Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees.

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (Anet ) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO2 (eCO2 ), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO2 on iWUE and its components Anet and stomatal conductance (gs ). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf-air vapour pressure difference (D). We expected smaller gs , but greater Anet , responses to eCO2 in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO2 in all PFTs, and that increases in Anet had stronger effects than reductions in gs . The USO model correctly captured stomatal behaviour with eCO2 across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g1 ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO2 conditions.

Elevated CO2 Impacts on Plant-Pollinator Interactions: A Systematic Review and Free Air Carbon Enrichment Field Study.

The impact of elevated CO2 (eCO2) on plant-pollinator interactions is poorly understood. This study provides the first systematic review of this topic and identifies important knowledge gaps. In addition, we present field data assessing the impact of eCO2 (150 ppm above ambient) on bluebell (Hyacinthoides non-scripta)-pollinator interactions within a mature, deciduous woodland system. Since 1956, only 71 primary papers have investigated eCO2 effects on flowering time, floral traits and pollination, with a mere 3 studies measuring the impact on pollination interactions. Our field experiment documented flowering phenology, flower visitation and seed production, as well as the abundance and phenology of dominant insect pollinators. We show that first and mid-point flowering occurred 6 days earlier under eCO2, but with no change in flowering duration. Syrphid flies and bumble bees were the dominant flower visitors, with peak activity recorded during mid- and late-flowering periods. Whilst no significant difference was recorded in total visitation or seed set between eCO2 and ambient treatments, there were clear patterns of earlier flowering under eCO2 accompanied by lower pollinator activity during this period. This has implications for potential loss of synchrony in pollination systems under future climate scenarios, with associated long-term impacts on abundance and diversity.

Comprehensive list of metabolites measured by DI-FTICR mass spectrometry in thyme plants with contrasting tolerance to drought.

This article contains data related to the main research entitled "Metabolomic approach reveals the biochemical mechanisms underlying drought stress tolerance in Thyme" (Moradi et al., 2017) [1]. Two thyme populations with contrasting drought tolerance were subjected to long term water deficit. Leaf samples harvested at the end of stress period and bi-phasic extraction carried out to get polar and non-polar fractions. Extracted samples were analyzed through Direct Infusion FT-ICR mass spectrometry. Date files comprise of four separate tables for all the putatively identified metabolites and their intensities in watered and droughted plants. P-values beside each m/z values indicate significances of difference between peak intensities of stressed and control conditions.

Metabolomic approach reveals the biochemical mechanisms underlying drought stress tolerance in thyme.

Thyme as a perennial herb has been recognized globally for its antimicrobial, antiseptic and spasmolytic effects. In this investigation, we have used non-targeted metabolite and volatile profiling combined with the morpho-physiological parameters in order to understand the responses at the metabolite and physiological level in drought sensitive and tolerant thyme plant populations. The results at the metabolic level identified the significantly affected metabolites. Significant metabolites belonging to different chemical classes consisting amino acids, carbohydrates, organic acids and lipids have been compared in tolerant and sensitive plants. These compounds may take a role through mechanisms including osmotic adjustment, ROS scavenging, cellular components protection and membrane lipid changes, hormone inductions in which the key metabolites were proline, betain, mannitol, sorbitol, ascorbate, jasmonate, unsaturated fatty acids and tocopherol. Regarding with volatile profiling, sensitive plants showed an increased-then-decreased trend at major terpenes apart from alpha-cubebene and germacrene-D. In contrast, tolerant populations had unchanged terpenes during the water stress period with an elevation at last day. These results suggesting that the two populations are employing different strategies. The combination of metabolite profiling and physiological parameters assisted to understand precisely the mechanisms of plant response at volatile metabolome level.

Trait Specific Expression Profiling of Salt Stress Responsive Genes in Diverse Rice Genotypes as Determined by Modified Significance Analysis of Microarrays.

Stress responsive gene expression is commonly profiled in a comparative manner involving different stress conditions or genotypes with contrasting reputation of tolerance/resistance. In contrast, this research exploited a wide natural variation in terms of taxonomy, origin and salt sensitivity in eight genotypes of rice to identify the trait specific patterns of gene expression under salt stress. Genome wide transcptomic responses were interrogated by the weighted continuous morpho-physiological trait responses using modified Significance Analysis of Microarrays. More number of genes was found to be differentially expressed under salt stressed compared to that of under unstressed conditions. Higher numbers of genes were observed to be differentially expressed for the traits shoot Na(+)/K(+), shoot Na(+), root K(+), biomass and shoot Cl(-), respectively. The results identified around 60 genes to be involved in Na(+), K(+), and anion homeostasis, transport, and transmembrane activity under stressed conditions. Gene Ontology (GO) enrichment analysis identified 1.36% (578 genes) of the entire transcriptome to be involved in the major molecular functions such as signal transduction (>150 genes), transcription factor (81 genes), and translation factor activity (62 genes) etc., under salt stress. Chromosomal mapping of the genes suggests that majority of the genes are located on chromosomes 1, 2, 3, 6, and 7. The gene network analysis showed that the transcription factors and translation initiation factors formed the major gene networks and are mostly active in nucleus, cytoplasm and mitochondria whereas the membrane and vesicle bound proteins formed a secondary network active in plasma membrane and vacuoles. The novel genes and the genes with unknown functions thus identified provide picture of a synergistic salinity response representing the potentially fundamental mechanisms that are active in the wide natural genetic background of rice and will be of greater use once their roles are functionally verified.

Improved techniques for measurement of nanolitre volumes of phloem exudate from aphid stylectomy.

BACKGROUND: When conducting aphid stylectomy, measuring accurate rates of phloem exudation is difficult because the volumes collected are in the nanolitre (nl) range. In a new method, exudate volume was calculated from optical measurement of droplet diameter as it forms on the tip of a severed aphid stylet. Evaporation was shown to decrease the accuracy of the measurement but was countered with the addition of water-saturated mineral oil. Volume measurements by optical estimation of the volume of a sphere suspended in oil was affected by the curvature of the oil surface. In contrast, measuring the exudate volume from optical measurement of droplet-diameter as formed on the tip of a severed aphid stylet, removes any inaccuracies due to oil surface curvature. A modified technique is proposed for measuring exudate volumes without oil by estimating the flow rate from photo-sequences of the collection period; a correction for evaporation is applied later. RESULTS: A change in oil volume of ±1.75% from an optimum volume of 285 µl had a statistically significant effect on droplet measurement, under or over-estimating droplet volume due to optical effects caused by the oil surface. Using microscope image capture and measurement software, a modified method for measuring phloem volume in air was developed, by reducing air exposure during measurement to approximately 5 s for each measurement. Phloem volumes were measured using both techniques with measurements in air being on average 19.9 nl less (SD 18.87, p<0.001) than those made in oil, and there was a strong linear relationship (R(2)=0.942) between the techniques. This linear relationship enabled the development of a correction equation with no significant difference at the 5% level between corrected volumes and actual volumes measured under oil. CONCLUSIONS: This study showed that oil has a significant role in countering evaporation but oil volume must be carefully optimised for optical measurement of droplets to ensure measurement accuracy. A linear correction factor was generated to correct the volumes measured in air for loss due to evaporation and the method provides for a much simpler alternative to previous approaches for measuring exudation rates and volumes from a cut aphid stylet.

Heat stress impedes development and lowers fecundity of the brown planthopper Nilaparvata lugens (Stål).

This study investigated the effects of sub-lethal high temperatures on the development and reproduction of the brown plant hopper Nilaparvata lugens (Stål). When first instar nymphs were exposed at their ULT(50) (41.8°C) mean development time to adult was increased in both males and females, from 15.2±0.3 and 18.2±0.3 days respectively in the control to 18.7±0.2 and 19±0.2 days in the treated insects. These differences in development arising from heat stress experienced in the first instar nymph did not persist into the adult stage (adult longevity of 23.5±1.1 and 24.4±1.1 days for treated males and females compared with 25.7±1.0 and 20.6±1.1 days in the control groups), although untreated males lived longer than untreated females. Total mean longevity was increased from 38.8±0.1 to 43.4±1.0 days in treated females, but male longevity was not affected (40.9±0.9 and 42.2±1.1 days respectively). When male and female first instar nymphs were exposed at their ULT(50) of 41.8°C and allowed to mate on reaching adult, mean fecundity was reduced from 403.8±13.7 to 128.0±16.6 eggs per female in the treated insects. Following exposure of adult insects at their equivalent ULT(50) (42.5°C), the three mating combinations of treated male x treated female, treated male x untreated female, and untreated male x treated female produced 169.3±14.7, 249.6±21.3 and 233.4±17.2 eggs per female respectively, all significantly lower than the control. Exposure of nymphs and adults at their respective ULT(50) temperatures also significantly extended the time required for their progeny to complete egg development for all mating combinations compared with control. Overall, sub-lethal heat stress inhibited nymphal development, lowered fecundity and extended egg development time.

Can tropical insects stand the heat? A case study with the brown planthopper Nilaparvata lugens (Stål).

The brown planthopper Nilaparvata lugens (Stål) is the most serious pest of rice across the world, especially in tropical climates. N. lugens nymphs and adults were exposed to high temperatures to determine their critical thermal maximum (CT(max)), heat coma temperature (HCT) and upper lethal temperature (ULT). Thermal tolerance values differed between developmental stages: nymphs were consistently less heat tolerant than adults. The mean (± SE) CT(max) of nymphs and adult females and males were 34.9±0.3, 37.0±0.2 and 37.4±0.2°C respectively, and for the HCT were 37.7±0.3, 43.5±0.4 and 42.0±0.4°C. The ULT50 values (± SE) for nymphs and adults were 41.8±0.1 and 42.5±0.1°C respectively. The results indicate that nymphs of N. lugens are currently living at temperatures close to their upper thermal limits. Climate warming in tropical regions and occasional extreme high temperature events are likely to become important limiting factors affecting the survival and distribution of N. lugens.

Brown planthopper (N. lugens Stal) feeding behaviour on rice germplasm as an indicator of resistance.

BACKGROUND: The brown planthopper (BPH) Nilaparvata lugens (Stal) is a serious pest of rice in Asia. Development of novel control strategies can be facilitated by comparison of BPH feeding behaviour on varieties exhibiting natural genetic variation, and then elucidation of the underlying mechanisms of resistance. METHODOLOGY/PRINCIPAL FINDINGS: BPH feeding behaviour was compared on 12 rice varieties over a 12 h period using the electrical penetration graph (EPG) and honeydew clocks. Seven feeding behaviours (waveforms) were identified and could be classified into two phases. The first phase involved patterns of sieve element location including non penetration (NP), pathway (N1+N2+N3), xylem (N5) [21] and two new feeding waveforms, derailed stylet mechanics (N6) and cell penetration (N7). The second feeding phase consisted of salivation into the sieve element (N4-a) and sieve element sap ingestion (N4-b). Production of honeydew drops correlated with N4-b waveform patterns providing independent confirmation of this feeding behaviour. CONCLUSIONS/SIGNIFICANCE: Overall variation in feeding behaviour was highly correlated with previously published field resistance or susceptibility of the different rice varieties: BPH produced lower numbers of honeydew drops and had a shorter period of phloem feeding on resistant rice varieties, but there was no significant difference in the time to the first salivation (N4-b). These qualitative differences in behaviour suggest that resistance is caused by differences in sustained phloem ingestion, not by phloem location. Cluster analysis of the feeding and honeydew data split the 12 rice varieties into three groups: susceptible, moderately resistant and highly resistant. The screening methods that we have described uncover novel aspects of the resistance mechanism (or mechanisms) of rice to BPH and will in combination with molecular approaches allow identification and development of new control strategies.

A mutation in amino acid permease AAP6 reduces the amino acid content of the Arabidopsis sieve elements but leaves aphid herbivores unaffected.

The aim of this study was to investigate the role of the amino acid permease gene AAP6 in regulating phloem amino acid composition and then to determine the effects of this altered diet on aphid performance. A genotype of Arabidopsis thaliana (L.) was produced in which the function of the amino acid permease gene AAP6 (At5g49630) was abolished. Plants homozygous for the insertionally inactivated AAP6 gene had a significantly larger mean rosette width than the wild type and a greater number of cauline leaves. Seeds from the aap6 mutant were also significantly larger than those from the wild-type plants. Sieve element (SE) sap was collected by aphid stylectomy and the amino acids derivatized, separated, and quantified using Capillary Electrophoresis with Laser Induced Fluorescence (CE-LIF). In spite of the large variation across samples, the total amino acid concentration of SE sap of the aap6 mutant plants was significantly lower than that of the wild-type plants. The concentrations of lysine, phenylalanine, leucine, and aspartic acid were all significantly lower in concentration in the aap6 mutant plants compared with wild-type plants. This is the first direct demonstration of a physiological role for an amino acid transporter in regulating SE composition in vivo. The amino acid availability in sieve element sap is thought to be the major limiting factor for aphid growth and reproduction. Despite the changes in their diet, the aphid Myzus persicae (Sulzer) displayed only small changes in feeding behaviour on mutant plants when measured using the Electronic Penetration Graph (EPG) technique. Salivation by the aphid into the SE (E1 phase) was increased on mutant plants but there was no significant effect on other feeding EPG behaviours, or in the rate of honeydew production. Consistent with the small effect on aphid feeding behaviour, there was only a small effect of reduced sieve element amino acid concentration on aphid reproduction. The data are discussed in relation to the regulation of phloem composition and the role of phloem amino acids in regulating aphid performance.

Analysis of mono-, di- and oligosaccharides by CE using a two-stage derivatization method and LIF detection.

A sensitive CE with LIF method has been developed for quantitative analysis of small carbohydrates. In this work, 17 carbohydrates including mono-, di- and oligosaccharides were simultaneously derivatized with 4-fluoro-7-nitrobenzofurazane (NBD-F) via a two-step reaction involving reductive amination with ammonia followed by condensation with NBD-F. Under the optimized derivatization conditions all carbohydrates were successfully derivatized within 2.5 h and separated within 15 min using borate buffer (90 mmol/L, pH 9.2). For sugar standards LODs were in the range of 49.7 to 243.6 nmol/L. Migration time and peak area reproducibility were better than RSD 0.1 and 3%, respectively. The method was applied to measure sugars in nanoliter volume samples of phloem sap obtained by stylectomy from wheat and to honeydew samples obtained from aphids feeding from wheat and willow.

Accumulation of calcium in the centre of leaves of coriander (Coriandrum sativum L.) is due to an uncoupling of water and ion transport.

The aim of this study is to understand the parameters regulating calcium ion distribution in leaves. Accumulation of ions in leaf tissue is in part dependent on import from the xylem. This import via the transpiration stream is more important for ions such as calcium that are xylem but not phloem mobile and cannot therefore be retranslocated. Accumulation of calcium was measured on bulk coriander leaf tissue (Coriandrum sativum L. cv. Lemon) using ion chromatography and calcium uptake was visualized using phosphor-images of (45)Ca(2+). Leaves of plants grown in hydroponics had elevated calcium in the centre of the leaf compared with the leaf margin, while K(+) was distributed homogeneously over the leaf. This calcium was shown to be localised to the mesophyll vacuoles using EDAX. Stomatal density and evapotranspiration (water loss per unit area of leaf) were equal at inner and outer sections of the leaf. Unequal ion distribution but uniformity of water loss suggested that there was a difference in the extent of uncoupling of calcium and water transport between the inner and outer leaf. Since isolated tissue from the inner and outer leaf were able to accumulate similar amounts of calcium, it is proposed that the spatial variation of leaf calcium concentration is due to differential ion delivery to the two regions rather than tissue/cell-specific differences in ion uptake capacity. There was a positive correlation between whole leaf calcium concentration and the difference in calcium concentration between inner and outer leaf tissue. Exposing the plants to increased humidity reduced transpiration and calcium delivery to the leaf and abolished this spatial variation of calcium concentration. Mechanisms of calcium delivery to leaves are discussed. An understanding of calcium delivery and distribution within coriander will inform strategies to reduce the incidence of calcium-related syndromes such as tip-burn and provides a robust model for the transport of ions and other substances in the leaf xylem.

A diurnal component to the variation in sieve tube amino acid content in wheat.

We have used high-sensitivity capillary electrophoresis coupled to a laser-induced fluorescence detection method to quantify 16 amino acids in wheat (Triticum aestivum) sieve tube (ST) samples as small as 2 nL collected by severing the stylets of feeding aphids. The sensitivity of the method was sufficient to determine a quantitative amino acid profile of individual STs without the need to bulk samples to produce larger volumes for analysis. This allowed the observation of the full range of variation that exists in individual STs. Some of the total concentrations of amino acids recorded are higher than those reported previously. The results obtained show variation in the concentrations of phenylalanine (Phe), histidine/valine (His/Val), leucine/isoleucine (Leu/Ile), arginine, asparagine, glutamine, tyrosine (Tyr), and lysine (Lys) across the ST samples. These could not be explained by plant-to-plant variation. Statistical analyses revealed five analytes (Tyr, Lys, Phe, His/Val, and Leu/Ile) that showed striking covariation in their concentrations across ST samples. A regression analysis revealed a significant relationship between the concentrations of Tyr, Lys, Phe, Leu/Ile, His/Val, asparagine, arginine, and proline and the time of collection of ST samples, with these amino acids increasing in concentration during the afternoon. This increase was confirmed to occur in individual STs by analyzing samples obtained from stylet bundles exuding for many hours. Finally, an apparent relationship between the exudation rate of ST sap and its total amino acid concentration was observed: samples containing higher total amino acid concentrations were observed to exude from the severed stylet bundles more slowly.

High susceptibility of Bt maize to aphids enhances the performance of parasitoids of lepidopteran pests.

Concerns about possible undesired environmental effects of transgenic crops have prompted numerous evaluations of such crops. So-called Bt crops receive particular attention because they carry bacteria-derived genes coding for insecticidal proteins that might negatively affect non-target arthropods. Here we show a remarkable positive effect of Bt maize on the performance of the corn leaf aphid Rhopalosiphum maidis, which in turn enhanced the performance of parasitic wasps that feed on aphid honeydew. Within five out of six pairs that were evaluated, transgenic maize lines were significantly more susceptible to aphids than their near-isogenic equivalents, with the remaining pair being equally susceptible. The aphids feed from the phloem sieve element content and analyses of this sap in selected maize lines revealed marginally, but significantly higher amino acid levels in Bt maize, which might partially explain the observed increased aphid performance. Larger colony densities of aphids on Bt plants resulted in an increased production of honeydew that can be used as food by beneficial insects. Indeed, Cotesia marginiventris, a parasitoid of lepidopteran pests, lived longer and parasitized more pest caterpillars in the presence of aphid-infested Bt maize than in the presence of aphid-infested isogenic maize. Hence, depending on aphid pest thresholds, the observed increased susceptibility of Bt maize to aphids may be either a welcome or an undesirable side effect.

Comparison of plant cell turgor pressure measurement by pressure probe and micromanipulation.

The conventional method of measuring plant cell turgor pressure is the pressure probe but applying this method to single cells in suspension culture is technically difficult and requires puncture of the cell wall. Conversely, compression testing by micromanipulation is particularly suited to studies on single cells, and can be used to characterise cell wall mechanical properties, but has not been used to measure turgor pressure. In order to demonstrate that the micromanipulation method can do this, pressure measurements by both methods were compared on single suspension-cultured tomato (Lycopersicon esculentum vf36) cells and generally were in good agreement. This validates further the micromanipulation method and demonstrates its capability to measure turgor pressure during water loss. It also suggests that it might eventually be used to estimate plant cell hydraulic conductivity.

Amino acid analysis by micellar electrokinetic chromatography with laser-induced fluorescence detection: application to nanolitre-volume biological samples from Arabidopsis thaliana and Myzus persicae.

Amino acids were derivatised with 4-fluoro-7-nitrobenzo-2,1,3-oxadiazol (NBD-F), separated by micellar electrokinetic chromatography (MEKC), and detected by argon-ion (488 nm) laser-induced fluorescence. The optimised MEKC background electrolyte conditions were: 40 mM sodium cholate, 5 mM beta-cyclodextrin in 20 mM aqueous borate buffer, pH 9.1, with 7% v/v acetonitrile. Using these conditions, 19 amino acids were separated within 17 min. The limits of detection were in the range of 7.6-42.2 pmol/mL and limits of quantitation from 0.05-0.14 nmol/mL. The method was systematically validated for injection volume error, migration time variation, calibration linearity, accuracy, precision, and recovery. Nanolitre volume samples of phloem sap of individual sieve element cells from the plant Arabidopsis thaliana and honeydew from the aphid Myzus persicae were directly analysed with this method. Quantitative amino acid concentrations in these two biological matrices were profiled for the first time. This method is particularly important because it allows the complete profile of the amino acids obtained from individual phloem elements, allowing cell to cell and plant to plant variation to be quantified, which to date has not been possible with Arabidopsis thaliana.

Cesium toxicity in Arabidopsis.

Cesium (Cs) is chemically similar to potassium (K). However, although K is an essential element, Cs is toxic to plants. Two contrasting hypotheses to explain Cs toxicity have been proposed: (1) extracellular Cs+ prevents K+ uptake and, thereby, induces K starvation; and (2) intracellular Cs+ interacts with vital K(+)-binding sites in proteins, either competitively or noncompetitively, impairing their activities. We tested these hypotheses with Arabidopsis (Arabidopsis thaliana). Increasing the Cs concentration in the agar ([Cs](agar)) on which Arabidopsis were grown reduced shoot growth. Increasing the K concentration in the agar ([K](agar)) increased the [Cs](agar) at which Cs toxicity was observed. However, although increasing [Cs](agar) reduced shoot K concentration ([K](shoot)), the decrease in shoot growth appeared unrelated to [K](shoot) per se. Furthermore, the changes in gene expression in Cs-intoxicated plants differed from those of K-starved plants, suggesting that Cs intoxication was not perceived genetically solely as K starvation. In addition to reducing [K](shoot), increasing [Cs](agar) also increased shoot Cs concentration ([Cs](shoot)), but shoot growth appeared unrelated to [Cs](shoot) per se. The relationship between shoot growth and [Cs](shoot)/[K](shoot) suggested that, at a nontoxic [Cs](shoot), growth was determined by [K](shoot) but that the growth of Cs-intoxicated plants was related to the [Cs](shoot)/[K](shoot) quotient. This is consistent with Cs intoxication resulting from competition between K+ and Cs+ for K(+)-binding sites on essential proteins.

Determining protein identity from sieve element sap in Ricinus communis L. by quadrupole time of flight (Q-TOF) mass spectrometry.

The phloem transport system is a complex tissue that primarily carries photoassimilate from source to sink. Its function depends on anucleate sieve elements (SE) supported by companion cells (CC). In this study, SE sap was sampled and the protein identity of soluble proteins was determined with the aim of understanding the function of proteins within the conduit. Unlike many plants, SE sap exudes from incisions in the bark of Ricinus communis and, although there is a greater possibility of contamination from tissues other than SE, sap can be obtained in sufficient quantities to separate proteins using 2D electrophoresis. Spots were excised for trypsin digest, then analysed by quadrupole time of flight (Q-TOF) mass spectrometry (MS) and database searched to determine sequence identity. Overall, 18 proteins were identified in the SE-enriched sap. Proteins identified that have not previously been identified directly from SE sap included a glycine-rich RNA-binding protein, metallothionein, phosphoglycerate mutase, and phosphopyruvate hydratase. The potential role of the identified protein in SE function is discussed. The protein identification in this study provides a first step towards the goal of a greater understanding of the function of proteins within the SE.

Solute is imported to elongating root cells of barley as a pressure driven-flow of solution.

This work relates solute import to elongating root cells in barley to the water relations of the symplastic pathway under conditions of varied plant K+ status. K+ is a major constituent of phloem sieve element (SE) sap, and as an osmoticum, it is believed to have a role in maintaining SE hydrostatic pressure and thus sap flow from source to sink tissue. The hypothesis that the solute import to elongating root cells is linked to pressure driven flow from the sieve tube is examined.Plants were grown in solutions containing either 0.05 mM (low K) or 2.05 mM (high K) K+ concentration. Solute import to the root elongation zone was estimated from biomass accumulation over time accounting for respiration and root elongation rate. SE sap K+ concentration was measured using X-ray microanalyses and osmotic pressure by picolitre osmometry. SE hydrostatic pressure was measured directly with a pressure probe glued onto an excised aphid stylet. Elongating root cell hydrostatic pressure was measured using a cell pressure probe.The low-K plants had lower SE K+ concentration and SE hydrostatic pressure compared to the high-K plants, but the elongating root cell hydrostatic pressure was similar in both treatments, thus the pressure difference between the SE and elongating root cells was less in the low-K plants compared to the high-K plants.The solute import rate to elongating root cells was lower in the low K plants and the reduction could be accounted for as a pressure driven solute flux, with a reduction both in the pressure difference between root sieve elements and elongating cells, and in the sap concentration.

Turgor, solute import and growth in maize roots treated with galactose.

It has been observed that extension growth in maize roots is almost stopped by exposure to 5 mm d-galactose in the root medium, while the import of recent photoassimilate into the entire root system is temporarily promoted by the same treatment. The aim of this study was to reconcile these two apparently incompatible observations. We examined events near the root tip before and after galactose treatment since the tip region is the site of elongation and of high carbon deposition in the root. The treatment rapidly decreased root extension along the whole growing zone. In contrast, turgor pressure, measured directly with the pressure probe in the cortical cells of the growing zone, rapidly increased by 0.15 MPa within the first hour following treatment, and the increase was maintained over the following 24 h. Both tensiometric measurements and a comparison of turgor pressure with local growth rate demonstrated that a rapid tightening of the cell wall caused the reduction in growth. Single cell sampling showed cell osmotic pressure increased by 0.3 MPa owing to accumulation of both organic and inorganic solutes. The corresponding change in cell water potential was a rise from -0.18 MPa to approximately zero. More mature cells at 14 mm from the root tip (just outside the growing region) showed a qualitatively similar response.