Temporal Patterns of Uptake, Flow and Utilization of Nitrate, Reduced Nitrogen and Carbon in a Leaf of Salt-Treated Castor Bean (Ricinus communis L.)

Changes in net photosynthesis, respiration, transpiration andcontents of total C, NO₃-N and reduced N were followed throughoutthe life of leaf 6 of nitrate-dependent plants of castor beanexposed to moderate salinity stress (71 mol m^–3 NaCl).Salt treatment was applied for measuring mineral flows in aparallel study (Jeschke and Pate, 1991b). Concurrent measurementswere made of solute composition and C: N molar ratios and concentrationsof reduced N and collected NO₃-N in phloem sap bleeding fromshallow incisions in the top and at the base of petioles andin xylem exudates from flaps of proximal leaf midribs followingpressurization of the root system. The resulting data were usedto construct empirical models of the respective economies ofC, total N, NO₃ and reduced N for a sequence of defined phasesof leaf life. Water use efficiency increased 3-fold from emergenceto a maximum of 1·5 mmol CO₂ mol^–1 H₂O before decliningto 0·5 mmol CO₂ mol^–1 H₂O at senescence. Xylemmolar ratios of C:N varied from 1·2–2·8,with nitrate always a smaller component than reduced N. Phloemsap C:N increased from 10–40 with leaf expansion and wasthen maintained in the range of 40–50 until falling steeplyto 20 at leaf senescence. Nitrate comprised less than 1% oftotal N in all phloem sap samples. The models of C uptake, flow,and utilization showed a major role of phloem import and thenincreasingly of laminar photosynthesis in providing C for leafgrowth. The carbon budget was thereafter characterized by ratesof phloem export closely matched to net rates of CO₂ fixationby the lamina. Corresponding data for total N depicted an earlymajor role of both xylem and phloem import, but the eventualdominance of xylem import as the N source for leaf growth. Cyclingof N by xylem to phloem exchange commenced before the leaf hadachieved maximum N content, and was the major contributor tophloem export until leaf senescence when mobilized N providedmost exported N. The nitrate economy of the leaf was characterizedby early establishment of tissue pools of the ion in the petioleand to a lesser extent in the lamina, continued high rates ofnitrate reduction in the lamina but negligible assimilationin the petiole, and a release through xylem of previously accumulatedNO₃ from petiole to lamina. Related data for reduced N illustratedthe much greater importance of this form of N than nitrate intransport, storage and cycling of N at all stages of leaf andpetiole life. Xylem to phloem interchanges of reduced N in petiolewere minimal in comparison with cycling through the lamina.The ratio of CO₂ reduction to NO₃ reduction in the lamina wasat first low (57 mol mol^–1) increasing to a peak valueof 294 during mature leaf functioning before declining to 190during the presenescence phase of leaf development. This patternreflected age-related effects on water use efficiency, changesin NO₃ levels in the xylem stream entering the lamina, and therelatively low photosynthetic performances of very young andsenescent laminae. Key words: Ricinus communis, leaf development, phloem transport, xylem transport, carbon, nitrogen, nitrate, reduced nitrogen, nitrate reduction, partitioning     

Partitioning of carbon and nitrogen and the nutrition of root and shoot apex in a nodulated legume

Empirically based models depicting exchanges of C, N, and H₂O in phloem and xylem among organs of nodulated white lupin (Lupinus albus cv Ultra) were constructed for the interval 51 to 58 days after sowing. Information was incorporated on the economy of C, N, and H₂O in plant parts, the solute composition of transport fluids collected at selected sites on the plant, and the photosynthetic inputs, transpirational losses, and translocatory activities of different age groups of leaflets and stem + petiole segments of the shoot. Partitioning of C and N showed preferential transfer of N to the shoot apex, which imported 13 milligrams C per milligram N, compared with 54 milligrams C per milligram N for the nodulated root. Leaves translocated assimilates at a C:N weight ratio of 43 to 59, and older leaves serving the roots produced the translocate most rich in N relative to C. The shoot apex was enriched with N, additional to its intake from leaves, by direct uptake of xylem fluid (C:N ratio, 2.4) and receipt of nitrogenous solutes transferred from xylem to upward-moving phloem streams in upper regions of the stem. The models for flow of N and H₂O indicated that xylem streams passing to leaves were substantially less rich in N than the adjacent stream moving through the body of the stem and that a progressive increase in concentration of N occurred within stem xylem elements from base to top of the shoot. This apparently resulted from an abstraction of N from xylem of departing leaf traces, possibly by xylem transfer cells, and a subsequent feedback of this N to xylem streams passing on up the shoot. Upper leaves and shoot apex, therefore, acquired more N from xylem per unit of H₂O transpired than lower parts of the shoot.     

Ionic Interactions of Petiole and Lamina During the Life of a Leaf of Castor Bean (Ricinus communis L.) Under Moderately Saline Conditions

Ion (K⁺, Na⁺, Mg²⁺, Ca²⁺ and Cl) flows and partitioning in thepetiole and lamina of leaf 6 of castor bean {Ricinus communisL.) plants growing in the presence of a mean of 71 mol m^–3NaCl were described by an empirical modelling technique. Thiscombined data on changes in ion contents of petiole and lamina,ion: carbon molar ratios of phloem bleeding sap and pressure-inducedxylem exudates of the leaf with previously described informationon the economies of C and N in identical leaf material. Datawere expressed as daily exchanges of ions in xylem and phloem,or depicted as models of ion balance and transport activityof petiole and lamina during four consecutive phases of leaflife. The early import phase was characterized by high intakeof K and Mg through phloem, and of Ca mainly through xylem,but only limited intake of Na and Cl. The next phase up to fullleaf expansion showed similar relative differences in xylemintake between ions and the onset of rapid phloem export fromthe lamina of K and Mg, some export of Na and Cl but scarcelyany of Ca. The next mature phase, marked by maximal photosynthesisand transpiration by the leaf, showed high xylem intake of allions in xylem. This was more than matched by phloem export ofMg and K, but by only fractional re-export of Na and Cl andagain very limited cycling through the leaf of Ca. The finalpre-senescence phase exhibited similar behaviour, but with generallygreater contribution to phloem transport from mobilization ofion reserves of the lamina. The petiole retained particularlylarge amounts of Na and Cl in its early growth, thereby protectingthe lamina from excessive entry of salt, but these petiolarpools, together with those or other nutrient ions, were laterpartially mobilized to the lamina via the xylem stream. Datawere discussed in relation to the relatively high salt toleranceexhibited by the species. Key words: Ricinus communis, xylem and phloem transport, ion balance, K+ economy, Na+ exclusion, NaCl-stress, salt tolerance, leaf development     

Transport of organic solutes in Phloem and xylem of a nodulated legume

Collections of xylem exudate of root stumps or detached nodules, and of phloem bleeding sap from stems, petioles, and fruits were made from variously aged plants of Lupinus albus L. relying on nodules for their N supply. Sucrose was the major organic solute of phloem, asparagine, glutamine, serine, aspartic acid, valine, lysine, isoleucine, and leucine, the principal N solutes of both xylem and phloem. Xylem sap exhibited higher relative proportions of asparagine, glutamine and aspartic acid than phloem sap, but lower proportions of other amino acids. Phloem sap of petioles was less concentrated in asparagine and glutamine but richer in sucrose than was phloem sap of stem and fruit, suggesting that sucrose was unloaded from phloem and amides added to phloem as translocate passed through stems to sinks of the plant. Evidence was obtained of loading of histidine, lysine, threonine, serine, leucine and valine onto phloem of stems but the amounts involved were small compared with amides. Analyses of petiole phloem sap from different age groups of leaves indicated ontogenetic changes and effects of position on a shoot on relative rates of export of sucrose and N solutes. Diurnal fluctuations were demonstrated in relative rates of loading of sucrose and N solutes onto phloem of leaves. Daily variations in the ability of stem tissue to load N onto phloem streams were of lesser amplitude than, or out of phase with fluctuations in translocation of N from leaves. Data were related to recent information on C and N transport in the species.     

Modelling the flow and partitioning of carbon and nitrogen in the holoparasite Cuscuta reflexa Roxb. and its host Lupinus albus L.: I. Methods for estimating net flows

Nodulated Lupinus albus L. was grown on quartz sand in the greenhouseand supplied with a N-free culture solution. Half the plantswere infected with Cuscuta reflexa Roxb. at 33 DAS. An empiricallybased modelling technique was developed to quantitatively depictuptake, flow and utilization of C and N in the host plant andbetween host and parasite over a 12 d period. The modellingincorporated C: N ratios of solutes in phloem and pressure-inducedxylem sap, net increments of C and N and respiratory lossesof C. For assessing the transfer of solutes from host phloemto Cuscuta it was not possible to use the C: N ratio of phloemsap close to the site of parasite attachment, a procedure whichwould have assumed non-specific withdrawal of phloem-borne solutes,since this would have implied unimpeded mass flow from hostto parasite. The relative intake of C and N by the parasiteby specific withdrawal of nitrogenous and carbonaceous solutesfrom the phloem was obtained independently by assuming thatxylem intake occurred non-specifically. Xylem import was thusobtained (a) from transpiration and tissue water increment ofCuscuta and the concentrations of N and C in xylem sap and (b)from the Ca²⁺ increment of Cuscuta and the ratios Ca: N andCa: C in lupin xylem sap, assuming that Ca²⁺ intake occurredsolely via xylem. By subtracting net xylem import from totaluptake of C and N by Cuscuta the methods resulted in comparableratios of C: N intake from the phloem. The average ratio (53.4)was smaller than the C:N ratio in host phloem (85.6) indicatingspecific withdrawal of solutes with a distinct preference forN. Using this ratio, modelling of flows of C and N was possibleand showed that Cuscuta abstracted C and N mainly from the hostphloem, but xylem supply was nutrient-dependent and amountedto 6.4% of the N but only 0.5% of the C demand. The resultsindicated that Cuscuta exerted a very strong sink and competedefficiently with the root, the major sink of L. albus, by attracting81% of the current photosynthate and more N (223%) than wascurrently fixed. The massive demand of the parasite led to lossesparticularly of N from leaves and the root and apart from causingcarbon losses it appeared to induce a sink-dependent stimulationof photosynthesis. In contrast, nitrogen fixation in the Cuscuta-infectedlupin was inhibited to 37% of the control. Key words: Cuscuta reflexa, Lupinus albus, carbon, nitrogen, phloem, xylem, transport, parasites, modelling     

Modelling of the Partitioning, Assimilation and Storage of Nitrate within Root and Shoot Organs of Castor Bean (Ricinus communis L.)

An experimentally-based modelling technique was developed todescribe quantitatively the uptake, flow, storage and utilizationof NO₃-N over a 9 d period in mid-vegetative growth of sandcultured castor bean (Ricinus communis L.) fed 12 mol m^–3nitrate and exposed to a mean salinity stress of 128 mol m^–3NaCl. Model construction used information on increments or lossesof NO₃-N or total reduced N in plant parts over the study periodand concentration data for NO₃-N and reduced (amino acid) Nin phloem sap and pressure-induced xylem exudates obtained fromstem, petiole and leaf lamina tissue at various levels up ashoot. The resulting models indicated that the bulk (87%) of incomingnitrate was reduced, 51% of this in the root, the remainderprincipally in the laminae of leaves. The shoot was 60% autotrophicfor N through its own nitrate assimilation, but was oversuppliedwith surplus reduced N generated by the root and fed to theshoot through the xylem. The equivalent of over half (53%) ofthis N returned to the root as phloem translocate and, mostly,then cycled back to the shoot via xylem. Nitrate comprised almosthalf of the N of most xylem samples, but less than 1% of phloemsap N. Laminae of leaves of different age varied greatly inN balance. The fully grown lower three leaves generated a surplusof reduced N by nitrate assimilation and this, accompanied byreduced N cycling by xylem to phloem exchange, was exportedfrom the leaf. Leaf 4 was gauged to be just self-sufficientin terms of nitrate reduction, while also cycling reduced N.The three upper leaves (5–7) met their N balance to varyingextents by xylem import, phloem import (leaves 6 and 7 only)and assimilation of nitrate. Petioles and stem tissue generallyshowed low reductase activities, but obtained most of theirN by abstraction from xylem and phloem streams. The models predictedthat nodal tissue of lower parts of the stem abstracted reducedN from the departing leaf traces and transferred this, but notnitrate, to xylem streams passing further up the shoot. As aresult, xylem sap was predicted to become more concentratedin N as it passed up the shoot, and to decrease the ratio ofNO₃-N to reduced N from 0·45 to 0·21 from thebase to the top of the shoot. These changes were reflected inthe measured N values for pressure-induced xylem exudates fromdifferent sites on the shoot. Transfer cells, observed in thexylem of leaf traces exiting from nodal tissue, were suggestedto be involved in the abstraction process. Key words: Ricinus communis, nitrogen, nitrate, nitrate reduction, partitioning, phloem, xylem, flow models     

Transport Exchange of Carbon,Nitrogen and Water in the Context of Whole Plant Growth and Functioning — Case History of a Nodulated Annual Legume

The economy of carbon, nitrogen and water during growth of nodulated, nitrogen-fixing plants of white lupin (Lupinus albus L.) was studied by measuring C, N and H₂O content of plant parts, concentrations of C and N in bleeding sap of xylem and phloem, transpirational losses of whole shoots and shoot parts, and daily exchanges of CO₂ between shoot and root parts and the surrounding atmosphere. Relationships were studied between water use and dry matter accumulation of shoot and fruits, and between net photosynthesis rate and leaf area, transpiration rate and nitrogen fixation. Conversion efficiencies were computed for utilization of net photosynthate for nitrogen fixation and for production of dry matter and protein in seeds. Partitioning of the plant's intake of C, N and H₂O was described in terms of growth, transpiration, and respiration of plant parts. An empirically-based model was developed to describe transport exchanges in xylem and phloem for a 10-day interval of growth. The model depicted quantitatively the mixtures of xylem and phloem streams which matched precisely the recorded amounts of C, N and H₂O assimilated, absorbed or consumed by the various parts of the plant. The model provided information on phloem translocation of carbon and nitrogen to roots from shoots, the cycling of carbon and nitrogen through leaves, the relationship between transpiration and nitrogen partitioning to shoot organs through the xylem, the relative amount of the plant's water budget committed to phloem translocation, and the significance of xylem to phloem transfer of nitrogen in stems as a means of supplying nitrogen to apical regions of the shoot.     

Root hypoxia reduces leaf growth : role of factors in the transpiration stream

This study examined the potential role of restricted phloem export, or import of substances from the roots in the leaf growth response to root hypoxia. In addition, the effects of root hypoxia on abscisic acid (ABA) and zeatin riboside (ZR) levels were measured and their effects on in vitro growth determined. Imposition of root hypoxia in the dark when transpirational water flux was minimal delayed the reduction in leaf growth until the following light period. Restriction of phloem transport by stem girdling did not eliminate the hypoxia-induced reduction in leaf growth. In vitro growth of leaf discs was inhibited in the presence of xylem sap collected from hypoxic roots, and also by millimolar ABA. Disc growth was promoted by sap from aerated roots and by 0.1 micromolar ZR. The flux of both ABA and ZR was reduced in xylem sap from hypoxic roots. Leaf ABA transiently increased twofold after 24 hours of hypoxia exposure but there were no changes in leaf cytokinin levels.     

Diurnal Patterns of Transport and Accumulation of Minerals in Fruiting Plants of Lupinus angustifolius L.

Fluctuations in mineral elements id xylem (tracheal) sap, fruitphloem sap, leaflets and dmloping fruits were studied in a fieldpopulation of Lupinus angustifolius L. by three-hourly samplingover a 39 h period. Elements usually reached maximum contentsor concentrations at or near noon, minimum levels during thenight. Amplitudes of diurnal fluctuations in minerals lay withinthe range ±4–33 per cent of the mean content ofleaflets, and ±17–157 per cent of the mean concentrationsin xylem and phloem sap. Most minerals elements fluctuatcd inphase with daily changes in sugar level of phloem sap and drymatter and carbohydrate fluctuations of leaflets, suggestinga coupling of translocation of photosynthate and minerals fromthe leaflets. Rates of import of minerals by shoots wereestimatedfrom shoot transpiration and mineral concentrations in trachealsap. Average day time rates of import of most elements were12–25 times those at night. Translocation of minerals,nitrogen and carbon to fruits also exhibited diurnal periodicity,average rates of import king three to seven times higher inthe day than at night. A model of transport based on the carbonand water economy of the fruit suggested that P, K, Fe, Zn,Mn and Cu were imported predominantly by phloem. Estimates ofvascular import accounted for 87–104 per cent of the fruit'sactual increment of these elements. Na and Ca were gauged tobe imported mainly by xylem, Mg almost equally by xylem andphloem. However, large discrepancies existed for these threeelements between estimated vascular import and actual intakeby the fruit. Lupinus angustifolius L., mineral transport, accumulation, fruits, xylem sap, phloem sap, transpiration     

Phloem loading in peach: Symplastic or apoplastic?

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2-month-old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with ³H-inulin, represented about 17% of the leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the symplastic loading of the phloem of this alditol. However, infiltration of ¹⁴C-labelled source leaves with 2 mMp-chloromercuribenzenesulfonic acid (PC-MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose-transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative symplastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.     

Detection and characterization of nitrogen circulation through the sieve tubes and xylem vessels of rice plants

Nitrogen movement through the xylem vessels and sieve tubes in rice plants was studied using xylem and phloem sap analysis in combination with stable and radioactive nitrogen isotope techniques.More than 90% of nitrogen was translocated in the sieve tubes of rice plants as amino acids. When 15N (99.6 atom%) was applied as a nitrate to the root, 15N first appeared in phloem sap of the leaf sheath within 10 minutes and increased to 37 atom% excess 5 hours after the experiment had started. In long-term experiments, 63% of nitrogen in the phloem sap of the leaf sheath and 15% in that of the uppermost internode came from nitrogen absorbed within the last 24 hours and 50 hours, respectively.To obtain information about the more rapid circulation of nitrogen in the plant, radioactive 13N was used as a tracer. A positron-emitting tracer imaging system was used to show that 13N was transferred to the leaf sheath within 8 minutes of its application to the roots. Analysis of the xylem sap of the leaf sheath showed that when the nitrate was applied to the roots, most of the nitrogen in the xylem was transported as a nitrate.These data showed that phloem and xylem sap analysis together with the stable and radioactive nitrogen techniques provide a good method for the detection of nitrogen cycles in plants.     

Economy of Carbon and Nitrogen in a Nodulated and Nonnodulated (NO(3)-grown) Legume

Partitioning and utilization of assimilated C and N were compared in nonnodulated, NO₃-fed and nodulated, N₂-fed plants of white lupin (Lupinus albus L.). The NO₃ regime used (5 millimolar NO₃) promoted closely similar rates of growth and N assimilation as in the symbiotic plants. Over 90% of the N absorbed by the NO₃-fed plants was judged to be reduced in roots. Empirically based models of C and N flow demonstrated that patterns of incorporation of C and N into dry matter and exchange of C and N among plant parts were essentially similar in the two forms of nutrition. NO₃-fed and N₂-fed plants transported similar types and proportions of organic solutes in xylem and phloem. Withdrawal of NO₃ supply from NO₃-fed plants led to substantial changes in assimilate partitioning, particularly in increased translocation of N from shoot to root. Nodulated plants showed a lower (57%) conversion of C or net photosynthate to dry matter than did NO₃-fed plants (69%), and their stems were only half as effective as those of NO₃-fed plants in xylem to phloem transfer of N supplied from the root. Below-ground parts of symbiotic plants consumed a larger share (58%) of the plants' net photosynthate than did NO₃-fed roots (50%), thus reflecting a higher CO₂ loss per unit of N assimilated (10.2 milligrams C/milligram N) by the nodulated root than by the root of the NO₃-fed plant (8.1 milligrams C/milligram N). Theoretical considerations indicated that the greater CO₂ output of the nodulated root involved a slightly greater expenditure for N₂ than for NO₃ assimilation, a small extra cost due to growth and maintenance of nodule tissue, and a considerably greater nonassimilatory component of respiration in root tissue of the symbiotic plant than in the root of the NO₃-fed plant.     

Cytokinins in the Phloem Sap of White Lupin (Lupinus albus L.)

Cytokinin-like activity in samples of xylem and phloem sap collected from field-grown plants of white lupin (Lupinus albus L.) over a period of 9 to 24 weeks after sowing was measured using the soybean hypocotyl callus bioassay following paper chromatographic separation. The phloem sap was collected from shallow incisions made at the base of the stem, the base of the inflorescence (e.g. stem top), the petioles, and the base and tip of the fruit. Xylem sap was collected as root exudate from the stump of plants severed a few centimeters above ground level. Concentration of cytokinin-like substances was highest in phloem sap collected from the base of the inflorescence and showed an increase over the entire sampling period (from week 10 [61 nanogram zeatin equivalents] to week 24 [407 nanogram zeatin equivalents]). Concentrations in the xylem sap and in the other phloem saps were generally lower. Relatively high concentrations of cytokinin-like substances in petiole phloem sap (70 to 130 nanogram zeatin equivalents per milliliter) coincided in time with high concentrations in sap from the base of the inflorescence (see above). Concentrations in sap (phloem or xylem) from the base of the stem were very much lower. This finding is consistent with movement of cytokinins from leaves into the developing inflorescence and fruit, rather than direct input to the fruit from xylem sap. However, an earlier movement of cytokinins from roots into leaves via the xylem cannot be ruled out. Sap collected at an 18-week harvest was additionally separated by sequential C₁₈ reversed-phase high performance liquid chromatography → NH₂ normal phase high performance liquid chromatography, bioassayed, and then analyzed by electron impact gas chromatography-mass spectrometry. Identification of zeatin riboside and dihydrozeatin as two of the major cytokinins in combined sap samples was accomplished by gas chromatography-mass spectrometry-selected ion monitoring.     

Modeling C and N transport to developing soybean fruits

Xylem sap and phloem exudates from detached leaves and fruit tips were collected and analyzed during early pod-fill in nodulated soybeans (Glycine max (L.) Merr. cv Wilkin) grown without (−N) and with (+N) NH₄NO₃. Ureides were the predominant from (91%) of N transported in the xylem of −N plants, while amides (45%) and nitrate (23%) accounted for most of the N in the xylem of +N plants. Amino acids (44%) and ureides (36%) were the major N forms exported in phloem from leaves in −N plants, but amides (63%) were most important in +N plants. Based on the composition of fruit tip phloem, ureides (55% and 33%) and amides (26% and 47%) accounted for the majority of N imported by fruits of −N and +N plants, respectively.

C:N weight ratios were lowest in xylem exudate (1.37 and 1.32), highest in petiole phloem (24.5 and 26.0), and intermediate in fruit tip exudate (12.6 and 12.1) for the −N and +N treatments, respectively. The ratios were combined with data on fruit growth and respiration to construct a model of C and N transport to developing fruits. The model indicates xylem to phloem transfer provides 35% to 52% of fruit N. Results suggest the phloem entering fruits oversupplies their N requirement so that 13% of the N imported is exported from fruit in the xylem.

     

Stomatal Closure in Flooded Tomato Plants Involves Abscisic Acid and a Chemically Unidentified Anti-Transpirant in Xylem Sap

We address the question of how soil flooding closes stomata of tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) plants within a few hours in the absence of leaf water deficits. Three hypotheses to explain this were tested, namely that (a) flooding increases abscisic acid (ABA) export in xylem sap from roots, (b) flooding increases ABA synthesis and export from older to younger leaves, and (c) flooding promotes accumulation of ABA within foliage because of reduced export. Hypothesis a was rejected because delivery of ABA from flooded roots in xylem sap decreased. Hypothesis b was rejected because older leaves neither supplied younger leaves with ABA nor influenced their stomata. Limited support was obtained for hypothesis c. Heat girdling of petioles inhibited phloem export and mimicked flooding by decreasing export of [14C]sucrose, increasing bulk ABA, and closing stomata without leaf water deficits. However, in flooded plants bulk leaf ABA did not increase until after stomata began to close. Later, ABA declined, even though stomata remained closed. Commelina communis L. epidermal strip bioassays showed that xylem sap from roots of flooded tomato plants contained an unknown factor that promoted stomatal closure, but it was not ABA. This may be a root-sourced positive message that closes stomata in flooded tomato plants.     

Dynamic changes in petiole specific conductivity in red maple (Acer rubrum L.), tulip tree (Liriodendron tulipifera L.) and northern fox grape (Vitis labrusca L.)

Diurnal variation in petiole specific hydraulic conductivity and simultaneous measurements of leaf water potential were recorded in red maple, tulip tree and fox grape. Petiole specific conductivity was determined from in situ measurements of water flow into the distal (leaf‐bearing) end of an attached petiole as a function of applied hydrostatic pressure and petiole dimensions. The hydraulic properties of the petiole dominated the measurements, indicating that this technique can be used for rapid estimates of petiole hydraulic conductivity. There was a significant decrease in petiole specific conductivity associated with increasingly more negative leaf water potentials in maple and tulip tree, but not in grape. Petiole specific conductivity increased during the afternoon while the plant was actively transpiring and the xylem sap was under tension. The recovery of petiole conductivity during the afternoon suggests that hydraulic conductivity reflects a dynamic balance between a loss of hydraulic conductivity with increasing water stress, and its restoration as tension within the xylem decreases. Three experimental manipulations were applied to red maple and tulip tree to examine the sensitivity of diurnal changes in petiole conductivity to various physiological perturbations. Both phloem girdling and application of HgCl₂ to the transpiration stream resulted in a marked decrease in the degree to which petiole specific conductivity recovered as xylem tension relaxed during the afternoon. Delivery of a surfactant to the xylem, however, did not significantly alter the relation between leaf water potential and petiole hydraulic conductivity.     

The Role of the Stem in the Partitioning of Na+ and K+ in Salt-Treated Barley

Hordeum vulgare cv. California Mariout was grown for 50 d insand culture at 100 mol m^–3 NaCl. Xylem sap was collectedthrough incisions at the base of individual leaves along thestem axis by applying pressure to the root system. K⁺ concentrationsin the xylem sap reaching individual leaves increased towardsthe apex, while concentrations of Na⁺, NO₃^–, and Cl^–declined. Phloem exudate was obtained by collecting into Li₂EDTAfrom the base of excised leaves. K/Na ratios of phloem exudatesincreased from older to younger leaves. K/Na ratios in xylem sap and phloem exudate were combined withchanges in ion content between two harvests (38 and 45 d aftergermination) and the direction of phloem export from individualleaves, to construct an empirical model of K⁺ and Na⁺ net flowswithin the xylem and phloem of the whole plant. This model indicatesthat in old leaves, phloem export of K⁺ greatly exceeded xylemimport. In contrast, Na⁺ export was small compared to importand Na⁺ once imported was retained within the leaf. The direction of export strongly depended on leaf age. Old,basal leaves preferentially supplied the root, and most of theK⁺ retranslocated to the roots was transferred to the xylemand subsequently became available to the shoot. Upper leavesexported to the apex. Young organs were supplied by xylem andphloem, with the xylem preferentially delivering Na⁺ , and thephloem most of the K⁺ . For the young ear, which was still coveredby the sheath of the flag leaf, our calculation predicts phloemimport of ions to such an extent that the surplus must havebeen removed by an outward flow in the xylem. Within the culm,indications for specific transfers of K⁺ and Na⁺ between xylemand phloem and release or absorption of these ions by the tissuewere obtained. The sum of these processes in stem internodes and leaves ledto a non-uniform distribution of Na⁺ and K⁺ within the shoot,Na⁺ being retained in old leaves and basal stem internodes,and K⁺ being available for growth and expansion of young tissues. Key words: Hordeum vulgare L., K⁺, Na⁺, stem, salt stress     

Patterns of Assimilate Production and Translocation in Muskmelon (Cucumis melo L.) : I. Diurnal Patterns

Continuous monitoring of steady-state carbon dioxide exchange rates in mature muskmelon (Cucumis melo L.) leaves showed diurnal patterns of photosynthesis and respiration that were translated into distinct patterns of accumulation and phloem export of soluble sugars and amino acids. Leaf soluble sugar patterns in general followed the pattern of photosynthetic activity observed in the leaf, whereas starch accumulated steadily throughout the light period. Sugar and starch levels declined through the dark phase. Phloem exudate analysis revealed that diurnal levels of the major transport sugars (stachyose and sucrose) in the phloem did not appear to correlate directly with the photosynthetic activity of the leaf but instead were inversely correlated with leaf starch accumulation and degradation. The amino acid pool in leaf tissues remained constant throughout the diurnal period; however, the relative contribution of individual amino acids to the total pool varied with the diurnal photosynthetic and respiratory activity of the leaf. In contrast, the phloem sap amino acid pool size was substantially larger in the light than in the dark, a result primarily due to enhanced export of glutamine, glutamate, and citrulline during the light period. The results indicate that the sugar and amino acid composition of cucurbit phloem sap is not constant but varies throughout the diurnal cycle in response to the metabolic activities of the source leaf.     

Uptake and Utilization of Xylem-borne Amino Compounds by Shoot Organs of a Legume

Amino compounds representative of the major N solutes of xylem sap were pulse-fed (10 to 20 minutes) singly in ¹⁴C-labeled form to cut transpiring shoots of white lupin (Lupinus albus L.). ¹⁴C distribution was studied by autoradiography and radioassays of phloem sap, leaflet tissues, and shoot parts harvested at intervals after labeling. Primary distribution of N by xylem was simulated using a 20-minute labeling pulse followed by a 30-minute chase in unlabeled xylem sap. Shoots fed ¹⁴C-labeled asparagine, glutamine, valine, serine, or arginine showed intense labeling of leaflet veins and marked retention (35 to 78%) of ¹⁴C by stem + petioles. Shoots fed ¹⁴C-labeled aspartic acid or glutamic acid showed heaviest ¹⁴C accumulation in interveinal regions of leaflets and low uptake (11 to 20%) of ¹⁴C by stem + petioles. Departing leaf traces were major sites of uptake of all amino compounds, and the implications of this were evaluated. Fruits acquired only 1 to 5% of the fed label directly from xylem, but more than doubled their intake during the period 30 to 160 minutes after feeding through receipt of ¹⁴C transferred from xylem to phloem in stem and leaves. ¹⁴C-Labeled asparagine and valine transferred directly from xylem to phloem, but the ¹⁴C of ¹⁴C-labeled aspartic acid and arginine appeared in phloem mainly as metabolic products of the fed compound. The labeling of the soluble pool of leaflets reflected these differences. The significance of heterogeneity in distribution and metabolism of xylem amino compounds in the shoot was discussed.     

Review article. Physicochemical aspects of ion relations and pH regulation in plants - a quantitative approach

A quantitative physicochemical approach to ion relations of biological solutions is presented, which applied fundamental laws of physical chemistry to these systems and allows analysis of dependent variables ([H⁺], [OH^-] and the dissociation state of partially dissociated ('weak') ions including carbonate species) in relation to independent variables (concentrations of strong and weak ions, dissociation constants and CO2 partial pressure). Within this concept the influence of strong (fully dissociated) ions is confined to their net unbalanced positive charge which is referred to as SID (strong ion difference). The SID concept is then applied to membrane transport processes and ion relations of xylem and phloem sap: simple transmembrane transport of protons between compartments cannot affect pH on either side of the membrane, because rather small deviations from electrical neutrality results in substantial changes of the membrane potential under natural conditions. Thus the membrane ATPases as electrogenic pumps cannot control the pH of adjacent compartments, but they energize secondary active transmembrane ion transport that results in pH changes. The SID approach is shown to be valid by matching pH values calculated from analysis of xylem and phloem saps with actual measured values. Sensitivity analysis based on the SID approach allows (1) to detect inconsistency in determination of composition in the analysed solutions and (2) quantitatively to analyse the influence of ion export or import and variations of pCO2 on pH and dissociation state of weak acids of complex biological solutions. The SID concept thus allows the evaluation of the contribution of a proposed pH-regulating or pH-affecting mechanism on a quantitative physicochemical basis.Key words: Electrical neutrality, membrane potential, pH regulation, phloem sap, SID, xylem sap.