Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Xylem”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 505 records · Page 28Linked to original sources
PubMed2001

Direct measurement of sodium and potassium in the transpiration stream of salt-excluding and non-excluding varieties of wheat.

The xylem-feeding insect Philaenus spumarius was used to analyse sodium and potassium fluxes in the xylem of intact, transpiring wheat plants. Two cultivars were compared: the salt-excluding (Chinese Spring) and the non-excluding (Langdon). Chinese Spring accumulated much less sodium in its leaves than the salt-sensitive Langdon. After 7 d in 150 mol m(-3) NaCl, the sodium concentration in the leaf sap of Langdon reached over 600 mol m(-3). This was some three-fold greater than that in Chinese Spring. Similar findings have previously been reported from these cultivars. The reduced ion accumulation was specific to sodium; accumulation of K(+) was unaffected by NaCl in Chinese Spring, such that it developed a much lower leaf Na(+)/K(+) ratio than Langdon. The spittlebug, P. spumarius was used to sample xylem sap from both cultivars. This approach showed that the leaf xylem sap of Chinese Spring had much lower levels of sodium than that of Langdon. In the 150 mol m(-3) NaCl treatment, sodium levels in the leaf xylem reached only 2-3 mol m(-3) in Chinese Spring, compared with 8-10 mol m(-3) in Langdon. Transpiration rates were found to be similar in the two varieties. The lower leaf xylem content alone was thus sufficient to account for the reduced accumulation of sodium in leaves of Chinese Spring. The mechanisms by which xylem sodium might be lowered are discussed and it is concluded that sodium is probably excluded from the xylem in the root of Chinese Spring.

Animals↗
PubMed2002

The contribution of internode and mesocotyl tissues to root-to-shoot signalling of abscisic acid.

The xylem of first internode of runner bean and of previously etiolated maize mesocotyl segments was perfused with media containing abscisic acid (ABA) or abscisic acid glucose ester (ABA-GE) in concentrations as they occur under stress conditions. ABA-GE passed through the internode and mesocotyl segments unchanged. Within 10 min the concentration of ABA-GE(xyl) rose to a level similar to that in the external perfusion medium. By contrast, 30-40 min passed before the concentration of free ABA in the xylem sap [ABA(xyl)] reached the level in the external medium. When ABA-free media were used, ABA was released from the xylem parenchyma to the xylem vessels resulting in an [ABA(xyl)] of 13-23 nM (runner bean internode) or 1-6 nM (maize mesocotyl). The total perimeter and, hence surface area, of the xylem elements was measured microscopically and from these measurements it was estimated that, in both bean internodes and maize hpyocotyls, the flux of ABA to the xylem was 1 pmol m(-2) s(-1). The ABA efflux from the stem and mesocotyl parenchyma into the xylem could be increased when the tissues were treated with tetcyclacis, an inhibitor of ABA degradation, but also by changing the pH from its normal value of about pH 5.8 to pH 7.0 and by adding 100 mM NaCl to the perfusion medium. If 100 nM ABA was added to the perfusion medium the above treatments had only small effects on the release of ABA from the tissues into the xylem.

Abscisic Acid↗
PubMed2003

Plasma membrane aquaporins are involved in winter embolism recovery in walnut tree.

In perennial plants, freeze-thaw cycles during the winter months can induce the formation of air bubbles in xylem vessels, leading to changes in their hydraulic conductivity. Refilling of embolized xylem vessels requires an osmotic force that is created by the accumulation of soluble sugars in the vessels. Low water potential leads to water movement from the parenchyma cells into the xylem vessels. The water flux gives rise to a positive pressure essential for the recovery of xylem hydraulic conductivity. We investigated the possible role of plasma membrane aquaporins in winter embolism recovery in walnut (Juglans regia). First, we established that xylem parenchyma starch is converted to sucrose in the winter months. Then, from a xylem-derived cDNA library, we isolated two PIP2 aquaporin genes (JrPIP2,1 and JrPIP2,2) that encode nearly identical proteins. The water channel activity of the JrPIP2,1 protein was demonstrated by its expression in Xenopus laevis oocytes. The expression of the two PIP2 isoforms was investigated throughout the autumn-winter period. In the winter period, high levels of PIP2 mRNA and corresponding protein occurred simultaneously with the rise in sucrose. Furthermore, immunolocalization studies in the winter period show that PIP2 aquaporins were mainly localized in vessel-associated cells, which play a major role in controlling solute flux between parenchyma cells and xylem vessels. Taken together, our data suggest that PIP2 aquaporins could play a role in water transport between xylem parenchyma cells and embolized vessels.

Amino Acid Sequence↗
PubMed1967

Distribution of an Indoleacetic Acid-oxidase-inhibitor in the Storage Root of Daucus carota.

Indoleacetic acid (IAA)-oxidase from both secondary phloem and xylem was dependent on 2,4-dichlorophenol for activity, and was enhanced by addition of Mn(2+). The pH optimum was 6.0 from both tissues. IAA-oxidase and its inhibitors were distributed differently in the secondary phloem and secondary xylem of carrot root. In the phloem a high IAA-oxidase activity was distributed uniformly along the radius but in the xylem a somewhat lower concentration decreased from the cambium. IAA-oxidase inhibitor in the phloem increased exponentially from a very low concentration near the cambium, whereas in the xylem an appreciable concentration was present near the cambium, decreasing linearly with distance from the cambium. Longitudinal gradients in the xylem parallel studies by other workers with the greatest IAA-destroying capacity present in older tissues. In the xylem inhibitor decreased and IAA-oxidase increased from the root apex. In the phloem IAA-oxidase was uniform, whereas the inhibitor increased in older tissue.The IAA-oxidase inhibitors in phloem and xylem may be different. In the xylem the IAA-oxidase inhibitor may be a lignin precursor present in young cells which disappears as lignification proceeds. In the phloem IAA-oxidase reacting with endogenous IAA appears to form a physiologically active product.

Journal Article↗
PubMed2005

Uptake and transport of calcium in plants.

Recently, research on Ca(2+) transport in plants has been focused on cellular and molecular level. But the uptake, transport and distribution are also very important for calcium to accomplish its function at whole plant level. There are many cells along the way of transport of Ca(2+) from root to shoot, and Ca(2+) passes either through the cytoplasm of cells linked by plasmodesmata (the symplast) or through the spaces between cells (the apoplast), which include Ca(2+) uptake by root cells, Ca(2+) transport from root cortex to and through the xylem, and then out of it into leaves or fruits. Ca(2+) channels, Ca(2+)/H(+) antiporter and Ca(2+)-ATPase play roles in the uptake and transport of Ca(2+) in root cells. To be transported from root surface to xylem, Ca(2+) needs to traverse endodermal cells and xylem parenchyma cells. Endodermal Casparian band, the main barrier for the apoplastic movement of ions into the stele, compels some Ca(2+) to enter root symplast through Ca(2+) channels in endodermal cells and then reach xylem parenchyma. Ca(2+)-ATPase may drive Ca(2+) into the stelar apoplast from xylem parenchyma. Some Ca(2+) effuses from endodermal cell and then get to xylem through apoplastic pathway. Ca(2+) is transported in plant xylem vessel in chelate form and the speed of water flow is the key factor Ca(2+) transport via xylem in trunk. There are both apoplastic and symplastic pathways of Ca(2+) transport in fruit or leaf tissue too.

Biological Transport↗
PubMed2006

Influence of soil texture on hydraulic properties and water relations of a dominant warm-desert phreatophyte.

We investigated hydraulic constraints on water uptake by velvet mesquite (Prosopis velutina Woot.) at a site with sandy-loam soil and at a site with loamy-clay soil in southeastern Arizona, USA. We predicted that trees on sandy-loam soil have less negative xylem and soil water potentials during drought and a lower resistance to xylem cavitation, and reach E(crit) (the maximum steady-state transpiration rate without hydraulic failure) at higher soil water potentials than trees on loamy-clay soil. However, minimum predawn leaf xylem water potentials measured during the height of summer drought were significantly lower at the sandy-loam site (-3.5 +/- 0.1 MPa; all errors are 95% confidence limits) than at the loamy-clay site (-2.9 +/- 0.1 MPa). Minimum midday xylem water potentials also were lower at the sandy-loam site (-4.5 +/- 0.1 MPa) than at the loamy-clay site (-4.0 +/- 0.1 MPa). Despite the differences in leaf water potentials, there were no significant differences in either root or stem xylem embolism, mean cavitation pressure or Psi(95) (xylem water potential causing 95% cavitation) between trees at the two sites. A soil-plant hydraulic model parameterized with the field data predicted that E(crit) approaches zero at a substantially higher bulk soil water potential (Psi(s)) on sandy-loam soil than on loamy-clay soil, because of limiting rhizosphere conductance. The model predicted that transpiration at the sandy-loam site is limited by E(crit) and is tightly coupled to Psi(s) over much of the growing season, suggesting that seasonal transpiration fluxes at the sandy-loam site are strongly linked to intra-annual precipitation pulses. Conversely, the model predicted that trees on loamy-clay soil operate below E(crit) throughout the growing season, suggesting that fluxes on fine-textured soils are closely coupled to inter-annual changes in precipitation. Information on the combined importance of xylem and rhizosphere constraints to leaf water supply across soil texture gradients provides insight into processes controlling plant water balance and larger scale hydrologic processes.

Circadian Rhythm↗
PubMed2000

Form of aluminium for uptake and translocation in buckwheat (Fagopyrum esculentum Moench).

The forms of Al for uptake by the roots and translocation from the root to the shoot were investigated in a buckwheat (Fagopyrum esculentum Moench, cv. Jianxi) that accumulates Al in its leaves. The Al concentration in the xylem sap was 15-fold higher in the plants exposed to AlCl3 than in those exposed to an Al-oxalate (1:3) complex, suggesting that the roots take up Al in the ionic form. The Al concentration in the xylem sap was 4-fold higher than that in the external solution after a 1-h exposure to AICl3 solution and 10-fold higher after a 2-h exposure. The Al concentration in the xylem sap increased with increasing Al concentration in the external solution. The Al uptake was not affected by a respiratory inhibitor, hydroxylamine, but significantly inhibited by the addition of La. These results suggest that Al uptake by the root is a passive process, and La3+ competes for the binding sites for Al3+ on the plasma membrane. The form of Al in the xylem sap was identified by 27Al-nuclear magnetic resonance analysis. The chemical shift of 27Al in the xylem sap was around 10.9 ppm, which is consistent with that of the Al-citrate complex. Furthermore, the dominant organic acid in the xylem sap was citric acid, indicating that Al was translocated in the form of Al-citrate complex. Because Al is present as Al-oxalate (1:3) in the root, the present data show that ligand exchange from oxalate to citrate occurs before Al is released to xylem.

Aluminum↗
PubMed2001

Compressive/tensile stresses and lignified cells as resistance components in joints between cladodes of Opuntia laevis (Cactaceae): responses to applied stresses.

Cactaceae are a diverse group of plants with a wide variety of morphologies and reproductive strategies. Many species have segmented stems in which terminal cladodes may be separated from main stem cladodes with varying amounts of resistance. Previous results demonstrated that lignified xylem cells in tensile portions of stem joints provide the main resistance to separation of cladodes within cactus plants. The purpose of the present study was to determine if stem joints of Opuntia laevis would produce additional lignified xylem cells in response to additional externally applied stresses. Normal average stress levels, which accompany the addition of a new cladode, were applied to 12 plants. In contrast, double the average stress levels were applied to 13 other plants. After exposure to the two stress regimens for 6 months, the amount and location of lignified xylem cells in joint segments were similar for both stress treatments. So, although the results support the hypothesis that lignified xylem cells act as the main resistance to stress at joints of cladodes, doubling the normal amount of applied stress was insufficient to alter the amount or location of lignified xylem cells in stem joints. These results indicate that normal amounts of lignified xylem cells can resist up to two times the normal amount of stress for 6 months without producing additional lignified xylem cells.

Journal Article↗
PubMed2003

Seasonal changes in stem radius and production of new tracheids in Norway spruce.

The progress of xylem formation in Norway spruce (Picea abies (L.) Karst.) was measured during one growing season in southern Finland. Stem radius was monitored continuously with band dendrometers, and the formation of new tracheids was determined by examination of small increment cores taken twice weekly. Tracheid production started in June and ceased in August. Xylem formation was fastest in early July, when 0.75-1.25 new tracheids were formed per day. The rate of xylem formation was significantly correlated with mean daily temperature. Synchronous fluctuations in tracheid and lumen diameters were observed at the same relative positions within each annual ring, but no relationship existed between the diameters and weather variables. The timing of changes in stem radius differed from the timing of actual xylem formation. Stem radius increased in April and May, and the fastest daily increments were recorded in June. Increases in stem radius slowed in July, but small increases were measured more than a month after xylem formation had ceased. Daily changes in stem radius were correlated with daily precipitation, reflecting changes in stem water content. Therefore, dendrometers are of dubious value for measuring the timing of actual xylem formation. Small increment cores proved to be useful in assessing actual xylem formation, but the method is laborious.

Finland↗
PubMed2001

A biophysical analysis of stem and root diameter variations in woody plants.

A comprehensive model of stem and root diameter variation was developed. The stem (or root) was represented using two coaxial cylinders corresponding with the mature xylem and the extensible tissues. The extensible tissues were assumed to behave as a single cell separated from the mature xylem by a virtual membrane. The mature xylem and the extensible tissues are able to dilate with temperature and grow. Moreover, the extensible tissues are able to shrink and swell according to water flow intensity. The model is mainly based on the calculation of water volume flows in the "single cell" that are described using the principles of irreversible thermodynamics. The elastic response to storage volume and plastic extension accompanying growth are described. The model simulates diameter variation due to temperature, solute accumulation, and xylem, water potential. The model was applied to the peach (Prunus persica) stem and to the plum (Prunus domestica x Prunus spinosa) root. The simulation outputs corresponded well with the diameter variation observed. The model predicts that variations of turgor pressure and osmotic potential are smaller than the variations of xylem water potential. It also demonstrates correlations between the xylem water potential, the turgor pressure, the elastic modulus, and the osmotic potential. The relationship between the diameter and the xylem water potential exhibits a substantial hysteresis, as observed in field data. A sensitivity analysis using the model parameters showed that growth and shrinkage were highly sensitive to the initial values of the turgor pressure and to the reflection coefficient of solutes. Shrinkage and growth were sensitive to elastic modulus and wall-yielding threshold pressure, respectively. The model was not sensitive to changes in temperature.

Biophysical Phenomena↗
PubMed1971

Evaluation of water stress control with polyethylene glycols by analysis of guttation.

The water relations of pepper plants (Capsicum frutescens L.) under conditions conducive to guttation were studied to evaluate the control of plant water stress with polyethylene glycols. The addition of polyethylene glycol 6000 to the nutrient solution resulted in water relations similar to those expected in soil at the same water potentials. Specifically, xylem pressure potential in the root and leaf became more negative during a 24-hour treatment period, while osmotic potential of the root xylem sap remained constant. The decrease in pressure potential was closely correlated with the decrease in osmotic potential of the nutrient solution. In contrast, the addition of polyethylene glycol 400 to the nutrient medium resulted in a reduction of osmotic potential in the root xylem sap; this osmotic adjustment in the xylem was large enough to establish an osmotic gradient for entry of water and cause guttation at a nutrient solution osmotic potential of -4.8 bars. Pressure potential in the root and leaf xylem became negative only at nutrient solution osmotic potentials lower than -4.8 bars. About half of the xylem osmotic adjustment in the presence of polyethylene glycol 400 was caused by increased accumulation of K(+), Na(+), Ca(2+), and Mg(2+) in the root xylem. These studies indicate that larger polyethylene glycol molecules such as polyethylene glycol 6000 are more useful for simulating soil water stress than smaller molecules such as polyethylene glycol 400.

Journal Article↗
PubMed1979

Assimilation and Transport of Nitrogen in Nonnodulated (NO(3)-grown) Lupinus albus L.

The response of nonnodulated white lupin (Lupinus albus L. cv. Ultra) plants to a range of NO(3) levels in the rooting medium was studied by in vitro assays of extracts of plant parts for NO(3) reductase (EC 1.6.6.1) activity, measurements of NO(3)-N in plant organs, and solute analyses of root bleeding (xylem) sap and phloem sap from stems and petioles. Plants were grown for 65 days with 5 millimolar NO(3) followed by 10 days with 1, 5, 15, or 30 millimolar NO(3). NO(3) reductase was substrate-induced in all tissues. Roots contained 76, 68, 62 and 31% of the total NO(3) reductase activity of plants fed with 1, 5, 15, and 30 millimolar NO(3), respectively. Stem, petioles, and leaflets contained virtually all of the NO(3) reductase activity of a shoot, the activity in extracts of fruits amounting to less than 0.3% of the total enzyme recovered from the plant. Xylem sap from NO(3)-grown nonnodulated plants contained the same organic solutes as from nodulated plants grown in the absence of combined N. Asparagine accounted for 50 to 70% and glutamine 10 to 20% of the xylem-borne N. The level of NO(3) in xylem sap amounted to 4, 13, 12, and 17% of the total xylem N at 1, 5, 15, and 30 millimolar NO(3), respectively. Xylem to phloem transfer of N appeared to be quantitatively important in supplying fruits and vegetative apices with reduced N, especially at low levels of applied NO(3). NO(3) failed to transfer in any quantity from xylem to phloem, representing less than 0.3% of the phloem-borne N at all levels of applied NO(3). Shoot organs were ineffective in storing NO(3). Even when NO(3) was supplied in great excess (30 millimolar level) it accounted for only 8% of the total N of stem and petioles, and only 2 and 1% of the N of leaflets and fruits, respectively.

Journal Article↗
PubMed1981

Direct and indirect measurements of Phloem turgor pressure in white ash.

Direct determinations and indirect calculations of phloem turgor pressure were compared in white ash (Fraxinus americana L.). Direct measurements of trunk phloem turgor were made using a modified Hammel-type phloem needle connected to a pressure transducer. Turgor at the site of the direct measurements was calculated from the osmotic potential of the phloem sap and from the water potential of the xylem. It was assumed that the water potentials of the phloem and xylem were close to equilibrium at any one trunk location, at least under certain conditions. The water potential of the xylem was determined from the osmotic potential of xylem sap and from the xylem tension of previously bagged leaves, measured with a pressure chamber. The xylem tension of bagged leaves on a branch adjacent to the site of the direct measurements was considered equivalent to the xylem tension of the trunk at that point. While both the direct and indirect measurements of phloem turgor showed clear diurnal changes, the directly measured pressures were consistently lower than the calculated values. It is not clear at present whether the discrepancy between the two values lies primarily in the calculated or in the measured pressures, and thus, the results from both methods as described here must be regarded as estimates of true phloem turgor.

Journal Article↗
PubMed1999

Biomechanics of the columnar cactus Pachycereus pringlei.

We report the longitudinal variations in stiffness and bulk density of tissue samples drawn from along the length of two Pachycereus pringlei plants measuring 3.69 and 5.9 m in height to determine how different tissues contribute to the mechanical stability of these massive vertical organs. Each of the two stems was cut into segments of uniform length and subsequently dissected to obtain and mechanically test portions of xylem strands, stem ribs, and a limited number of pith and cortex samples. In each case, morphometric measurements were taken to determine the geometric contribution each tissue likely made to the ability of whole stems to resist bending forces. The stiffness of each xylem strand increased basipetally toward the base of each plant where stiffness sharply decreased, reaching a magnitude comparable to that of strands 1 m beneath the stem apex. The xylem was anisotropic in behavior, i.e., its stiffness measured in the radial and in the tangential directions differed significantly. Despite the abrupt decrease in xylem strand stiffness at the stem base, the contribution made by this tissue to resist bending forces increased exponentially from the tip to the base of each plant due to the accumulation of wood. A basipetal increase in the stiffness of the pith (and, to limited extent, that of the cortex) was also observed. In contrast, the stiffness of stem rib tissues varied little as a function of stem length. These tissues were stiffer than the xylem in the corresponding portions of the stem along the upper two-fifths of the length of either plant. Tissue stiffness and bulk density were not significantly correlated within or across tissue types. However, a weak inverse relationship was observed for these properties in the case of the xylem and stem rib tissues. We present a simple formula that predicts when stem ribs rather than the xylem strands serve as the principal stiffening agents in stems. This formula successfully predicted the observed aspect ratio of the stem ribs (the average quotient of the radial and tangential dimensions of rib transections), and thus provided circumstantial evidence that the ribs are important for mechanical stability for the distal and younger regions of the stems examined.

Journal Article↗
PubMed2005

[Root activity and nitrogen assimilation of rice (Oryza sativa) under free-air CO2 enrichment].

With Free-Air CO2 Enrichment(FACE) technique, this paper studied the root activity and amino acid(aa) synthesis of rice (Oryza sativa )at low N(LN, 150 kgN x hm(-2)) and normal N(NN, 250 kgN x hm(-2)) under ambient air and elevated atmospheric CO2 (Ambient + 200 micromol x mol(-1)). Under elevated CO2, the xylem exudates per hill changed little, while the xylem exudates per stem declined by 1.4% - 21.7% as the result of greater tiller numbers. At tillering and heading stages, elevated CO2 increased aa N/inorganic N in xylem exudates by 11.1% - 143.1%, but did not affect the aa concentration in xylem exudates and the total amount of aa in roots significantly. However, at 35 days after heading, the aa N/inorganic N ratio decreased by 38.1% (LN) and 29.2% (NN) under elevated CO2. FACE also declined the aa concentration in xylem exudates by 34.0% (LN) and 44.7% (NN), and the total amount of aa by 50.8% (LN) and 40.0% (NN), which meant the retarded capability of aa synthesis in roots. N amendment led to a decrease of aa N/inorganic N in xylem exudates by 19.5% (Ambient) and 36.8% (FACE) at heading stage, as the result of unaffected aa and increased inorganic N concentration. There existed a significantly antagonistical CO2 x N interaction on aa N/inorganic N at heading stage.

Air↗
PubMed2003

APL regulates vascular tissue identity in Arabidopsis.

Vascular plants have a long-distance transport system consisting of two tissue types with elongated cell files, phloem and xylem. Phloem has two basic cell types, enucleate sieve elements and companion cells. Xylem has various lignified cell types, such as tracheary elements, the differentiation of which involves deposition of elaborate cell wall thickenings and programmed cell death. Until now, little has been known about the genetic control of phloem-xylem patterning. Here we identify the ALTERED PHLOEM DEVELOPMENT (APL) gene, which encodes a MYB coiled-coil-type transcription factor that is required for phloem identity in Arabidopsis. Phloem is established through asymmetric cell divisions and subsequent differentiation. We show that both processes are impaired by a recessive apl mutation. This is associated with the formation of cells that have xylem characteristics in the position of phloem. The APL expression profile is consistent with a key role in phloem development. Ectopic APL expression in the vascular bundle inhibits xylem development. Our studies suggest that APL has a dual role both in promoting phloem differentiation and in repressing xylem differentiation during vascular development.

Arabidopsis↗
PubMed2002

Dry matter accumulation in citrus fruit is not limited by transport capacity of the pedicel.

The vascularization of the pedicel in Marisol clementine (Citrus clementina Hort. ex Tanaka) has been characterized in relation to fruit growth. Phloem and xylem formation occurred during the first half of the period of fruit growth. Phloem cross-sectional area reached its maximum value by the end of fruitlet abscission, 78 d after anthesis (DAA), shortly after the rate of accumulation of dry matter in fruitlets reached its maximum value. Secondary xylem formation occurred until day 93, well after the end of fruitlet abscission. At fruit maturity, xylem accounted for 42-46 % of the cross-section of the pedicel. Vessels differentiated in this late-formed xylem. Formation of phloem and early xylem was directly related to fruitlet size (and growth rate). Differences in the rate of formation of conductive tissues in the pedicel of the developing fruitlets followed rather than preceded the differences in growth rate. Specific mass transfer (SMT) in the phloem was highest in the fastest growing fruitlets, and peaked during the late stages of fruitlet abscission (72-78 DAA) and during the main period of fruit growth (107-121 DAA). Application of a synthetic auxin to developing fruits, either at the end of flowering (2,4-D) or by day 64 after flowering (2,4-DP), increased the growth rate of the fruit and fruit size at maturity (8-13 % increase in fruit diameter at maturity). These auxin applications also enhanced the formation of conductive tissues in the pedicel, with a specific effect on phloem formation. Applying auxin at flowering resulted in a reduction in the phloem SMT by days 72-78, whereas auxin application on day 64 increased this parameter. Despite this difference in behaviour, which resulted from the different time-course of the growth response of the fruit to auxin applications, these applications increased fruit size to a similar extent. Severing 37 % of the phloem of the pedicel during the main period of fruit growth resulted in an increase in the specific mass transfer in the phloem but had no influence on fruit growth. These observations demonstrate that the transport capacity in the phloem of the pedicel does not limit fruit growth and, within the limits of our experiments, an increase in demand by the fruit appeared to be matched by an increase in SMT. The dependence of late xylem formation (after the period of fruitlet abscission) on fruitlet growth was demonstrated in Salustiana orange [Citrus sinensis (L.) Osbeck] by means of controlling fruit growth through the manipulation of leaf area. Fruit growth at this time was more closely related to leaf area than to carbohydrate levels, suggesting that it may be limited by current photosynthesis.

2,4-Dichlorophenoxyacetic Acid↗
PubMed2001

How the roots contribute to the ability of Phaseolus vulgaris L. to cope with chilling-induced water stress.

Intact plants and stem-girdled plants of Phaseolus vulgaris grown hydroponically were exposed to 5 degrees C for up to 4 d; stem girdling was used to inhibit the phloem transport from the leaves to the roots. After initial water stress, stomatal closure and an amelioration of root water transport properties allowed the plants to rehydrate and regain turgor. Chilling augmented the concentration of abscisic acid (ABA) content in leaves, roots and xylem sap. In intact plants stomatal closure and leaf ABA accumulation were preceded by a slight alkalinization of xylem sap, but they occurred earlier than any increase in xylem ABA concentration could be detected. Stem girdling did not affect the influence of chilling on plant water relations and leaf ABA content, but it reduced slightly the alkalinization of xylem sap and, principally, prevented the massive ABA accumulation in root tissues and the associated transport in the xylem that was observed in non-girdled plants. When the plants were defoliated just prior to chilling or after 10 h at 5 degrees C, root and xylem sap ABA concentration remained unchanged throughout the whole stress period. When the plants were chilled under conditions preventing the occurrence of leaf water deficit (i.e. at 100% relative humidity), there were no significant variations in endogenous ABA levels. The increase in root hydraulic conductance in chilled plants was a response neither to root ABA accretion, nor to some leaf-borne chemical signal transported downwards in the phloem, nor to low temperature per se, as indicated by the results of the experiments with defoliated or girdled plants and with plants chilled at 100% relative humidity. It was concluded that the root system contributed substantially to the bean's ability to cope with chilling-induced water stress, but not in an ABA-dependent manner.

Abscisic Acid↗