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PubMed1991

Transport and Metabolism of 1-Aminocyclopropane-1-carboxylic Acid in Sunflower (Helianthus annuus L.) Seedlings.

Transport and metabolism of [2,3-(14)C] 1-aminocyclopropane-1-carboxylic acid (ACC) from roots to shoots in 4-day-old sunflower (Helianthus annuus L.) seedlings were studied. [(14)C]ACC was detected in, and (14)C(2)H(4) was evolved from, shoots 0.5 hours after [(14)C]ACC was supplied to roots. Ethylene emanation from the shoots returned to normal levels after 6 hours. The roots showed a similar pattern, although at 24 hours ethylene emanation was still slightly higher than in those plants that did not receive ACC. [(14)C]N-malonyl-ACC (MACC) was detected in both tissues at all times sampled. [(14)C]MACC levels surpassed [(14)C]ACC levels in the shoot at 2 hours, whereas [(14)C]MACC levels in the root remained below [(14)C]ACC levels until 6 hours, after which they were higher. Thin-layer chromatography analysis identified [(14)C] ACC in 1-hour shoot extracts, and [(14)C]MACC was identified in root tissues at 1 and 12 hours after treatment. [(14)C]ACC and [(14)C] MACC in the xylem sap of treated seedlings were identified by thin-layer chromatography. Xylem transport of [(14)C]ACC in treated seedlings, and transport of ACC in untreated seedlings, was confirmed by gas chromatography-mass spectrometry. Some evidence for the presence of [(14)C]MACC in xylem sap in [(14)C]ACC-treated seedlings is presented. A substantial amount of radioactivity in both ACC and MACC fractions was detected leaking from the roots over 24 hours. A second radiolabeled volatile compound was trapped in a CO(2)-trapping solution but not in mercuric perchlorate. Levels of this compound were highest after the peak of ACC levels and before peak MACC levels in both tissues, suggesting that an alternate pathway of ACC metabolism was operating in this system.

Journal Article↗
PubMed2002

The putative plasma membrane Na(+)/H(+) antiporter SOS1 controls long-distance Na(+) transport in plants.

The salt tolerance locus SOS1 from Arabidopsis has been shown to encode a putative plasma membrane Na(+)/H(+) antiporter. In this study, we examined the tissue-specific pattern of gene expression as well as the Na(+) transport activity and subcellular localization of SOS1. When expressed in a yeast mutant deficient in endogenous Na(+) transporters, SOS1 was able to reduce Na(+) accumulation and improve salt tolerance of the mutant cells. Confocal imaging of a SOS1-green fluorescent protein fusion protein in transgenic Arabidopsis plants indicated that SOS1 is localized in the plasma membrane. Analysis of SOS1 promoter-beta-glucuronidase transgenic Arabidopsis plants revealed preferential expression of SOS1 in epidermal cells at the root tip and in parenchyma cells at the xylem/symplast boundary of roots, stems, and leaves. Under mild salt stress (25 mM NaCl), sos1 mutant shoot accumulated less Na(+) than did the wild-type shoot. However, under severe salt stress (100 mM NaCl), sos1 mutant plants accumulated more Na(+) than did the wild type. There also was greater Na(+) content in the xylem sap of sos1 mutant plants exposed to 100 mM NaCl. These results suggest that SOS1 is critical for controlling long-distance Na(+) transport from root to shoot. We present a model in which SOS1 functions in retrieving Na(+) from the xylem stream under severe salt stress, whereas under mild salt stress it may function in loading Na(+) into the xylem.

Arabidopsis↗
PubMed1992

Ultrastructural Localization of a Bean Glycine-Rich Protein in Unlignified Primary Walls of Protoxylem Cells.

A polyclonal antibody was used to localize a glycine-rich cell wall protein (GRP 1.8) in French bean hypocotyls with the indirect immunogold method. GRP 1.8 could be localized mainly in the unlignified primary cell walls of the oldest protoxylem elements and also in cell corners of both proto- and metaxylem elements. In addition, GRP 1.8 was detected in phloem using tissue printing. The labeled primary walls of dead protoxylem cells showed a characteristically dispersed ultrastructure, resulting from the action of hydrolases during the final steps of cell maturation and from mechanical stress due to hypocotyl growth. Primary walls of living protoxylem and adjacent parenchyma cells were only weakly labeled. This was true also for the secondary walls of proto- and metaxylem cells, which in addition showed high background labeling. Inhibition of lignification with a specific and potent inhibitor of phenylalanine ammonia-lyase did not lead to enhanced labeling of secondary walls, showing that lignin does not mask the presence of GRP 1.8 in these walls. Dictyosomes of living proto- and metaxylem cells were not labeled, but dictyosomes of xylem parenchyma cells without secondary walls, adjacent to strongly labeled protoxylem elements, were clearly labeled. These observations suggest that GRP 1.8 is not produced by xylem vessels but by xylem parenchyma cells that export the protein to the wall of protoxylem vessels.

Journal Article↗
PubMed1994

Novel vascular cell-specific genes whose expression is regulated temporally and spatially during vascular system development.

We have isolated three cDNA clones (TED2, TED3, and TED4) for genes expressed preferentially in cells that redifferentiate into tracheary elements from mesophyll cells isolated from leaves of Zinnia elegans. Sequence analyses of TED clones revealed that TED2 encodes a hydrophobic polypeptide with a significant similarity to the guinea pig lens-specific protein (zeta-crystallin) and that the deduced polypeptide of TED3 may be a novel cell wall protein. In situ hybridization of the TED probes with young Zinnia seedlings showed that expression of the three TED genes was restricted to vascular cells and regulated in a temporal and spatial manner during vascular development. TED3 transcripts were localized specifically to a few cells that are to differentiate or are differentiating into tracheary elements in all organs examined. TED4 transcripts were present mainly in the immature primary xylem both of cotyledons and of the boundary region between the root and hypocotyl and in the procambium of roots. In contrast, TED2 transcripts accumulated not only in immature primary xylem cells but also in immature phloem cells both in roots and in the boundary region between the root and hypocotyl. In addition, TED2 transcripts were expressed in the procambium cells of roots. In cotyledons, TED2 transcripts did not accumulate in xylem or phloem cells but only in two regions that might form a new vein just outside the phloem of the main leaf vein. Taken together, our findings indicate that TED2, TED3, and TED4 can be novel and efficient markers for development of the vascular system.

Amino Acid Sequence↗
PubMed2006

Ecological physiology of Pereskia guamacho, a cactus with leaves.

The specialized physiology of leafless, stem-succulent cacti is relatively well understood. This is not true, however, for Pereskia (Cactaceae), the 17 species of leafy trees and shrubs that represent the earliest diverging lineages of the cacti. Here we report on the water relations and photosynthesis of Pereskia guamacho, a small tree of the semiarid scrubland of Venezuela's Caribbean coast. Sapwood-specific xylem conductivity (Ksp) is low when compared to other vessel-bearing trees of tropical dry systems, but leaf-specific xylem conductivity is relatively high due to the high Huber value afforded by P. guamacho's short shoot architecture. P. guamacho xylem is not particularly vulnerable to drought-induced cavitation, especially considering the high leaf water potentials maintained year round. This is confirmed by the lack of significant variation exhibited in Ksp between wet and dry seasons. In the rainy season, P. guamacho exhibited C3-like patterns of stomatal conductance, but during a prolonged drought we documented nocturnal stomatal opening with a concomitant accumulation of titratable acid in leaves. This suggests that P. guamacho can perform drought-induced crassulacean acid metabolism (CAM photosynthesis), although delta 13C values imply that most carbon is assimilated via the C3 pathway. P. guamacho leaves display very low stomatal densities, and maximum stomatal conductance is low whether stomata open during the day or night. We conclude that leaf performance is not limited by stem hydraulic capacity in this species, and that water use is conservative and tightly regulated at the leaf level.

Biological Evolution↗
PubMed2006

Silicon deposition in the root reduces sodium uptake in rice (Oryza sativa L.) seedlings by reducing bypass flow.

Sodium chloride reduces the growth of rice seedlings, which accumulate excessive concentrations of sodium and chloride ions in their leaves. In this paper, we describe how silicon decreases transpirational bypass flow and ion concentrations in the xylem sap in rice (Oryza sativa L.) seedlings growing under NaCl stress. Salt (50 mM NaCl) reduced the growth of shoots and roots: adding silicate (3 mM) to the saline culture solution improved the growth of the shoots, but not roots. The improvement of shoot growth in the presence of silicate was correlated with reduced sodium concentration in the shoot. The net transport rate of Na from the root to shoot (expressed per unit of root mass) was also decreased by added silicate. There was, however, no effect of silicate on the net transport of potassium. Furthermore, in salt-stressed plants, silicate did not decrease the transpiration, and even increased it in seedlings pre-treated with silicate for 7 d prior to salt treatment, indicating that the reduction of sodium uptake by silicate was not simply through a reduction in volume flow from root to shoot. Experiments using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), an apoplastic tracer, showed that silicate dramatically decreased transpirational bypass flow in rice (from about 4.2 to 0.8%), while the apparent sodium concentration in the xylem, which was estimated indirectly from the flux data, decreased from 6.2 to 2.8 mM. Direct measurements of the concentration of sodium in xylem sap sampled using Philaenus spumarius confirmed that the apparent reduction was not a consequence of sodium recycling. X-ray microanalysis showed that silicon was deposited in the outer part of the root and in the endodermis, being more obvious in the latter than in the former. The results suggest that silicon deposition in the exodermis and endodermis reduced sodium uptake in rice (Oryza sativa L.) seedlings under NaCl stress through a reduction in apoplastic transport across the root.

Biological Transport↗
PubMed2005

Sap flow rates and sapwood density are critical factors in within- and between-tree variation in CO2 efflux from stems of mature Dacrydium cupressinum trees.

Measurements of CO2 efflux from stems and branches, sap velocity, and respiratory activity of excised wood cores were conducted in Dacrydium cupressinum trees that differed in diameter, age, and canopy emergence. The objective of this study was to determine if consistent linkages exist among respiratory production of CO2 within stems, xylem transport of CO2, and the rate of CO2 diffusing from stem surfaces. Stem CO2 efflux was depressed during periods of sap flow compared with the efflux rate expected for a given stem temperature and was positively correlated with sapwood density. By contrast, no significant relationships were observed between CO2 efflux and the respiratory activity of wood tissues. Between 86 and 91% of woody tissue respiration diffused to the atmosphere over a 24-h period. However, at certain times of the day, xylem transport and internal storage of CO2 may account for up to 13-38% and 12-18%, respectively, of woody tissue respiration. These results demonstrate that differences in sap flow rates and xylem anatomy are critically important for explaining within- and between-tree variation in CO2 efflux from stems.

Carbon Dioxide↗
PubMed2007

Relating root structure and anatomy to whole-plant functioning in 14 herbaceous Mediterranean species.

This study investigated the relationships between root structure and anatomy and whole-plant functioning in herbaceous species. Fourteen annual and perennial species representative of a Mediterranean old-field succession were grown in monocultures in a common-garden experiment. Whole-plant functioning was assessed by inherent relative growth rate (RGR(max)), measured in standardized conditions, and maximum height (H(max)). Root tissue density (TMD(r)), considered as a major component of root structure, was measured on roots harvested within in-growth cores. Anatomical characteristics were analysed on cross-sectional areas (CSA). TMD(r) was correlated positively with H(max) and negatively with RGR(max). Root CSA explained interspecific variation in H(max) but not that in TMD(r) and RGR(max). Root xylem CSA and xylem proportion in root CSA were positively correlated with TMD(r) and H(max) and negatively with RGR(max). Mean xylem vessel CSA did not account for variations in TMD(r), H(max) and RGR(max). These results suggested that RGR(max) and H(max) are constrained by opposite root structural and anatomical traits, which have potential links with hydraulic conductance, support and longevity.

Magnoliopsida↗
PubMed2005

Upstream migration of Xylella fastidiosa via pilus-driven twitching motility.

Xylella fastidiosa is a xylem-limited nonflagellated bacterium that causes economically important diseases of plants by developing biofilms that block xylem sap flow. How the bacterium is translocated downward in the host plant's vascular system against the direction of the transpiration stream has long been a puzzling phenomenon. Using microfabricated chambers designed to mimic some of the features of xylem vessels, we discovered that X. fastidiosa migrates via type IV-pilus-mediated twitching motility at speeds up to 5 mum min(-1) against a rapidly flowing medium (20,000 mum min(-1)). Electron microscopy revealed that there are two length classes of pili, long type IV pili (1.0 to 5.8 mum) and short type I pili (0.4 to 1.0 mum). We further demonstrated that two knockout mutants (pilB and pilQ mutants) that are deficient in type IV pili do not twitch and are inhibited from colonizing upstream vascular regions in planta. In addition, mutants with insertions in pilB or pilQ (possessing type I pili only) express enhanced biofilm formation, whereas a mutant with an insertion in fimA (possessing only type IV pili) is biofilm deficient.

Bacterial Proteins↗
PubMed1996

XYLOGENESIS: INITIATION, PROGRESSION, AND CELL DEATH.

Xylem cells develop from procambial or cambial initials in situ, and they can also be induced from parenchyma cells by wound stress and/or a combination of phytohormones in vitro. Recent molecular and biochemical studies have identified some of the genes and proteins involved in xylem differentiation, which have led to an understanding of xylem differentiation based on comparisons of events in situ and in vitro. As a result, differentiation into tracheary elements (TEs) has been divided into two processes. The "early" process involves the origination and development of procambial initials in situ. In vitro, the early process of transdifferentiation involves the dedifferentiation of cells and subsequent differentiation of dedifferentiated cells into TE precursor cells. The "late" process, observed both in situ and in vitro, involves a variety of events specific to TE formation, most of which have been observed in association with secondary wall thickenings and programmed cell death. In this review, I summarize these events, including coordinated expression of genes that are involved in secondary wall formation.

Journal Article↗
PubMed2005

A genomic approach to investigate developmental cell death in woody tissues of Populus trees.

BACKGROUND: Poplar (Populus sp.) has emerged as the main model system for molecular and genetic studies of forest trees. A Populus expressed sequence tag (EST) database (POPULUSDB) was previously created from 19 cDNA libraries each originating from different Populus tree tissues, and opened to the public in September 2004. We used this dataset for in silico transcript profiling of a particular process in the woody tissues of the Populus stem: the programmed death of xylem fibers. RESULTS: One EST library in POPULUSDB originates from woody tissues of the Populus stem where xylem fibers undergo cell death. Analysis of EST abundances and library distribution within the POPULUSDB revealed a large number of previously uncharacterized transcripts that were unique in this library and possibly related to the death of xylem fibers. The in silico analysis was complemented by a microarray analysis utilizing a novel Populus cDNA array with a unigene set of 25,000 sequences. CONCLUSIONS: In silico analysis, combined with the microarray analysis, revealed the usefulness of non-normalized EST libraries in elucidating transcriptional regulation of previously uncharacterized physiological processes. The data suggested the involvement of two novel extracellular serine proteases, nodulin-like proteins and an Arabidopsis thaliana OPEN STOMATA 1 (AtOST1) homolog in signaling fiber-cell death, as well as mechanisms responsible for hormonal control, nutrient remobilization, regulation of vacuolar integrity and autolysis of the dying fibers.

Apoptosis Regulatory Proteins↗
PubMed2004

Plant uptake and translocation of highly weathered, soil-bound technical chlordane residues: data from field and rhizotron studies.

It has been observed that plants are susceptible to uptake from soil and in planta transport of technical chlordane, in spite of its hydrophobicity and sequestration within the soil matrix due to weathering. Field and rhizotron studies were conducted with Cucurbitaceae planted in highly weathered, chlordane-contaminated soil to investigate details of soil-to-plant contaminant uptake. In the field-work, Cucurbita pepo L. (zucchini) was grown in soil at four levels of chlordane contamination: Clean (<limits of quantitation, 5 ng/g), low (average, 370 ng/g), medium (average, 1,951 ng/g), and high (average, 4,572 ng/g). The analysis of plant tissues (root, stem, leaf, and fruit) resulted in the detection of chlordane consistently at the highest concentration in the root tissue at each level of soil contamination. As the soil chlordane concentration increased, the average chlordane concentration in the root tissue increased as follows: Clean, 370 ng/g; low, 8,130 ng/g; medium, 21,800 ng/g; high, 29,400 ng/g. Further analysis of the field-grown plants showed distinct differences in both the proportional distribution of chlordane among the plant tissues and the pattern of the chlordane residues in each tissue type. These differences are attributed to plant uptake from soil versus uptake from air. In the rhizotron studies, uptake of chlordane residues by C. pepo L. was compared with that of another Cucurbitaceae, Cucumis sativus L. (cucumber). Xylem sap from the rhizotron-grown plants was collected and analyzed for chlordane, in addition to determination of chlordane residues in soil, roots, and aerial plant tissue. Component fractions and enantiomer fractions of both chiral and achiral chlordanes were followed through soil, root, xylem sap, and aerial tissue compartments. They indicate that the xenobiotic residues translocate enantioselectively from the soil matrix into and through the plant environment with genera-specific patterns. The determination of chlordanes at ng/g concentration explicitly for the first time in the xylem sap of plants grown in contaminated soil confirms the presence of a soil-sequestered and highly hydrophobic organic contaminant within the aqueous plant environment.

Air↗
PubMedDate not supplied

Soil and tree-ring chemistry response to liming in a sugar maple stand.

An evaluation of the impact of dolomitic lime [CaMg(CO3)2] on soils (five years after treatment) and sapwood chemistry (after four growing seasons) was realized for a Ca-deficient sugar maple stand at the lake Clair watershed. The effect on humus chemistry was significant: exchangeable Mg and Ca, effective acidity (EA), base saturation (BSe), pH, and effective cation exchange capacity (CECe) significantly increased, while exchangeable Fe significantly decreased. In the B horizon, liming increased exchangeable Ca, Mg, and Mn concentrations while decreasing other acid cations. No significant temporal trends in element concentrations in tree rings could be detected, although the lime treatment significantly changed the average xylem Mg and Mn concentrations as well as the average Mg/Mn and Ca/Mn ratios of the sapwood. The absence of temporal trends in rings from the last 20 yr implied a significant re-equilibration of elements through the sapwood. Significant relationships were found between averaged xylem Ca/Mn and Mg/Mn ratios and exchangeable humus Ca, Mg, Mn, Al, Fe, and H+ concentration, EA, CECe, and BSe, suggesting that the average xylem Ca/Mn and Mg/Mn ratios are strong indicators of the soil acid-base status.

Acer↗
PubMed2006

Soil acidity and manganese in declining and nondeclining sugar maple stands in Pennsylvania.

For decades, the hardwood forests of northern Pennsylvania have been subjected to chronic atmospheric loading of acidifying agents. On marginal, high-elevation, unglaciated sites, sugar maples (Acer saccharum Marsh.) have experienced severe decline symptoms and mortality. Accelerated soil acidification, base cation leaching, and increased availability of toxic metals have been suggested as predisposing factors contributing to this decline. Manganese, an essential micronutrient, is also a potentially phytotoxic metal that may be a factor associated with poor sugar maple health on soils vulnerable to acidification from anthropogenic sources. We measured Mn levels in four compartments of the soil-tree system (soil, foliage, xylem wood, and sap) on three sugar maple stands in northern Pennsylvania. Two stands were classified as declining and one was in good health. Negative correlations were found between soil pH and Mn levels in the soil, foliage, sap, and xylem wood. Levels of Mn in these pools were consistently higher on declining sites, which correspondingly exhibited lower levels of Ca and Mg. Species differences between red maple (Acer rubrum L.) and sugar maple at the two declining sites suggested different tolerances to excessive Mn. Molar ratios of Mg/Mn and Ca/Mn were different among sites and showed potential as indicators of soil acidification. Significant correlations among soil, sap, foliage, and xylem wood Mn were also noted. These results show clear Mn differences among sites and, when viewed with recent Mn toxicity experiments and other observational studies, suggest that excessive Mn may play a role in the observed decline and mortality of sugar maple.

Acer↗
PubMed2005

Simulation of phytoremediation of a TNT-contaminated soil using the CTSPAC model.

Knowledge of water movement in the plant-xylem system and contaminant bioavailability in the soil environment is crucial to evaluate the success of phytoremediation practices. This study investigated the removal of 2,4,6-trinitrotoluene (TNT) from a contaminated sandy soil by a single poplar (Populus fastigiata) tree through the examinations of temporal variations of xylem water potential, root water uptake, and soil TNT bioavailability. A mathematical model, CTSPAC (Coupled Transport of water, heat, and solutes in the Soil-Plant-Atmosphere Continuum), was modified for the purpose of this study. The model was calibrated using laboratory measurements before its application. Our simulations show that the xylem water potential was high in the roots and low in the leaves with a potential head difference of 3.55 cm H2O, which created a driving force for water flow and chemical transport upward from the roots through the stem to the leaves. The daily average root water uptake rate was 25 cm3 h(-1) when an equilibrium condition was reached after 24 h. Our simulations further reveal that no TNT was found in the stem and leaves and only about 1% of total TNT mass was observed in the roots due to the rapid biodegradation and transformation of TNT into its daughter products. About 13% of the soil TNT was removed by the poplar tree, resulting mainly from root uptake since TNT is a recalcitrant compound. In general, the soil TNT bioavailability decreased with time due to the depletion of soil solution TNT by the poplar tree. A constant bioavailability (i.e., 3.1 x 10(-6)) was obtained in 14 d in which the soil TNT concentration was about 10 mg L(-1). Our study suggests that CTSPAC is a useful model to simulate phytoremediation of TNT-contaminated sites.

Biodegradation, Environmental↗
PubMedDate not supplied

[A source of high frequency variations in the tracheid size in annual tree rings of conifers].

Statistical analysis of high-frequency variations in the radial dimensions of tracheids was carried out using precise measurements of five radial rows in each annual tree ring for several coniferous species: Larix gmelinii, L. leptolepis, L. sibirica, Picea abies, Pinus sylvestris, P. rigida, and P. densiflora. More than 25 tree rings with a varying number of cells and width were measured for each species. High-frequency variations in radial dimensions were found to have a regular (cyclic) characteristic and accumulated 5 to 11% of the total variability. The cyclic pattern was identified using Fourier analysis of time-related or successive series statistical procedure. The first order autocorrelations (-0.45 to -0.74) and the mean cycle of high-frequency variations (2.1-2.5) did not depend on the number of cells in the annual ring, i.e., they are determined by internal causes. The cyclic pattern of high-frequency variation in tracheid dimensions is used to interpret the seasonal mechanism of xylem formation. Specifically, (1) high-frequency variations in the tracheid radial size take place during the last asymmetrical division of the xylem mother cell before transition to the elongation zone, (2) the final tracheid radial size is mostly determined within the zone of division due to the last asymmetrical division, and (3) acceleration of cell cycle from the initial to the periphery of the cambial zone. The results obtained are also discussed with respect to the mechanisms of xylem differentiation.

Cycadopsida↗
PubMed2000

Root pressure and specific conductivity in temperate lianas: exotic Celastrus orbiculatus (Celastraceae) vs. native Vitis riparia (Vitaceae).

The exotic temperate liana (woody vine) Celastrus orbiculatus has become a weed in Michigan, occurring in many of the same habitats as the native liana Vitis riparia. However, C. orbiculatus frequently develops into extensive monospecific infestations, while V. riparia does not. Freezing-induced embolism may be responsible for limiting liana distribution. Root pressure has been observed in numerous tropical lianas and temperate species of Vitis and has been implicated as vital to the recovery of xylem function in wide vessels following winter freezes. For both of these co-occurring lianas we investigated root pressure and water conductance as possible explanatory factors for their differential spread. According to our hypothesis, C. orbiculatus should have produced greater or more frequent root pressures than V. riparia. However, the reverse proved true, indicating that root pressure is not a prerequisite for weedy proliferation of C. orbiculatus. Additionally, the seasonal patterns of specific conductivity of stem xylem indicate that each species responds differently to environmental constraints. Vitis riparia establishes conductivity early in the growing season, before the leaves emerge, using root pressure to reverse embolism, but loses conductivity with the first freeze in early autumn. Celastrus orbiculatus is slow to establish conductivity, depending on new wood production, but leafs out sooner than V. riparia and maintains green leaves after the first freeze. Vulnerability curves of xylem to cavitation caused by water stress for the two species indicate that they respond similarly to dehydration. These results indicate that root pressures are not responsible for the invasive success of C. orbiculatus and suggest that other factors must be key to its prolific invasion.

Journal Article↗
PubMed2001

Transport and accumulation rates of abscisic acid and aldehyde oxidase activity in Pisum sativum L. in response to suboptimal growth conditions.

Pea plants (Pisum sativum L.) grown initially in nutrient solutions with adequate nitrogen supply (4 mM NO3-) were transferred to solutions containing salt (50 or 100 mM NaCl), ammonium (4 mM) or a low nitrogen supply (0.4 mM NO3-). No changes of abscisic acid (ABA) content were found in roots of stressed pea plants 9 d after the beginning of the treatments; however, accumulation of ABA in the leaves was observed. Old leaves accumulated ABA to a higher extent than young leaves. Accumulation of ABA in leaves of ammonium-fed plants and plants grown under low nitrogen supply occurred in the absence of both increased ABA xylem loading rate and enhanced aldehyde oxidase (AO, EC 1.2.3.1) activity in roots. Enhanced leaf AO activity was observed in all treatments, with the highest increase in old leaves. Among the three AO isoforms (AO-1, AO-2 and AO-3) detected in extracts of pea leaves, the lowest one AO-3 (highest mobility in the gel) correlated with ABA production and showed the highest increment in response to the treatments. The increase of AO activity detected in leaves after 2 weeks of stress application was less prominent than after 9 d, suggesting a transient enhancement of ABA production following the onset of stress. An increase of ABA xylem loading rate as well as AO root activity 4 d and 9 d after application of the treatments was observed only in salt-treated plants followed by a decrease after 14 d in 100 mM NaCl. Decreased cytokinin (trans-zeatin riboside) delivery rate into the xylem sap was observed in all treatments. The role of abscisic acid and cytokinins as positive and negative growth signals, as well as the involvement of root-generated ABA on ABA accumulation in leaves is discussed.

Abscisic Acid↗