area and productivity of 5 poplar clones during their establishment year B Saugier M Mousseau TS Barigah J Guittet R Ceulemans 3 1 INRA, Station de Recherches Forestières, BP 709, 97387 Kourou cedex; Université de Paris XI, Laboratoire d’Écologie Végétale, Centre d’Orsay, 2 Bâtiment 362, 91405 Orsay cedex, France; 3 d’Anvers, Département Biologie, UIA, Universiteitsplein 1, B-2610 Wilrijk, Belgium Université 24 March 3 November 1993; (Received accepted 1994) Summary — The stem volume and biomass (stem + branches) production, net photosynthesis of mature leaves and leaf area production of 5 poplar (Populus) clones, Populus trichocarpa x deltoides (Raspalje and Beaupré), Populusx euramericana (Robusta) and P trichocarpa (Columbia River and Fritzi Pauley), were studied during the first year of growth in an experimental high density plantation (15 600 plants ha Significant differences were found in volume production, woody biomass production, ). -1 total leaf area and net photosynthesis. Above-ground biomass production was 3.5 times higher in Raspalje than in Robusta. The best performing clones (Raspalje, Beaupré) were those with large leaves, high leaf area index and high photosynthetic rates. A positive relationship between leaf photo- synthetic capacity and above-ground biomass production was also noted for 4 of the 5 clones. The euramerican clone Robusta was an exception, showing high photosynthetic rates, but low biomass pro- duction. This discrepancy was mainly due to the lower leaf area of this clone, and possibly also due to a larger carbon allocation to below-ground biomass (Barigah, 1991). The root/shoot ratios at the end of the first season in the clones Raspalje and Robusta were 1.23 and 1.79, respectively. photosynthesis / leaf area / biomass production / Populus net Résumé — Photosynthèse, surface foliaire et productivité de 5 clones de peuplier dans leur pre- mière année. Des plants issus de boutures de 5 clones de peuplier (Populus trichocarpa x deltoides (Raspalje et Beaupré), P x euramericana (Robusta) et P trichocarpa (Columbia River et Fritzi Pauley) ont été cultivés en peuplement dense (15 600 tiges ha Des mesures d’assimilation de CO et de crois- ). -1 2 sance (surface foliaire, volume de tiges, biomasse aérienne) ont été réalisées sur les jeunes plants. L’ac- cumulation de biomasse du clone le plus performant (Raspalje) représentait 3,5 fois celle observée dans le clone le moins performant (Robusta). Les clones les plus performants (Raspalje, Beaupré) étaient également caractérisés par une surface foliaire importante et une assimilation nette foliaire élevée. Les différences de surface foliaire entre clones étaient liées à des différences de surface individuelle des feuilles et non au nombre de feuilles par arbre, qui était quasi constant. La biomasse aérienne était posi-
tivement corrélée à la capacité photosynthétique foliaire pour 4 clones. Cependant le clone Robusta, de capacité de production faible, présentait une photosynthèse foliaire élevée. Cette faible production de biomasse aérienne chez Robusta était due à un faible développement foliaire et probablement aussi à un investissement en biomasse racinaire important (Barigah, 1991) ; le rapport de la bio- masse racinaire à la biomasse aérienne était respectivement de 1,23 et de 1,79 pour les clones Ras- palje et Robusta. photosynthèse foliaire / surface foliaire / production de biomasse / Populus INTRODUCTION parable leaves in different genotypes, to phenological and physiological changes dur- ing the growing season, and to the distri- Plant productivity depends on the interac- bution of photosynthates within the tree. For tion of light intercepting the leaf area of a example, some poplar clones retain green plant and the intensity of the CO assimila- 2 leaves late in the fall with measurable a tion process taking place in those leaves. after frosts, photosynthetic production even The production of forest stands has been thus contributing significantly to a late sea- shown to be strongly correlated with total stem diameter increment (Nelson et al, son annual intercepted irradiance (Linder, 1984; 1982) and root growth (Isebrands and Nel- Beadle and Long, 1985). Differences in the son, 1983). amount of leaf area displayed or in the inten- sity of the photosynthetic rate will result in In addition to photosynthetic rate, leaf different biomass productivity rates. area is also a very important determinant of biomass productivity. Comparing differ- Photosynthetic capacity is known to vary ent spruce (Picea abies) provenances Gross widely among tree species, usually being and Hettesheimer (1983) found a negative higher in deciduous than in coniferous trees correlation between leaf area and both (Ceulemans and Saugier, 1991).In several biomass production of the trees and CO 2 tree species, intensive selection for assimilation rate. The relationship between increased biomass productivity has resulted biomass productivity and its determining in hybrids demonstrating heterosis for photo- factors may thus be complicated. Never- synthetic performance (Isebrands et al, theless, variability in plant genotypes accord- 1988). Moreover, a positive correlation ing to plant branchiness and leaf distribu- between photosynthetic capacity and biomass productivity has already been tion, position and orientation within the crown demonstrated for poplar hybrids (Ceule- could strongly influence the efficiency of mans and Impens, 1983; Michael et al, conversion of solar energy into biomass pro- 1990), larch hybrids (Matyssek and Schulze, duction (Isebrands and Nelson, 1982; Ise- 1987) and different provenances of loblolly brands and Michael, 1986). However, direct pine (Boltz et al, 1986). linear relationships between biomass pro- duction and solar radiation intercepted by However, in many other cases, net the foliage have been demonstrated in agri- rate measurements have photosynthesis cultural crops (Monteith, 1981) as well as been found to be poorly correlated with in forest stands (Linder, 1984; Leverenz and growth rate and productivity, such as in the Hinckley, 1990). Although this simple rela- of Populus grandidentata, P tremu- case tionship appears robust in young planta- loides and P smithii (Okafo and Hanover, tions, its general and empirical approach 1978; Reighard and Hanover, 1990). These conflicting results are due to the difficulty of have been criticized (Byrne et al, 1986; measuring the gas exchange rate on com- Agren et al, 1991).
for detailed measurements (height, diameter, leaf In this study, photosynthetic capacity, dimensions, number of leaves, photosynthesis, leaf area development, and biomass pro- stem height and diameter at 22 cm above the duction rates of different kinds of poplar ground). Measurements of young stem diameter (Populus) clones were compared during at 22 cm above the ground was found to be a their first year of growth. good compromise between the need for a mea- surement of the diameter close to the ground and the necessity to eliminate stem distorsion caused by the connection of the roots. These 4 trees were MATERIALS AND METHODS chosen from the 9 interior trees and had one border row around them. Stem volume index was Five poplar clones were used: 2 fast-growing and calculated from height (H) and diameter (D) mea- high-producing interamerican P trichocarpa x P surements as D To estimate total leaf area H. 2 deltoides hybrid clones (Raspalje and Beaupré); per tree (main stem), 80 leaves of surrounding 2 native American clones P trichocarpa (Columbia trees were harvested at different heights to mea- River and Fritzi Pauley); and 1 Populus x sure their leaf area, using a ΔT leaf area meter euramericana clone (Robusta), which is often (Delta-T Devices, Burwell, Cambridge, UK), and referred to as the reference clone. The latter is their dimensions (length and width). The allometric the result of a spontaneous hybridization between relationship between leaf dimensions and leaf P deltoides and a European P nigra, presumably area (table II) was then applied to monitor leaf the poplar clone Italica. The origin, sex, parentage area development of the 4 trees per clone. At the and provenances (table I) of these clones have end of the first growing season, all trees including previously been described (Ceulemans and the border ones were harvested, because no Impens, 1983; Ceulemans, 1990). border effect was found between the plants in the first year for height or for volume index (Van Hecke Hardwood cuttings of each of the 5 clones et al, unpublished data). Leaf biomass and leaf were planted on 8 April, 1987 in Orsay (48°50’N, area index (LAI) were estimated using leaf mass 2°20’E) near Paris, France, in monoclonal plots of per area data collected during the growing season. 4 x 4 m on a 0.8 x 0.8 m planting pattern (ie a Wood volume (stems and branches) was mea- tree density of 1.56 plants per m All plots were ). 2 sured by immersion in water, and wood biomass irrigated and fertilized. During the first growing was measured at harvest after oven-drying at season 4 trees per clone were monitored weekly
Measurements were made on several sunny days the growing season. The data were throughout plotted in a CO assimilation (A) versus PPFD 2 graph and were fitted using rectangular hyper- bola equation (A /(α•PPFD max {α•PPFD•A + = A where a is the photochemical efficiency, )}; max and A is the asymptotic value of A at satu- max rating irradiance. Leaf photosynthetic capacity was defined here as the PPFD-saturated net pho- at an atmospheric CO concentra- 2 tosynthesis tion of 350 vpm. Differences among clones in photosynthetic capacity were assessed using a t- test after comparing confidence intervals at the 95% level. RESULTS Growth patterns 80°C for 15 d. Since the dimensions of the plots were rather small, these biomass values were only used to compare the performance of the height after the first growing The total tree various clones and were not representative of ranged from 1.8 m for clone Robusta the biomass production of real stands. season to 3.5 m for clone Beaupré (table III). The 2 Leaf net photosynthetic rates and incident P trichocarpa x P deltoides clones (Beaupré photosynthetic photon flux density (PPFD) were measured in the field using an ADC Parkinson and Raspajle) were superior to the other leaf chamber connected to a portable CO ana- 2 clones with regard to tree height, while lyzer (ADC Company Ltd, Hoddedson, UK) in an clones Columbia River, Fritzi Pauley and open system arrangement. The leaf chamber was Robusta had similar heights around 2.0 m. supplied with an air mixture of a known CO con- 2 Stem volume index values (fig 1) increased centration from a compressed air cylinder, and for all clones from the beginning of the the CO drop in the chamber was 79 ± 21 vpm. 2 To avoid differences in photosynthetic rates due growing season until mid-October (day 288), to the variation of the CO concentration, which 2 except for clone Robusta (Barigah, 1991) ranged from 360 to 385 vpm in the air contained which ended extension growth early in in different gas cylinders, net photosynthesis at September (day 259). At the end of the first (A was calculated using the formula: 350 vpm ) 350 growing season, the ranking of the clones in terms of stem volume index was in agree- ment with that observed in height growth This formula assumes a linear relationship except for clones Columbia River and Fritzi between net photosynthesis (A) and CO con- 2 Pauley. centration (C) (Gaastra, 1959), and a constant CO compensation point (Γ). This relationship 2 Clone Beaupré had the highest wood vol- was established in the laboratory at 22°C and is production (732 cm table III), but the , 3 ume rather insensitive to variations in r, since a dif- highest biomass (stem + branches) was pro- ference of 20 vpm in Γ ronly caused a 2% variation duced at the end of the first season by clone in A using Γ equal to 60 vpm. 350 Raspalje, a branchy clone (table III). The Only fully expanded leaves having maximum fasted growing clone Raspalje produced 3.5 photosynthetic rates (Barigah, 1991) were used times more woody biomass than the slowest for gas exchange measurements and all experi- growing clone Robusta. ments were performed on single attached leaves.
The proportion of biomass allocation to since A increased only slightly between 1 300 and 2 000 μmol m s the values -2 -1 the leaves was nearly the same for all , clones, ranging from 28% of total biomass recorded over this range were considered as for clone Beaupré to 36% in clone Robusta the maximum net photosynthesis by taking (table III). The ratio stem volume mean value of individual photosynthesis rate index/actual wood volume almost constant of several leaves. (0.41) among genotypes, which confirms highest values of photosynthetic The the relevance of using D an index of H as 2 capacity (defined as A at saturating PPFD wood production. CO were observed for clones and 350 vpm ) 2 Beaupré, Raspalje and Robusta (between 25.0 and 27.2 μmol m s Significantly -2 -1 ). Photosynthetic characteristics lower values of A were found in the 2 P tri- Columbia River and Fritzi chocarpa clones, Pauley (17.5 and 19.2 μmol m s respec- -2 -1 , The relationships between CO assimila- 2 tion rate (A) and PPFD did not show a very tively). Differences among clones Beaupré, clear saturation level, even at PPFD values Raspalje and Robusta not significant at were of 2 000 μmol m s (fig 2). However, -2 -1 the p= 0.05 level.
Leaf area characteristics production becomes evident. The P tri- chocarpa clones, Columbia River and Fritzi Pauley, had the lowest photosynthetic rates Clones Raspalje and Beaupré had the high- as well as a low leaf area production (thus, values per tree at the end of the est leaf area low LAI), resulting in a low biomass pro- growing season (table III); the lowest values ductivity (fig 3, table III). observed in Robusta and the values in were For 4 out of the 5 poplar clones the maxi- Columbia River and Fritzi Pauley were inter- mum net photosynthesis was significantly mediate. At mid-August of the first year LAI correlated with above-ground biomass pro- values were 2.75 and 2.95 in clones duction (fig 3). Net photosynthetic rate has Beaupré and Raspalje, respectively, and often been reported not to be correlated with only 0.8 for clone Robusta. Significant dif- yield (Ledig, 1969; Gifford and Evans, 1981); ferences in the leaf area distribution over for these weak correlations the reasons main stem and branches (table III) were to be inadequate or varying nitrogen seems observed for the studied clones. and water supply, lack of standardisation of The results (table III) showed that in all photosynthetic measurements (eg, leaf age), clones more than half of the total leaf area plant density, and number of comparable was produced on the main stem (the branch replications. The high maximum net photo- leaves were not numerous and were smaller synthesis values of the P trichocarpa x P than the main stem ones). However Bari- deltoides clones were of a comparable order gah (1991) observed early in September of magnitude to those previously reported 1989 that the branch leaf area was 3 times for similar poplar hybrids (Isebrands et al, higher than the main stem leaf area in clone 1988; Ceulemans, 1990), while the low pho- Raspalje and 1.4 times in clone Robusta. tosynthetic performance of the 2 P tri- Clone Robusta had the largest number of chocarpa clones (Columbia River and Fritzi leaves on the main stem after the first grow- Pauley) is also in agreement with previous ing season (64 leaves), and clone Fritzi observations (Ceulemans, 1990). Pauley the smallest (48 leaves), but clone Clone Robusta was the only clone that Robusta had the smallest average individual combined a rather high photosynthetic rate leaf area with 66 cm versus 201 cm for 2 2 (comparable to clones Beaupré and Ras- clone Raspalje and 254 cm for clone 2 palje) with a low volume and a low above- Beaupré (table III). ground biomass production (fig 3). This can be mainly explained by its low leaf area pro- duction and low LAI, but also by the fact DISCUSSION that the clone Robusta had a proportionally larger allocation to below-ground biomass. In terms of woody biomass and stem volume For example, at the end of the first growing productivity, the 2 P trichocarpa x P del- season the root/shoot ratio was 1.23 for toides clones Beaupré and Raspalje, were clone Raspalje and 1.79 for Robusta (Bari- clearly superior to the other 3 clones. The gah, 1991).Similar observations (weak cor- higher productivity of these 2 clones can be relation between net photosynthetic rate explained by both their significantly larger and wood biomass productivity, and signifi- leaf area production (thus, higher LAI) and cant differences in root/shoot ratio) have their higher photosynthetic performance. already been made for the same clones Indeed by ranking the different parameters (Impens, 1988) as well as for other poplar reported in table III, the correlation between clones and species (Okafo and Hanover, net photosynthesis, leaf area and biomass 1978; Reighard and Hanover, 1990). The
and Hanover, 1990) and in other Reighard in ecological significance of the difference like Malus, Prunus, Acerand Pinus the root/shoot ratio is still uncertain as there genera (Heim et al, 1979; Kramer, 1986; Loescher is very little knowledge about the specific et al, 1990). Furthermore, Blake and Raita- roles root compounds play in tree survival, nen (1981) and Afocel (1983) considered growth and development (Loescher et al, the first growth cycle for cuttings to be poorly 1990). Cannell et al (1988) found that, com- productive due to greater biomass alloca- pared to willow trees (Salix viminalis), bal- tion to root establishment than to above- sam poplar (P trichocarpa) stored much ground biomass structures. more biomass in their roots than above ground (the above-ground biomass and As the high root/shoot ratio observed in below-ground biomass were respectively clone Robusta was not directly reflected in its 14 t ha and 3 t ha for the willow, and -1 -1 above-ground growth, the abundant reserves 8 t ha and 4 t ha for the poplar). Cannell -1 -1 stored in the root system of Robusta might et al (1988) stated that the abundance of be the support for the high root respiration biomass found in the roots of the balsam rate observed in this clone (Barigah, 1991) and/or for drought adaptation or resistance to poplar was a clonal characteristic, but in diseases. However, these factors were not fact this characteristic is also very common monitored in this study. in the Populus genus (Isebrands, 1982;
clones due to late leaf duration of The best performing clones (Beaupré some and Raspalje) seem to be those which not senescence, guarantee high above-ground only develop the largest leaf area and have growth in poplar. A significant difference in the largest LAI, but also those with the high- leaf area duration between the clones est individual leaf size (fig 4). The high total Robusta (568 m d m and Beaupré (927 2 -2 ) biomass production of clone Raspalje might 2 -2 m d m during their second growing sea- ) thus mainly be due to its large total and indi- son has been reported previously (Nelson vidual leaf area (Ridge et al, 1986). Clone and Isebrands, 1983; Mau and Impens, Raspalje produced only slightly not more 1989; Ceulemans et al, 1993). branches and leaves than clone Beaupré, In breeding and selection programmes but also a much higher leaf area per tree. for fast-growing and highly productive poplar However, individual leaf size of clone clones, attention should be paid to a number Beaupré was slightly larger than that of Ras- of physiological, morphological and envi- palje (table III, fig 4). The larger number of ronmental factors (Magnussen, 1985; Ceule- branches in clone Raspalje seemed to result mans et al, 1987), to soil water regime and in a larger biomass production, although its nutrient availability (Garbaye, 1979; Gar- main stem volume production was slightly baye, 1980; Hinckley et al, 1990) as well as inferior to that of clone Beaupré. It thus to the inheritance of late retention of green seems that poplar clones with a larger indi- leaves in the fall with a measurable photo- vidual leaf area and a high number of leaves synthetic production even after frosts (Nel- on their branches might have considerable et al, 1982). son advantages in developing a high total leaf conclusion, we believe that high net In area per tree early and rapidly during their first growing season, and consequently a photosynthetic rates, in combination with LAI. large leaf area production and duration, led high high biomass production of fast- to the striking feature of the fast-growing P tri- A growing clones Beaupré and Raspalje dur- chocarpa x P deltoides hybrids remains their ing their establishment year. large individual leaf size (fig 4B). Earlier experiments with a variety of these hybrids have already shown that stem volume and ACKNOWLEDGMENTS stem biomass production were more closely related to individual leaf size than to the number of leaves produced per tree (Ridge This research was carried out within the frame- et al, 1986). The correlation between woody work of a EEC research project (contract EN3B- 0114-B/GDF to I Impens, UIA). The authors thank biomass and individual leaf size (fig 4B) B Legay, JY Pontailler, J Liebert and JM Dreuil- might suggest that the inheritance of fast- laux for their help with collection of the experi- growing, large leaves cause the observed mental data. R Ceulemans is a Senior Research increase in stem biomass (and stem vol- Associate of the Belgian National Fund for Scien- ume) of the P trichocarpa x P deltoides tific Research (NFWO, Brussels). hybrids. However, this relationship needs to be examined over a wide range of F F , 12 and backcross material so that the mecha- REFERENCES nisms associated with it can be understood. The positive correlation between net Afocel (1983) Production de Biomasse : Taillis à Courte photosynthesis and first year (above-ground) Rotation. Assoc For Cell, Nangis, France, 214 p biomass production for 4 of the 5 study Agren GI, McMurtrie RE, Parton WJ, Pastor J, Shugart the extended leaf well clones, HH (1991) State-of-the-art of models of produc- as as area
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