W. Dieter Jeschke

Ion Circulation via Phloem and Xylem Between Root and Shoot of Nodulated White Lupin

The exchange rates of mineral cations in the xylem and phloem between root and shoot of white lupin (Lupinus albus L., cv. Ultra) were measured using nodulated plants grown in a defined liquid culture medium low in Na and lacking nitrogen. Harvests were taken at 39 and 49 days after sowing and plant parts analysed for C, N, and the mineral cations K(+), Mg(++), Ca(++), and Na(+). Respiration losses of carbon by nodulated roots were assessed using Pettenkofer assemblies, and concentrations of C, N, and cations in xylem and phloem assayed by collecting root bleeding xylem sap and phloem sap of stem base during the day and night at several times throughout the study period. Flow rates of C and N between root and shoot were determined as in earlier modeling studies (Pate et al., 1979 b), using data on consumption of C by roots, increments of C and N in shoot and root dry matter and, C : N ratios in xylem and phloem sap. Using ratios of cation : C in xylem and phloem, net flow of the various ions between shoot and root were computed. The data showed substantial return of K(+) and Mg(++) from shoot to root with phloem translocate. This return flow provided roots with more K(+) or Mg(++) than was required for growth. It was estimated that 76 % or 87 % of the phloem-borne K(+) and Mg(++) respectively reentered the xylem and was thus circulated within the plant. Rates of return flow to roots and circulation within the plant were very small for Ca(++) and for Na(+) under the conditions of the experiment.

Partitioning of K+, Na+, Mg++ , and Ca++ through Xylem and Phloem to Component Organs of Nodulated White Lupin under Mild Salinity

Summary Rates of cation (K + , Na + , Mg ++ , Ca ++ ) transport via xylem and phloem and of cation exchange among component organs of effectively nodulated white lupin ( Lupinus albus L. cv. Ultra) were determined during an 8-day period at flowering under mild salinity (10 mol m −3 NaCl). Carbon, nitrogen and mineral cations were analyzed in roots, leaflets, stem, and petiole tissue of each of four strata of leaves of the shoot, in root bleeding (xylem) sap and in phloem saps obtained from stems and petioles of the corresponding strata. Respiration of roots and respiration and photosynthesis of stem segments and leaves of each stratum of the shoot were measured. Carbon and nitrogen flows within the shoot and between root and shoot were then estimated as in earlier studies (Pate et al., 1979 b). Using these flow profiles, ratios of each cation to carbon in transport fluids and increments of ions in plant parts during the study period and patterns of partitioning of individual cations were estimated. The data revealed high rates of transport in phloem and xylem within the shoot for K + , and to a lesser extent also for Mg ++ and Na + . Substantial rates of cycling of K + and Mg ++ between shoot and root were observed. K + was translocated preferentially towards young, growing organs, but Na + mainly deposited in stems (partly in exchange for K + ) and transported preferentially to the root. Acropetal translocation of K + in phloem occurred preferentially towards the inflorescence, in xylem principally towards lateral branches. The flow model for K + suggested progressive enrichment of the upward moving xylem stream with K + through mobilization of K + from mature stem tissues, combined with selective phloem to xylem transfer of K + in the stem vasculature. Translocation of Na + to the root was partly due to preferential xylem to phloem transfer within stem tissue. Basipetal transport of Na + was particularly evident when the external supply of NaCl was removed. Phloem mobility of Na + is discussed in relation to the relatively limited salt tolerance of white lupin.

Effects of NaCl Salinity on Growth, Development, Ion Transport and Ion Storage in White Lupin (Lupinus albus L. cv. Ultra)

Summary Effectively nodulated plants of white lupin were grown in silica sand culture and exposed to 1, 5, 10, 25 and 40 mol m −3 NaCl. Dry matter gains of shoot and root were almost linearly decreased with increasing external NaCl, relative growth rate of roots being more affected than that of shoots. Concentrations of Na + and Cl - in root bleeding (xylem) sap and shoot tissues increased proportional to applied NaCl, indicating limited control over entry of Na + and Cl - into root and xylem stream. K: Na ratios were higher in leaflets than in adjoining petioles and stem segments, and in younger than older parts of the shoot, suggesting capacities for Na + retention in stems and selectivity in K + mobilization to young tissues. However, phloem sap of stems and leaf petioles from different sites on the shoot showed dramatic increases in Na + and Cl - with increasing salt in the medium, so non-uniformity in distribution of K + and Na + was all but lost at higher salinities (25 and 40 mol m -3 NaCl). Total amino acid levels in xylem sap increased with increasing salinity, those in phloem increasing or decreasing depending on site of collection. Asparagine contributed most to these changes, but serine increased greatly in all phloem sap samples with increasing applied salt. Proline was only a minor constituent of xylem and phloem, even at the highest salinity. The relatively high sensitivity of the species to salt was ascribed to limited capacity of mature tissues to accommodate or eliminate Na + and Cl - through acquisition of succulence, synthesis of counter osmotica, or abscission of leaves, combined with relatively unrestricted access of Na + and Cl - to both xylem and phloem. Results are discussed in relation to published data for other salt sensitive or salt tolerant species.

Modeling of Sodium and Potassium Flows via Phloem and Xylem in the Shoot of Salt-stressed Barley

Summary Hordeum vulgare, cv. California Mariout was grown in liquid culture in presence of NaCl (1to 100 mol m-3). 14 and 19 days after sowing, i.e. at the three to four leaf stage, plants were harvested, and increments of K+ and Na+ during this interval were determined in individual organs. In parallel, K/Na ratios of phloem exudate and xylem sap were measured. Phloem exudation was induced by employing Li2EDTA; xylem sap was obtained by applying pressure to the root system. Efficient control of ion flows across the roots was checked by measuring solute vs. volume flow. Using ion increments and K/Na ratios of the sap net flows and partitioning of K+ and Na+ within the shoots were calculated. Although all K+ flows in salt-treated barley (100 mol m-3 NaCl) were greatly reduced relative to the control, there was substantial retranslocation of K+ from mature to young leaves and recirculation of K+ via phloem from shoot to roots. Part of this recirculated K+ was transferred from phloem to xylem within the root and returned to the shoot via xylem. Translocation of K+ in the phloem exceeded that of Na+ despite high external Na+ concentrations. The relation of K+ translocation and of low phloem mobility of Na+ to salt tolerance of barley is discussed.

Modelling of the Uptake, Flow and Utilization of C, N and H2O within Whole Plants of Ricinus communis L. Based On Empirical Data

Summary An empirically based modelling technique was used to quantitatively depict uptake, flow and utilization of C, N and H2O for a 9 day period in mid vegetative growth of NO3-fed castor bean (Ricinus communis L.) exposed to a mean salinity stress of 128 mol m-3 NaCl. The models incorporated data on C : N weight ratios of solutes of phloem sap and pressure-induced xylem exudates of leaves, stem internodes and petioles, net increments or losses of C, N and H2O in plant parts, transpirational losses of shoot organs and respiratory losses of C from shoot parts and root. A computational technique was developed to assess the extent of xylem to phloem transfer of C and N within internally sited organs (stem segments and petioles) in addition to traffic of C and N through these organs to terminal organs (root, shoot apex and leaves). Molar ratios of inputs of H2O: C : N were 4803: 22: 1. Half of the net daytime photosynthetic gain of C by the shoot was translocated initially to the root, 33 % was lost in root respiration, 17% in shoot night respiration, 8% cycled through the root system and 40% was finally incorporated into shoot dry matter, 10% into root dry matter. The corresponding N budget showed 93% initial transfer in xylem from root to leaf laminae, 32% backflow in phloem from shoot to root and 15 % cycled through the root. The water budget involved a 98% loss in transpiration, 2% incorporation into tissues and 2% commitment to phloem transport. Xylem to phloem exchanges of C and N in shoot mostly involved transfer from xylem to phloem. The models predicted a steep upward gradient in decreasing water use efficiency with leaf age, due principally to poor CO2 fixation in relation to water loss by youngest leaves. Comparison of fluxes of N and H2O between stem internode segments with those passing out to leaves indicated progressive lateral abstraction of N from leaf traces serving lower leaves and subsequent passage of this N to the xylem of cauline traces serving upper regions of the shoot. Data were compared with earlier obtained information on C, N and H2O partitioning in nodulated white lupin (Lupinus albus L.).

Role of Stems in Transport, Storage, and Circulation of Ions and Metabolites by the Whole Plant

Publisher Summary This chapter demonstrates that stems do not function in a passive manner when mediating the transport activities mentioned above. First, because stems are mostly inactive in photosynthesis or uptake of minerals from the environment, their requirements for extension and secondary thickening must derive from lateral uptake of solutes from transport fluids passing through their structure. It provides several examples in which integrated exchanges of this nature comprise vital and quantitatively important elements of plant functioning. The plan of this chapter is to examine, first, some of the principal sites and tissue types within stems that are likely to be committed to short-distance exchanges between transport channels,to engage in storage of specific solutes, or insoluble reserves. Second, a brief account is given of an empirically based modeling procedure that has been designed for the study of uptake, partitioning, and utilization of nutrient elements in whole plants. Third, employing this same approach, case studies are presented to highlight the vital role of the stem in partitioning and storage of nutrients. Last, a summary of the major conclusions from the chapter is presented, with the goal of highlighting how little is still known of the potential role of stems in regulating the traffic of solutesthus, shaping temporal and spatial growth and storage within other parts of the plant body.

Solute flows from Hordeum vulgare to the hemiparasite Rhinanthus minor and the influence of infection on host and parasite nutrient relations

Using the facultative root hemiparasite Rhinanthus minor and Hordeum vulgare L. as a host, the flows and partitioning of mineral nutrients within the host, the parasite and between host and parasite have been studied during the study period 41-54 d after planting, i.e approximately 30-43 d after successful attachment of the parasite to the host. In parasitising Rhinanthus shoot growth was 12-fold, but root growth only 2-fold increased compared to non-parasitising plants. Conversely, in the Hordeum host, shoot dry matter growth was clearly reduced, by 33% in leaf laminae and by 52% in leaf sheaths, whereas root growth was only slightly reduced as a consequence of parasitism. Growth-dependent increments of total nitrogen (N) and phosphorus (P) and of potassium (K), calcium (Ca) and magnesium (Mg) in parasitising Rhinanthus shoot were strongly increased, particularly increments of total N and P, which were 18 and 42 times, respectively, higher than in solitary Rhinanthus. However, increments of the above mineral nutrients in leaf sheaths of parasitised Hordeum vulgare were more strongly decreased than in leaf laminae in response to parasitic attack. Estimation of the flows of nutrients revealed that Rhinanthus withdrew from the host xylem sap about the same percentage of each nutrients: 18% of total N, 22% of P and 20% of K. Within the host almost all net flows of nutrient ions were decreased due to parasitism, but retranslocation from shoot to root was somewhat increased for all nutrients. Quantitative information is provided to show that the substantially increased growth in the shoot of attached Rhinanthus and the observed decrease in Hordeum shoot growth after infection were related to strongly elevated supply of nitrogen and phosphorus in the parasite and to incipient deficiency of these nutrients in the parasitised host. The flows of nutrients between host and parasite are discussed in terms of low selectivity of nutrient abstraction from the host xylem by the hemiparasite Rhinanthus minor.

Does legume nitrogen fixation underpin host quality for the hemiparasitic plant Rhinanthus minor

The high quality of leguminous hosts for the parasitic plant Rhinanthus minor (in terms of growth and fecundity), compared with forbs (non-leguminous dicots) has long been assumed to be a function of the legumes ability to fix atmospheric nitrogen (N) from the air and the potential for direct transfer of compatible amino compounds to the parasite. Using associations between Rhinanthus minor and Vicia faba (Fabaceae) that receive N either exclusively via symbiotic associations with rhizobia supplying organic N fixed from N(2) or exclusively through the supply of inorganic nitrate to the substrate, the underlying reasons for the quality of legumes as hosts for this parasite are unravelled. It is shown that sole dependence of the host, V. faba, on N fixation results in lower growth of the attached parasite than when the host is grown in a substrate supplied exclusively with inorganic N. In contrast, the host plants themselves achieved a similar biomass irrespective of their N source. The physiological basis for this is investigated in terms of N and abscisic acid (ABA) partitioning, haustorial penetration, and xylem sap amino acid profiles. It is concluded that legume N fixation does not underpin the quality of legumes as hosts for Rhinanthus but rather the well-developed haustorium formed by the parasite, coupled with the lack of defensive response of the host tissues to the invading haustorium and the presence of sufficient nitrogenous compounds in the xylem sap accessible to the parasite haustoria, would appear to be the primary factors influencing host quality of the legumes.

Effects of Potassium Withdrawal on Nitrate Transport and on the Contribution of the Root to Nitrate Reduction in the Whole Plant

Summary The effect of K + as a countercation to NO 3 − on the reduction of NO 3 − in the root has been investigated using castor bean ( Ricinus communis L.) and barley ( Hordeum vulgare L.) grown with reciprocal splitroots. One part of the root system was supplied with a nutrient solution lacking potassium but containing NaNO 3 instead of KNO 3 (−K+N roots), whereas the other side obtained potassium but no nitrate (+K−N roots). The contribution of the roots to nitrate reduction was assessed firstly by analyzing the proportion of reduced to total nitrogen in the xylem sap and secondly by measuring nitrate reductase activity (NRA) in vivo and in vitro in the roots. −K+N roots of Ricinus contributed more to nitrate reduction in the whole plant than control roots, as shown by a higher ratio of reduced to total nitrogen in xylem sap and by a 2.3- to 2.8-fold higher in vivo NRA. This minus K stimulation of NRA was found also in roots of Hordeum in the NRA measured in vivo (1.9-fold) as well as in vitro (1.6-fold). The minus K stimulation in −K+N roots was reversible within 4 or 6 h and it decreased with a half-time of 43 or 75 min after supply of external K + or Rb + , respectively. The development of the minus K stimulation, however, was slow. It occurred with a time lag of 4 days and was completed after 10 days exposure to minus K conditions, suggesting that regrowth of new root tissue was involved. NRA in +K−N roots was, as expected, low. These findings suggest that availability of easily permeating countercations to nitrate, such as potassium, plays a role in the regulation of the partitioning of nitrate reduction between shoot and root in the whole plant.

Interactions Between Rhinanthus minor and Its Hosts: A Review of Water, Mineral Nutrient and Hormone Flows and Exchanges in the Hemiparasitic Association

At the ecological level, the effects of the facultative root hemiparasite Rhinanthus minor on the structure and functioning of its host communities are relative well described; yet until recently, the mechanistic basis for parasitic plant-driven community change and the physiological basis for the host-parasite interaction were poorly understood. Empirical incremental flow models, based on the increase in water, mineral nutrients, carbon assimilates or phytohormones between two defined time points, have been successfully employed to investigate the physiology of resource acquisition by- and distribution within host-parasitic plant associations. In this study we review the application of these empirical flow models to Rhinanthus-host associations showing the extent of and physiological basis of resource abstraction from the host and how this is profoundly influenced by soil nutrient status. We show that Rhinanthus primarily abstracts water and mineral nutrients via the apoplastic pathway through direct lumen-lumen connections with little resource acquisition via symplastic pathways. Nutrient status of the soil is shown to significantly influence the resource acquisition. We also investigate the hormonal regulation of resource acquisition by Rhinanthus showing pivotal roles for the key for the phytohormones abscisic acid (ABA) and cytokinins.