Journal of Experimental Botany

Popp, Marianne;Lied, Wolfgang;Meyer, Andreas J.;Richter, Andreas;Schiller, Petra;Schwitte, Hildegard

doi: 10.1093/jxb/47.10.1469pmid: N/A

Abstract In search of a reliable drying method, which might be used even under field conditions, microwave drying was compared to freeze-drying of plant material. Leaves of Ananas comosus and Avicennia germinans as well as buds and phloem of Acer pseudoplatanus were used and checked for one or more of the following substances: sugars, sugar alcohols, organic and amino acids, total nitrogen, and glycinebetaine. With most samples good agreement was achieved between the two drying methods. Only in the case of the Ananas comosus leaves, which exhibited low pH and high water content, did appreciable differences occur in organic and amino acids. Besides that, sucrose was the compound most susceptible to alterations, which was especially evident when leaves of Sambucus nigra were dried in the two different compartments (condenser compartment, drying bell jar) of the freeze-dryer in use. For Ananas comosus leaf samples it was shown that microwaving can also be used prior to extraction of tissue sap. Microwave, freeze-drying, drying method, tissue sap, organic solutes © Oxford University Press

Dodd, I.C.;Stikic, R.;Davies, W.J.

doi: 10.1093/jxb/47.10.1475pmid: N/A

Abstract There is now substantial evidence that chemical regulation of shoot physiology occurs in droughted plants in the field. The evidence that ABA may play a role in such regulation is considered, and topics of relevance to the worker interested in determining the ABA relations of plants in the field; such as the methods used for ABA quantification, the relevance of quantifying ABA in various plant tissues, methods of xylem sap collection and timing of sap collection are reviewed. A possible role of tissue sensitivity to ABA in controlling the diurnal changes in stomatal conductance and leaf growth rate seen in the field is also considered. ABA, drought, stomatal conductance, leaf growth, hormonal sensitivity, xylem sap © Oxford University Press

Rennenberg, Heinz;Schneider, Stephan;Weber, Paul

doi: 10.1093/jxb/47.10.1491pmid: N/A

Abstract Due to anthropogenic activities trees that often occupy nitrogen-poor environments are exposed to high loads of atmospheric nitrogen. Nitrogen influx into the leaves by dry and wet deposition may then contribute a considerable fraction to total nitrogen required for growth and development of trees. This uncontrolled influx does not only affect nitrogen, but also sulphur metabolism, because of regulatory interactions of nitrogen and sulphur assimilation by inter-pathway control. To assess the metabolic consequences of nitrogen influx via the leaves, analysis of its influence on the uptake of nitrogen and sulphur compounds by the roots and the distribution of these compounds within the trees are required. Techniques that may be used for this purpose under field conditions include (a) depletion of artificial soil water for the analysis of root uptake, (b) xylem sap extraction by pressure and displacement, (c) xylem sap flow analysis by heat balance methods, and (d) phloem sap exudation by extraction of bark segments. To quantify the contribution of gaseous nitrogen influx into the leaves to total nitrogen nutrition, additional trace gas exchange studies by dynamic chamber techniques are required. In the present report, these techniques are discussed with special emphasis on the application under field conditions. Dry deposition, nitrogen and sulphur uptake, phloem sap, xylem sap © Oxford University Press

Goodman, A.M.;Ennos, A.R.

doi: 10.1093/jxb/47.10.1499pmid: N/A

Abstract Despite numerous studies of the effects of mechanical stimulation on plant shoots, the response of roots to mechanical stimulation has largely been neglected. In this study the effects of shoot flexure on the morphology and mechanics of two contrasting species of herbaceous angiosperm, growing in a glasshouse were compared: maize (Zea mays), a monocot; and sunflower (Helianthus annuus L.) a dicot. Mechanical stimulation affected the root more than the shoot components. Root systems of mechanicallystressed sunflowers had a greater angle of spread and increased root number. As well as large morphological and weight effects, with increases over the control of 33% in the length of rigid root and 38% in the dry weight of lateral roots, in sunflowers, there were also mechanical effects. In both species roots of flexed plants were more rigid, stronger and composed of stiffer material and their root systems also provided greater anchorage strength. In contrast, there was only a small reduction in shoot weight and shoot height in flexed plants and no effects on mechanical properties. There were differences in behaviour between species; maize root morphology responded less than that of sunflowers to mechanical stimulation. The basal diameter of roots increased by only 8% compared with 16% in sunflowers, though the roots of both species showed similar increases in material stiffness. This difference is related to the lack of secondary thickening in the monocots compared with the dicot sunflowers. Thigmomorphogenesis, Helianthus annuus L., Zea mays, anchorage, lodging © Oxford University Press

Crook, M.J.; Ennos, A.R.

doi: 10.1093/jxb/47.10.1509pmid: N/A

Abstract The anchorage of deep rooted 16-year-old larch trees, Larix europea × japonica, has been studied by combining winching tests with analyses of strain around the base of the trunk and root system and mechanical tests on individual roots. These showed that anchorage is provided by the laterals which emerge from around the stem base, sinker roots which emerge along their length, and tap roots positioned directly underneath the bole. During anchorage failure the leeward laterals are bent and eventually break close to their base, whilst the windward laterals are pulled out of the ground, with their sinker roots intact. After initially being confined by the soil and bending, the tap root rotates in the soil. Anchorage failure is similar when the soil is dry as when it is wet, but failure occurs closer to the trunk. Strain measurements along the lateral roots revealed that the stresses were highest close to the trunk and that these regions of the roots contribute most to tree stability. The two major components of anchorage were found to be the resistance of leeward laterals to bending and the resistance of tap roots and windward sinkers to uprooting. Bending tests on leeward laterals revealed that they provide around 25% of tree anchorage. Almost 75% of the anchorage strength must, therefore, be provided by the windward sinkers and tap roots. Anchorage strength of roots was positively correlated to their cross-sectional area. The vertical orientation of the sinkers makes the anchorage system of larch more efficient than the plate system formed by Sitka spruce on waterlogged soils and means that no root-soil plate is formed. Anchorage, root architecture, sinker roots, root bending strength, windthrow This content is only available as a PDF. © Oxford University Press © Oxford University Press

Pritchard, Jeremy

doi: 10.1093/jxb/47.10.1519pmid: N/A

Abstract The aim of the present study was to quantify osmotic pressures directly in the translocation pathway, from leaf to growing root tip, in order to understand the forces driving solutes from a source to a sink. Solutes move through the translocation pathway down an osmotically derived turgor gradient. Accordingly aphid stylectomy and single cell sampling techniques have been combined to examine the osmotic pressure of root phloem and growing root cells. Sieve tube sap was obtained from shoots and, for the first time, roots of barley seedlings using aphid stylectomy. Vacuolar sap was also obtained from a variety of cells in leaf and root tissues using single cell sampling methods. Osmotic pressure of sieve tube sap from roots and shoots was measured at high temporal resolution (within min) and over long periods of time (up to 24 h). Osmotic pressure did not change significantly in the minutes immediately following excision, suggesting that confidence can be placed in the assumption that stylet exudate is representative of sieve tube sap in vivo. There were no differences in the osmotic pressure of sieve tube sap from shoots (1.24±0.26 MPa, n = 10) or roots (1.42±0.15 MPa, n = 13). However, osmotic pressure of sap from root cortical cells (0.71±0.09, n = 12) was about 0.7 MPa lower than that of the sieve elements from roots, this difference may be maintained by consumption of incoming solutes at the root tip. Results are discussed in the context of pressure driven flow in the phloem and symplastic contact between root tip cells and sieve tube. It is hoped that the approach described here will provide important insights into the nature of the relationship between root cell extension and assimilate supply through the phloem. Phloem, sieve tube, aphid, root, barley, osmotic pressure, translocation © Oxford University Press

Griffiths, Allen;Bray, Elizabeth A.

doi: 10.1093/jxb/47.10.1525pmid: N/A

Abstract Plant responses to water deficit are dynamic and varied, requiring co-ordination between the shoot and root. Among these responses are alterations in gene expression. The expression of four genes, le4, le16, le20, and le25, which require increased ABA content for expression, was studied in tomato plants in which the root systems were divided between two large pots to impose water deficit gradually and to control signals from the root in response to soil drying without inducing a signal from the shoot. One group of plants had one-half of the roots watered, another group had both halves watered, and another group had neither halves watered. In unwatered plants, the expression of le4 and le25 correlated with ABA content, and that of le16 and le20 occurred before a detectable increase in leaf ABA content. The contrasting patterns of expression indicate a difference in sensitivity of these genes to ABA or an additional signalling mechanism. Ample evidence indicates that shoot processes such as stomatal closure are controlled by signals from the root. This study demonstrates that genes may also be induced in the shoot by signals from the root. Shoots of plants in which only half of the roots were watered showed no decrease in relative water content and no increase in ABA content; however, three of the four genes, le4, le16, and le20, were induced. Root-to-shoot communication plays a role in changes in gene expression and in alterations in physiological processes. Abscisic acid, water deficit, gene expression, split-root plants, long-distance signal © Oxford University Press

Pedersen, A.L.;Feldner, H.C.;Rosendahl, L.

doi: 10.1093/jxb/47.10.1533pmid: N/A

Abstract Experiments were carried out to investigate if drought stress affects the ability of bacteroids from soybean (Glycine max L.) root nodules to utilize proline and malate to support nitrogenase activity. The bacteroids were isolated in sub-ambient oxygen and nitrogenase activity was measured by acetylene reduction. Nitrogenase activity supported by proline was 8-fold higher in bacteroids from drought-stressed nodules than in bacteroids from control nodules. In contrast to the results with proline there was no significant response to drought stress in the rate of bacteroid nitrogenase activity supported by malate. The effect of drought stress on transport of proline and malate across the symbiosome membrane was investigated by incubation of symbiosomes isolated in sub-ambient oxygen with radioactive tracers. Drought stress tended to increase the rate of proline uptake relative to a minor decrease in malate uptake into symbiosomes in response to drought. There was no indication of a saturable camer in the symbiosome membrane for either substrate at concentrations in the range 0.1-2 mM. The rate of malate uptake into symbiosomes was twice as high as the rate of proline uptake at all substrate levels tested. The protein composition of the symbiosome membrane was altered in response to drought stress and these changes may relate .to the permeability of the symbiosome membrane. Drought stress, nitrogenase activity, proline, soybean nodules, symbiosome membrane, transport © Oxford University Press

Duan, Xiangrong;Neuman, Dawn S.;Reiber, Janet M.;Green, Craig D.;Saxton, Arnold M.;Augé, Robert M.

doi: 10.1093/jxb/47.10.1541pmid: N/A

Abstract During drying, mycorrhizal plants often maintain higher stomatal conductance (gs) than similarly-sized and -nourished non-mycorrhizal plants, but the mechanism of mycorrhizal influence remains unclear. Several hydraulic and non-hydraulic factors previously implicated in control of stomatal behavior during drought were measured, to learn which are affected when roots of cowpea (Vigna unguiculata [L.] Walp. cv. White Acre) are extensively colonized by Glomus intraradices Schenck and Smith isolate UT143. At low soil water contents (Θ), mycorrhizal plants maintained higher gs, transpiration and shoot water potential (ψ) than non-mycorrhizal plants. These higher foliar water status characters were associated with lower xylemsap abscisic acid concentrations ([ABA]) and lower ABA fluxes to leaves in mycorrhizal plants at low soil Θ. Stomatal conductance was most closely correlated with xylem-sap [ABA], ABA flux to leaves and shoot ψ. Stomatal conductance was not correlated with xylemsap concentrations of calcium or zeatin riboside equivalents, or with xylem-sap pH, nor were these xylem-sap constituents affected by mycorrhizal symbiosis. Stomata of mycorrhizal and non-mycorhizal leaves showed similar sensitivities to ABA, whether leaves were intact or detached. It is concluded that mycorrhizal fungi probably increased the capability of root systems to scavenge water in drier soil, resulting in less strain to foliage and hence higher gs, and shoot ψ at particular soil Θ. Abscisic acid, cytokinins, Glomus intraradices © Oxford University Press

Setter, Timothy Louis;Laureles, Eufrocino Vidonia

doi: 10.1093/jxb/47.10.1551pmid: N/A

Abstract Adverse effects of elongation growth on tolerance to complete submergence for up to 14 d were evaluated in rice seedlings of cultivars which differed in submergence tolerance. There is a good negative correlation between per cent survival and elongation growth of genotypes during complete submergence (r = − 0.81). When elongation growth underwater is minimized by application of a gibberellin biosynthesis inhibitor, per cent survival increases by as much as 50 times for one cultivar. These effects are likely related to elongation growth since (i) addition of gibberellin had the opposite effect by reducing survival, and (ii) when the elongation inhibitor and gibberellin were added together, there was no effect on elongation growth and the per cent survival did not change. A GA-deficient mutant of rice which had little elongation ability during submergence showed a high level of submergence tolerance when plants were submerged at equal initial dry weights and carbohydrate levels relative to a submergence-tolerant cultivar. These results are consistent with the hypothesis that elongation growth competes with maintenance processes for energy and hence reduces survival during submergence. The impact of these findings is that in environments where elongation ability is not required, there is a potential to increase submergence tolerance of agriculturally important cultivars by selecting for least elongation, at least during periods of complete submergence. Furthermore, this trade-off between stimulated elongation growth and submergence tolerance will have important ecological consequences for the distribution of plant species in different flood-prone environments. Gibberellin, growth, Oryza sativa, rice, submergence © Oxford University Press