The lack of information on the ways seasonal drought modifies the CO2 exchange between Neotropical rainforest ecosystems and the atmosphere and the resulting carbon balance hinders our ability to precisely predict how these ecosystems will respond as global environmental changes force them to face increasingly contrasting conditions in the future. To address this issue, seasonal variations in daily net ecosystem productivity (NEPd) and two main components of this productivity, daily total ecosystem respiration (REd) and daily gross ecosystem productivity (GEPd), were estimated over 2 years at a flux tower site in French Guiana, South America (511605400N, 5215404400W). We compared seasonal variations between wet and dry periods and between dry periods of contrasting levels of intensity (i.e. mild vs. severe) during equivalent 93-day periods. During the wet periods, the ecosystem was almost in balance with the atmosphere (storage of 9.0 gCm_2). Seasonal dry periods, regardless of their severity, are associated with higher incident radiation and lower REd combined with reduced soil respiration associated with low soil water availability. During the mild dry period, as is normally the case in this region, the amount of carbon stored in the ecosystem was 32.7 gCm_2. Severe drought conditions resulted in even lower REd, whereas the photosynthetic activity was only moderately reduced and no change in canopy structure was observed. Thus, the severe dry period was characterized by greater carbon storage (64.6 gCm_2), emphasizing that environmental conditions, such as during a severe drought, modify the CO2 exchange between Neotropical rainforest ecosystems and the atmosphere and potentially the resulting carbon balance

Canopy CO concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO]) throughout the canopy (0.02-38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO] in the upper and middle canopy decreased on average 7-10 μmol mol below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO] did not differ between the wet and dry seasons. In contrast, [CO] below 0.7 m were generally higher during the dry season, resulting in larger [CO] gradients. Supporting this observation, soil CO efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO] immediately above the forest floor of about 50␣μmol mol. Temporal and spatial variations in [CO] were reflected in changes of δC and δO values. Tight relationships were observed between δC and δO of canopy CO during both seasons (r > 0.86). The most depleted δC and δO values were measured immediately above the forest floor (δC = -16.4‰; δO = 39.1‰ SMOW). Gradients in the isotope ratios of CO between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δC, and between 1.0‰ and 3.5‰ for δO. The δC and calculated c /c of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δC within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c /c. Plotting 1/[CO] vs. the corresponding δC ratios resulted in very tight, linear relationships (r = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δC of respired sources were close to the measured δC of soil respired CO and to the δC of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δ, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season.

No abstract

Poplar is the first forest tree genome to be decoded. As an initial step to the comprehensive analysis of poplar proteome, we described reference 2-D-maps for eight tissues/organs of the plant, and the functional characterization of some proteins. A total of 398 proteins were excised from the gels. About 91.2% were identified by nanospray LC-MS/MS, based on comparison with 260,000 Populus sp. ESTs. In comparison, reliable PMFs were obtained for only 51% of the spots by MALDI-TOF-MS, from which 43% (83 spots) positively matched gene models of the Populus trichocarpa genome sequence. Among these 83 spots, 58% matched with the same proteins as identified by LC-MS/MS, 21.7% with unknown function proteins and 19.3% with completely different functions. In the second phase, we studied the effect of drought stress on poplar root and leaf proteomes. The function of up- and down-regulated proteins is discussed with respect to the physiological response of the plants and compared with transcriptomic data. Some important clues regarding the way poplar copes with water deficit were revealed.

Classical quantitative genetics and quantitative trait dissection analysis (QTL) approaches were used in order to investigate the genetic determinism of wood cellulose carbon isotope composition ( δ δ δ δ 13 C, a time integrated estimate of water use efficiency) and of diameter growth and their relationship on adult trees (15 years) of a forest tree species (maritime pine). A half diallel experimental set-up was used to (1) estimate heritabilities for δ δ δ δ 13 C and ring width and (2) to decompose the phenotypic δ δ δ δ 13 C/growth correlation into its genetic and environmental components. Considerable variation was found for δ δ δ δ 13 C (range of over 3‰) and for ring width (range of over 5 mm) and significant heritabilities (narrow sense 0·17/0·19 for δ δ δ δ 13 C and ring width, respectively, 100% additivity). The significant phenotypic correlation between δ δ δ δ 13 C and ring width was not determined by the genetic component, but was attributable to environmental components. Using a genetic linkage map of a full-sib family, four significant and four suggestive QTLs were detected for δ δ δ δ 13 C, the first for δ δ δ δ 13 C in a forest tree species, as far as known to the authors. Two significant and four suggestive QTLs were found for ring width. No co-location of QTLs was found between δ δ δ δ 13 C and growth.

Genetic variation for intrinsic water use efficiency (W i) and related traits was estimated in a full-sib family of Quercus robur L. over 3 years. The genetic linkage map available for this F1 family was used to locate quantitative trait loci (QTL) for W i, as estimated by leaf carbon stable isotope composition (δ 13C) or the ratio of net CO2 assimilation rate (A) to stomatal conductance to water vapour (g w) and related leaf traits. Gas exchange measurements were used to standardize estimates of A and g w and to model the sensitivity of g w to leaf-to-air vapour pressure deficit (sgVPD). δ 13C varied by more than 3‰ among the siblings, which is equivalent to 40% variation of W i. Most of the studied traits exhibited high clonal mean repeatabilities (>50%; proportion of clonal mean variability in global variance). Repeatabilities for δ 13C, leaf mass per area (LMA) and leaf nitrogen content were higher than 70%. For δ 13C, ten QTLs were detected, one of which was detected repeatedly for all 3 years and consistently explained more than 20% of measured variance. Four genomic regions were found in which co-localizing traits linked variation in W i to variations in leaf chlorophyll and nitrogen content, LMA and sgVPD. A positive correlation using clonal means between δ 13C and A/g w, as well as a co-localisation of QTL detected for both traits, can be seen as validation of the theoretical model linking the genetic architecture of these two traits

SVMMARYGrowth, COj assimilation rate {A), leaf conductance {g), transpiration efficiency (W = ratio biomass production/plant water use) and carbon isotope discrimination (A) were assessed in maritime pine (Pinus pinaster Ait.) and pedunculate oak (Quercus robur L.) grown on a sand-peat mixture with three levels of fertilization : FlOO, optimal complete fertilization; F25, 25 "o of the optima! fertilizer supply; FO, no fertilization. Leaf phosphorus (P) and potassium (K) concentrations were affected little by the diminishing nutrient availability. Reduced fertilization decreased plant nitrogen (N) concentration in both species but leaf N concentration was less affected in oak than in pine. In pine W was markedly reduced in response to reduced leaf or whole plant N concentration, which was consistent with the sharp decrease also observed for plant intrinsic water-use efficiency (ratio A/g) both at the instantaneous (gas exchange data) and time-integrated (A/g derived from A measurements) levels. In this species, lowered W in the N deficient conditions was primarily associated with enhanced values of g. The existence of such a stomatal response pattern, confirmed by the increase in plant transpiration between FlOO and F25, has not been reported before. In oak, both A and g were decreased in F25 and FO as compared with FlOO. W was not affected -and instantaneous as well as time-integrated A/g values were oni}' slightly decreased -in relation to decreasing plant N concentration. For FlOO, no difference in W was noticed between pine and oak though the A values were 2-6°^ lower in oak. We speculate that this discrepancy was linked with higher plant-carbon losses through processes like respiration, fine-root mortality or root exudation in oak. The isotopic approach proved useful for assessing the effects of nutritional status on W^ but has to be used with caution w hen comparing different species.Key words: Pinuspinaster (maritime pine), Quercus robur [pedunculate o-dk), nitrogen deficiency, gas exchange and water-use efficiency, carbon isotope discrimination.

To test if some leaf parameters are predictors of productivity in a range of Populus deltoides (Bartr.) Marsh. x P. nigra L. clones, we assessed leaf traits and productivity in 2-month-old rooted cuttings from 31 clones growing in 4-l pots in a greenhouse, under conditions of controlled temperature and optimal irrigation. We evaluated four groups of variables describing (1) productivity (total biomass), (2) leaf growth (total leaf number increment and total leaf area increment rate), (3) leaf structure (specific leaf area and nitrogen and carbon contents) and (4) carbon isotope discrimination (delta), which is negatively correlated with time-integrated water-use efficiency. High-yielding clones did not necessarily display high leaf growth rates, but they displayed a larger total leaf area, lower specific leaf area and lower leaf nitrogen concentration than clones with low productivity. Total leaf area was mainly controlled by maximal individual leaf area and total leaf area increment rate (r = 0.51 and 0.56, respectively). Carbon isotope discrimination did not correlate with total biomass, but it was associated with total number of leaves and total leaf area increment rate (r = 0.39 and 0.45, respectively). Therefore, leaf area and specific leaf area were better indicators of productivity than leaf growth traits. The observed independence of delta from biomass production provides opportunities for selecting poplar clones combining high productivity and high water-use efficiency.