Effet de l’interaction lumiere-salinite sur l’activite du photosysteme ii des feuilles excisees de maïs

La diminution de la croissance vegetative observée chez nombreuses plantes, soumises au stress salin et exposées à des conditions naturelles d’ensoleillement, est souvent associée à une baisse de leur activité photosynthétique. Cependant les mécanismes de l’inhibition photosynthétique sont encore peu étudiés. Le photosystème II (PS II) est considéré un des facteurs clé dans la réponse de la photosynthèse des feuilles aux stress environnementaux. L’association de la lumière et du stress salin parait avoir des effets synergétiques sur l’activité photochimique du PSII conduisant, ainsi à la photoinhibition. L’objectif de cette etude est de diagnostiquer l’effet de l’interaction lumière- salinité sur l’activité photochimique du photosystème II au cours de la photosynthèse. Le materiel vegetal est constitué de deux variétés de maïs (Zea mays L.) fourrager: Aristo et Arper. Des feuilles détachées de plantes cultivées sur milieu témoin (eau distilée), sont incubées pendant 6 heures dans des solutions salines à différentes concentrations (0, 100, 200 et 300 mM NaCl), soit en absence ou en présence de lumière (1000 μmol m-2 s-1). Puis, leurs teneurs en sodium ont été déterminées. Pour vérifiers’il y’aura récupération de leur activité photochimique, d’autres feuilles qui ont été mises à absorber du NaCl dans une solution de 300 mM à la lumière pendant 4 heures, sont transferees dans de l’eau distillée en obscurité ou en lumière. Les resultants montrent une stabilité du rendement quantique maximal (Fv/Fm) pour les feuilles mises à absorber du NaCl à l’obscurité.Par contre l’éclairement associé avec la salinité engender une photoinhibition qui se manifeste par une diminution du rendement quantique maximal du PSII. Cette photoinhibition, due à une accumulation excessive des ions Na+dans les tissus foliaires, est réversible. En effet, en absence d’un seul des facteurs de l’association lumière-salinité le PSII récupère son activité photochimique.Mots Clés: fluorescence chlorophyllienne, feuillesexcisées, stress lumineux, chlorure de sodium, Zea maysEnglish AbstractThe decline in growth observed in many plants, subjected to salt stress and exposed to sunlight conditions, is often associated with a decrease in their photosynthetic activity. No clear mechanisms of the inhibited photosynthesis have emerged since photosystem II (PS II) is considered to play a key role in the response of leaf photosynthesis to environmental  perturbations. The combination of light and salt stress appears to have synergistic effects on the photochemical activity of PSII driving, and to photoinhibition. The objective of this study was to evaluate the reversible effect of salinity and light interaction on maximal quantum efficiency of photosystem II (Fv/Fm.). In this experiment, detached leaves of two forage maize (Zea mays L.) varieties, Aristo and Arper were placed during 6 hours in solutions of different concentrations of NaCl (0, 100, 200 and 300 mM) and subjected to light (1000 μmol m-2 s-1) or obscurity. Then, their contents of sodium were determined. In order to verify the photo-inhibition reversibility, other leaves which were incubated in a solution of 300 Mm NaCl, during 4 hours were transferred in distilled water and also subjected to light or to obscurity. Results indicate that leaves which had been put to absorb NaCl in obscurity showed no change in maximal efficiency of PSII (Fv/Fm). Nevertheless, light treatment associated with salinity generates a photo-inhibition of PSII manifested by a significant decrease in maximal efficiency of PSII. This photo-inhibition, due to an excessive accumulation of sodium in leaves, is reversible. It is quite sufficient to eliminate only one factor of the association light-salinity for the PSII activity resume.Keywords: Light stress, sodium chloride, Zea may

First-principles density functional theory study of strained wurtzite InP and InAs

International audienceWe report on semilocal and hybrid density functional theory study of strained wurtzite crystals of InAs and InP. The crystal-field splitting has a large and nonlinear dependence on strain for both crystals. Moreover, the study of the electronic deformation potentials reveals that the well-known quasi-cubic approximation fails to reproduce the electronic features of the non-ideal c/a ratio. This theoretical study is of crucial importance for the simulation of self-assembled InAs/InP nanowires

Selective scattering between Floquet-Bloch and Volkov states in a topological insulator

The coherent optical manipulation of solids is emerging as a promising way to engineer novel quantum states of matter. The strong time periodic potential of intense laser light can be used to generate hybrid photon-electron states. Interaction of light with Bloch states leads to Floquet-Bloch states which are essential in realizing new photo-induced quantum phases. Similarly, dressing of free electron states near the surface of a solid generates Volkov states which are used to study non-linear optics in atoms and semiconductors. The interaction of these two dynamic states with each other remains an open experimental problem. Here we use Time and Angle Resolved Photoemission Spectroscopy (Tr-ARPES) to selectively study the transition between these two states on the surface of the topological insulator Bi2Se3. We find that the coupling between the two strongly depends on the electron momentum, providing a route to enhance or inhibit it. Moreover, by controlling the light polarization we can negate Volkov states in order to generate pure Floquet-Bloch states. This work establishes a systematic path for the coherent manipulation of solids via light-matter interaction.Comment: 21 pages, 6 figures, final version to appear in Nature Physic

Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry

The advent of Dirac materials has made it possible to realize two dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity - the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.Comment: Nature Communications, in press (12 pages, 6 figures

A new critical curve for the Lane-Emden system

We study stable positive radially symmetric solutions for the Lane-Emden system $-\Delta u=v^p$ in $\R^N$, $-\Delta v=u^q$ in $\R^N$, where $p,q\geq 1$. We obtain a new critical curve that optimally describes the existence of such solutions.Comment: 13 pages, 1 figur

Chemical composition and antibacterial activity of essential oils from the medicinal plant Mentha cervina L. grown in Portugal

Mentha cervina is a medicinal plant traditionally used in Portugal in folk medicine, in different gastric disorders and inflammations of the respiratory tract. In order to validate those traditional uses, M. cervina essential oils (EOs) were characterized by GC and GC–MS and their antimicrobial activity was tested against 23 bacterial strains (including multiresistant strains). The EOs were dominated by the monoterpenes pulegone (52–75%), isomenthone (8–24%), limonene (4–6%), and menthone (1–2%). The antibacterial activity of these EOs was compared to that of the main components standards. The most effective antibacterial activity was expressed by the EOs against the Gram-negative bacteria, Escherichia coli and Acinetobacter baumanni, with MIC values of 1 mg/ml. The EOs complex mixtures were more active than the individual aromatic components supporting the hypothesis that the EOs antibacterial activity is a function of the synergistic effect of their different aromatic components. These results show the potential role of M. cervina EOs as antibacterial agents and validate the traditional use of this plant

Altermagnetic lifting of Kramers spin degeneracy

Lifted Kramers spin-degeneracy has been among the central topics of condensed-matter physics since the dawn of the band theory of solids. It underpins established practical applications as well as current frontier research, ranging from magnetic-memory technology to topological quantum matter. Traditionally, lifted Kramers spin-degeneracy has been considered to originate from two possible internal symmetry-breaking mechanisms. The first one refers to time-reversal symmetry breaking by magnetization of ferromagnets, and tends to be strong due to the non-relativistic exchange-coupling origin. The second mechanism applies to crystals with broken inversion symmetry, and tends to be comparatively weaker as it originates from the relativistic spin-orbit coupling. A recent theory work based on spin-symmetry classification has identified an unconventional magnetic phase, dubbed altermagnetic, that allows for lifting the Kramers spin degeneracy without net magnetization and inversion-symmetry breaking. Here we provide the confirmation using photoemission spectroscopy and ab initio calculations. We identify two distinct unconventional mechanisms of lifted Kramers spin degeneracy generated by the altermagnetic phase of centrosymmetric MnTe with vanishing net magnetization. Our observation of the altermagnetic lifting of the Kramers spin degeneracy can have broad consequences in magnetism. It motivates exploration and exploitation of the unconventional nature of this magnetic phase in an extended family of materials, ranging from insulators and semiconductors to metals and superconductors, that have been either identified recently or perceived for many decades as conventional antiferromagnets

Comparison of Ion Balance and Nitrogen Metabolism in Old and Young Leaves of Alkali-Stressed Rice Plants

BACKGROUND: Alkali stress is an important agricultural contaminant and has complex effects on plant metabolism. The aim of this study was to investigate whether the alkali stress has different effects on the growth, ion balance, and nitrogen metabolism in old and young leaves of rice plants, and to compare functions of both organs in alkali tolerance. METHODOLOGY/PRINCIPAL FINDINGS: The results showed that alkali stress only produced a small effect on the growth of young leaves, whereas strongly damaged old leaves. Rice protected young leaves from ion harm via the large accumulation of Na(+) and Cl(-) in old leaves. The up-regulation of OsHKT1;1, OsAKT1, OsHAK1, OsHAK7, OsHAK10 and OsHAK16 may contribute to the larger accumulation of Na(+) in old leaves under alkali stress. Alkali stress mightily reduced the NO(3)(-) contents in both organs. As old leaf cells have larger vacuole, under alkali stress these scarce NO(3)(-) was principally stored in old leaves. Accordingly, the expression of OsNRT1;1 and OsNRT1;2 in old leaves was up-regulated by alkali stress, revealing that the two genes might contribute to the accumulation of NO(3)(-) in old leaves. NO(3)(-) deficiency in young leaves under alkali stress might induce the reduction in OsNR1 expression and the subsequent lacking of NH(4)(+), which might be main reason for the larger down-regulation of OsFd-GOGAT and OsGS2 in young leaves. CONCLUSIONS/SIGNIFICANCE: Our results strongly indicated that, during adaptation of rice to alkali stress, young and old leaves have distinct mechanisms of ion balance and nitrogen metabolism regulation. We propose that the comparative studies of young and old tissues may be important for abiotic stress tolerance research