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The spittlebug Philaenus spumarius feeds from mature xylem at the full hydraulic tension of the transpiration stream
- M. MALONE, R. WATSON, J. PRITCHARD
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- Journal:
- The New Phytologist / Volume 143 / Issue 2 / August 1999
- Published online by Cambridge University Press:
- 01 August 1999, pp. 261-271
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- August 1999
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Feeding dynamics in the spittlebug Philaenus spumarius were analysed to determine the food source. Evidence from previous reports indicates that this species and similar insects feed from the xylem. However, it is not clear whether they extract sap from the main transpiration stream, or from some subsidiary compartment of xylem, such as immature or cavitated vessels. In feeding from such subsidiary compartments, the insects could avoid the large hydraulic tensions thought to prevail in xylem on the main transpiration stream. Philaenus, for example, is shown here to initiate feeding, and to feed rapidly, from plants in which the full xylem tension is estimated at −10 bar. From measurements of feeding rates and feeding patterns in stressed and unstressed plants, it is shown that Philaenus cannot feed from subsidiary xylem compartments. It is concluded that this insect feeds directly from the main transpiration stream, and at the full hydraulic tension which prevails there. The anatomy of the feeding pump in Philaenus appears consistent with this conclusion. Spittlebugs therefore offer an experimental tool for quasi- non-invasive sampling of the transpiration stream.
Simultaneous collection of xylem sap from Rhinanthus minor and the hosts Hordeum and Trifolium: hydraulic properties, xylem sap composition and effects of attachment
- W. E. SEEL, W. D. JESCHKE
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- Journal:
- The New Phytologist / Volume 143 / Issue 2 / August 1999
- Published online by Cambridge University Press:
- 01 August 1999, pp. 281-298
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- August 1999
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The facultative hemiparasite Rhinanthus minor was grown alone, or with either Hordeum vulgare or Trifolium alpestre as a host. All plants were fed 5 mM nitrate. In the parasites, successful attachment led to dramatic increases in growth (particularly with barley as host) and in tissue concentrations of total N, NO3−, H2PO4− and K+, as well as moderate increases in Cl− and SO42−. Mg2+ and Ca2+ concentrations either remained unchanged, increased or decreased, depending on the host. Xylem sap was collected from leaf veins (barley) or the stem (Rhinanthus and clover), by applying pneumatic pressure to the rooted soil to raise the soil water potential so that in vivo xylem sap exuded from small incisions once the applied pressure balanced the tension exerted by transpiration. High balancing pressures were needed for unattached Rhinanthus, whereas simultaneous collection of xylem sap from attached Rhinanthus and its host, at a much lower balancing pressure, was possible only when the parasite was enclosed in polyethylene film and high transpiration abolished. From plots of xylem sap volume flow versus applied pressure, the hydraulic conductivity Lp and the overall hydraulic resistance r were obtained. The value of r was high in unattached Rhinanthus and dramatically decreased by attachment to a host, highlighting the improved access to water gained by Rhinanthus when parasitizing a host. For the parasites, attachment resulted in greatly increased concentrations and solute flow rates in xylem of K+, NO3−, H2PO4− and amino acids, and decreased concentrations of Ca2+ and Mg2+. Estimates of NO3− reduction in the parasite showed that 99% was reduced in unattached plants, 85% in those attached to clover and 52% in those attached to barley. In the parasitized hosts, ion concentrations in xylem sap were somewhat increased relative to unparasitized controls. Conversely, xylem sap amino-acid concentrations in infected hosts were decreased. Glutamine (Gln) was the principal amino acid in xylem sap of unattached Rhinanthus, but after attachment to both hosts asparagine (Asn) predominated. In clover, Asn was the major transport amino acid, suggesting massive transfer of Asn from host to parasite. In unparasitized barley, however, Gln was the major xylem sap amino acid, but unexpectedly, in this species Asn was induced as the principal amino acid when parasitized. The results are discussed in terms of mutual host–parasite interactions.
Quantification of vessel embolisms by direct observation: a comparison of two methods
- J. S. PATE, M. J. CANNY
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- Journal:
- The New Phytologist / Volume 141 / Issue 1 / January 1999
- Published online by Cambridge University Press:
- 01 January 1999, pp. 33-43
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- January 1999
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When freshly cut segments of naturally decorticated steles of horizontal roots of the Australian grass tree (Xanthorrhea preissii) are subjected to gentle suction, bubbles of gas sometimes appear, as well as liquid, in capillaries attached to the aspirated ends of the steles. We tested the hypothesis that this gas comes from vessels embolized in the intact xylem stream, and that the gas volume extruded is therefore an effective measure of the extent of this embolism. To do this, twin samples were taken from individual roots of X. preissii in the field, one was fast-frozen intact for subsequent estimation of vessel embolisms in the cryo-scanning electron microscope (CSEM), the other 15-cm segment rapidly assessed for volume of gas aspirated into a standard micropipette tube. The two measures mutually confirmed one another by showing a strong positive correlation between numbers of embolized vessels and extracted gas volume. Similar gas volumes were obtained from replicate root segments excised directly from a root when the ends of the segment were frozen before excision, and aspiration conducted after subsequent thawing of the ends under water. The pattern of changes in embolisms during unstressed conditions in early summer, shown by both CSEM and aspiration, indicated almost no embolisms before dawn, followed by a rapid rise to a peak in mid morning, than a progressive loss of embolisms in late afternoon. It was also shown that the amount of embolism did not change with time after excision of the roots up to at least 30 min. A comparison of changes in leaf transpiration with gas volumes in steles during a 24-h cycle at peak transpiration stress in mid summer showed rapid rates of transpiration in early morning and late afternoon, with an intervening period of low water loss during the rest of the day. Numbers of embolisms rose to an early morning peak, followed by apparent repair of these before noon. There was a second spate of embolisms in late afternoon, followed by complete refilling of all xylem with liquid by an hour or so after dusk. All vessels then remained fully recharged until the following dawn. We believe that aspiration is a direct and reliable technique, which offers a simple, inexpensive means of assessing the relative extent of embolism of vessels in xylem, and a means to test earlier findings by the other direct method of the CSEM. In a broad context, the technique should provide new opportunities for evaluating water relations of the xylem of whole plants.
Metabolic responses of Norway spruce (Picea abies) trees to long-term forest management practices and acute (NH4)2SO4 fertilization: transport of soluble non-protein nitrogen compounds in xylem and phloem
- PAUL WEBER, HEIKE STOERMER, ARTHUR GEßLER, STEPHAN SCHNEIDER, DOMINIK VON SENGBUSCH, ULRIKE HANEMANN, HEINZ RENNENBERG
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- Journal:
- The New Phytologist / Volume 140 / Issue 3 / November 1998
- Published online by Cambridge University Press:
- 01 November 1998, pp. 461-475
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- November 1998
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From laboratory experiments with seedlings and young trees of Norway spruce (Picea abies L. Karst.), a cycling pool of soluble non-protein N compounds is thought to be indicative of the N-nutritional status of trees. In order to test whether this assumption can be transferred to mature trees grown in the field, xylem sap and phloem exudate were collected from spruce trees in two remote forest stands: (1) a N-limited stand (Villingen site), and (2) a stand where trees are sufficiently supplied with N from the soil (Schluchsee site). Trees at these sites were c. 80–100 (Villingen site) and c. 40–60 (Schluchsee site) yr old. In addition to untreated control areas, one entire watershed area at both sites was subjected to (NH2)2SO4 fertilization to the soil.
In the xylem sap of the spruce roots at both sites Gln, Asp and Arg were the dominant total soluble non-protein nitrogen (TSNN) compounds. In the xylem sap of the trunk and the twigs Arg was virtually absent and Gln plus Asp dominated TSNN. On average, TSNN in the xylem sap of trees at the Schluchsee site was 1·5–2-fold higher than those at Villengen. Highest TSNN contents in the xylem sap were found during growth and development of current year tissues, while the lowest TSNN contents were found in summer. At the Villingen site (NH4)2SO4 fertilization caused an increase in the Gln content in the xylem sap of all tree sections analysed as well as an increase in the Arg content in the xylem sap of the roots. At the Schluchsee site only a small increase in TSNN contents of the xylem was observed, mostly in the xylem sap of the roots. In phloem exudates TSNN contents were much higher in trees on the Schluchsee than on the Villingen site. The seasonal pattern of TSNN in phloem exudates was similar to the seasonal pattern found in the xylem sap. During spring and early summer Gln was the predominant TSNN compound in phloem exudates, but during late summer and autumn Arg became predominant. At the Villingen site (NH4)2SO4 fertilization caused a significant increase in TSNN contents in phloem exudates of twigs and roots, but at Schluchsee an increase in TSNN was found only in phloem exudates of the roots. At both field sites Arg that was not transported to the shoot by xylem transport, was allocated from the leaves to the roots by phloem transport and was cycled within the root system by both xylem and phloem transport. From these results it is calculated that shoot-to-root signalling by long-distance transport of amino compounds can also contribute to the regulation of N-nutrition of mature spruce trees. Apparently, the internal cycling of individual N compounds within spruce trees differs considerably.
Soluble N compounds in trees exposed to high loads of N: a comparison between the roots of Norway spruce (Picea abies) and beech (Fagus sylvatica) trees grown under field conditions
- ARTHUR GESSLER, STEPHAN SCHNEIDER, PAUL WEBER, ULRIKE HANEMANN, HEINZ RENNENBERG
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- Journal:
- The New Phytologist / Volume 138 / Issue 3 / March 1998
- Published online by Cambridge University Press:
- 01 March 1998, pp. 385-399
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- March 1998
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During the growing session of 1995, the total soluble non-protein nitrogen (TSNN) composition and contents of mycorrhizal fine roots, xylem sap and phloem exudates of roots from a coniferous (Picea abies L.(Karst)) and a deciduous (Fagus sylvatica L.) tree species were analysed at a field site (‘Höglwald’, Germany) exposed to high loads of N. In April, TSNN in fine roots of spruce and beech trees amounted to 16 μmol N g−1 f. wt and 23·3 μmol N g−1 f. wt, respectively. It decreased to 9·2 μmol N g−1 f. wt and 18·1 μmol N g−1 f. wt, respectively, after bud break in June. The seasonal maximum of TSNN in fine roots of spruce was observed in July (32·7 μmol N g−1 f. wt) followed by a decline of c. 30% until the end of the growing season in September. TSNN in fine roots of beech trees showed a further decline between June and July, when its seasonal minimum was determined (15·6 μmol N g−1 f. wt), and increased to c. 29 μmol N g−1 f. wt until September. In spruce roots Gln and Arg were the most abundant TSNN compounds during the entire growing season. In roots of beech Asn played an important role alongside Gln and Arg, especially in April, when it was the most abundant TSNN compound. Other proteinogenic and non-proteinogenic N compounds comprised c. 20–30% of TSNN. Nitrate made up <1%, and ammonium <7% of TSNN in the fine roots of both species.
In April, TSNN in the xylem sap of roots of spruce and beech trees amounted to 3·4 and 8·6 μmol N ml−1, respectively. In roots of spruce trees xylem sap TSNN increased after bud break up to 12·7 μmol N ml−1 in July. At the end of the growing season TSNN had declined again to 3·9 μmol N ml−1. TSNN in the root xylem sap of beech trees decreased after bud break until July (2·4 μmol N ml−1 in July) followed by a slight increase until September (2·9 μmol N ml−1). Arg, Gln and Asp were the most abundant TSNN compounds in the xylem sap of spruce trees contributing together c. 90% to TSNN. The same TSNN compounds prevailed in the root xylem sap of beech trees in April and July, whereas in June and September Asp was replaced by Asn comprising 57% of TSNN in June. In addition to the N compounds mentioned above, a number of other proteinogenic and non-proteinogenic amino compounds were found in root xylem sap of both species. In either species, nitrate and ammonium were present in small amounts, contributing <1% and <4% to TSNN, respectively. Apparently, inorganic N taken up by the mycorrhizal roots is mainly assimilated in root tissues or by the mycorrhiza and N uptake by the roots is largely adapted to the assimilatory capacity of this organ.
In phloem exudates of spruce roots, TSNN amounted to 10·7 μmol N g−1 f. wt in April, increased in June to 23·4 μmol N g−1 f. wt and decreased again until September to a seasonal minimum of 4·8 μmol N g−1 f. wt. In contrast to spruce, TSNN content in phloem exudates of beech roots showed a seasonal maximum (c. 20 μmol N g−1 f. wt) in April with a subsequent decrease in June after bud break (c. 2 μmol N g−1 f. wt). A fourfold increase in July was followed by a decrease in September, when TSNN in phloem exudates of beech roots amounted to 4·3 μmol N g−1 f. wt. Arg was the most abundant N compound in the phloem of roots from spruce trees and made up c. 60–85% of TSNN during the entire growing season. In beech trees the seasonal course of TSNN correlated with the relative abundance of Arg. Arg comprised 69 and 57% of TSNN in April and July, respectively, but contributed <20% in June and September. Besides Arg, other proteinogenic and non-proteinogenic amino compounds could be detected in the phloem of both species. In addition, nitrate and ammonium were present in considerable amounts.
From these results and a previous report on TSNN in above-ground parts of spruce and beech at the same site, a whole-plant model for the cycling of TSNN in both species is proposed. Differences in the location of storage pools are assumed to be responsible for the differences in the seasonal course of TSNN composition and contents observed between the two tree species.