Similar soil drying-induced stomatal closure in soybean genotypes varying in abscisic acid accumulation and stomatal sensitivity to abscisic acid.

Different soybean cultivars (Williams 82 , Union , Jindou 21 , Long Huang 1 , Long Huang 2 ) were exposed to drying soil, to investigate whether endogenous abscisic acid (ABA) concentrations and leaf water relations regulated stomatal behaviour. We measured ABA concentrations in xylem and tissue of the first and second trifoliate leaves respectively; stomatal conductance (gs ) and leaf water potential (Ψleaf ) in both leaves; and water content in soil. Cultivar variation in leaf area and g s caused different rates of soil drying, but g s and Ψ leaf declined similarly with soil drying in all cultivars. Variation in leaf xylem ABA concentration better explained stomatal responses than foliar ABA concentration in some cultivars, and was highly correlated with stomatal conductance. Xylem ABA concentration in well-watered soil was highest in Union , and in drying soil was lowest in Jindou 21 and Long Huang 2 , although the latter had the highest foliar ABA concentrations. Jindou 21 accumulated lower xylem ABA concentrations than other cultivars as soil moisture or Ψ leaf decreased, but its stomatal sensitivity to xylem ABA was greater. Because cultivars varied in both ABA accumulation and stomatal sensitivity to ABA, but had similar stomatal sensitivity to Ψ leaf , leaf water relations seem more important in regulating stomatal closure of soybean.

Bi-directional, long-distance hormonal signalling between roots and shoots of soil water availability.

While the importance of plant water relations in determining crop response to soil water availability is difficult to over-emphasise, under many circumstances, plants maintain their leaf water status as the soil dries yet shoot gas exchange and growth is restricted. Such observations lead to development of a paradigm that root-to-shoot signals regulate shoot physiology, and a conceptual framework to test the importance of different signals such as plant hormones in these physiological processes. Nevertheless, shoot-to-root (hormonal) signalling also plays an important role in regulating root growth and function and may dominate when larger quantities of a hormone are produced in the shoots than the roots. Here, we review the evidence for acropetal and basipetal transport of three different plant hormones (abscisic acid, jasmonates, strigolactones) that have antitranspirant effects, to indicate the origin and action of these signalling systems. The physiological importance of each transport pathway likely depends on the specific environmental conditions the plant is exposed to, specifically whether the roots or shoots are the first to lose turgor when exposed to drying soil or elevated atmospheric demand, respectively. All three hormones can interact to influence each other's synthesis, degradation and intracellular signalling to augment or attenuate their physiological impacts, highlighting the complexity of unravelling these signalling systems. Nevertheless, such complexity suggests crop improvement opportunities to select for allelic variation in the genes affecting hormonal regulation, and (in selected crops) to augment root-shoot communication by judicious selection of rootstock-scion combinations to ameliorate abiotic stresses.

Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks.

Mesophyll conductance (gmCO2 ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O3 ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O3 and drought on gmCO2 are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O3 (ambient [35.2 nmol mol-1 as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO2 inside a leaf. The combination of O3 and WD significantly decreased net photosynthetic rate (PN ) regardless of the species. The reduction of photosynthesis was associated with a decrease in gmCO2 and stomatal conductance (gsCO2 ) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (Vcmax ) and maximum electron transport rate (Jmax ) with decreased diffusive conductance parameters. The reduction of gmCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between gmCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between gsCO2 and ABA under high O3 and WD due to impaired stomatal physiological behaviour, indicating that the reduction of PN was related to gmCO2 rather than gsCO2 . The results suggest that gmCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O3 pollution.

Climate and species stress resistance modulate the higher survival of large seedlings in forest restorations worldwide.

Seedling planting plays a key role in active forest restoration and regeneration of managed stands. Plant attributes at outplanting can determine tree seedling survival and consequently early success of forest plantations. Although many studies show that large seedlings of the same age within a species have higher survival than small ones, others report the opposite. This may be due to differences in environmental conditions at the planting site and in the inherent functional characteristics of species. Here, we conducted a global-scale meta-analysis to evaluate the effect of seedling size on early outplanting survival. Our meta-analysis covered 86 tree species and 142 planting locations distributed worldwide. We also assessed whether planting site aridity and key plant functional traits related to abiotic and biotic stress resistance and growth capacity, namely specific leaf area and wood density, modulate this effect. Planting large seedlings within a species consistently increases survival in forest plantations worldwide. Species' functional traits modulate the magnitude of the positive seedling size-outplanting survival relationship, showing contrasting effects due to aridity and between angiosperms and gymnosperms. For angiosperms planted in arid/semiarid sites and gymnosperms in subhumid/humid sites the magnitude of the positive effect of seedling size on survival was maximized in species with low specific leaf area and high wood density, characteristics linked to high stress resistance and slow growth. By contrast, high specific leaf area and low wood density maximized the positive effect of seedling size on survival for angiosperms planted in subhumid/humid sites. Results have key implications for implementing forest plantations globally, especially for adjusting nursery cultivation to species' functional characteristics and planting site aridity. Nursery cultivation should promote large seedlings, especially for stress sensitive angiosperms planted in humid sites and for stress-resistant species planted in dry sites.

Soil moisture heterogeneity regulates water use in Populus nigra L. by altering root and xylem sap phytohormone concentrations.

Soil moisture heterogeneity in the root zone is common both during the establishment of tree seedlings and in experiments aiming to impose semi-constant soil moisture deficits, but its effects on regulating plant water use compared with homogenous soil drying are not well known in trees. Pronounced vertical soil moisture heterogeneity was imposed on black poplar (Populus nigra L.) grown in soil columns by altering irrigation frequency, to test whether plant water use, hydraulic responses, root phytohormone concentrations and root xylem sap chemical composition differed between wet (well-watered, WW), and homogeneously (infrequent deficit irrigation, IDI) and heterogeneously dry soil (frequent deficit irrigation, FDI). At the same bulk soil water content, FDI plants had greater water use than IDI plants, probably because root abscisic acid (ABA) concentration was low in the upper wetter layer of FDI plants, which maintained root xylem sap ABA concentration at basal levels in contrast with IDI. Soil drying did not increase root xylem concentration of any other hormone. Nevertheless, plant-to-plant variation in xylem jasmonic acid (JA) concentration was negatively related to leaf stomatal conductance within WW and FDI plants. However, feeding detached leaves with high (1200 nM) JA concentrations via the transpiration stream decreased transpiration only marginally. Xylem pH and sulphate concentration decreased in FDI plants compared with well-watered plants. Frequent deficit irrigation increased root accumulation of the cytokinin trans-zeatin (tZ), especially in the dry lower layer, and of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), in the wet upper soil layer. Root hormone accumulation might explain the maintenance of high root hydraulic conductance and water use in FDI plants (similar to well-watered plants) compared with IDI plants. In irrigated tree crops, growers could vary irrigation scheduling to control water use by altering the hormone balance.

A customizable method to characterize Arabidopsis thaliana transpiration under drought conditions.

BACKGROUND: Characterization of the dynamic response of plant transpiration to decreasing soil water content in a reproducible way is required for the correct phenotyping of traits related to water saving strategies. Nowadays, an increasing number of automated high throughput platforms are available, but their development requires a great economic investment and it is not always desirable/feasible to outsource these analyses. We propose a medium-throughput protocol to characterize transpiration responses to decreasing soil moisture in a quantitative and highly reproducible way with a minimum investment of resources. RESULTS: The quantitative characterization of plant responses to a decreasing soil water content using our phenotyping platform has showed high reproducibility between different experiments. The proposed irrigation strategy allowed us to harvest plants ranging from a well-watered condition to the loss-of-turgor point in a predictable and controlled way. Coupling this protocol with hormone profiling allows investigation of hormonal responses (metabolite accumulation as well as plant sensitivity) to water stress. As a proof-of-concept, we have characterized the dynamic responses of leaf transpiration to decreasing soil water contents in an abscisic acid (ABA) deficient genotype (aba1-1) as well as in genotypes with altered sensitivity to ABA (abi1-1 and hab1-1abi1-1), which are insensitive and hypersensitive to ABA, respectively. CONCLUSIONS: This protocol allows for assessment of quantitative differences in rosette transpiration responses to water depletion in both ABA biosynthesis mutants and genotypes with altered sensitivity to the hormone. Data indicate a correlation between ABA levels and/or hormone perception and growth rate and/or water content. The protocol guarantees the correct application of water stress to adult plants, which is essential to understand responses of mutants and/or natural accessions.

Facing Climate Change: Biotechnology of Iconic Mediterranean Woody Crops.

The Mediterranean basin is especially sensitive to the adverse outcomes of climate change and especially to variations in rainfall patterns and the incidence of extremely high temperatures. These two concurring adverse environmental conditions will surely have a detrimental effect on crop performance and productivity that will be particularly severe on woody crops such as citrus, olive and grapevine that define the backbone of traditional Mediterranean agriculture. These woody species have been traditionally selected for traits such as improved fruit yield and quality or alteration in harvesting periods, leaving out traits related to plant field performance. This is currently a crucial aspect due to the progressive and imminent effects of global climate change. Although complete genome sequence exists for sweet orange (Citrus sinensis) and clementine (Citrus clementina), olive tree (Olea europaea) and grapevine (Vitis vinifera), the development of biotechnological tools to improve stress tolerance still relies on the study of the available genetic resources including interspecific hybrids, naturally occurring (or induced) polyploids and wild relatives under field conditions. To this respect, post-genomic era studies including transcriptomics, metabolomics and proteomics provide a wide and unbiased view of plant physiology and biochemistry under adverse environmental conditions that, along with high-throughput phenotyping, could contribute to the characterization of plant genotypes exhibiting physiological and/or genetic traits that are correlated to abiotic stress tolerance. The ultimate goal of precision agriculture is to improve crop productivity, in terms of yield and quality, making a sustainable use of land and water resources under adverse environmental conditions using all available biotechnological tools and high-throughput phenotyping. This review focuses on the current state-of-the-art of biotechnological tools such as high throughput -omics and phenotyping on grapevine, citrus and olive and their contribution to plant breeding programs.

Applying 'drought' to potted plants by maintaining suboptimal soil moisture improves plant water relations.

Pot-based phenotyping of drought response sometimes maintains suboptimal soil water content by applying high-frequency deficit irrigation (HFDI). We examined the effect of this treatment on water and abscisic acid (ABA) relations of two species (Helianthus annuus and Populus nigra). Suboptimal soil water content was maintained by frequent irrigation, and compared with the effects of withholding water and with adequate irrigation. At the same average whole-pot soil moisture, frequent irrigation resulted in larger soil water content gradients, lower root and xylem ABA concentrations ([X-ABA]), along with higher transpiration rates or stomatal conductance, compared with plants from which water was withheld. [X-ABA] was not uniquely related to transpiration rate or stomatal conductance, as frequently irrigated plants showed partial stomatal closure compared with well-watered controls, without differing in [X-ABA] and, in H. annuus, [ABA]leaf. In two P. nigra genotypes differing in leaf area, the ratio between leaf area and root weight in the upper soil layer influenced the soil water content of this layer. Maintaining suboptimal soil water content alters water relations, which might become dependent on root distribution and leaf area, which influences soil water content gradients. Thus genotypic variation in 'drought tolerance' derived from phenotyping platforms must be carefully interpreted.

The importance of soil drying and re-wetting in crop phytohormonal and nutritional responses to deficit irrigation.

Soil drying and re-wetting (DRW) occurs at varying frequencies and intensities during crop production, and is deliberately used in water-saving irrigation techniques that aim to enhance crop water use efficiency. Soil drying not only limits root water uptake which can (but not always) perturb shoot water status, but also alters root synthesis of phytohormones and their transport to shoots to regulate leaf growth and gas exchange. Re-wetting the soil rapidly restores leaf water potential and leaf growth (minutes to hours), but gas exchange recovers more slowly (hours to days), probably mediated by sustained changes in root to shoot phytohormonal signalling. Partial rootzone drying (PRD) deliberately irrigates only part of the rootzone, while the remainder is allowed to dry. Alternating these wet and dry zones (thus re-wetting dry soil) substantially improves crop yields compared with maintaining fixed wet and dry zones or conventional deficit irrigation, and modifies phytohormonal (especially abscisic acid) signalling. Alternate wetting and drying (AWD) of rice can also improve yield compared with paddy culture, and is correlated with altered phytohormonal (including cytokinin) signalling. Both PRD and AWD can improve crop nutrition, and re-wetting dry soil provokes both physical and biological changes which affect soil nutrient availability. Whether this alters crop nutrient uptake depends on competition between plant and microbes for nutrients, with the rate of re-wetting determining microbial dynamics. Nevertheless, studies that examine the effects of soil DRW on both crop nutritional and phytohormonal responses are relatively rare; thus, determining the cause(s) of enhanced crop yields under AWD and PRD remains challenging.

Local root abscisic acid (ABA) accumulation depends on the spatial distribution of soil moisture in potato: implications for ABA signalling under heterogeneous soil drying.

Patterns of root abscisic acid (ABA) accumulation ([ABA]root), root water potential (Ψroot), and root water uptake (RWU), and their impact on xylem sap ABA concentration ([X-ABA]) were measured under vertical partial root-zone drying (VPRD, upper compartment dry, lower compartment wet) and horizontal partial root-zone drying (HPRD, two lateral compartments: one dry, the other wet) of potato (Solanum tuberosum L.). When water was withheld from the dry compartment for 0-10 d, RWU and Ψroot were similarly lower in the dry compartment when soil volumetric water content dropped below 0.22cm(3) cm(-3) for both spatial distributions of soil moisture. However, [ABA]root increased in response to decreasing Ψroot in the dry compartment only for HPRD, resulting in much higher ABA accumulation than in VPRD. The position of the sampled roots (~4cm closer to the surface in the dry compartment of VPRD than in HPRD) might account for this difference, since older (upper) roots may accumulate less ABA in response to decreased Ψroot than younger (deeper) roots. This would explain differences in root ABA accumulation patterns under vertical and horizontal soil moisture gradients reported in the literature. In our experiment, these differences in root ABA accumulation did not influence [X-ABA], since the RWU fraction (and thus ABA export to shoots) from the dry compartment dramatically decreased simultaneously with any increase in [ABA]root. Thus, HPRD might better trigger a long-distance ABA signal than VPRD under conditions allowing simultaneous high [ABA]root and relatively high RWU fraction.

Two potato (Solanum tuberosum) varieties differ in drought tolerance due to differences in root growth at depth.

To test the hypothesis that root growth at depth is a key trait explaining some genotypic differences in drought tolerance in potato (Solanum tuberosum L.), two varieties (Horizon and Maris Piper) differing in drought tolerance were subjected to different irrigation regimes in pots in a glasshouse and in the field under a polytunnel. In the glasshouse, both cultivars showed similar gas exchange, leaf water potential, leaf xylem ABA concentration and shoot biomass independently of whether plants were grown under well watered or water deficit conditions. Under well watered conditions, root growth was three-fold higher in Horizon compared with Maris Piper, 3 weeks after emergence. Water deficit reduced this difference. In the polytunnel, applying 60% or less irrigation volume compared with full irrigation significantly decreased tuber yield in Maris Piper but not in Horizon. This was coincident with the higher root density of Horizon in deep soil layers (>40cm), where water content was stable. The results suggest that early vigorous root proliferation may be a useful selection trait for maintaining yield of potato under restricted irrigation or rainfall, because it rapidly secures access to water stored in deep soil layers. Although selecting for vigorous root growth may assist phenotyping screening for drought tolerance, these varieties may require particular environmental or cultural conditions to express root vigour, such as sufficiently deep soils or sufficient water shortly after emergence.

Long-distance abscisic acid signalling under different vertical soil moisture gradients depends on bulk root water potential and average soil water content in the root zone.

To determine how root-to-shoot abscisic acid (ABA) signalling is regulated by vertical soil moisture gradients, root ABA concentration ([ABA](root)), the fraction of root water uptake from, and root water potential of different parts of the root zone, along with bulk root water potential, were measured to test various predictive models of root xylem ABA concentration [RX-ABA](sap). Beans (Phaseolus vulgaris L. cv. Nassau) were grown in soil columns and received different irrigation treatments (top and basal watering, and withholding water for varying lengths of time) to induce different vertical soil moisture gradients. Root water uptake was measured at four positions within the column by continuously recording volumetric soil water content (θv). Average θv was inversely related to bulk root water potential (Ψ(root)). In turn, Ψ(root) was correlated with both average [ABA](root) and [RX-ABA](sap). Despite large gradients in θv, [ABA](root) and root water potential was homogenous within the root zone. Consequently, unlike some split-root studies, root water uptake fraction from layers with different soil moisture did not influence xylem sap (ABA). This suggests two different patterns of ABA signalling, depending on how soil moisture heterogeneity is distributed within the root zone, which might have implications for implementing water-saving irrigation techniques.

Water-use efficiency in cork oak (Quercus suber) is modified by the interaction of water and light availabilities.

We studied the interaction of light and water on water-use efficiency in cork oak (Quercus suber L.) seedlings. One-year-old cork oak seedlings were grown in pots in a factorial experiment with four light treatments (68, 50, 15 and 5% of full sunlight) and two irrigation regimes: well watered (WW) and moderate drought stress (WS). Leaf predawn water potential, which was measured at the end of each of two cycles, did not differ among the light treatments. Water-use efficiency, assessed by carbon isotope composition (delta(13)C), tended to increase with increasing irradiance. The trend was similar in the WW and WS treatments, though with lower delta(13)C in all light treatments in the WW irrigation regime. Specific leaf area increased with decreasing irradiance, and was inversely correlated with delta(13)C. Thus, changes in delta(13)C could be explained in part by light-induced modifications in leaf morphology. The relationship between stomatal conductance to water vapor and net photosynthesis on a leaf area basis confirmed that seedlings in higher irradiances maintained a higher rate of carbon uptake at a particular stomatal conductance, implying that shaded seedlings have a lower water-use efficiency that is unrelated to water availability.