Mesophyll diffusion conductance to CO(2) is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g(m), and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance. Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.

Summary Both photosynthesis (A) and stomatal conductance (g s) respond to changing irradiance, yet stomatal responses are an order of magnitude slower than photosynthesis, resulting in noncoordination between A and g s in dynamic light environments.Infrared gas exchange analysis was used to examine the temporal responses and coordination of A and g s to a step increase and decrease in light in a range of different species, and the impact on intrinsic water use efficiency was evaluated.The temporal responses revealed a large range of strategies to save water or maximize photosynthesis in the different species used in this study but also displayed an uncoupling of A and g s in most of the species. The shape of the guard cells influenced the rapidity of response and the overall g s values achieved, with different impacts on A and W i. The rapidity of g s in dumbbell‐shaped guard cells could be attributed to size, whilst in elliptical‐shaped guard cells features other than anatomy were more important for kinetics.Our findings suggest significant variation in the rapidity of stomatal responses amongst species, providing a novel target for improving photosynthesis and water use.

A protocol is described to isolate small quantities of highly purified cellulose for isotopic analysis of 10–100 mg samples of secondary (Pinus sylvestris L.) and primary (Rubus idaeus L.) plant cell wall material. Elemental analysis of 350 cellulose samples isolated from pine wood samples estimated the relative carbon content to be ca. 43.7% ± 1.2%. This value indicates that the cellulose quality is high and that the protocol is highly reproducible. High‐performance anion exchange chromatography with pulsed amperometric detection of hydrolysis products quantified the purity of the cellulose as ca. 99% for wood cellulose and primary cell wall cellulose. DRIFT spectroscopy corroborated this purity and found no evidence of cellulose degradation. Carbon isotopic composition of the purified cellulose using mass spectrometry was measured with an accuracy of 0.11‰ (standard deviation). The method is rapid (56 samples may be routinely processed within 8 h) and requires only standard laboratory equipment and chemicals. Copyright © 2000 John Wiley & Sons, Ltd.

Summary• The stomatal control of transpiration is one of the major strategies by which plants cope with water stress. Here, we investigated the genetic architecture of the rootstock control of scion transpiration-related traits over a period of 3 yr.• The rootstocks studied were full sibs from a controlled interspecific cross (Vitis vinifera cv. Cabernet Sauvignon · Vitis riparia cv. Gloire de Montpellier), onto which we grafted a single scion genotype. After 10 d without stress, the water supply was progressively limited over a period of 10 d, and a stable water deficit was then applied for 15 d. Transpiration rate was estimated daily and a mathematical curve was fitted to its response to water deficit intensity. We also determined d13 C values in leaves, transpiration efficiency and water extraction capacity. These traits were then analysed in a multienvironment (year and water status) quantitative trait locus (QTL) analysis.• Quantitative trait loci, independent of year and water status, were detected for each trait. One genomic region was specifically implicated in the acclimation of scion transpiration induced by the rootstock. The QTLs identified colocalized with genes involved in water deficit responses, such as those relating to ABA and hydraulic regulation.• Scion transpiration rate and its acclimation to water deficit are thus controlled genetically by the rootstock, through different genetic architectures.

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

BackgroundThe Fagaceae family comprises about 1,000 woody species worldwide. About half belong to the Quercus family. These oaks are often a source of raw material for biomass wood and fiber. Pedunculate and sessile oaks, are among the most important deciduous forest tree species in Europe. Despite their ecological and economical importance, very few genomic resources have yet been generated for these species. Here, we describe the development of an EST catalogue that will support ecosystem genomics studies, where geneticists, ecophysiologists, molecular biologists and ecologists join their efforts for understanding, monitoring and predicting functional genetic diversity.ResultsWe generated 145,827 sequence reads from 20 cDNA libraries using the Sanger method. Unexploitable chromatograms and quality checking lead us to eliminate 19,941 sequences. Finally a total of 125,925 ESTs were retained from 111,361 cDNA clones. Pyrosequencing was also conducted for 14 libraries, generating 1,948,579 reads, from which 370,566 sequences (19.0%) were eliminated, resulting in 1,578,192 sequences. Following clustering and assembly using TGICL pipeline, 1,704,117 EST sequences collapsed into 69,154 tentative contigs and 153,517 singletons, providing 222,671 non-redundant sequences (including alternative transcripts). We also assembled the sequences using MIRA and PartiGene software and compared the three unigene sets. Gene ontology annotation was then assigned to 29,303 unigene elements. Blast search against the SWISS-PROT database revealed putative homologs for 32,810 (14.7%) unigene elements, but more extensive search with Pfam, Refseq_protein, Refseq_RNA and eight gene indices revealed homology for 67.4% of them. The EST catalogue was examined for putative homologs of candidate genes involved in bud phenology, cuticle formation, phenylpropanoids biosynthesis and cell wall formation. Our results suggest a good coverage of genes involved in these traits. Comparative orthologous sequences (COS) with other plant gene models were identified and allow to unravel the oak paleo-history. Simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs) were searched, resulting in 52,834 SSRs and 36,411 SNPs. All of these are available through the Oak Contig Browser http://genotoul-contigbrowser.toulouse.inra.fr:9092/Quercus_robur/index.html.ConclusionsThis genomic resource provides a unique tool to discover genes of interest, study the oak transcriptome, and develop new markers to investigate functional diversity in natural populations.

A comparative genetic and QTL mapping was performed between Quercus robur L. and Castanea sativa Mill., two major forest tree species belonging to the Fagaceae family. Oak EST-derived markers (STSs) were used to align the 12 linkage groups of the two species. Fifty-one and 45 STSs were mapped in oak and chestnut, respectively. These STSs, added to SSR markers previously mapped in both species, provided a total number of 55 orthologous molecular markers for comparative mapping within the Fagaceae family. Homeologous genomic regions identified between oak and chestnut allowed us to compare QTL positions for three important adaptive traits. Colocation of the QTL controlling the timing of bud burst was significant between the two species. However, conservation of QTL for height growth was not supported by statistical tests. No QTL for carbon isotope discrimination was conserved between the two species. Putative candidate genes for bud burst can be identified on the basis of colocations between EST-derived markers and QTL.T HE genetic basis and evolution of adaptive traits that evolve in response to selection are still largely unknown. Because of the prominent neo-Darwinian view that pointed out the major role of mutations with small effects (infinitesimal model) (Fisher 1930), the study of the genetic basis of adaptation has received little attention until recently (Orr and Coyne 1992). At the same time, Barton and Turelli (1989) reviewed theories and experimental results on evolutionary quantitative genetics. Most of the quantitative traits can evolve in response to selection because the additive variance represents a significant part of their phenotypic variance. Nevertheless, the number of loci involved, the magnitude of their effects, the type of gene action (additivity, dominance, epistasis, and pleiotropy), and the existence of genotypeby-environment interaction effect remain unknown for many traits of adaptive significance. In particular, the number of their underlying loci and the magnitude of the allelic effects are key factors of the evolution of adaptive traits. Orr and Coyne (1992) showed that Fisher's model was incomplete and that mutations with large effects were sometimes involved in adaptation. Indeed, on the basis of Orr's (1998) recent theoretical work, adaptation seems to involve many loci of small and moderate effect but also a few loci of large effect, giving rise to an L-shaped distribution of factors fixed during adaptive evolution. Moreover, from an evolutionary point of view, in a complex organism, adaptation would occur mainly with mutations of intermediate effects that permit it to achieve an appropriate tradeoff between an acceptable probability of fixation and an acceptable probability to be favorable (Orr 2000).Quantitative trait locus (QTL) studies were expected to provide new insights into fundamental questions regarding the genetic basis of quantitative traits and adaptation (Mitchell-Olds 1995). Despite some biases in QTL analysis, such as the underestimation of the number ...