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Where stomatal closure as a result of drought coincides with exposure to high photosynthetically active radiation, leaves are subjected to excess incident radiation relative to the available intracellular CO2. The rate of electron production therefore can exceed the rate of electron use in the Calvin cycle. Reactive oxygen species (ROS), such as the superoxide anion (O2ï‚·ï€), hydrogen peroxide (H2O2), the hydroxyl radical (HOï‚·), and singlet oxygen (1O2), are therefore produced, particularly in the chloroplasts, which are both the main producers and targets of ROS . The ROS react with proteins and lipids, causing damage to cellular structures and metabolism, particularly those associated with photosynthesis . This situation will ultimately damage the photosynthetic apparatus, unless either photoprotective mechanisms are available to down-regulate photosynthesis, or the decline in CO2 assimilation coincides with an increase in the strength of another sink for the absorbed radiation. Photoprotective mechanisms include thermal dissipation in the xanthophylls or lutein cycles , while alternative sinks include photorespiration or the Mehler peroxidase reaction, in which electrons are transferred from reduced ferredoxin to O2ï‚·ï€ . ROS accumulation under such conditions depends on the balance between ROS synthesis and ROS dissipation (discussed below).
Unfortunately, ROS synthesis, dissipation, and damage associated with ROS accumulation has not been quantified under clearly defined levels/duration of irradiance and levels of water deficit , leaving the impact of this system poorly understood as yet. Also, ROS are generally considered to lead to photodamage, but Nishiyama et al. argue that the impact of ROS is more related to inactivation of repair of photodamaged PSII than to the photodamage itself. Another complication in understanding the role of ROS in drought-stressed plants is that in addition to causing damage, ROS also act as signal molecules that activate multiple defence responses: Increased ROS production and the high redox state of the electron transport chain during water deficit induce expression of genes coding for components of energy-dissipating and regulation systems in chloroplasts, which assists in acclimation .
Scavenging of reactive oxygen species
Plants use both enzymatic and non-enzymatic antioxidant defence mechanisms to scavenge ROS. The enzymatic system includes superoxide dismutases (SOD), which act as the first line of defence against superoxide radicals as they catalyse the dismutation of superoxide radicals to H2O2 and O2 . The subsequent defences are mostly concerned with depleting the resulting H2O2 before it can be converted to the highly reactive (and extremely damaging) hydroxyl radical (HOï‚·) by the Fenton reaction, in the presence of ferrous (Fe2+) ions (Mittler 2002). The enzymes involved include catalase (CAT), guaiacol-type peroxidases (POD), and enzymes of the ascorbate-glutathione cycle (Mittler 2002), such as ascorbate peroxidase (APX). In the process of converting O2ï‚·ï€ to H2O2 and O2, ascorbic acid (AsA) is oxidised to form the monohydroascorbate radical (MDA) that is reduced back to AsA by either reduced ferredoxin or by NADPH, catalysed by MDA reductase (MDAR). Dehydroascorbate (DHA) is produced when MDAR fails to reduce MDA to AsA, and is reduced to AsA by DHA reductase (DHAR). Alternatively, in the glutathione peroxidase (GPX) cycle, glutathione (GSH) is required to restore AsA as the electron donor ; glutathione reductase (GR) catalyses the NADPH-dependent reduction of oxidised glutathione to its reduced form . Polyphenol oxidase isoenzymes, located mainly in the thylakoid lumen, oxidise o-diphenolic substrates to o-quinones, and are therefore involved in the metabolism of phenols, which have a non-enzymatic antioxidant action. In another cycle, the catalase cycle , catalases - heme-containing enzymes particularly abundant in the glyoxysomes - destroy the H2O2 generated by oxidases involved in the ï¢-oxidation of fatty acids, and in the peroxisomes of green leaves, where they scavenge the H2O2 arising from the oxidation of the photorespiratory-produced glycolate.
Changes of expression and activities of antioxidant enzymes have been detected in many species of plants in response to adverse environmental conditions, such as water deficit and other abiotic, biotic and developmental stimuli . Sofo et al. found that the activities of SOD, APX, CAT, and POD increased in relation to the severity of drought stress in both leaves and roots of olive (grown under high temperature and irradiance). In particular, a marked increase in APX activity was found in leaves of plants during severe drought stress. The authors suggested that up-regulation of the antioxidant system might be an important attribute linked to drought tolerance, which could limit cellular damage caused by ROS during water deficit. APX in the roots, in contrast, showed reduced levels of activity, possibly indicating that APX activity could be attributed mainly to the chloroplast-located enzyme (chlAPX) of leaf tissues. The huge increase in APX activity in the leaves under drought could explain how the chloroplasts were sufficiently protected against reactive oxygen species to maintain high electron transport rates.
Over-expression of one or more ROS-scavenging enzymes in various compartments has been shown to relieve oxidative stress . Eltayeb et al. found that over-expression of a MDAR gene in tobacco resulted in enhanced tolerance of PEG-induced water stress; the authors suggested this may be due to increased levels of AsA which mainly resulted from the enhanced activity of MDAR.
Accurate characterisation of the complex stress tolerance phenotypes of transgenic plants (over-)expressing a variety of antioxidant enzymes has been identified as a significant challenge in understanding antioxidant defences . To date, assessment of the behaviour of mutants with altered ROS-scavenging capacity has focussed on stress-factors other than drought. Thus much work is still needed to better understand the role of ROS and ROS-scavenging in drought tolerance.
Osmotic adjustment relates to the lowering of osmotic potential due to the net accumulation of solutes in response to water deficits . Osmotic adjustment is often induced during drought , with solutes accumulating, resulting in the maintenance of a higher turgor potential at a given leaf water potential . Different types of compatible solutes can be responsible - various sugars, organic acids, amino acids, sugar alcohols, and ions. Concentrations of soluble sugars (sucrose, glucose, and fructose) are altered by drought - in general concentrations increase - although under severe dehydration they may decrease . Soluble sugars act as signalling molecules under stress , interact with hormones, and modify the expression of genes involved in photosynthetic metabolism - generally resulting in a reduction in source activity such as photoassimilate export and an increase in sink activity such as production of lipids and proteins .
Osmotic adjustment in plant cells can aid the maintenance of water uptake and cell turgor during stress, and therefore can allow a plant to continue growth during water deficit, since zero turgor occurs at a lower water potential in osmotically adjusted leaf tissue. However, where water supply is not replenished, continued abstraction of water will ultimately be detrimental - and thus osmotic adjustment is not always advantageous . Engineering osmoprotectant synthesis pathways into model plant species has led to significant (albeit modest) improvements in stress tolerance; adding multiple genes to increase osmoprotectant flux in response to stress may be more beneficial .
Developments in molecular biology have opened up the possibility of exploring the role of diverse molecules in drought tolerance. Many molecular adjustments have been found during drought stress, and comparison of drought-tolerant and non-drought tolerant lines has been used to indicate whether or not the extent of such adjustments may in some way be related to drought tolerance. It has been suggested, for example, that microRNAs may play a role in drought tolerance in maize . MicroRNAs are small RNA molecules that are important regulators of gene expression at the post-transcriptional level by repressing mRNA expression . The expression of a wide range of genes is altered during drought. Some are involved in the processes mentioned above e.g. osmotic adjustment. Dehydrins are amongst the most frequently observed proteins induced by dehydration, and may help stabilise membranes or proteins during stress . A relationship has been suggested between both water-soluble inositolpolyphophates and membrane lipid polyphosphoinositides and drought stress . The activity of phospholipase D (PLD), which regulate the production of phosphatidic acid - a key class of lipid mediators in plant response to environmental stress, increases under drought . PLDï¡1 is particularly interesting with respect to drought tolerance, since it promotes stomatal closure and reduces water loss.
The method of imposing drought in many molecular papers, however, limits their application to 'real' drought situations. For example, in the above-mentioned microRNA publication , 'drought' actually involved dehydration by removing plants from soil and leaving them on filter paper - and Pinheiro and Chaves found that results from such experiments are very different to those where plants are droughted in soil/growing media). The lack of measurement of plant water relations in many molecular studies also means comparisons cannot be made across studies. A specific disadvantage of transcriptomic analysis is that in most comparisons protein abundance correlated very poorly with gene expression , which can be particularly problematic in stress physiology, where sometimes only a small portion of the transcripts representing a specific subset of genes are actively translated. Deyholos pointed out another problem with many transcriptomic studies: they tend to focus on young tissue, which may not be the most relevant tissue in 'real' crops in the field. Nonetheless, collaboration between ecophysiologists, agronomists, and molecular biologists in improving these investigations should be encouraged and is essential to optimise our understanding of plant responses to drought. In particular, the generation of mapping populations of contrasting cultivars or ecotypes has provided powerful new resources for dissecting the genetic basis of differences in drought tolerance and/or water use efficiency . 'Model' plants, however, are still not necessarily well understood in relation to physiological responses to water stress - this needs rapid correction in order to fully exploit the wealth of genetic and genomic data available for such plants.
As highlighted in the introduction, the response of plants to drought is a huge topic. Water stress has an impact on many processes e.g. inflorescence development that are outside the scope of this chapter. Several different processes interact. To give some examples, Kadioglu & Terzi highlight links between ROS-scavenging, osmolyte accumulation, and leaf rolling in dehydration avoidance; AQP down-regulation during drought stress may be a response to a cascade of events triggered initially by ROS accumulation ; while ABA induces transcription factors that regulate the expression of PIP AQPs .
Exploiting the impact of drought stress on plant physiology
Exploiting stomatal closure - deficit irrigation and partial rootzone drying
It has now been known for some time that chemical compounds synthesised in drying roots can act as long-distance signals, which induce stomatal closure in the leaf or restrict leaf growth via arrest of meristematic development. As a result, in some cases, stomatal closure can occur without significant changes in shoot water status. This occurs where plant water potential is buffered by controlling stomatal aperture via feed-forward mechanisms. Plants that show this response are said to be 'isohydric'. Even where the leaf water potential is similar, plants exposed to a water deficit will have a lower xylem water potential - and it is this that controls leaf growth - on account of a reduced gradient of water potential from the roots through to the leaves due to reduced water flux ; this difference between xylem and leaf water potential may result in reduced leaf growth in the plants experiencing a water deficit. Careful manipulation of soil water availability to induce a mild water deficit allows minimisation of the impact of the deficit on shoot water status . This has been exploited in 'Deficit Irrigation' strategies, where less than 100% of crop evapotranspiration is replaced by irrigation, and in variations on Deficit Irrigation, one of which is known as Partial Rootzone Drying, in which case water is applied only to one side of the roots, so that the other side is exposed to drying soil, with the side being irrigated switched at intervals. Such techniques have allowed water savings without reducing yield .
The potential of exploiting plant responses to dry soil/substrate is not confined to food production. In landscaping under semi-arid conditions, transplantation is more likely to result in successful growth if the plants have been pre-conditioned to dry conditions. Thus deficit irrigation is increasingly being used in the production of ornamental nursery stock with reduced shoot height and/or leaf area, increased root-collar diameter, root growth potential, and root:shoot ratio, increased osmotic adjustment and water use efficiency, and low stomatal conductance, leaf water and turgor potentials, and relative water content . Variations on the idea of deficit irrigation can be applied to this end - for example Bañón et al. exposed Nerium oleander seedlings to both deficit irrigation and low air humidity on the nursery, prior to transplant, with the result that mortality after transplant was reduced from 92% to 32% compared to control plants; Franco et al. highlight that microclimate management during the nursery phase can be an effective means of producing high-quality seedlings capable of withstanding transplant shock and capable of rapid establishment in arid landscapes. Even where water is not limiting, deficit irrigation can be used to control the size and quality of hardy ornamental nursery stock , particularly where the application of deficit irrigation is combined with an efficient method of sensing plant water requirements that is suited to application on the nursery .
In addition to the impact of long-distance signalling, mild water deficits also exert direct or indirect impacts on yield and the quality of harvested products: for example, reduced leaf growth may improve the light environment around fruit while the fruit are developing.