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PubMed2004

The ascorbic acid redox state controls guard cell signaling and stomatal movement.

H(2)O(2) serves an important stress signaling function and promotes stomatal closure, whereas ascorbic acid (Asc) is the major antioxidant that scavenges H(2)O(2). Dehydroascorbate reductase (DHAR) catalyzes the reduction of dehydroascorbate (oxidized ascorbate) to Asc and thus contributes to the regulation of the Asc redox state. In this study, we observed that the level of H(2)O(2) and the Asc redox state in guard cells and whole leaves are diurnally regulated such that the former increases during the afternoon, whereas the latter decreases. Plants with an increased guard cell Asc redox state were generated by increasing DHAR expression, and these exhibited a reduction in the level of guard cell H(2)O(2). In addition, a higher percentage of open stomata, an increase in total open stomatal area, increased stomatal conductance, and increased transpiration were observed. Guard cells with an increase in Asc redox state were less responsive to H(2)O(2) or abscisic acid signaling, and the plants exhibited greater water loss under drought conditions, whereas suppressing DHAR expression conferred increased drought tolerance. Our analyses suggest that DHAR serves to maintain a basal level of Asc recycling in guard cells that is insufficient to scavenge the high rate of H(2)O(2) produced in the afternoon, thus resulting in stomatal closure.

Ascorbic Acid↗
PubMed2000

The political economy of complex emergency and recovery in northern Ethiopia.

During the 1980s Ethiopia experienced the effects of conflict, drought and famine on a scale far greater than many CPEs elsewhere. In May 1991, after the decisive defeat of the military dictatorship of Mengistu Haile Mariam by the Ethiopian Peoples' Revolutionary Democratic Front (EPRDF) and after decades of civil war, drought and famine, Ethiopia faced the prospects of peace and of much needed development. This paper explores both Ethiopia's experience of conflict and humanitarian intervention in areas of Tigray held by the Tigray Peoples' Liberation Front (TPLF) during the 1980s, and its experience of post-conflict rehabilitation and reconstruction in the 1990s. It first deals with the roots of the conflicts within Ethiopia: political marginalisation, heavy state intervention and highly extractive relations between state and peasants, inappropriate and failed development policies, ethnic identity and the politicisation of ethnicity. The Mengistu regime's counter-insurgency measures are then contrasted with the policies and programmes of the TPLF, Ethiopia's most effective opposition movement and the leading element in the EPRDF, and its achievements in mobilising popular support: its establishment of democratically elected structures of local governance and its famine relief distribution programme.

Disasters↗
PubMed2002

Emergency seed aid in Kenya: some case study insights on lessons learned during the 1990s.

This article reviews the effectiveness of seed-aid distributions in Kenya during the 1990s. It analyses the internal process and effects, i.e. the performance of the aid itself as well as the external process and effects, i.e. how seed-aid intervention affected farmers' broader agricultural management strategies. During the drought emergency of 1997, Kenyan farmers favourably judged many of the immediate seed-aid features such as crop and variety appropriateness and seed quality--even through the overarching goals of the seed assistance were muddled, ranging from assistance to the poor, to generalised gift-giving to stimulating progressive farming practice. However, the longer term analyses, drawn from recollections of a decade of relief activity, showed no concrete evidence that seed aid, per se, had strengthened their farming systems, nor that those who have received it once were less likely to receive it again. Thus, while seed aid has been promoted to lessen the effects of an 'acute' stress, drought, Kenyan farmers, in practice, have been experiencing much wider, 'chronic' seed system problems. This article ends by exploring this distinction between acute and chronic seed system stress and suggests a range of interventions appropriate to each.

Agriculture↗
PubMed2004

The cost of melanization: butterfly wing coloration under environmental stress.

Evolutionary studies typically focus on adaptations to particular environmental conditions, thereby often ignoring the role of possible constraints. Here we focus on the case of variation in dorsal wing melanization in a satyrine butterfly Pararge aegeria. Because melanin is a complex polymer, its synthesis may be constrained if ambient conditions limit the resource budget. This hypothesis was tested by comparing melanization among butterflies that fed as larvae on host grasses experiencing different drought-stress treatments. Treatment differences were validated both at the level of the host plant (nitrogen, carbonate, and water content) and of the butterfly (life-history traits: survival, development time, and size at maturity). Melanization rate was measured as average gray value of the basal dorsal wing area. This area, close to the thorax, is known to be functionally significant for basking in order to thermoregulate. Individuals reared on drought-stressed host plants developed paler wings, and development of darker individuals was slower and less stable as estimated by their level of fluctuating asymmetry. These results provide evidence that melanin is indeed costly to synthesize, and that differences in environmental quality can induce phenotypic variation in wing melanization. Therefore, studies dealing with spatial and/or temporal patterns of variation in wing melanization should not focus on adaptive explanations alone, but rather on a cost-benefit balance under particular sets of environmental conditions.

Animal Nutritional Physiological Phenomena↗
PubMed2004

Changes in root water flow properties of solution culture-grown trembling aspen (Populus tremuloides) seedlings under different intensities of water-deficit stress.

To study the effects of water-deficit stress on root water flow properties in trembling aspen (Populus tremuloides Michx.), seedlings were grown in solution culture and subjected to water-deficit stress by placing their roots in sealed high humidity chambers. After 17 h of stress treatment, seedlings showed mild stress (MS) symptoms with a decline in shoot water potentials. Within 20 h, shoot water potentials rapidly declined, and severe stress (SS) symptoms were present. Root hydraulic conductivity (L(pr)) increased more than two-fold and the relative concentration of apoplastic tracer dye trisodium 3-hydroxy-5, 8, 10-pyrenetrisulphonate (PTS(3)) in xylem exudate decreased by 73.6% in MS seedlings. Conversely, L(pr) decreased (55.3%) and PTS(3) increased (28.6%) in SS seedlings. Treatment of roots with 0.1 mM mercuric chloride decreased root volume flow density (J(v)) by about 29.0% in control and MS plants with no decrease measured in SS seedlings. Mercuric chloride also increased PTS(3) concentration in xylem exudate of control (59%) and MS (86%) seedlings with no change observed in SS plants. The results suggest that aquaporin-mediated transport is important in the regulation of root water flow under drought stress and that root water flow properties are strongly affected by the stress level. Regulation of root water flow may represent an important drought-stress resistance mechanism.

Journal Article↗
PubMed2005

Poly(ADP-ribose) polymerase in plants affects energy homeostasis, cell death and stress tolerance.

Plants contain two genes that code for poly(ADP-ribose) polymerase (PARP): parp1 and parp2. Both PARPs are activated by DNA damage caused by, example reactive oxygen species. Upon activation polymers of ADP-ribose are synthesized on a range of nuclear enzymes using NAD(+) as substrate. Here, we show that in plants stresses such as drought, high light and heat activate PARP causing NAD(+) breakdown and ATP consumption. When the PARP activity is reduced by means of chemical inhibitors or by gene silencing, cell death is inhibited and plants become tolerant to a broad range of abiotic stresses like high light, drought and heat. Plant lines with low poly(ADP-ribosyl)ation activity maintain under stress conditions their energy homeostasis by reducing NAD(+) breakdown and consequently energy consumption. The higher energy-use efficiency avoids the need for a too intense mitochondrial respiration and consequently reduces the formation of reactive oxygen species. From these results it can be concluded that breeding or engineering for a high energy-use efficiency under stress conditions is a valuable, but until today nearly unexploited, approach to enhance overall stress tolerance of crops.

Arabidopsis↗
PubMed2006

CYP707A3, a major ABA 8'-hydroxylase involved in dehydration and rehydration response in Arabidopsis thaliana.

Abscisic acid (ABA) catabolism is one of the determinants of endogenous ABA levels affecting numerous aspects of plant growth and abiotic stress responses. The major ABA catabolic pathway is triggered by ABA 8'-hydroxylation catalysed by the cytochrome P450 CYP707A family. Among four members of Arabidopsis CYP707As, the expression of CYP707A3 was most highly induced in response to both dehydration and subsequent rehydration. A T-DNA insertional cyp707a3-1 mutant contained higher ABA levels in turgid plants, which showed a reduced transpiration rate and hypersensitivity to exogenous ABA during early seedling growth. On dehydration, the cyp707a3-1 mutant accumulated a higher amount of stress-induced ABA than the wild type, an event that occurred relatively later and was coincident with slow drought induction of CYP707A3. The cyp707a3 mutant plants exhibited both exaggerated ABA-inducible gene expression and enhanced drought tolerance. Conversely, constitutive expression of CYP707A3 relieved growth retardation by ABA, increased transpiration, and a reduction of endogenous ABA in both turgid and dehydrated plants. Taken together, our results indicate that CYP707A3 plays an important role in determining threshold levels of ABA during dehydration and after rehydration.

Abscisic Acid↗
PubMed2006

Yeast complementation reveals a role for an Arabidopsis thaliana late embryogenesis abundant (LEA)-like protein in oxidative stress tolerance.

A functional cloning approach using the oxidant-sensitive yeast mutant, Deltayap1, was employed to identify plant genes involved in tolerance of oxidative stress. In this screen, we identified an Arabidopsis late embryogenesis-abundant (LEA)-like protein, AtLEA5, which increased the tolerance of Deltayap1 cells to the oxidants H(2)O(2), diamide, menadione and tert-butyl hydroperoxide. Unlike canonical LEAs, AtLEA5 is constitutively expressed in roots and reproductive organs but not in seeds. In leaves of short-day grown plants, AtLEA5 transcripts exhibited a diurnal pattern of regulation, where transcripts were repressed in the light and abundant in the dark. Expression of AtLEA5 in leaves was induced by oxidants, ABA and dehydration. Use of abi1-1 (ABA-insensitive) and aba1-1 (ABA-deficient) Arabidopsis mutants indicated that drought induction of AtLEA5 required ABA synthesis but was independent of the ABI1 gene product. Abscisic acid and H(2)O(2) induction of AtLEA5 was also independent of the OXI1 protein kinase. Constitutive overexpression of AtLEA5 resulted in increased root growth and shoot biomass, both in optimal conditions and under H(2)O(2) stress. However, in comparison with wild type, photosynthesis in overexpressing plants was more susceptible to drought. These features suggest that AtLEA5 has a unique function among LEA proteins in that it plays a specific role in protection against oxidative stress involving decreased photosynthesis. This protein functions as part of a complex network of defences that contribute to robustness of plants under stress by minimizing the negative effects of oxidation.

Amino Acid Sequence↗
PubMed2004

Biochemical and molecular responses to water stress in resurrection plants.

A small group of angiosperms, known as resurrection plants, can tolerate extreme dehydration. They survive in arid environments because they are able to dehydrate, remain quiescent during long periods of drought, and then resurrect upon rehydration. Dehydration induces the expression of a large number of transcripts in resurrection plants. Gene products with a putative protective function such as LEA proteins have been identified; they are expressed at high levels in the cytoplasm or in chloroplasts upon dehydration and/or ABA treatment of vegetative tissue. An increase in sugar concentration is usually observed at the onset of desiccation in vegetative tissue of resurrection plants. These sugars may be effective in osmotic adjustment or they may stabilize membrane structures and proteins. Regulatory genes such as a protein translation initiation factor, homeodomain-leucine zipper genes and a gene probably working as a regulatory RNA have been isolated and characterized. The knowledge of the biochemical and molecular responses that occur during the onset of drought may help to improve water stress tolerance in plants of agronomic importance.

Journal Article↗
PubMed2004

Over-expression of a transcription factor regulating ABA-responsive gene expression confers multiple stress tolerance.

Unfavourable environmental conditions, such as drought, freezing and high salinity, are major limiting factors of plant productivity. Plants perceive and respond adaptively to such 'abiotic stress' conditions, and the adaptive process is controlled mainly by the phytohormone, abscisic acid (ABA). The hormone, whose level increases under various stress conditions, functions as a signal to trigger adaptive responses that include changes in gene expression patterns. We have recently reported transcription factors that regulate the ABA-responsive gene expression. As ABA mediates adaptation to several common abiotic stresses, we investigated whether the transcription factors are involved in various stress responses. Here, we report that over-expression of ABF3, one of the factors, confers tolerance to chilling, freezing, high temperature and oxidative stress in addition to drought. Our results indicate that ABF3 is involved in multiple stress responses, and that it may be a useful genetic resource for the engineering of plant stress tolerance.

Journal Article↗
PubMed2006

El Niño platforms: participatory disaster response in Peru.

Climate change is expected to lead to greater extremes (droughts and floods) in river regimes around the world. While the number of major calamities is predicted to rise, the efforts of the public sector, experts and local stakeholders are badly coordinated. Consequently, aid does not reach target groups, resulting in unnecessary losses. Hence, there is a need for more participatory and integrative approaches. To ensure a more concerted response to climate-induced disasters, stakeholders could coordinate and negotiate within Multi-Stakeholder Platforms. Such roundtables are increasingly being established for vision-building and integrated water resource management, but could be employed in disaster management as well. After discussing the advantages and disadvantages of participation, this article trace the rise of and the problems facing two 'El Niño' platforms: one in Ica, a city on the Peruvian coast that flooded unexpectedly in January 1998, and one in Ayacucho, which saw a climate change-induced drought around the same time. The issue of internal and external legitimacy receives particular emphasis.

Community Networks↗
PubMed2005

Irradiance and temperature affect the competitive interference of blackberry on the physiology of European beech seedlings.

The potential negative influence of competition from early successional species like blackberry (Rubus fruticosus) may be decisive for the natural regeneration success of drought-sensitive beech (Fagus sylvatica), especially in the light of climate change. With a split plot glasshouse experiment, we investigated the influence of two air temperature and irradiance levels on the competitive interference of blackberry on the water, nitrogen (N) and carbon (C) balance of beech seedlings under moderate drought. When increased temperature was accompanied by low irradiance the biomass, root-to-shoot ratio, N uptake and assimilation rates of blackberry were lower compared with beech, either grown alone or with blackberry. By contrast, when elevated temperature and high irradiance were combined, the root-to-shoot ratio and specific N uptake rate of blackberry were substantially increased, while the N acquisition of beech was impaired. Under lower temperature, with either full light or shade, the presence of blackberry had no significant effects on beech, for almost all tested parameters. Under elevated air temperature beech was impaired by the presence of blackberry at high irradiance. These findings emphasize the interacting effects between environmental factors and competition on the establishment of beech regeneration, which should be considered for future forest management in the frame of climate change.

Fagus↗
PubMed2005

Evolution and maintenance of pollen-colour dimorphisms in Nigella degenii: habitat-correlated variation and morph-by-environment interactions.

Dimorphism in pollen colour is rare among flowering plants, but occurs in two geographically and morphologically distinct subspecies of Nigella degenii (Ranunculaceae). We evaluated the role of genotype-by-environment interactions in the maintenance of two pollen morphs within each of these subspecies. Morph frequencies in a number of populations were related to current habitat conditions, and an extensive common-garden experiment involving both optimal and stressful conditions (drought and nutrient deficiency) was carried out. The putatively derived (dark) pollen morph of N. degenii ssp. barbro has a higher frequency on slopes facing north or east than on slopes facing south or west. Plants of the dark morph also have a higher mortality under drought stress or nutrient deficiency. Data available for N. degenii ssp. jenny provide little evidence for habitat-correlated variation in morph frequency or morph-specific differences in fitness under optimal and stressful growth conditions. Our results suggest that morph-by-environment interactions in mortality could contribute to the maintenance of pollen-colour dimorphisms in N. degenii ssp. barbro.

Biological Evolution↗
PubMed2006

Differential modulation of host plant delta13C and delta18O by native and nonnative arbuscular mycorrhizal fungi in a semiarid environment.

Native, drought-adapted arbuscular mycorrhizal fungi (AMF) often improve host-plant performance to a greater extent than nonnative AMF in dry environments. However, little is known about the physiological basis for this differential plant response. Seedlings of Olea europaea and Rhamnus lycioides were inoculated with either a mixture of eight native Glomus species or with the nonnative Glomus claroideum before field transplanting in a semiarid area. Inoculation with native AMF produced the greatest improvement in nutrient and water status as well as in long-term growth for both Olea and Rhamnus. Foliar delta18O measurements indicated that native AMF enhanced stomatal conductance to a greater extent than nonnative AMF in Olea and Rhamnus.delta13C data showed that intrinsic water-use efficiency in Olea was differentially stimulated by native AMF compared with nonnative AMF. Our results suggest that modulation of leaf gas exchange by native, drought-adapted AMF is critical to the long-term performance of host plants in semiarid environments. delta18O can provide a time-integrated measure of the effect of mycorrhizal infection on host-plant water relations.

Carbon↗
PubMed2006

Below-ground interactions with arbuscular mycorrhizal shrubs decrease the performance of pinyon pine and the abundance of its ectomycorrhizas.

Few studies have examined how below-ground interactions among plants affect the abundance and community composition of symbiotic mycorrhizal fungi. Here, we combined observations during drought with a removal experiment to examine the effects of below-ground interactions with arbuscular mycorrhizal (AM) shrubs on the growth of pinyon pines (Pinus edulis), and the abundance and community composition of their ectomycorrhizal (EM) fungi. Shrub density was negatively correlated with pinyon above- and below-ground growth and explained 75% of the variation in EM colonization. Consistent with competitive release, pinyon fine-root biomass, shoot length and needle length increased with shrub removal. EM colonization also doubled following shrub removal. EM communities did not respond to shrub removal, perhaps because of their strikingly low diversity. These results suggest that below-ground competition with AM shrubs negatively impacted both pinyons and EM fungi. Similar competitive effects may be observed in other ecosystems given that drought frequency and severity are predicted to increase for many land interiors.

Ecosystem↗
PubMed2006

Light rings in Chinese pine (Pinus tabulaeformis) in semiarid areas of north China and their palaeo-climatological potential.

Light rings in conifer trees are characterized by a light-coloured, narrow latewood band of thin-walled tracheids. Most reports on light rings have been for subarctic and subalpine regions, and little is known about their occurrence in semiarid areas. Dendrochronological methods were used to date the occurrence of light rings in Chinese pine (Pinus tabulaeformis) in the semiarid region of north China. The anatomical and chemical characteristics and the potential environmental controls of their formation were investigated. Light rings in Chinese pine were dated to the year of their formation. The wall thickness and lumen diameter of the wood cells of light rings and reference rings were distinctly different. However, the configuration of the light-ring latewood cell walls was normal, although they were thinner than average, and their lignification had been completed normally. The climate characteristics that result in light-ring formation appear to be ongoing severe drought from the previous autumn to July of the current year in conjunction with a warm summer, suggesting that light rings can be used as indicators for past drought events.

Biological Evolution↗
PubMed2007

The rice genome encodes two vacuolar invertases with fructan exohydrolase activity but lacks the related fructan biosynthesis genes of the Pooideae.

* Fructans are believed to contribute to cold and drought tolerance in several plant families (Poaceae, Asparagaceae and Asteraceae), but it is not clear why the ability to accumulate these polymers is found in some genera (e.g. Triticum) but not in others (e.g. Oryza). * As fructan biosynthesis enzymes (FBEs) evolved from vacuolar invertases (VINs), we searched the rice genome sequence for genes related to both FBE and VIN genes of wheat and other members of the Pooideae. We compared them at the levels of exon-intron structure, protein sequence, and the enzymatic properties of recombinant proteins after expression in the yeast Pichia pastoris. * We found that rice possesses two VIN genes (OsVIN1 and OsVIN2) and no FBE genes. FBE genes appear to have arisen in the Pooideae by a series of gene duplications from an ancestor of wheat TaVIN3. Recombinant TaVIN2, OsVIN1 and OsVIN2 behaved as invertases with no FBE activity, but possessed high fructan exohydrolase activity, especially OsVIN1. * The engineering of fructan accumulation into rice for greater stress tolerance could founder on endogenous exohydrolases, but the fact that OsVIN1 transcripts are absent from peduncles of well watered and drought-stressed plants removes one potential obstacle to this endeavour.

Amino Acid Sequence↗
PubMed2007

Shifts from competition to facilitation between a foundation tree and a pioneer shrub across spatial and temporal scales in a semiarid woodland.

* Theoretical and empirical research has supported the hypothesis that plant-plant interactions change from competition to facilitation with increasing abiotic stress. However, the consistency of such changes has been questioned in arid and semiarid ecosystems. * During a drought in the semiarid south-western USA, we used observations and a field experiment to examine the interactions between juveniles of a foundation tree (Pinyon pine, Pinus edulis) and a common shrub (Apache plume, Fallugia paradoxa) in replicated areas of high and low stress. * The presence of F. paradoxa reduced P. edulis performance at low-stress sites, but had the opposite effect at high-stress sites. However, the intensity of the interactions depended on temporal variation in climate and age of P. edulis. Both above- and below-ground factors contributed to competition, while only above-ground factors contributed to facilitation. * These results support the hypothesis that interactions can change from competition to facilitation as abiotic stress increases in semiarid environments. A shift from competition to facilitation may be important for the recovery of P. edulis and other foundation species that have experienced large-scale mortality during recent droughts.

Adaptation, Physiological↗