Senescence Of Barley Research Articles

N6-methyladenosine (m6A) RNA modification as a metabolic switch between plant cell survival and death in leaf senescence.

Crop losses caused by climate change and various (a)biotic stressors negatively affect agriculture and crop production. Therefore, it is vital to develop a proper understanding of the complex response(s) to (a)biotic stresses and delineate them for each crop plant as a means to enable translational research. In plants, the improvement of crop quality by m6A editing is believed to be a promising strategy. As a reaction to environmental changes, m6A modification showed a high degree of sensitivity and complexity. We investigated differences in gene medleys between dark-induced leaf senescence (DILS) and developmental leaf senescence in barley, including inter alia RNA modifications active in DILS. The identified upregulated genes in DILS include RNA methyltransferases of different RNA types, embracing enzymes modifying mRNA, tRNA, and rRNA. We have defined a decisive moment in the DILS model which determines the point of no return, but the mechanism of its control is yet to be uncovered. This indicates the possibility of an unknown additional switch between cell survival and cell death. Discoveries of m6A RNA modification changes in certain RNA species in different stages of leaf senescence may uncover the role of such modifications in metabolic reprogramming. Nonetheless, there is no such data about the process of leaf senescence in plants. In this scope, the prospect of finding connections between the process of senescence and m6A modification of RNA in plants seems to be compelling.

Near-infrared in vivo measurements of photosystem I and its lumenal electron donors with a recently developed spectrophotometer.

In photosynthesis research, non-invasive in vivo spectroscopic analyses have been used as a practical tool for studying photosynthetic electron transport. Klas-NIR spectrophotometer has been recently developed by Klughammer and Schreiber (Photosynth Res 128:195-214, 2016) for in vivo measurements of redox changes of P700, plastocyanin (Pcy) and ferredoxin (Fd). Here we show examples using the Klas-NIR spectrophotometer for the evaluation of the redox states and quantities of these components in plant leaves and cyanobacterial suspensions. The redox poise under light of the electron transport components is different in leaves from higher plants compared with cyanobacteria. During a short illumination with an actinic light, P700, Pcy, and Fd are kept reduced in barley leaves but are oxidized in cyanobacteria. During far-red light illumination, P700 and Pcy are mostly oxidized in the leaves but are partially kept reduced in cyanobacteria. In the cyanobacterium, Thermosynechococcus elongatus, which has no Pcy but uses cytochrome c6 (cyt c6) as the electron donor to photosystem I, a cyt c6 signal was detected in vivo. To show the potential of Klas-NIR spectrophotometer for studying different developmental stages of a leaf, we performed measurements on fully mature and early senescing barley leaves. Pcy content in leaves decreased during senescence at an early stage. The Pcy loss was quantitatively analyzed using Klas-NIR spectrophotometer, giving absolute ratios of Pcy to PSI of 2.5 and 1.6 in younger and older leaves, respectively. For quantification of the signals in vivo, in vitro data (Sétif et al. in Photosynth Res142:307-319, 2019) obtained with Klas-NIR spectrophotometer were used.

Characterization of S40-like proteins and their roles in response to environmental cues and leaf senescence in rice

BackgroundSenescence affects the quality and yield of plants by regulating different traits of plants. A few members of S40 gene family, the barley HvS40 and the Arabidopsis AtS40–3, have been shown to play a role in leaf senescence in Barley and Arabidopsis. Although we previously reported that S40 family exist in most of plants, up to now, no more function of S40 members in plant has been demonstrated. The aim of this study was to provide the senescence related information of S40 gene family in rice as rice is a major crop that feeds about half of the human population in the world.ResultsA total of 16 OsS40 genes were identified from the genome database of Oryza sativa L. japonica by bioinformatics analysis. Phylogenetic analysis reveals that the 16 OsS40 proteins are classified into five groups, and 4 of the 16 members belong to group I to which also the HvS40 and AtS40–3 is assigned. S40 genes of rice show high structural similarities, as 13 out of the 16 genes have no intron and the other 3 genes have only 1 or 2 introns. The expression patterns of OsS40 genes were analyzed during natural as well as stress-induced leaf senescence in correspondence with senescence marker genes. We found that 6 of them displayed differential but clearly up-regulated transcript profiles under diverse situations of senescence, including darkness, nitrogen deficiency, hormone treatments as well as pathogen infection. Furthermore, three OsS40 proteins were identified as nuclear located proteins by transient protoplast transformation assay.ConclusionsTaking all findings together, we concluded that OsS40–1, OsS40–2, OsS40–12 and OsS40–14 genes have potential regulatory function of crosstalk among abiotic, biotic and developmental senescence in rice. Our results provide valuable baseline for functional validation studies of the rice S40 genes in rice leaf senescence.

Silicon transcriptionally regulates sulfur and ABA metabolism and delays leaf senescence in barley under combined sulfur deficiency and osmotic stress

Several studies have revealed that both sulfur (S) and silicon (Si) nutrition play a pivotal role in alleviating drought stress tolerance in plants. Additionally, Si application has also been reported to mitigate plant nutrient deficiency. However, to date there is no report that evaluates the performance and relevance of Si nutrition under S deficiency. In the present study, we explored the role of supplemental Si on S metabolism under combined S deficiency and osmotic stress. Barley plants were simultaneously subjected to polyethylene glycol (PEG)-induced osmotic stress under low or high S supply and Si nutrition. The results showed that Si supply regulated S and ABA metabolism at the transcriptional level under combined S deficiency and osmotic stress. The induction of the genes involved in S metabolism further contributed to the balance of redox potential by lowering the GSSG/GSH ratio and reducing H2O2 level in shoots. The transcriptional regulation of the genes involved in ABA biosynthesis and degradation pathways by Si reduced the level of shoot ABA under combined stress. Additionally, Si resulted in delayed leaf senescence under combined stresses by increasing chlorophyll levels, suppressing the expression of barley senescence gene HvS40, and reducing IAA levels. Therefore, shoots showed enhanced tolerance to osmotic stress under S deficiency as supported by higher sucrose concentrations, higher relative water content and higher shoot biomass. In roots under combined stress, Si induced the expression of the sulfur transporter HvST1;1 which resulted in a higher uptake of SO42− and NO3−. The higher root NO3- increased both glutamine and proline concentration. The higher sucrose in roots also increased root biomass, reflecting the tolerance of roots to osmotic stress. Our investigation reveals an important role of Si in transcriptionally regulating S and ABA metabolism under collective stress conditions manifested by S deficiency and osmotic stress.

Phloem transport of assimilates in relation to flowering time and senescence in barley grown with different availabilities of nitrogen and phosphorus

ABSTRACTUnderstanding the way in which N and P availability affects the transport of sugar and amino acids is crucial to improve grain quality and yield. Thus, in the present study, two greenhouse and field experiments were conducted with barley plants grown with different N and P availabilities to assess the dynamics of the phloem transport of assimilates in relation to the beginning of flowering and senescence. The phloem transport of assimilates decreased before the beginning of protein degradation in all treatments, but the onset of flowering and senescence varied according to the N and P availability, as evidenced by the concentrations of proteins, amino acids, and sugar and the gene expression of senescence-related proteases and all glutamine synthetase isoforms. In N-deficient plants, the phloem transport decreased before flowering, but only when P was not limiting; in N- and P-sufficient plants it decreased at flowering; and in P-deficient plants it decreased after flowering. Therefore, only N-deficient but P-sufficient plants have a post-anthesis period with high export rate of assimilates. This alteration of phenology in relation to phloem leads to important consequences in assimilate utilization, as shown by the higher yield and N content of the former compared to P-deficient plants.

Metabolite Profiling for Leaf Senescence in Barley Reveals Decreases in Amino Acids and Glycolysis Intermediates

Leaf senescence is a long developmental phase important for plant performance and nutrient management. Cell constituents are recycled in old leaves to provide nutrients that are redistributed to the sink organs. Up to now, metabolomic changes during leaf senescence have been mainly studied in Arabidopsis (Arabidopsis thaliana L.). The metabolite profiling conducted in barley (Hordeum vulgare L.) during primary leaf senescence under two nitrate regimes and in flag leaf shows that amino acids, hexose, sucrose and glycolysis intermediates decrease during senescence, while minor carbohydrates accumulate. Tricarboxylic acid (TCA) compounds changed with senescence only in primary leaves. The senescence-related metabolite changes in the flag leaf were globally similar to those observed in primary leaves. The effect of senescence on the metabolite changes of barley leaves was similar to that previously described in Arabidopsis except for sugars and glycolysis compounds. This suggests a different role of sugars in the control of leaf senescence in Arabidopsis and in barley.

A Potential Role of Flag Leaf Potassium in Conferring Tolerance to Drought-Induced Leaf Senescence in Barley.

Terminal drought stress decreases crop yields by inducing abscisic acid (ABA) and premature leaf senescence. As potassium (K) is known to interfere with ABA homeostasis we addressed the question whether there is genetic variability regarding the role of K nutrition in ABA homeostasis and drought tolerance. To compare their response to drought stress, two barley lines contrasting in drought-induced leaf senescence were grown in a pot experiment under high and low K supply for the analysis of flag leaves from the same developmental stage. Relative to the drought-sensitive line LPR, the line HPR retained more K in its flag leaves under low K supply and showed delayed flag leaf senescence under terminal drought stress. High K retention was further associated with a higher leaf water status, a higher concentration of starch and other primary carbon metabolites. With regard to ABA homeostasis, HPR accumulated less ABA but higher levels of the ABA degradation products phaseic acid (PA) and dehydro-PA. Under K deficiency this went along with higher transcript levels of ABA8′-HYDROXYLASE, encoding a key enzyme in ABA degradation. The present study provides evidence for a positive impact of the K nutritional status on ABA homeostasis and carbohydrate metabolism under drought stress. We conclude that genotypes with a high K nutritional status in the flag leaf show superior drought tolerance by promoting ABA degradation but attenuating starch degradation which delays flag leaf senescence. Flag leaf K levels may thus represent a useful trait for the selection of drought-tolerant barley cultivars.

Meeting Report: Plant Science Student Conference (PSSC) 2015--Young researchers in green biotechnology.

Meeting Report: Plant Science Student Conference (PSSC) 2015--Young researchers in green biotechnology.

Changes in production of reactive oxygen species in illuminated thylakoids isolated during development and senescence of barley

This paper presents a detailed analysis of thylakoids isolated from secondary barley leaves harvested 18, 22, 25, 29, 32, 35 and 39 days after sowing (DAS). Goal of the analysis was to investigate the production of different reactive oxygen species (ROS) during development and senescence of barley. Generation of superoxide anion (O2−) and hydrogen peroxide (H2O2) increases during development of barley reaching the highest value right after the onset of senescence (between 25 and 29 DAS), thereafter the levels of both ROS start to decrease until 35 DAS when production of H2O2 increases again. In comparison with O2− and H2O2, generation of singlet oxygen (1O2) showed continuous production of low amounts thought the duration of experiment. Oxidative damage to the thylakoid membrane was assessed by measuring lipid peroxidation. Results showed gradual increase in lipid peroxidation with progress of plant development with highest increase occurring at the late stages of senescence. A possible factor contributing to the elevation in the production of ROS could be an increase in membrane fluidity observed in our previous study. Fluidization of the membrane, allows for better penetration of oxygen inside the membrane, which can lead to an increase in the production of ROS. Indeed, the production of ROS started to increase together with observed fluidization of the membrane from 22 to 29 DAS. Thereafter, production of ROS started to decline till 35th DAS. On the last day of the measurement, chl is at 25% of its initial value, lipid peroxidation reaches the highest value and H2O2 increases again.

Identification of genomic regions involved in tolerance to drought stress and drought stress induced leaf senescence in juvenile barley.

BackgroundPremature leaf senescence induced by external stress conditions, e.g. drought stress, is a main factor for yield losses in barley. Research in drought stress tolerance has become more important as due to climate change the number of drought periods will increase and tolerance to drought stress has become a goal of high interest in barley breeding. Therefore, the aim is to identify quantitative trait loci (QTL) involved in drought stress induced leaf senescence and drought stress tolerance in early developmental stages of barley (Hordeum vulgare L.) by applying genome wide association studies (GWAS) on a set of 156 winter barley genotypes.ResultsAfter a four weeks stress period (BBCH 33) leaf colour as an indicator of leaf senescence, electron transport rate at photosystem II, content of free proline, content of soluble sugars, osmolality and the aboveground biomass indicative for drought stress response were determined in the control and stress variant in greenhouse pot experiments. Significant phenotypic variation was observed for all traits analysed. Heritabilities ranged between 0.27 for osmolality and 0.61 for leaf colour in stress treatment and significant effects of genotype, treatment and genotype x treatment were estimated for most traits analysed. Based on these phenotypic data and 3,212 polymorphic single nucleotide polymorphisms (SNP) with a minor allele frequency >5 % derived from the Illumina 9 k iSelect SNP Chip, 181 QTL were detected for all traits analysed. Major QTLs for drought stress and leaf senescence were located on chromosome 5H and 2H. BlastX search for associated marker sequences revealed that respective SNPs are in some cases located in proteins related to drought stress or leaf senescence, e.g. nucleotide pyrophosphatase (AVP1) or serine/ threonin protein kinase (SAPK9).ConclusionsGWAS resulted in the identification of many QTLs involved in drought stress and leaf senescence of which two major QTLs for drought stress and leaf senescence were located on chromosome 5H and 2H. Results may be the basis to incorporate breeding for tolerance to drought stress or leaf senescence in barley breeding via marker based selection procedures.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0524-3) contains supplementary material, which is available to authorized users.

Selection of reference genes for qPCR- and ddPCR-based analyses of gene expression in Senescing Barley leaves.

Leaf senescence is a tightly regulated developmental or stress-induced process. It is accompanied by dramatic changes in cell metabolism and structure, eventually leading to the disintegration of chloroplasts, the breakdown of leaf proteins, internucleosomal fragmentation of nuclear DNA and ultimately cell death. In light of the global and intense reorganization of the senescing leaf transcriptome, measuring time-course gene expression patterns in this model is challenging due to the evident problems associated with selecting stable reference genes. We have used oligonucleotide microarray data to identify 181 genes with stable expression in the course of dark-induced senescence of barley leaf. From those genes, we selected 5 candidates and confirmed their invariant expression by both reverse transcription quantitative PCR and droplet digital PCR (ddPCR). We used the selected reference genes to normalize the level of the expression of the following senescence-responsive genes in ddPCR assays: SAG12, ICL, AGXT, CS and RbcS. We were thereby able to achieve a substantial reduction in the data variability. Although the use of reference genes is not considered mandatory in ddPCR assays, our results show that it is advisable in special cases, specifically those that involve the following conditions: i) a low number of repeats, ii) the detection of low-fold changes in gene expression or iii) series data comparisons (such as time-course experiments) in which large sample variation greatly affects the overall gene expression profile and biological interpretation of the data.

Differences in gene expression between natural and artificially induced leaf senescence in barley

Senescence is the last step of leaf development in the life span of an annual plant. Senescence can be induced prematurely by treating leaf tissues with jasmonic acid methyl ester (methyl jasmonate, MeJA). During both senescence programmes, drastic changes occur at the biochemical, cellular and ultra-structural levels that were compared here for primary leaves of barley (Hordeum vulgare L.). Our findings indicate that both types of senescence are similar with respect to the morphological changes including the loss of chlorophyll, disintegration of thylakoids, and formation of plastoglobules. However, the time elapsed for reaching senescence completion was different and ranged from 7 to 8 days for artificially senescing, MeJA-treated plants to 7–8 weeks for naturally senescing plants. Pulse-labelling studies along with RNA and protein gel blot analyses showed differential changes in the expression of both plastid and nuclear genes coding for photosynthetic proteins. Several unique messenger products accumulated in naturally and artificially senescing, MeJA-treated leaves. Detailed expression and crosslinking studies revealed that pheophorbide a oxygenase (PAO), a previously implicated key enzyme of chlorophyll breakdown, is most likely not rate-limiting for chlorophyll destruction under both senescence conditions. Metabolite profiling identified differential changes in the composition of carotenoid derivatives and prenyl-lipids to occur in naturally senescing and artificially senescing plants that underscored the differences between both senescence programmes.

Contribution of the drought tolerance-related stress-responsive NAC1 transcription factor to resistance of barley to Ramularia leaf spot.

Summary NAC proteins are plant transcription factors that are involved in tolerance to abiotic and biotic stresses, as well as in many developmental processes. Stress‐responsive NAC1 (SNAC1) transcription factor is involved in drought tolerance in barley and rice, but has not been shown previously to have a role in disease resistance. Transgenic over‐expression of HvSNAC1 in barley cv. Golden Promise reduced the severity of Ramularia leaf spot (RLS), caused by the fungus Ramularia collo‐cygni, but had no effect on disease symptoms caused by Fusarium culmorum, Oculimacula yallundae (eyespot), Blumeria graminis f. sp. hordei (powdery mildew) or Magnaporthe oryzae (blast). The HvSNAC1 transcript was weakly induced in the RLS‐susceptible cv. Golden Promise during the latter stages of R. collo‐cygni symptom development when infected leaves were senescing. Potential mechanisms controlling HvSNAC1‐mediated resistance to RLS were investigated. Gene expression analysis revealed no difference in the constitutive levels of antioxidant transcripts in either of the over‐expression lines compared with cv. Golden Promise, nor was any difference in stomatal conductance or sensitivity to reactive oxygen species‐induced cell death observed. Over‐expression of HvSNAC1 delayed dark‐induced leaf senescence. It is proposed that mechanisms controlled by HvSNAC1 that are involved in tolerance to abiotic stress and that inhibit senescence also confer resistance to R. collo‐cygni and suppress RLS symptoms. This provides further evidence for an association between abiotic stress and senescence in barley and the development of RLS.

Members of the barley NAC transcription factor gene family show differential co-regulation with senescence-associated genes during senescence of flag leaves.

The senescence process of plants is important for the completion of their life cycle, particularly for crop plants, it is essential for efficient nutrient remobilization during seed filling. It is a highly regulated process, and in order to address the regulatory aspect, the role of genes in the NAC transcription factor family during senescence of barley flag leaves was studied. Several members of the NAC transcription factor gene family were up-regulated during senescence in a microarray experiment, together with a large range of senescence-associated genes, reflecting the coordinated activation of degradation processes in senescing barley leaf tissues. This picture was confirmed in a detailed quantitative reverse transcription-PCR (qRT-PCR) experiment, which also showed distinct gene expression patterns for different members of the NAC gene family, suggesting a group of ~15 out of the 47 studied NAC genes to be important for signalling processes and for the execution of degradation processes during leaf senescence in barley. Seven models for DNA-binding motifs for NAC transcription factors were designed based on published motifs, and available promoter sequences of barley genes were screened for the motifs. Genes up-regulated during senescence showed a significant over-representation of the motifs, suggesting regulation by the NAC transcription factors. Furthermore, co-regulation studies showed that genes possessing the motifs in the promoter in general were highly co-expressed with members of the NAC gene family. In conclusion, a list of up to 15 NAC genes from barley that are strong candidates for being regulatory factors of importance for senescence and biotic stress-related traits affecting the productivity of cereal crop plants has been generated. Furthermore, a list of 71 senescence-associated genes that are potential target genes for these NAC transcription factors is presented.

Identification of WHIRLY1 as a Factor Binding to the Promoter of the Stress- and Senescence-Associated Gene HvS40

HvS40 is a marker gene for leaf senescence in barley. It is expressed during dark-induced as well as age-dependent senescence and in response to biotic and abiotic stress factors. Its expression is also induced by hormones involved in the control of stress responses and senescence processes. Run-on-transcription assays with isolated nuclei showed that its expression during dark-induced senescence is regulated at the transcriptional level. Its promoter contains two elicitor response elements (ERE) consisting of two W-boxes. Binding of proteins to these motifs changed during senescence. A 24-kDa protein binding to ERE2 of nonsenescent leaves was identified as WHIRLY1.

1H, 13C, 15N backbone and side chain NMR resonance assignments for the N-terminal RNA recognition motif of the HvGR-RBP1 protein involved in the regulation of barley (Hordeum vulgare L.) senescence

Leaf senescence is an important process in the developmental life of all plant species. Senescence efficiency influences important agricultural traits such as grain protein content and plant growth, which are often limited by nitrogen use. Little is known about the molecular mechanisms regulating this highly orchestrated process. To enhance our understanding of leaf senescence and its regulation, we have undertaken the structural and functional characterization of previously unknown proteins that are involved in the control of senescence in barley (Hordeum vulgare L.). Previous microarray analysis highlighted several barley genes whose transcripts are differentially expressed during senescence, including a specific gene which is greater than 40-fold up-regulated in the flag leaves of early- as compared to late-senescing near-isogenic barley lines at 14 and 21 days past flowering (anthesis). From inspection of its amino acid sequence, this gene is predicted to encode a glycine-rich RNA-binding protein herein referred to as HvGR-RBP1. HvGR-RBP1 has been expressed as a recombinant protein in Escherichia coli, and preliminary NMR data analysis has revealed that its glycine-rich C-terminal region [residues: 93-162] is structurally disordered whereas its N-terminal region [residues: 1-92] forms a well-folded domain. Herein, we report the complete (1)H, (13)C, and (15)N resonance assignments of backbone and sidechain atoms, and the secondary structural topology of the N-terminal RNA recognition motif (RRM) domain of HvGR-RBP1, as a first step to unraveling its structural and functional role in the regulation of barley leaf senescence.

Senescence-associated Barley NAC (NAM, ATAF1,2, CUC) Transcription Factor Interacts with Radical-induced Cell Death 1 through a Disordered Regulatory Domain

Senescence in plants involves massive nutrient relocation and age-related cell death. Characterization of the molecular components, such as transcription factors (TFs), involved in these processes is required to understand senescence. We found that HvNAC005 and HvNAC013 of the plant-specific NAC (NAM, ATAF1,2, CUC) TF family are up-regulated during senescence in barley (Hordeum vulgare). Both HvNAC005 and HvNAC013 bound the conserved NAC DNA target sequence. Computational and biophysical analyses showed that both proteins are intrinsically disordered in their large C-terminal domains, which are transcription regulatory domains (TRDs) in many NAC TFs. Using motif searches and interaction studies in yeast we identified an evolutionarily conserved sequence, the LP motif, in the TRD of HvNAC013. This motif was sufficient for transcriptional activity. In contrast, HvNAC005 did not function as a transcriptional activator suggesting that an involvement of HvNAC013 and HvNAC005 in senescence will be different. HvNAC013 interacted with barley radical-induced cell death 1 (RCD1) via the very C-terminal part of its TRD, outside of the region containing the LP motif. No significant secondary structure was induced in the HvNAC013 TRD upon interaction with RCD1. RCD1 also interacted with regions dominated by intrinsic disorder in TFs of the MYB and basic helix-loop-helix families. We propose that RCD1 is a regulatory protein capable of interacting with many different TFs by exploiting their intrinsic disorder. In addition, we present the first structural characterization of NAC C-terminal domains and relate intrinsic disorder and sequence motifs to activity and protein-protein interactions.

Differential Response of Salt-Tolerant and Susceptible Barley Genotypes to Salinity Stress

The performance of selected salt-tolerant genotypes of barley [Hordeum vulgare (L.)], derived using field and greenhouse assessment methods, was evaluated under greenhouse and field conditions. Two barley genotypes, one salt-tolerant and one salt-sensitive, selected from each of the methods, were used. This study was conducted under both saline and non-saline field conditions, as well as under greenhouse conditions with salinized solution culture at 0 mM (control), 100, 200, 300, and 400 mM NaCl. Days to heading, days to maturity, plant height, number of grains per spike, grain weight per spike, 1000 grain weight, number of spikes per square meter, grain yield, harvest index and some physiological characters were measured. Plant dry weight and accumulated Na+, K+, and Ca2+ were measured 20 days after salinity treatments. The field salinity significantly reduced (P < 0.01) the means of all traits averaged on two tested genotypes. Under greenhouse conditions, the salt-tolerant genotype ‘Afzal’ produced the highest dry weight and K+/Na+ ratio under salt-stress conditions (200 mM). The influence of NaCl on parameters including protein and chlorophyll concentrations and membrane permeability was investigated in young and old leaves of cultivars, differing in salt resistance. NaCl-induced senescence also involved specific modifications, such as a transient increase in soluble protein concentration recorded in salt-resistant genotypes only. Alteration of membrane permeability appeared as one of the first symptoms of senescence in barley leaves and allowed discrimination among cultivars after only the seventh day of stress. Since a higher tolerance to salinity in Afzal was apparently associated with a lower concentration of Na+ and a higher K+/Na+ ratio of the shoots, this trait could probably be used for yield improvement of barley cultivars under saline conditions. It is can concluded from our findings that a relationship exists between genotype-specific salinity tolerance and the accumulation of proline in the leaves and roots along with higher uptake of sodium and chloride, lower decrease in the water and osmotic potential, less inhibition in the yield under salinity stress.

Aspects of programmed cell death during early senescence of barley leaves: possible role of nitric oxide

Leaf senescence is a highly coordinated process which involves programmed cell death (PCD). Early stages of leaf senescence occurring during normal leaf ontogenesis, but not triggered by stress factors, are less well known. In this study, we correlated condensation of chromatin and nuclear DNA (nDNA) fragmentation, two main features of PCD during early senescence in barley leaves, with the appearance of nitric oxide (NO) within leaf tissue. With the help of the alkaline version of the comet assay, together with measurements of nDNA fluorescence intensity, we performed a detailed analysis of the degree of nDNA fragmentation. We localised NO in vivo and in situ within the leaf and photometrically measured its concentration with the NO-specific fluorochrome 4-amino-5-methylamino-2',7'-difluorofluorescein. We found that both nDNA fragmentation and chromatin condensation occurred quite early during barley leaf senescence and always in the same order: first nDNA fragmentation, in leaves of 6-day-old seedlings, and later chromatin condensation, in the apical part of leaves from 10-day-old seedlings. PCD did not start simultaneously even in neighbouring cells and probably did not proceed at the same rate. NO was localised in vivo and in situ within the cytoplasm, mainly in mitochondria, in leaves at the same stage as those in which chromatin condensation was observed. Localisation of NO in vascular tissue and in a large number of mesophyll cells during the senescence process might imply its transport to other parts of the leaf and its involvement in signalling between cells. The fact that the highest concentration of NO was found in the cytoplasm of mesophyll cells in the earliest stage of senescence and lower concentrations were found during later stages might suggest that NO plays an inductive role in PCD.

Evidence of Low Land Surface Thermal Infrared Emissivity in the Presence of Dry Vegetation

Land surface emissivity in the thermal infrared usually increases when the vegetation amount increases, reaching values that are larger than 0.98. During an experiment in Morocco over dry barley crops, it was found that emissivity may be significantly lower than 0.98 at full cover and that in some situations, it might decrease with increasing amount of vegetation, which was unexpected. Older data acquired in Barrax, Spain, over senescent barley also exhibited emissivity values lower than 0.98. The decrease of emissivity was also observed by means of simulations done with our land surface emissivity model developed earlier. The main reason for such behavior might be found in low leaf emissivity due to leaf dryness. This letter also stresses that knowledge on leaf and canopy emissivities and on their variation as a function of water content is still very limited