In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought — such as phenology, root size and depth, hydraulic conductivity and the storage of reserves — are those associated with plant development and structure, and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype–phenotype gap to be bridged, which is essential for faster progress in stress biology research.

Plants are often subjected to periods of soil and atmospheric water deficit during their life cycle. The frequency of such phenomena is likely to increase in the future even outside today's arid/semi-arid regions. Plant responses to water scarcity are complex, involving deleterious and/or adaptive changes, and under field conditions these responses can be synergistically or antagonistically modified by the superimposition of other stresses. This complexity is illustrated using examples of woody and herbaceous species mostly from Mediterranean-type ecosystems, with strategies ranging from drought-avoidance, as in winter/spring annuals or in deep-rooted perennials, to the stress resistance of sclerophylls. Differences among species that can be traced to different capacities for water acquisition, rather than to differences in metabolism at a given water status, are described. Changes in the root : shoot ratio or the temporary accumulation of reserves in the stem are accompanied by alterations in nitrogen and carbon metabolism, the fine regulation of which is still largely unknown. At the leaf level, the dissipation of excitation energy through processes other than photosynthetic C-metabolism is an important defence mechanism under conditions of water stress and is accompanied by down-regulation of photochemistry and, in the longer term, of carbon metabolism.

Methanogenesis has traditionally been assumed to occur only in anoxic environments, yet there is mounting, albeit indirect, evidence of methane (CH 4 ) production in oxic marine and freshwaters. Here we present the first direct, ecosystem-scale demonstration of methanogenesis in oxic lake waters. This methanogenesis appears to be driven by acetoclastic production, and is closely linked to algal dynamics. We show that oxic water methanogenesis is a significant component of the overall CH 4 budget in a small, shallow lake, and provide evidence that this pathway may be the main CH 4 source in large, deep lakes and open oceans. Our results challenge the current global understanding of aquatic CH 4 dynamics, and suggest a hitherto unestablished link between pelagic CH 4 emissions and surface-water primary production. This link may be particularly sensitive to widespread and increasing human influences on aquatic ecosystem primary productivity.

ORCID ID: 0000-0002-9663-5371 (RN).Accumulation of anthocyanins in the exocarp of red grapevine (Vitis vinifera) cultivars is one of several events that characterize the onset of grape berry ripening (véraison). Despite our thorough understanding of anthocyanin biosynthesis and regulation, little is known about the molecular aspects of their transport. The participation of ATP binding cassette (ABC) proteins in vacuolar anthocyanin transport has long been a matter of debate. Here, we present biochemical evidence that an ABC protein, ABCC1, localizes to the tonoplast and is involved in the transport of glucosylated anthocyanidins. ABCC1 is expressed in the exocarp throughout berry development and ripening, with a significant increase at véraison (i.e., the onset of ripening). Transport experiments using microsomes isolated from ABCC1-expressing yeast cells showed that ABCC1 transports malvidin 3-Oglucoside. The transport strictly depends on the presence of GSH, which is cotransported with the anthocyanins and is sensitive to inhibitors of ABC proteins. By exposing anthocyanin-producing grapevine root cultures to buthionine sulphoximine, which reduced GSH levels, a decrease in anthocyanin concentration is observed. In conclusion, we provide evidence that ABCC1 acts as an anthocyanin transporter that depends on GSH without the formation of an anthocyanin-GSH conjugate.

No abstract

Abstract. A study to assess the effects of the Partial Rootzone Drying (PRD) irrigation strategy in comparison to other irrigation systems was carried out in southern Portugal in two field-grown grapevines varieties, Moscatel and Castelão. We addressed the question of whether by regulating growth and plant water use, the PRD system would enable an equilibrated vegetative development, leading to a favourable capture of solar radiation for photoassimilate production and, at the same time to provide an optimum environment for fruit maturation. Three irrigation schemes were applied in addition to the non-irrigated (NI) vines: partial root drying (PRD), 50% of crop evapotranspiration (ETc), supplied to only one side of the root system while the other one was allowed to dry, alternating sides every 15 days; deficit irrigated (DI), 50% ETc supplied, half to each side of the root system and full irrigated (FI, 100% ETc). During the whole season FI plants of both varieties exhibited a high leaf predawn water potential (ψ pd , ca -0.2 MPa) while a progressive decline was observed in NI plants, reaching ψ pd values near -0.7 MPa at the end of August. PRD and DI presented intermediate values. PRD vines exhibited a stronger control over vegetative growth as compared with DI and FI plants. This was expressed by lower values of total leaf area at harvest, leaf layer number, canopy wideness and water shoots number, allowing a higher light interception at the cluster zone that induced an improvement in some berry quality characteristics. Watering had no significant effects on sugar accumulation in the berries but led to a favourable increase in the must titratable acidity, mainly in Castelão. Whereas in DI and FI treatments berry skin anthocyanins and phenols content were always lower than in NI, in PRD there was either no reduction or the reduction was much lower than in the other irrigation treatments. Water use efficiency (WUE) was increased by about 80% in PRD and DI when compared with FI, as a result of almost similar yields in the three treatments. Yield gains of irrigated plants in relation to NI were modest, explained by the rainy spring in both years.

Knowledge on variety traits and physiological responses to stress is still scarce in Vitis vinifera L., limiting the optimisation of irrigation and breeding for high water use efficiency. We have characterised five grapevine varieties using thermal imaging, leaf gas exchange, leaf morphology and carbon isotope composition. Plants of the varieties Aragonez, Trincadeira, Cabernet Sauvignon, Syrah and Touriga Nacional were grown in field conditions. Two experiments were performed. In Experiment I (2006), vines of Aragonez and Trincadeira were either well irrigated (WI, 80% ETc), non-irrigated but rain fed (NI) or subjected to regulated deficit irrigation (RDI, 40% ETc) and studied along the summer season. In Experiment II (2006 and 2007), vines of the five varieties were subjected to RDI (30–40% ETc) and studied at veraison. In Experiment I, leaf temperature (Tleaf) correlated negatively with stomatal conductance (gs) and leaf water potential (Ψpd). The inverse relationship between gs and Tleaf was highly significant in the afternoon. In Experiment II, the different genotypes showed different Tleaf for similar Ψpd. Stomatal density did not correlate with gs suggesting that varieties have different stomatal control. Our results show that combined measurements of canopy temperature and Ψpd can aid in better understanding of stomatal regulation in different grapevine varieties. Such variation in stomatal regulation should be taken into account in determining irrigation strategies.

Improvements in water use efficiency of crops are essential under the scenarios of water scarcity predicted by global change models for the Mediterranean region. In recent years, deficit irrigation, including partial root drying (PRD), has been proposed as an irrigation technique to improve water use efficiency and standardize grapevine yield and quality. The objective of this study was to evaluate the impact of deficit irrigation on photosynthetic responses of field grown grapevines of cv. Moscatel and Castelão. The treatments were: full irrigation (FI), corresponding to 100% ET c ; non-irrigated, but rain fed (NI) and partial root zone drying (PRD) and deficit irrigation (DI), both corresponding to an irrigation of 50% ET c . While in the DI treatment water was applied to both sides of the root system, in the PRD treatment, water was supplied to only one side of the root system, alternating sides periodically. In both cultivars, PRD and DI vines showed intermediate pre-dawn leaf water potential (c pd ) values (around À0.4 MPa) while FI vines, showed c pd around À0.2 MPa during the growing season. NI showed the lowest c pd , reaching À0.6 MPa in Moscatel and À0.8 MPa in Castelão, at the end of growing season. Water status of PRD vines remained closer to FI than DI, especially at midday. Photosynthetic rates and fluorescence parameters of the deficit irrigation treatments (PRD and DI) did not show significant differences from FI vines, for most of the season. In Moscatel, although PRD generally showed g s , relative stomatal limitation (RSL) and intrinsic water use efficiency (A/g s ) values closer to NI than DI, most of the differences between PRD and DI were not statistically significant. Our results showed a stability of the photosynthetic machinery in grapevines under low-to-moderate water availabilities, as demonstrated by the maintenance of the activity of three Calvin Cycle enzymes and of the V cmax values. However, a decline in J max was observed in NI vines, that can be a result either of a decrease in ATP production or, alternatively, of decreased mesophyll conductance to CO 2 diffusion. In general terms, stomatal limitation of photosynthesis is likely to be dominant in non-irrigated plants. Deficit irrigation had no negative impact on CO 2 assimilation, despite less water application than in full-irrigation. Differences among varieties may be related to differences in sensitivity of stomata, shoot growth and/or the interaction between rootstock and cultivar to soil water availability. #