Biophysical and molecular determinants of vulnerability to cavitation in young poplars

Poster + Communication oraleTrees acclimate to changing environment. Water transport occurs under negative pressure from the roots to the leaves in the vascular system of the xylem. During water stress, the level of negative pressure can increase drastically and lead to the rupture of the water column. This cavitation event can be critical for the tree. Vulnerability to cavitation (VC) is a highly variable hydraulic trait that depends on the specie and the growing conditions. The anatomical parameters that drive the phenomena are still unknow. Actually, researches focus on the pits structures that are the hydraulic pathway between vessels. In this work, we investigated the impact of different growth conditions on pit structure of young poplars and the relationship with the VC. Xylem formed under stress conditions was analyzed using a multidisciplinary approach: - Ecophysiology: stomatal conductance, transpiration, leaf water potential, VC, primary and secondary growth were measured. - Wood anatomy was investigated at the tissue and cell wall level by optical and TEM microscopy. This approach was associated with X-ray microtomography observations to characterize the spatial distribution of cavitation. - Molecular analysis: We hypothezed that genes involved in VC may be involved in the biosynthesis of the cell wall and pits. We investigated the local transcriptome of the tissues. Preliminary results showed lower stomatal conductance, transpiration and leaf water potential for stressed plants than control plants. X-ray microtomography observations indicated that wood formed under water stress condition is more resistant to embolism. We will discuss the relationship between growth speed and VC

Modelling the mechanical behaviour of pit membranes in bordered pits with respect to cavitation resistance in angiosperms

Background and Aims Various correlations have been identified between anatomical features of bordered pits in angiosperm xylem and vulnerability to cavitation, suggesting that the mechanical behaviour of the pits may play a role. Theoretical modelling of the membrane behaviour has been undertaken, but it requires input of parameters at the nanoscale level. However, to date, no experimental data have indicated clearly that pit membranes experience strain at high levels during cavitation events. Methods Transmission electron microscopy (TEM) was used in order to quantify the pit micromorphology of four tree species that show contrasting differences in vulnerability to cavitation, namely Sorbus aria, Carpinus betulus, Fagus sylvatica and Populus tremula. This allowed anatomical characters to be included in a mechanical model that was based on the Kirchhoff–Love thin plate theory. A mechanistic model was developed that included the geometric features of the pits that could be measured, with the purpose of evaluating the pit membrane strain that results from a pressure difference being applied across the membrane. This approach allowed an assessment to be made of the impact of the geometry of a pit on its mechanical behaviour, and provided an estimate of the impact on air-seeding resistance. Key Results The TEM observations showed evidence of residual strains on the pit membranes, thus demonstrating that this membrane may experience a large degree of strain during cavitation. The mechanical modelling revealed the interspecific variability of the strains experienced by the pit membrane, which varied according to the pit geometry and the pressure experienced. The modelling output combined with the TEM observations suggests that cavitation occurs after the pit membrane has been deflected against the pit border. Interspecific variability of the strains experienced was correlated with vulnerability to cavitation. Assuming that air-seeding occurs at a given pit membrane strain, the pressure predicted by the model to achieve this mechanical state corresponds to experimental values of cavitation sensitivity (P50). Conclusions The results provide a functional understanding of the importance of pit geometry and pit membrane structure in air-seeding, and thus in vulnerability to cavitation

Comparison of photosynthesis and antioxidant performance of several Citrus and Fortunella species (Rutaceae) under natural chilling stress

TreesInternational audienceCitrus plants originate from southeastern Asia, in a large area with various climates characterized by a broad range of temperatures. Some species have been diversified in temperate climates, others in subtropical climates. Temperature is assumed to be a key factor in citrus species adaptation and diversification of basic cellular functions. In a field experiment, the tolerance of the three fundamental Citrus species C. medica L., C. reticulata Blanco and C. maxima (Burm.) Merr., and Fortunella japonica (Thunb.) Swing. to photooxidative stress caused by seasonal climatic changes was evaluated on adult trees by measuring net photosynthesis (Pnet), stomatal conductance (Gs), maximum photosynthesis (Pmax) and chlorophyll fluorescence (Fv/Fm). In addition, seasonal changes in oxidative status, antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) and antioxidant metabolites (ascorbate and glutathione) were monitored. Mandarin and pummelo appeared to be the most tolerant, showing the lowest down-regulation of photosynthetic parameters, and the lowest accumulation of oxidized compounds associated with efficiency of their antioxidant system. Kumquat showed intermediate behaviour, with a large diminution of photosynthetic parameters and marked accumulation of hydrogen peroxide, whereas the malondialdehyde content remained low, with a strong induction of glutathione synthesis. Finally, citron appeared to be the most sensitive genotype with a marked decrease in photosynthetic performance, the largest accumulation of oxidative parameters, insufficient induction of antioxidant enzymes and down-regulation of ascorbate and glutathione synthesis

Physiological and biochemical response to photooxidative stress of the fundamental citrus species

International audienceDespite the economic importance of citrus, insights on the genetic response to stress are scarce. The aim of the present study was to compare fundamental citrus species for their response to photooxidative stress. The experiment was conducted under orchard conditions on three fundamental citrus species C. medica L., C. reticulata Blanco and C. maxima (Burm.) Merr., and on Fortunella japonica (Thunb.) Swing. We examined their respective net photosynthesis (Pnet), stomatal conductance (Gs) and chlorophyll fluorescence (Fv/Fm) on sun-acclimated leaves and shade-acclimated leaves returned under natural sunlight irradiance. To compare the respective response mechanism, we analyzed changes in oxidative status (hydrogen peroxide (H2O2) and malondialdehyde (MDA)), reactive oxygen species (ROS)-scavenging enzymes (superoxide dismutase (SOD), catalase, ascorbate peroxidase), recycling enzymes (monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) and antioxidant metabolites (ascorbate and glutathione). Kumquat and pummelo exposed lower down-regulation and full recovery of photosynthetic parameters, lower accumulation of oxidized compounds associated with greater production of reduced glutathione (Gsh) and enhanced activity of the three ROS scavenging enzymes, especially SOD. Citron and mandarin showed a marked decrease and incomplete recovery in photosynthetic performance, mainly in Pnet and Fv/Fm, larger accumulation of oxidative parameters, slighter induction of antioxidant enzymes and down-regulation of reduced ascorbate (Asa) and Gsh synthesis. These results suggest that kumquat and pummelo have a greater tolerance to photooxidative stress than citron and mandarin

Could thioredoxin h be involved in early response to gravitropic stimulation of poplar stems?

The perception of gravity is essential for plant development. Trees constantly develop specialized woody tissues, termed « reaction wood » to correct inclined branch and stem growth in order to adopt an optimal position. Despite the economical impact of reaction wood occurrence and itsimportance from a developmental point of view, the perception and response to the gravitational stimulus have not been extensively studied in woody species in which primary and secondary growth occur. Using complementary approaches (proteomics, qRT-PCR, immunolocalization), we have compared straight polar stems to stems that were inclined at 35° from the vertical axis for periods of time varying from 10 min to 6 hours depending on the experiments. The proteomics approach revealed that thirty six percent of the identified proteins that were differentially expressed after gravistimulation were established as potential Thioredoxin targets. qRT-PCR indicated an early induction of Thioredoxin h expression following gravistimulation. In situ immunolocalization indicated that Thioredoxin h protein co-localized with the amyloplasts located in the endodermalcells which may be specialized in gravity perception. These investigations suggest the involvement of Thioredoxin h in the first events of signal transduction in inclined poplar stems, leading to reaction wood formation

Nutrient Deficiency Tolerance in Citrus Is Dependent on Genotype or Ploidy Level

Plants require essential minerals for their growth and development that are mainly acquired from soil by their roots. Nutrient deficiency is an environmental stress that can seriously affect fruit production and quality. In citrus crops, rootstock/scion combinations are frequently employed to enhance tolerance to various abiotic stresses. These tolerances can be improved in doubled diploid genotypes. The aim of this work was to compare the impact of nutrient deficiency on the physiological and biochemical response of diploid (2x) and doubled diploid (4x) citrus seedlings: Volkamer lemon, Trifoliate orange × Cleopatra mandarin hybrid, Carrizo citrange, Citrumelo 4475. Flhorag1 (Poncirus trifoliata + and willow leaf mandarin), an allotetraploid somatic hybrid, was also included in this study. Our results showed that depending on the genotype, macronutrient and micronutrient deficiency affected certain physiological traits and oxidative metabolism differently. Tetraploid genotypes, mainly Flhorag1 and Citrumelo 4475, appeared resistant compared to the other genotypes as indicated by the lesser decrease in photosynthetic parameters (Pnet, Fv/Fm, and Gs) and the lower accumulation of oxidative markers (MDA and H2O2) in roots and leaves, especially after long-term nutrient deficiency. Their higher tolerance to nutrient deficiency could be explained by better activation of their antioxidant system. For the other genotypes, tetraploidization did not induce greater tolerance to nutrient deficiency

La vie des arbres

https://www.lamontagne.fr/sermentizon-63120/actualites/la-nature-sexpose-au-chateau-daulteribe_14389530/La nature s'expose au château d'Aulteribe. À l’occasion du vernissage de l’exposition de photographies « A table », Stéphane Herbette, professeur à l’Université Clermont Auvergne, botaniste et spécialiste de la résistance à la sécheresse des arbres, a animé une conférence sur la vie de l’arbre, vendredi 13 octobre, au château d’Aulteribe, en présence de plus de 50 personnes.Appuyé par de nombreuses photos, schémas et résultats d’études scientifiques, Stéphane Herblette a expliqué comment cet être se développe tout au long de la vie, comment il se nourrit et comment il fait face aux contraintes de l’environnement. Pour terminer, il a détaillé l’importance de l’arbre pour les autres êtres vivants, en particulier, les chauves-souris, les oiseaux qui y trouvent refuge et tous les insectes xylophages qui se nourrissent de bois mort.Cette conférence marquait le début d’une exposition-photo d’animaux sur le thème « A table » par l’association Culture et patrimoine de Sermentizon et venait compléter ce que les photographes amateurs ne pouvaient pas montrer comme l’alimentation des végétaux

Impact de la sécheresse sur les arbres

Conférence grand public donnée à l'occasion de la fête de la nature organisée par la ville de Courpière 6312

Vulnérabilité du xylème à la cavitation : de l’écophysiologie aux bases génétiques

Diplôme : HD