Root Hydraulic Conductivity Research Articles

Reassessing the Role of Potassium in Tomato Grown with Water Shortages

Potassium (K) is closely related to plant water uptake and use and affects key processes in assimilation and growth. The aim of this work was to find out to what extent K supply and enhanced compartmentation might improve water use and productivity when tomato plants suffered from periods of water stress. Yield, water traits, gas exchange, photosynthetic rate and biomass partition were determined. When plants suffered dehydration, increasing K supply was associated with reduction in stomatal conductance and increased water contents, but failed to protect photosynthetic rate. Potassium supplements increased shoot growth, fruit setting and yield under water stress. However, increasing the K supply could not counteract the great yield reduction under drought. A transgenic tomato line with enhanced K uptake into vacuoles and able to reach higher plant K contents, still showed poor yield performance under water stress and had lower K use efficiency than the control plants. With unlimited water supply (hydroponics), plants grown in low-K showed greater root hydraulic conductivity than at higher K availability and stomatal conductance was not associated with leaf K concentration. In conclusion, increasing K supply and tissue content improved some physiological features related to drought tolerance but did not overcome yield restrictions imposed by water stress.

Tomato Rootstocks Mediate Plant-Water Relations and Leaf Nutrient Profiles of a Common Scion Under Suboptimal Soil Temperatures.

Environments with short growing seasons and variable climates can have soil temperatures that are suboptimal for chilling-sensitive crops. These conditions can adversely affect root growth and physiological performance thus impairing water and nutrient uptake. Four greenhouse trials and a field study were conducted to investigate if rootstocks can enhance tomato performance under suboptimal soil temperatures (SST). In a controlled greenhouse environment, we exposed four commercial rootstocks (Estamino, Maxifort, RST-04-106-T, and Supernatural) grafted with a common scion (cv. BHN-589) to optimal (mean: 24°C) and SST (mean: 13.5°C) and compared their performance with the non-grafted BHN-589 cultivar. Several root and shoot physiological traits were evaluated: root hydraulic conductivity and conductance, root anatomy, leaf gas exchange, leaf δ13C, shoot C and N, and biomass. Under field conditions, the same five phenotypes were evaluated for canopy growth, normalized difference vegetation index (NDVI), leaf nutrients, biomass, and yield. Under SST, root hydraulic conductivity (Lp) and conductance (KR), stomatal conductance (gs), and plant biomass decreased. Hydrostatic Lp decreased more than osmotic Lp (Lp∗hyd: 39–65%; Lp∗os: 14–40%) and some of the reduced conductivity was explained by the increased cortex area of primary roots observed under SST (67–140%). Under optimal soil temperatures, all rootstocks conferred higher gs than the non-grafted cultivar, but only two rootstocks maintained higher gs under SST. All phenotypes showed greater reductions in shoot biomass than root biomass resulting in greater (∼20%) root-to-shoot ratios. In the field, most grafted phenotypes increased early canopy cover, NDVI, shoot biomass, and fruit yield. Greenhouse results showed that Lp∗os may be less affected by SST than Lp∗hyd and that reductions in Lp may be offset by enhanced root-to-shoot ratios. We show that some commercial rootstocks possess traits that maintained better rates of stomatal conductance and shoot N content, which can contribute toward better plant establishment and improved performance under SST.

Rhizosphere plant-microbe interactions under water stress.

Climate change, with its extreme temperature, weather and precipitation patterns, is a major global concern of dryland farmers, who currently meet the challenges of climate change agronomically and with growth of drought-tolerant crops. Plants themselves compensate for water stress by modifying aerial surfaces to control transpiration and altering root hydraulic conductance to increase water uptake. These responses are complemented by metabolic changes involving phytohormone network-mediated activation of stress response pathways, resulting in decreased photosynthetic activity and the accumulation of metabolites to maintain osmotic and redox homeostasis. Phylogenetically diverse microbial communities sustained by plants contribute to host drought tolerance by modulating phytohormone levels in the rhizosphere and producing water-sequestering biofilms. Drylands of the Inland Pacific Northwest, USA, illustrate the interdependence of dryland crops and their associated microbiota. Indigenous Pseudomonas spp. selected there by long-term wheat monoculture suppress root diseases via the production of antibiotics, with soil moisture a critical determinant of the bacterial distribution, dynamics and activity. Those pseudomonads producing phenazine antibiotics on wheat had more abundant rhizosphere biofilms and provided improved tolerance to drought, suggesting a role of the antibiotic in alleviation of drought stress. The transcriptome and metabolome studies suggest the importance of wheat root exudate-derived osmoprotectants for the adaptation of these pseudomonads to the rhizosphere lifestyle and support the idea that the exchange of metabolites between plant roots and microorganisms profoundly affects and shapes the belowground plant microbiome under water stress.

The Singular Molecular Conformation of Humic Acids in Solution Influences Their Ability to Enhance Root Hydraulic Conductivity and Plant Growth.

Some studies have reported that the capacity of humic substances to improve plant growth is dependent on their ability to increase root hydraulic conductivity. It was proposed that this effect is directly related to the structural conformation in solution of these substances. To study this hypothesis, the effects on root hydraulic conductivity and growth of cucumber plants of a sedimentary humic acid and two polymers—polyacrylic acid and polyethylene glycol—presenting a molecular conformation in water solution different from that of the humic acid have been studied. The results show that whereas the humic acid caused an increase in root hydraulic conductivity and plant growth, both the polyacrylic acid and the polyethylene glycol did not modify plant growth and caused a decrease in root hydraulic conductivity. These results can be explained by the different molecular conformation in water solution of the three molecular systems. The relationships between these biological effects and the molecular conformation of the three molecular systems in water solution are discussed.

A redundant hydraulic function of root hairs in barley plants grown in hydroponics.

The root hair-less brb of Hordeum vulgare L. (bald root barley) mutant was used to assess the significance that root hairs have for the hydraulic properties of roots and response to a limited supply of mineral nutrients in plants grown on hydroponics. The barley brb mutant and its parent wild-type (H. vulgare cv. Pallas) were grown under nutrient sufficient control conditions, and under conditions of low supply of P and N. Plants were analysed when they were 14-18 days old. Root hydraulic conductivity (Lp) was determined for excised root systems and intact transpiring plants, and cell Lp was determined through cell pressure probe measurements. The formation of Casparian bands and suberin lamellae was followed through staining of cross-sections. The presence or absence of root hairs had no effect on the overall hydraulic response of plants to nutritional treatments. Root and cell Lp did not differ between the two genotypes. The most apparent difference between brb and wild-type plants was the consistently reduced formation of apoplastic barriers in brb plants. Any hydraulic function of root hairs can be redundant in barley, at least under the hydroponic conditions tested.

Zinc regulates the hydraulic response of maize root under water stress conditions

Zinc (Zn) is involved in plant growth and stress resistance and is known to increase crop yield. Here, we investigated the effect of Zn on water absorption in the roots of maize (Zea mays L.), a crop which is sensitive to Zn deficiency, during water stress conditions. Seedlings of the maize variety “Zhengdan 958” were cultivated with 0.1 or 6 μM ZnSO4·7H2O. To simulate drought stress, three-week-old seedlings were exposed to 15% polyethylene glycol (PEG). Root growth parameters, root antioxidant enzyme activity, root hydraulic conductivity, root aquaporin gene expression, root and leaf anatomy structure, leaf water potential, chlorophyll content, leaf area, and gas exchange parameters were measured. Under water stress, moderate Zn treatment promoted root growth; maintained root and leaf anatomy structural integrity. Moderate Zn significantly increased roots hydraulic conductivity (51%) and decreased roots antioxidant enzyme activity (POD: −11.1%, CAT: −35.1%, SOD: −3.1%) compared with low-level Zn under water stress. The expression of ZmPIP1;1, ZmPIP1;2, and ZmPIP2;2 was significantly higher with moderate Zn treatment than that of low-level Zn treatment. The leaf water potential, chlorophyll content, leaf area, and gas exchange parameters with moderate Zn treatment increased significantly under water stress compared with low-level Zn treatment. The moderate concentration of Zn improved root hydraulic conductivity in maize and increased resistance to simulated drought conditions by maintaining root structural integrity, decreasing antioxidant enzyme activity, and increasing aquaporin gene expression. Moderate Zn application increased root water absorption and leaf transpiration, thereby maintaining maize water balance under water stress conditions.

The State of the Art in Modeling Waterlogging Impacts on Plants: What Do We Know and What Do We Need to Know

AbstractModels are key tools in our quest to better understand the impacts of soil waterlogging on plant growth and crop production. Here, we reviewed the state of the art of modeling approaches and compared the conceptual design of these models with recent experimental findings. We show that many models adopt an aeration stress (AS) principle where surplus water reduces air‐filled porosity, with implications for root growth. However, subsequent effects of AS within each model vary considerably. In some cases, AS inhibits biomass accumulation (e.g. AquaCrop), altering processes prior to biomass accumulation such as light interception (e.g. APSIM), or photosynthesis and carbohydrate accumulation (e.g. SWAGMAN Destiny). While many models account for stage‐dependent waterlogging effects, few models account for experimentally observed delays in phenology caused by waterlogging. A model intercomparison specifically designed for long‐term waterlogged conditions (APSIM‐Oryza) with models developed for dryland conditions with transient waterlogging would advance our understanding of the current fitness for purpose of exsiting frameworks for simulating transient waterlogging in dryland cropping systems. Of the point‐based dynamic models examined here, APSIM‐Soybean and APSIM‐Oryza simulations most closely matched with the observed data, while GLAM‐WOFOST achieved the highest performance of the spatial‐regional models examined. We conclude that future models should incorporate waterlogging effects on genetic tolerance parameters such as (1) phenology of stress onset, (2) aerenchyma, (3) root hydraulic conductance, (4) nutrient‐use efficiency, and (5) plant ion (e.g. Fe/Mn) tolerance. Incorporating these traits/effects into models, together with a more systematic model intercomparison using consistent initialization data, will significantly improve our understanding of the relative importance of such factors in a systems context, including feedbacks between biological factors, emergent properties, and sensitive variables responsible for yield losses under waterlogging.

The apple DNA-binding one zinc-finger protein MdDof54 promotes drought resistance

DNA-binding one zinc-finger (Dof) proteins constitute a family of transcription factors with a highly conserved Dof domain that contains a C2C2 zinc-finger motif. Although several studies have demonstrated that Dof proteins are involved in multiple plant processes, including development and stress resistance, the functions of these proteins in drought stress resistance are largely unknown. Here, we report the identification of the MdDof54 gene from apple and document its positive roles in apple drought resistance. After long-term drought stress, compared with nontransgenic plants, MdDof54 RNAi plants had significantly shorter heights and weaker root systems; the transgenic plants also had lower shoot and root hydraulic conductivity, as well as lower photosynthesis rates. By contrast, compared with nontransgenic plants, MdDof54-overexpressing plants had higher photosynthesis rates and shoot hydraulic conductivity under long-term drought stress. Moreover, compared with nontransgenic plants, MdDof54-overexpressing plants had higher survival percentages under short-term drought stress, whereas MdDof54 RNAi plants had lower survival percentages. MdDof54 RNAi plants showed significant downregulation of 99 genes and significant upregulation of 992 genes in response to drought, and 366 of these genes were responsive to drought. We used DAP-seq and ChIP-seq analyses to demonstrate that MdDof54 recognizes cis-elements that contain an AAAG motif. Taken together, our results provide new information on the functions of MdDof54 in plant drought stress resistance as well as resources for apple breeding aimed at the improvement of drought resistance.

Drought resistance in oat involves ABA-mediated modulation of transpiration and root hydraulic conductivity

Drought is one of the most important constraints to crop productivity worldwide. Control of plant responses to drought is very complex. The mechanisms and their intensity may differ between species and/or genotypes ultimately conditioning tolerance or susceptibility. We explore here the strategy set up by two oat cultivars to cope with drought based on root morphological, anatomical, physiological and molecular studies. A dramatic and rapid abscisic acid increase in the susceptible genotype resulted in a tight and rapid reduction of stomatal conductance. Despite of this, leaf water potential decreased concomitantly due to a decrease in root hydraulic conductivity. By contrast, the resistant genotype, showed a mild and slow increase in abscisic acid that allowed maintaining transpiration longer. This response was linked to an increase in root hydraulic conductance through an increase in total root length and in the length of the thinnest roots as well as a rise in root conductivity. This was also coupled with anatomical changes leading to a reduction of metabolic cost. These changes allowed the resistant genotype to maintain higher water potential reducing drought symptoms and promoting growth under water deficit conditions.

Effects of Plant Growth Promoting Rhizobacteria on the Content of Abscisic Acid and Salt Resistance of Wheat Plants.

Although salinity inhibits plant growth, application of appropriate rhizosphere bacteria can diminish this negative effect. We studied one possible mechanism that may underlie this beneficial response. Wheat plants were inoculated with Bacillus subtilis IB-22 and Pseudomonas mandelii IB-Ki14 and their consequences for growth, water relations, and concentrations of the hormone abscisic acid (ABA) were followed in the presence of soil salinity. Salinity alone increased ABA concentration in wheat leaves and roots and this was associated with decreased stomatal conductance, but also with chlorophyll loss. Bacterial treatment raised ABA concentrations in roots, suppressed accumulation of leaf ABA, decreased chlorophyll loss, and promoted leaf area and transpiration. However, water balance was maintained due to increased water uptake by inoculated plants, brought about in part by a larger root system. The effect may be the outcome of ABA action since the hormone is known to maintain root extension in stressed plants. Root ABA concentration was highest in salt-stressed plants inoculated with B. subtilis and this contributed to greater root hydraulic conductivity. We conclude that bacteria can raise salt resistance in wheat by increasing root ABA, resulting in larger root systems that can also possess enhanced hydraulic conductivity thereby supporting better-hydrated leaves.

Comparison of root water uptake models in simulating CO<sub>2</sub> and H<sub>2</sub>O fluxes and growth of wheat

Abstract. Stomatal regulation and whole plant hydraulic signaling affect water fluxes and stress in plants. Land surface models and crop models use a coupled photosynthesis–stomatal conductance modeling approach. Those models estimate the effect of soil water stress on stomatal conductance directly from soil water content or soil hydraulic potential without explicit representation of hydraulic signals between the soil and stomata. In order to explicitly represent stomatal regulation by soil water status as a function of the hydraulic signal and its relation to the whole plant hydraulic conductance, we coupled the crop model LINTULCC2 and the root growth model SLIMROOT with Couvreur's root water uptake model (RWU) and the HILLFLOW soil water balance model. Since plant hydraulic conductance depends on the plant development, this model coupling represents a two-way coupling between growth and plant hydraulics. To evaluate the advantage of considering plant hydraulic conductance and hydraulic signaling, we compared the performance of this newly coupled model with another commonly used approach that relates root water uptake and plant stress directly to the root zone water hydraulic potential (HILLFLOW with Feddes' RWU model). Simulations were compared with gas flux measurements and crop growth data from a wheat crop grown under three water supply regimes (sheltered, rainfed, and irrigated) and two soil types (stony and silty) in western Germany in 2016. The two models showed a relatively similar performance in the simulation of dry matter, leaf area index (LAI), root growth, RWU, gross assimilation rate, and soil water content. The Feddes model predicts more stress and less growth in the silty soil than in the stony soil, which is opposite to the observed growth. The Couvreur model better represents the difference in growth between the two soils and the different treatments. The newly coupled model (HILLFLOW–Couvreur's RWU–SLIMROOT–LINTULCC2) was also able to simulate the dynamics and magnitude of whole plant hydraulic conductance over the growing season. This demonstrates the importance of two-way feedbacks between growth and root water uptake for predicting the crop response to different soil water conditions in different soils. Our results suggest that a better representation of the effects of soil characteristics on root growth is needed for reliable estimations of root hydraulic conductance and gas fluxes, particularly in heterogeneous fields. The newly coupled soil–plant model marks a promising approach but requires further testing for other scenarios regarding crops, soil, and climate.

Aquaporins are main contributors to root hydraulic conductivity in pearl millet [Pennisetum glaucum (L) R. Br.

Pearl millet is a key cereal for food security in arid and semi-arid regions but its yield is increasingly threatened by water stress. Physiological mechanisms relating to conservation of soil water or increased water use efficiency can alleviate that stress. Aquaporins (AQP) are water channels that mediate root water transport, thereby influencing plant hydraulics, transpiration and soil water conservation. However, AQP remain largely uncharacterized in pearl millet. Here, we studied AQP function in root water transport in two pearl millet lines contrasting for water use efficiency (WUE). We observed that these lines also contrasted for root hydraulic conductivity (Lpr) and AQP contribution to Lpr. The line with lower WUE showed significantly higher AQP contribution to Lpr. To investigate AQP isoforms contributing to Lpr, we developed genomic approaches to first identify the entire AQP family in pearl millet and secondly, characterize the plasma membrane intrinsic proteins (PIP) gene expression profile. We identified and annotated 33 AQP genes in pearl millet, among which ten encoded PIP isoforms. PgPIP1-3 and PgPIP1-4 were significantly more expressed in the line showing lower WUE, higher Lpr and higher AQP contribution to Lpr. Overall, our study suggests that the PIP1 AQP family are the main regulators of Lpr in pearl millet and may possibly be associated with mechanisms associated to whole plant water use. This study paves the way for further investigations on AQP functions in pearl millet hydraulics and adaptation to environmental stresses.

Photosynthetically active radiation impacts significantly on root and cell hydraulics in barley (Hordeum vulgare L.).

Photosynthetically active radiation (PAR) affects transpirational water loss, yet we do not know through which mechanisms root water uptake is adjusted in parallel. Here, we exposed hydroponically grown barley plants to three levels of PAR [Normal (control), Low, High] and focused on the role which aquaporins (AQPs), apoplastic barriers (Casparian bands, suberin lamellae) and root morphology play in the adjustment of root hydraulic conductivity (Lp). Plants were analyzed when they were 14-18 days (d) old. Root and cell Lp, which involves AQP activity, was determined through exudation and cell pressure probe measurements, respectively. Gene expression of AQPs was analyzed through qPCR. The formation of apoplastic barriers was studied through staining of cross-sections. The rate of transpirational water loss per plant and unit leaf area increased in response to high-PAR and decreased in response to low-PAR treatments, both during day and night. Hydraulic conductivity in roots decreased significantly at organ and cell level in response to Low-PAR, and increased (organ) or did not change (cell level) in response to High-PAR. The formation of apoplastic barriers was little affected by PAR. Gene expression of AQPs tended to be highest in the Low-PAR treatment. Lateral roots, showing few apoplastic barriers, contributed the least in Low- and the most to root surface area in High-PAR plants. It is concluded that barley plants which experience changes in shoot transpirational water loss in response to PAR adjust root water uptake through changes in root Lp, and that these changes are mediated through altered AQP activity and root morphology.

Aluminum-induced stomatal closure is related to low root hydraulic conductance and high ABA accumulation

Many studies ask how aluminum (Al) reduces the root growth, but as Al is mostly retained in the root system, physiological explanations for the also expected Al-induced decrease in stomatal conductance (gs) are unclear, mainly in well-watered conditions. We exposed tomato plants (Solanum lycopersicum) to 0, 25, 50 and 100 μM Al in nutrient solution to investigate whether Al impairs root hydraulic conductance (Lpr), affecting leaf water potential (Ψleaf) and possibly inducing abscisic acid (ABA) accumulation in roots and/or leaves. We also measured ABA delivery rate, xylem sap pH and the root/leaf area ratio in order to explain the low gs in plants exposed to Al. Declines in Lpr and gs were proportional to the increase in Al concentration, and all Al treatments similarly decreased Ψleaf, indicating the plant’s attempt to reduce water loss through transpiration while accumulating more ABA. Despite Al-induced increases in root ABA, the root-to-shoot delivery of ABA did not enhance, but Al caused root xylem sap alkalization. Despite the stability of root/leaf area ratio across a range of Al concentrations (0, 25 and 50 μM Al), the leaf hydration and stomatal opening was not conserved. Here we provide the first evidence that decreases in Lpr and increases in ABA might explain Al-induced stomatal closure.

Properties of root water transport in canola (Brassica napus) subjected to waterlogging at the seedling, flowering and podding growth stages

The objective was to clarify the role of root water transport in waterlogged canola plants. We examined the hypothesis that waterlogging triggers root suberization and lignification, which reduces the effectiveness of apoplastic bypass and increases the dependence of roots on the aquaporin-mediated water transport, and that this effect varies depending on the growth stage. Brassica napus plants at the seedling, flowering and podding growth stages were waterlogged for up to 8 days. Growth, gas exchange, leaf water potentials, and root hydraulic conductance were measured in addition to relative contributions of the aquaporin-mediated and apoplastic root water transport, gene expression levels of plasma membrane intrinsic proteins (BnPIPs), and lignin and suberin deposition in roots. Waterlogging decreased dry weights, gas exchange, leaf water potentials and root hydraulic conductivity more in plants at the seedling stage than other growth stages. It also accelerated root suberization and lignification resulting in an increase of a relative contribution of aquaporin-mediated water transport. The transcript levels of BnPIPs increased with an increasing duration of waterlogging. Effects on root water transport played an important role in the sensitivity of canola seedlings to waterlogging. Increased relative contribution of aquaporin-mediated water transport in waterlogged plants was accompanied by enhanced root suberization and lignification. The evidence points to the importance of maintaining functional aquaporins in plant survival of waterlogging.

Accession difference in leaf photosynthesis, root hydraulic conductance and gene expression of root aquaporins under salt stress in barley seedlings

ABSTRACT Soil salinity causes considerable losses of crop productivity. Barley (Hordeum vulgare) is one of the most salt-tolerant Gramineae crops. Previously, we found that net photosynthetic rate (A n) was kept remarkably higher in the salt-tolerant barley accession OUE812 than in the salt-sensitive accession OUC613 after heading under salt stress due to the low level of salt accumulation in leaves. Here we grew seedlings in Hoagland solution with 100 mM NaCl (salt treatment) or without added NaCl (control), and compared A n, stomatal conductance (g s), salt accumulation in leaves, root hydraulic conductance and gene expression of root aquaporins between the accessions under salt stress for a few days. A n, g s and root hydraulic conductance of the plants with salt treatment decreased significantly in OUC613 compared to OUE812 with no accession difference in salt accumulation in leaves at 2 days after the onset of treatment (DAT). The reduction in root hydraulic conductance in OUC613 was caused by the reduction of the root hydraulic conductivity (Lp r). Salt treatment also decreased the transcript levels of some plasma membrane intrinsic aquaporin genes (HvPIPs) in OUC613 and, on the contrary, increased those of some HvPIPs in OUE812, resulting in a large difference between OUC613 and OUE812 in the transcript levels at 2 DAT. The accession difference in HvPIPs expression and thus Lp r was closely associated with the accession difference in A n and g s under the short-term salt stress.

Exogenous 2-(3,4-Dichlorophenoxy) triethylamine alleviates salinity stress in maize by enhancing photosynthetic capacity, improving water status and maintaining K+/Na+ homeostasis

BackgroundSoil salinity restricts plant growth and productivity. 2-(3,4-dichlorophenoxy) triethylamine (DCPTA) can alleviate salinity stress in plants. However, the mechanism of DCPTA-mediated salinity tolerance has not been fully clarified. We aimed to investigate its role in enhancing photosynthetic capacity, improving water status, maintaining K+/Na+ homeostasis and alleviating salinity stress in maize (Zea mays L.).ResultsIn present study, maize seedlings were grown in nutrient solutions with a combination of NaCl (0, 150 mM) and DCPTA (0, 20, 100, and 400 μM). And photosynthesis, water status, ion homeostasis and the expression of genes involved in ion uptake and transport were evaluated in the maize seedlings. The results demonstrated that DCPTA alleviated the growth inhibition of maize seedlings exposed to salinity stress by increasing the net photosynthetic rate (Pn) and the quantum efficiency of photosystem II (PSII) photochemistry. DCPTA improved the root hydraulic conductivity, which help maintained the water status. A relatively high K+ concentration but a relatively low Na+ concentration and the Na+/K+ ratio were observed in the presence of DCPTA under salinity stress. Additionally, DCPTA altered the expression of four genes (ZmSOS1, ZmHKT1, ZmNHX1 and ZmSKOR) that encode membrane transport proteins responsible for K+/Na+ homeostasis.ConclusionsDCPTA improved the salinity tolerance of maize may be associated with enhanced photosynthetic capacity, maintenance of water status and altered expression of genes involved in ion uptake and transport.

Transpiration difference under high evaporative demand in chickpea (Cicer arietinum L.) may be explained by differences in the water transport pathway in the root cylinder.

Terminal drought substantially reduces chickpea yield. Reducing water use at vegetative stage by reducing transpiration under high vapor pressure deficit (VPD), i.e. under dry/hot conditions, contributes to drought adaptation. We hypothesized that this trait could relate to differences in a genotype's dependence on root water transport pathways and hydraulics. Transpiration rate responses in conservative and profligate chickpea genotypes were evaluated under increasing VPD in the presence/absence of apoplastic and cell-to-cell transport inhibitors. Conservative genotypes ICC 4958 and ICC 8058 restricted transpiration under high VPD compared to the profligate genotypes ICC 14799 and ICC 867. Profligate genotypes were more affected by aquaporin inhibition of the cell-to-cell pathway than conservative genotypes, as measured by the root hydraulic conductance and transpiration under high VPD. Aquaporin inhibitor treatment also led to a larger reduction in root hydraulic conductivity in profligate than in conservative genotypes. In contrast, blockage of the apoplastic pathway in roots decreased transpiration more in conservative than in profligate genotypes. Interestingly, conservative genotypes had high early vigour, whereas profligate genotypes had low early vigour. In conclusion, profligate genotypes depend more on the cell-to-cell pathway, which might explain their higher root hydraulic conductivity, whereas water-saving by restricting transpiration led to higher dependence on the apoplastic pathway. This opens the possibility to screen for conservative or profligate chickpea phenotypes using inhibitors, itself opening to the search of the genetic basis of these differences.

The expression response of plasma membrane aquaporins to salt stress in tomato plants

Aquaporin isoforms have distinct functions, and show differential responses to stress conditions among different plant species. In tomato (Solanum lycopersicum), the functions of individual aquaporin isoforms under environmental stresses remain largely unknown. As a step toward understanding their functions under stress conditions, the expression response of plasma membrane intrinsic proteins (PIPs) to salt stress was investigated. Tomato seedlings were subjected to 150 mM NaCl for 1 h and 24 h, and water uptake and SlPIP expressions were investigated. The results showed that the root hydraulic conductivity (Lpr) was decreased under salt stress. Salt stress for 1 h had no effect or increased the expressions of most of the highly expressed SlPIPs in roots and leaves. Salt stress inhibited cell-to-cell water transport. Protein phosphatase and endocytosis inhibitors partially inhibited the Lpr decrease under salt stress. After a 24-h salt stress, the expressions of most SlPIPs were decreased in both roots and leaves, and a recovery treatment in control nutrient solution demonstrated a partial or full expression restoration of most previously downregulated SlPIPs. The expressions of SlPIP isoforms highly varied in both roots and leaves, and across different stress durations. SlPIP1;1, SlPIP1;3, SlPIP1;7, SlPIP2;10 and SlPIP2;12 demonstrated consistent expression responses to salt stress and recovery treatment in both roots and leaves, implying their important roles in regulating water transport. Our results suggest that SlPIPs are involved in mediating water transport in tomato, and their regulations are associated with stress duration and tissue type. At early stage of salt stress, the inhibition of Lpr in tomato might be due to the decrease of SlPIP activity; while at later stress period, the Lpr reduction might be mainly attributed to the decrease of SlPIP expression.

Effect of traditional soybean breeding on water use strategy in arid and semi-arid areas

Abstract Water is the main factor determining yield performance under drought conditions. Traditional soybean breeding has significantly increased grain yield under drought, but its effect on water use strategies and associated traits are not well understood. Field and pot experiments with new cultivars and landrace soybean genotypes were undertaken to identify the effect of soybean breeding on water use strategies and related leaf and root functional traits under different water regimes. The new cultivars, on average, had 33.9 %, 45.1 % and 169 % higher grain yields and 47.9 %, 98.4 % and 244 % higher water use efficiencies for grain yield (WUEG) but 8.8 %, 38.1 % and 19.6 % lower water use than the landraces in the field and pot experiments 1 and 2, respectively. Stomatal conductance decreased in the new cultivars at higher soil water contents than the landraces during soil drying. The new cultivars, on average, had a 22.9 % lower root fraction, 21.5 % lower leaf fraction, 20.6 % lower stem fraction and 69.8 % higher pod fraction than landraces, which had 21.7 % and 26.8 % lower root hydraulic conductance at the R2 and R6 stages, respectively, than the landraces. Our results show that (1) traditional soybean breeding has changed the water use strategy from profligate to conservative and improved yield performance and WUEG, (2) the high sensitivity of stomatal conductance to water stress is coordinated with lower root hydraulic conductance to limit water use, and (3) selecting new cultivars with high grain yield but low water use may a new way to improve yield performance under water-limited conditions.