Short-term physiological and biometrical responses of Lepidium sativum seedlings exposed to PET-made microplastics and acid rain.

Plastics enter in terrestrial natural system primarily by agricultural purposes, while acid rain is the result of anthropogenic activities. The synergistic effects of microplastics and acid rain on plant growth are not known. In this study, different sizes of polyethylene terephthalate (PET) and acid rain are tested on Lepidium sativum, in two separate experimental sets. In the first one we treated plants only with PET, in the second one we used PET and acid rain together. In both experimentations we analyzed: i) plant biometrical parameters (shoot height, leaf number, percentage inhibition of seed germination, fresh biomass), and ii) oxidative stress responses (hydrogen peroxide; ascorbic acid and glutathione). Results carried out from our experiments highlighted that different sizes of polyethylene terephthalate are able to affect plant growth and physiological responses, with or without acid rain supplied during acute toxicity (6 days). SHORT DESCRIPTION: This study showed that different sizes of PET microplastics affect physiological and biometrical responses of Lepidum sativum seedlings, with or without acid rain; roots and leaves responded differently.

Effect of various abiotic stressors on some biochemical indices of Lepidium sativum plants.

In this study, the regulation of ascorbate peroxidase (APX) specific activity, anthocyanin, carotenoid, hydrogen peroxide, lipid peroxidation, and protein levels in cress leaves in response to different abiotic stresses were investigated. The total APX specific activity was significantly elevated after 9 days of drought treatment, short-term (2 h) exposure to 10, 100 and 370 µE of light, long-term exposure (at least 6 days) to 100 mM NaCl versus the specific APX activity in the controls. Furthermore, a significant change in total APX activity was detected in response to treatment with different temperatures; this change was an early response to 4 °C and 30 °C for a maximum of 4 h, while short-term exposure to 35 °C did not change total APX activity. The results of the present study revealed that plants have a wide range of mechanisms to cope with different stresses that possibly involve morphological changes. The results indicated that Lepidium sativum plants launch common protective pathways only under drought, salinity and high light stresses, while other protective mechanisms/strategies could be responsible for increasing the plants tolerance towards temperature and low light. Future studies will investigate changes in the photosynthetic quantum yield and specific target metabolites, proteins, and nonenzymatic antioxidants.

Addition of wood sawdust during the co-composting of sewage sludge and wheat straw influences seeds germination.

The objective of this study was to investigate the effect of addition of sawdust co-composted with sewage sludge and wheat straw on seeds germination. Two mixtures were piled and composted over 90 days. The first mixture (C1) was composed of sewage sludge and wheat straw, while the second mixture (C2) was composed of sewage sludge, wheat straw and wood sawdust. The results showed that the physicochemical parameters of both composts, i.e., temperature (> 55 °C in the thermophilic phase), moisture content (~ 30%). pH (6.73 for C1 and 7.19 for C2) and EC values (1.81 mS cm-1 for C1 and 1.32 mS cm-1 for C2) reached the required maturity standard. The values of C/N were below 12 indicating a high degree of maturity. Also, no bacterial pathogens were detected in the final composts. The concentration of total heavy metals has been reduced allowing the elimination of sewage sludge toxicity, confirmed by the germination index, which reached over 80%. Strong positive correlations were noticed between total Kjeldahl nitrogen of C2 and germination indexes of all the studied species. The obtained results indicate that the addition of wood sawdust increases the nitrogen content leading to slightly alkaline compost which influences seeds germination by reducing the phytotoxicity of sewage sludge.

Rhizophagus irregularis influences As and P uptake by alfafa and the neighboring non-host pepperweed growing in an As-contaminated soil.

It was documented that arbuscular mycorrhiza fungi (AMF) play an important role in protecting host plants against arsenic (As) contamination. However, most terrestrial ecosystems contain a considerable number of nonmycorrhizal plants. So far little information is available for the interaction of such non-host plants with AMF under As contaminations. By using a dual compartment cultivation system with a plastic board or a nylon mesh separating roots of non-host pepperweed from roots of the AM-host alfafa plants, avoiding direct root competition, the two plant species were grown separately or partially separated (with rhizosphere effects) in the presence or absence of the AMF Rhizophagus irregularis in As-contaminated soil. The results indicated that mycorrhiza caused phosphorus (P) concentration decrease in the non-host pepperweed, but promoted the P concentration of the AM host alfafa. Mycorrhiza is potentially helpful for non-host pepperweed to adapt to As contamination by decreasing root As concentration and showing no suppressing effect on biomass production. The study provides further evidence for the protective effects of AMF on non-host plants against As contamination, and improved our understanding of the potential role of AMF for non-host plant adaptation to As contaminated soils.

Anaesthetics stop diverse plant organ movements, affect endocytic vesicle recycling and ROS homeostasis, and block action potentials in Venus flytraps.

Background and Aims: Anaesthesia for medical purposes was introduced in the 19th century. However, the physiological mode of anaesthetic drug actions on the nervous system remains unclear. One of the remaining questions is how these different compounds, with no structural similarities and even chemically inert elements such as the noble gas xenon, act as anaesthetic agents inducing loss of consciousness. The main goal here was to determine if anaesthetics affect the same or similar processes in plants as in animals and humans. Methods: A single-lens reflex camera was used to follow organ movements in plants before, during and after recovery from exposure to diverse anaesthetics. Confocal microscopy was used to analyse endocytic vesicle trafficking. Electrical signals were recorded using a surface AgCl electrode. Key Results: Mimosa leaves, pea tendrils, Venus flytraps and sundew traps all lost both their autonomous and touch-induced movements after exposure to anaesthetics. In Venus flytrap, this was shown to be due to the loss of action potentials under diethyl ether anaesthesia. The same concentration of diethyl ether immobilized pea tendrils. Anaesthetics also impeded seed germination and chlorophyll accumulation in cress seedlings. Endocytic vesicle recycling and reactive oxygen species (ROS) balance, as observed in intact Arabidopsis root apex cells, were also affected by all anaesthetics tested. Conclusions: Plants are sensitive to several anaesthetics that have no structural similarities. As in animals and humans, anaesthetics used at appropriate concentrations block action potentials and immobilize organs via effects on action potentials, endocytic vesicle recycling and ROS homeostasis. Plants emerge as ideal model objects to study general questions related to anaesthesia, as well as to serve as a suitable test system for human anaesthesia.

Potassium, not lepidimoide, is the principal 'allelochemical' of cress-seed exudate that promotes amaranth hypocotyl elongation.

Background and Aims: Imbibed cress ( Lepidium sativum L.) seeds exude 'allelochemicals' that promote excessive hypocotyl elongation and inhibit root growth in neighbouring competitors, e.g. amaranth ( Amaranthus caudatus L.) seedlings. The major hypocotyl promoter has recently been shown not to be the previously suggested acidic disaccharide, lepidimoic acid (LMA), a fragment of the pectic polysaccharide domain rhamnogalacturonan-I. The nature of the hypocotyl promoter has now been re-assessed. Methods: Low-molecular weight cress-seed exudate (LCSE) was fractionated by high-voltage electrophoresis, and components with different charge:mass ratios were tested for effects on dark-grown amaranth seedlings. Further samples of LCSE were size-fractionated by gel permeation chromatography, and active fractions were analysed electrophoretically. Key Results: The LCSE strongly promoted amaranth hypocotyl elongation. The active principle was hydrophilic and, unlike LMA, stable to hot acid. After electrophoresis at pH 6·5, the only fractions that strongly promoted hypocotyl elongation were those with a very high positive charge:mass ratio, migrating towards the cathode 3-4 times faster than glucosamine. Among numerous naturally occurring cations tested, the only one with such a high mobility was potassium. K + was present in LCSE at approx. 4 m m , and pure KCl (1-10 m m ) strongly promoted amaranth hypocotyl elongation. No other cation tested (including Na + , spermidine and putrescine) had this effect. The peak of bioactivity from a gel permeation chromatography column exactly coincided with the peak of K + . Conclusions: The major 'allelopathic' substance present in cress-seed exudate that stimulates hypocotyl elongation in neighbouring seedlings is the inorganic cation, K + , not the oligosaccharin LMA.

Promotion of testa rupture during garden cress germination involves seed compartment-specific expression and activity of pectin methylesterases.

Pectin methylesterase (PME) controls the methylesterification status of pectins and thereby determines the biophysical properties of plant cell walls, which are important for tissue growth and weakening processes. We demonstrate here that tissue-specific and spatiotemporal alterations in cell wall pectin methylesterification occur during the germination of garden cress (Lepidium sativum). These cell wall changes are associated with characteristic expression patterns of PME genes and resultant enzyme activities in the key seed compartments CAP (micropylar endosperm) and RAD (radicle plus lower hypocotyl). Transcriptome and quantitative real-time reverse transcription-polymerase chain reaction analysis as well as PME enzyme activity measurements of separated seed compartments, including CAP and RAD, revealed distinct phases during germination. These were associated with hormonal and compartment-specific regulation of PME group 1, PME group 2, and PME inhibitor transcript expression and total PME activity. The regulatory patterns indicated a role for PME activity in testa rupture (TR). Consistent with a role for cell wall pectin methylesterification in TR, treatment of seeds with PME resulted in enhanced testa permeability and promoted TR. Mathematical modeling of transcript expression changes in germinating garden cress and Arabidopsis (Arabidopsis thaliana) seeds suggested that group 2 PMEs make a major contribution to the overall PME activity rather than acting as PME inhibitors. It is concluded that regulated changes in the degree of pectin methylesterification through CAP- and RAD-specific PME and PME inhibitor expression play a crucial role during Brassicaceae seed germination.

DELAY OF GERMINATION 1 mediates a conserved coat-dormancy mechanism for the temperature- and gibberellin-dependent control of seed germination.

Seed germination is an important life-cycle transition because it determines subsequent plant survival and reproductive success. To detect optimal spatiotemporal conditions for germination, seeds act as sophisticated environmental sensors integrating information such as ambient temperature. Here we show that the delay of germination 1 (DOG1) gene, known for providing dormancy adaptation to distinct environments, determines the optimal temperature for seed germination. By reciprocal gene-swapping experiments between Brassicaceae species we show that the DOG1-mediated dormancy mechanism is conserved. Biomechanical analyses show that this mechanism regulates the material properties of the endosperm, a seed tissue layer acting as germination barrier to control coat dormancy. We found that DOG1 inhibits the expression of gibberellin (GA)-regulated genes encoding cell-wall remodeling proteins in a temperature-dependent manner. Furthermore we demonstrate that DOG1 causes temperature-dependent alterations in the seed GA metabolism. These alterations in hormone metabolism are brought about by the temperature-dependent differential expression of genes encoding key enzymes of the GA biosynthetic pathway. These effects of DOG1 lead to a temperature-dependent control of endosperm weakening and determine the optimal temperature for germination. The conserved DOG1-mediated coat-dormancy mechanism provides a highly adaptable temperature-sensing mechanism to control the timing of germination.

Mobile organic compounds in biochar - a potential source of contamination - phytotoxic effects on cress seed (Lepidium sativum) germination.

Biochar can be contaminated during pyrolysis by re-condensation of pyrolysis vapours. In this study two biochar samples contaminated by pyrolysis liquids and gases to a high degree, resulting in high volatile organic compound (high-VOC) content, were investigated and compared to a biochar with low volatile organic compound (low-VOC) content. All biochar samples were produced from the same feedstock (softwood pellets) under the same conditions (550 °C, 20 min mean residence time). In experiments where only gaseous compounds could access germinating cress seeds (Lepidium sativum), application amounts ranging from 1 to 30 g of high-VOC biochar led to total inhibition of cress seed germination, while exposure to less than 1 g resulted in only partial reduction. Furthermore, leachates from biochar/sand mixtures (1, 2, 5 wt.% of biochar) induced heavy toxicity to germination and showed that percolating water could dissolve toxic compounds easily. Low-VOC biochar didn't exhibit any toxic effects in either germination test. Toxicity mitigation via blending of a high-VOC biochar with a low-VOC biochar increased germination rate significantly. These results indicate re-condensation of VOCs during pyrolysis can result in biochar containing highly mobile, phytotoxic compounds. However, it remains unclear, which specific compounds are responsible for this toxicity and how significant re-condensation in different pyrolysis units might be.

Role of a respiratory burst oxidase of Lepidium sativum (cress) seedlings in root development and auxin signalling.

Reactive oxygen species are increasingly perceived as players in plant development and plant hormone signalling pathways. One of these species, superoxide, is produced in the apoplast by respiratory burst oxidase homologues (rbohs), a family of proteins that is conserved throughout the plant kingdom. Because of the availability of mutants, the focus of research into plant rbohs has been on Arabidopsis thaliana, mainly on AtrbohD and AtrbohF. This study investigates: (i) a different member of the Atrboh family, AtrbohB, and (ii) several rbohs from the close relative of A. thaliana, Lepidium sativum ('cress'). Five cress rbohs (Lesarbohs) were sequenced and it was found that their expression patterns were similar to their Arabidopsis orthologues throughout the life cycle. Cress plants in which LesarbohB expression was knocked down showed a strong seedling root phenotype that resembles phenotypes associated with defective auxin-related genes. These transgenic plants further displayed altered expression of auxin marker genes including those encoding the auxin responsive proteins 14 and 5 (IAA14 and IAA5), and LBD16 (LATERAL ORGAN BOUNDARIES DOMAIN16), an auxin-responsive protein implicated in lateral root initiation. It is speculated that ROS produced by rbohs play a role in root development via auxin signalling.

Simple biotoxicity tests for evaluation of carbonaceous soil additives: establishment and reproducibility of four test procedures.

Biochar derived from pyrolysis has received much attention recently as a soil additive to sequester carbon and increase soil fertility. Hydrochar, a brown, coal-like substance produced via hydrothermal carbonization, has also been suggested as a beneficial soil additive. However, before soil application, both types of char need to be tested for potential toxic effects. The aim of this study was to develop simple, inexpensive, and easy-to-apply test procedures to identify negative effects of chars but not to provide false-negative results. The following tests, based partly on ISO norm biotoxicity test procedures, were chosen: (i) cress germination test for gaseous phytotoxic emissions; (ii) barley germination and growth test; (iii) salad germination test; and (iv) earthworm avoidance test for toxic substances. Test reproducibility was ensured by carrying out each test procedure three times with the same biochar. Several modifications were necessary to adapt the tests for biochars/hydrochars. The tested biochar did not induce negative effects in any of the tests. In contrast, the beet-root chip hydrochar showed negative effects in all tests. In an extension to the regular procedure, a regrowth of the harvested barley shoots without further nutrient additions yielded positive results for the hydrochar, which initially had negative effects. This implies that the harmful substance(s) must have been degraded or they were water soluble and leached. Tests with a biochar and hydrochar showed that the proposed modified quick-check test procedures provide a fast assessment of risks and effects of char application to soils within a short period of time (<2 wk).

Phytotoxicity of low-weight carboxylic acids.

Presence of low-weight carboxylic acids (LWCAs) can be the reason for phytotoxicity of green manures, treated bio-waste or digestates from biogas production applied to soils. As the phytotoxic concentrations of LWCA are poorly known, this work presents data on six acids (C(1)C(6): formic, acetic, propionic, butyric, valeric, and caproic). Phytotoxicity was measured in acute (72 or 120 h) and subchronic (21 d) assays for seed germination, seedling elongation, and plant growth for garden cress Lepidium sativum and ryegrass Lolium multiflorum. The dose-response relationship was modeled using Weibull model. Results showed a trend that toxicity of LWCA increases with the length of the carbon chain, formic acid (C(1)) being the least and caproic acid (C(6)) the most toxic. EC50 values in the acute seed germination of cress ranged between 1.9 and 4.2mM and for ryegrass between 1.8 and 6.4mM. In subchronic assays EC50 values for germination were in a range from 11 to 46mMkg(-1)dm for cress, and from 18 to 127 mM kg(-1) dm for ryegrass. EC50 values for early seedling growth of cress based on acute assays ranged from 0.7 to 2.3mM and that of ryegrass from 1.2 to 1.8mM. Range of EC50 values for shoot biomass of cress was between 8 and 40 mM kg(-1) dm and of ryegrass between 12 and 93 mM kg(-1) dm.

Chemical composition and possible in vitro phytotoxic activity of Helichrsyum italicum (Roth) Don ssp. italicum.

The chemical composition of the essential oil of Helichrysum italicum (Roth) Don ssp. italicum, collected in the National Park of Cilento and Diano Valley, Southern Italy, was studied by means of GC and GC/MS. Forty four compounds of 45 constituents were identified in the oil, mainly oxygenated sesquiterpenes. The essential oil was evaluated for its potential in vitro phytotoxic activity against germination and early radicle elongation of radish and garden cress. The radicle elongation of radish was significantly inhibited at the highest doses tested, while germination of both seeds was not affected.

Changes in nutritional value and cytotoxicity of garden cress germinated with different selenium solutions.

The selenium supply in almost all European countries is below the recommended daily intake, and different strategies are followed to fortify foods. In the present work, the influence of germination of garden cress ( Lepidium sativum cv. Ogrodowa) in different selenium solutions (Na(2)SeO(3) and Na(2)SeO(4)) on Se uptake, total antioxidant capacity, glucosinolates, protein, and amino acids was studied. Cytotoxicity in HL-60 human leukemic cell line was also assessed. The addition of selenite (Na(2)SeO(3)) or selenate (Na(2)SeO(4)) led to a significant increment in Se uptake in garden cress sprouts, and the highest Se content was observed at 8 mg/L in both inorganic Se solutions (36-38 microg/g of dm). The Se-enriched sprouts presented a large total antioxidant capacity (142-157 mumol of Trolox/g of dm), total glucosinolate content (99-124 microg/g of dm), protein (36-37% dm), and total essential amino acid content (40-41 g/100 g of protein), and no cytotoxicity on HL-60 human leukemic cells was observed. Garden cress sprouts obtained with selenite solution at 8 mg/L presented the best nutritional qualities and might provide a substantial proportion of Se in European diets. Bearing in mind the high nutritional value of sprouts, these may serve for the production of functional foods.

Chemical, microbial and physical properties of manufactured soils produced by co-composting municipal green waste with coal fly ash.

Increasing proportions of coal fly ash were co-composted with municipal green waste to produce manufactured soil for landscaping use. Only the 100% green waste treatment reached a thermophilic composting phase (50 degrees C) which lasted for 6 days. The 25% and 50% ash treatments reached 36-38 degrees C over the same period while little or no self-heating occurred in the 75% and 100% ash treatments. Composted green waste had a low bulk density and high total and macro-porosity. Addition of 25% ash to green waste resulted in a 75% increase in available water holding capacity. As the proportions of added ash in the composts increased, the organic C, soluble C, microbial biomass C, basal respiration and activities of beta-glucosidase, L-asparaginase, alkali phosphatase and arylsulphatase enzymes in the composted products all decreased. It could be concluded that addition of fly ash to green waste at a proportion higher than 25% did not improve the quality parameters of manufactured soil.

Assessment of the impact of excessive chemical additions to municipal wastewaters and comparison of three technologies in the removal performance of pathogens and toxicity.

The removal efficiencies of pathogens such as Salmonella (S), helminths ova (H), protozoan cysts (P), total coliforms (TC), faecal coliforms (FC) and faecal streptococci (FS) by three treatment processes: aerated lagoon (AL), activated sludge (AS) and anaerobic membrane bioreactor (MBR) were evaluated by means of standard microbiological numeration methods. The micro-toxicity and phyto-toxicity of wastewaters were monitored by LUMIStox and germination index (GI) of Lepidium sativum tests. The results of municipal wastewaters receiving industrial effluents such as Sfax and Mahres were compared with other municipal wastewaters receiving mainly domestic effluents such as Ksour-Essaf. The anaerobic MBR allowed an effective removal of 100% for all the microorganisms tested. The average removal of TC, FC, FS, S, H and P was 1.65log(10), 1.42log(10), 1.23log(10), 0.91log(10), 52.23% and 76.15% in AL system and 0.62-0.84log(10), 0.87-0.93log(10), 0.71-0.78log(10), 0.81-2.71log(10), 59-74.1% and 59.84-72.2% in AS processes, respectively. LUMIStox and GI of L. sativum tests demonstrated that Ksour-Essaf wastewater (KW) was fairly toxic, while Sfax wastewater (SW) exhibited a high toxicity. This toxicity caused the inhibition of the anaerobic consortia when the MBR was fed with the SW. Moreover, the COD and the micro-toxicity increased during the day, parallel to the industrial and domestic activities resulting in the toxic character of SW during the day. SW treated in the AL remained toxic whereas the toxicity of KW treated in the AS or in anaerobic MBR decreased considerably. However, the anaerobic MBR showed a high efficiency in removing toxicity for both SW and KW. The quality of the anaerobic MBR permeate largely conforms with the microbiological WHO guidelines for unrestricted irrigation.

Application of Toxkit microbiotests for toxicity assessment in soil and compost.

The potential of Toxkit microbiotests to detect and analyze pollution in agricultural soil and the quality of compost was studied. The toxicity tests used included seed germination biotests using cress salad (Lepidum sativum L.), tomato (Lycopersicum esculentum L.), and cucumber (Cucumis sativus L.), and the Toxkit microbiotests included those with microalgae (Selenastrum capricornutum), protozoa (Tetrahymena thermophila), crustaceans (Daphnia magna, Thamnocephalus platyurus, and Heterocypris incongruens), and rotifers (Brachionus calyciflorus). Experiments on compost were undertaken in a modified solid-state fermentation system (SSF) and under field conditions (in a windrow). To promote the composting process, two strains of Trichoderma (Trichoderma lignorum and Trichoderma viride), as well as a nitrification association that regulated the nitrogen-ammonification and nitrification processes were applied.

[Effect of changes in gravity on the quantity and localization of fibrillarin in the nucleolus of cress root meristem]. / Vliianie izmenennoi gravitatsii na kolichestvo i lokalizatsiiu fibrillarina v iadryshkakh kornevoi meristemy kress-salata.

For the first time, in altered gravity, with antifibrillarin antibodies and immunogold microscopy fibrillarin that is one of the most important proteins of rRNA processing was localized. The quantitative study of the density of gold particles in the nuclelous, under clinorotation both the transition zone FC-DFC and the dense fibrillar component were less labeled as compared to the control. Obtained data allow us to suppose the lowering of the rRNA processing level in the nucleoli under the influence of altered gravity.

Decontamination of seeds for seed sprout production by high hydrostatic pressure.

Garden cress, sesame, radish, and mustard seeds immersed in water were treated with high pressure (250, 300, 350, and 400 MPa) for 15 min at 20 degrees C. After treatment, percentages of seeds germinating on water agar were recorded for up to 11 days. Of the seeds tested, radish seeds were found to be the most pressure sensitive, with seeds treated at 250 MPa reaching 100% germination 9 days later than untreated control seeds did. Garden cress seeds, on the other hand, were the most pressure resistant, with seeds treated at 250 MPa reaching 100% germination 1 day later than untreated control seeds did. Garden cress sprouts from seeds treated at 250 and 300 MPa also took about 1 day longer to reach average sprout length than sprouts from untreated control seeds did, indicating that sprout growth was not retarded once germination had occurred. Garden cress seeds were inoculated with suspensions of seven different bacteria (10(7) CFU/ml) and processed with high pressure. Treatment at 300 MPa (15 min, 20 degrees C) resulted in 6-log reductions of Salmonella Typhimurium, Escherichia coli MG1655, and Listeria innocua, > 4-log reductions of Shigella flexneri and pressure-resistant E. coli LMM1010, and a 2-log reduction of Staphylococcus aureus. Enterococcus faecalis was virtually not inactivated. For suspensions of the gram-positive bacteria, similar levels of inactivation in water in the absence of garden cress seeds were found, but the inactivation of E. coil LMM1010 and S. flexneri in water in the absence of garden cress seeds was significantly less extensive. These data suggest that garden cress seeds contain a component that acts synergistically with high hydrostatic pressure against gram-negative bacteria.

Basipetal propagation of gravity-induced surface pH changes along primary roots of Lepidium sativum L.

While there is ample evidence for a role of auxin in root gravitropism, the seeming rapidity of gravi-induced changes in electrical parameters has so far been an argument against auxin being a primary signal in gravitropic signal transmission. To address this problem, we re-investigated the effect of gravistimulation on membrane voltages of Lepidium sativum L. and Vigna mungo L. root cells. In our hands, gravistimulation did not induce changes in membrane voltage in cells of the root cap statenchyma, root meristem or apical elongation zone that can be correlated with the orientation of the cells relative to the gravity vector. While these results challenge a model of rapid electrically based signal transmission, there is evidence for a slower signal propagation along gravistimulated L. sativum roots. Using multiple proton-selective microelectrodes to simultaneously measure surface pH on opposite root flanks at different distances from the root tip, we observed gravi-induced asymmetric pH changes at the surface of all investigated root zones. Upon gravistimulation, the surface pH decreased on the physically upper root flank and increased on the lower flank. The pH asymmetry appeared first [2.1+/-0.4 min (mean +/- SD) after tilting] at the root cap and then - with incrementing lag times - at the meristem (after 2.5+/-0.3 min at 300 micro m from root tip; after 3.7+/-0.4 min at 700 micro m) and apical elongation zone (4.8+/-0.5 min at 1,000 micro m), suggesting a basipetal progression of differential surface acidification at a rate of 250-350 micro m min(-1), consistent with reported auxin transport rates.