Journal of Experimental Botany, Vol. 52, No. 364, pp. 2143-2150,
November 1, 2001
© 2001 Oxford University Press
Original Papers |
Effect of top excision and replacement by 1-naphthylacetic acid on partition and flow of potassium in tobacco plants
1 Department of Plant Nutrition, China Agricultural University, Yuanmingyuan West Road 2, Beijing 100094, PR China
2 Julius-von-Sachs-Institut für Biowissenschaften, Lehrstuhl Botanik I, Universität Würzburg, Mittlerer Dallenbergweg 64, D-97082 Würzburg, Germany
Received 24 April 2001; Accepted 26 June 2001
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Abstract |
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The effect of removal of the shoot apex of 92-d-old tobacco plants and its replacement by 1-naphthylacetic acid (NAA) on sink–source relationships and on the flows and partitioning of potassium and water has been studied over a short-term period of 7 d (intact control plants) or 8 d (decapitated and NAA-treated plants). For determining flows an upper, middle and lower stratum of three leaves each were analysed. Within the study period three new leaves were formed in control plants and 57.7% of the total dry matter increment during the experimental period was allocated to the apex and these newly formed leaves. An even higher proportion of the K+ taken up (93.8%) was deposited in these organs and this was imported via xylem (72%) and phloem (28%). Only 18.7% and 9.8% of the total dry matter increment were found in the previously present upper leaves and the roots, respectively, and substantial net K+ export occurred from middle and lower leaves and roots. Decapitation removed the dominant phloem sink and caused marked changes in sink–source relationships. After decapitation the net increase in root dry matter was twice that of control plants. 56.2% of the total net increments in dry matter and 70% of the absorbed K+ were deposited in upper leaves (below the excised apex). There was only slight net K+ export from the middle leaves. Application of NAA on the cut surface of the stem stump did not change the growth of plants that much, apart from a substantial increase in stem growth, correspondingly it stimulated the partitioning of K+ into the upper leaves and most dramatically into the stem, which deposited 64.5% or 27% of the K+ uptake, respectively. In these plants K+ uptake was increased and the K+ concentrations in upper, middle and lower leaves were increased from 4.7, 5.4 and 5.6 to 5.1, 6.1 and 6.1% of dry matter, respectively. Possible mechanisms of this effect of NAA on the improvement of K+ concentration in tobacco leaves are discussed in detail.
Key words: Decapitation, 1-naphthylacetic acid, phloem and xylem transport, potassium, sink–source relationship, tobacco.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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The plant apex is not only an important source of auxin production, but also a centre of growth and metabolism in higher plants. Assimilates synthesized in leaves and mineral nutrients taken up by roots are necessary for maintaining the growth and development of the apex. There is much circumstantial evidence to imply effects of IAA on the regulation of assimilate and nutrient translocation and partitioning in plants (Davies and Wareing, 1965
; Patrick, 1982; Thomas, 1986). The results of growing apex excision and replacement by an exogenous source of IAA indicated that assimilate translocation towards auxin-treated tissues may involve both ‘local’ effects of the growth substance on sink activity and/or assimilate unloading, and ‘remote’ effects along the phloem pathway between source and sink. An enhanced accumulation of endogenous nitrogen, phosphorus and potassium in the IAA-treated areas has been shown (Phillips, 1968).
It is well known that potassium (K) is one of the most abundant cations in cells of higher plants and plays vital roles in plant growth and development. Potassium has the property of high phloem mobility and, as a result, of a high degree of reutilization by retranslocation via phloem (Marschner, 1995). It has been shown that the partitioning and the amount of phloem retranslocation of K+ from the shoot and cycling through the root are very different in various plants (tomato: Amstrong and Kirkby, 1979; white lupin: Jeschke et al., 1987; wheat: Cooper and Clarkson, 1989; castor bean: Jeschke and Pate, 1991b; tobacco: Hibberd et al., 1999). This partitioning and retranslocation of K+ in plants can be changed when plants grow under saline conditions (Jeschke et al., 1987; Jeschke and Pate, 1991b; Wolf et al., 1991) or are infected by parasitic angiosperms (Hibberd et al., 1999).
In tobacco production, the tops of the plants are excised when the first flower of the inflorescence is blossoming. Although this is beneficial for getting high leaf yield with good quality, in terms of the plant's physiology this is likely to cause changes in the sink–source relationships of the tobacco plant, which will result in changes in distribution and translocation of assimilates and mineral nutrients. Recently, field experiments (CJ Li, unpublished results) indicated that leaf K+ concentration could be improved markedly when auxin, or more specifically the auxin-analogue NAA, was applied to the cut surfaces of stem stumps after decapitation. However, to what extent translocation and circulation of potassium between the different plant organs is altered after excision of the apex and replacing it by a source of exogenous auxin is not clear. The present experiments were conducted to repeat the field experiments of NAA application under controlled laboratory conditions and to quantify the translocation and circulation of potassium in the xylem and phloem in tobacco plants following decapitation and replacement of the apex by an exogenous source of auxin in the form of NAA. The aim of the paper was to elucidate the possible mechanisms for the improved leaf K+ concentration by the application of auxin after decapitation.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Plant growth
Tobacco seeds (Nicotiana tabacum L. K 326) were germinated in a mixture of 60% (w/w) peat culture substrate, 20% (w/w) ground maize stalk and 20% (w/w) perlite, and grown in a seedbed in a naturally illuminated glasshouse until the normal transplanting stage for tobacco production in the field (more than 2 months). Afterwards, they were washed with tap water until all substrates were removed from roots, and then transferred into 2.1 l pots (one plant per pot) containing quartz sand (0.25–0.5 mm in diameter). The plants were watered daily, initially with a quarter-strength Hoagland nutrient solution containing (in mM for full strength): 1 KH2PO4, 5 KNO3, 5 Ca(NO3)2, 2 MgSO4, 4.6x10-2 H3BO3, 7.65x10-4 ZnSO4.7H2O, 3.2x10-4 CuSO4, 1.6x10-5 (NH4)6MoO24, 9x10-3 MnCl2.4H2O, and 3.7x10-2 FeEDTA. After 3 d full strength solution was provided. The pots were watered daily with an excess of the nutrient solution, and the draining solution was removed. Plants were grown in a growth room with a 14 h photoperiod at a light intensity 210–250 µE m-2 s-1 provided by reflector sunlight dysprosium lamps (DDF 400, made in a bulb factory in Nanjing, China). The first harvest was made 92 d after sowing and 24 d after transferring to the constant environmental conditions; the second harvest was performed 7 d and 8 d later.
Treatments and harvest procedures
For the harvests, plants were grouped into four groups of six plants each with similar size and development. One group was used for the first harvest, the remaining three groups for the second harvest. On the day of the first harvest, these latter three groups of plants were treated as follows: (a) intact plants, no treatment (control); (b) apices were excised above the youngest unfolded leaf, no. 9 from the base (decapitated); (c) apices were excised in the same way above leaf no. 9 and immediately a paste of 30 mM 1-naphthylacetic acid (NAA) and Tween 20 in lanolin was applied to the decapitated stem stump (decapitated+NAA). In both, decapitated and NAA-treated plants lateral bud outgrowths were excised as they appeared.
Leaves were numbered in ascending order, starting with the lowest mature leaf, which was designated as leaf 1. Smaller leaves, which had already senesced, were removed. The youngest unfolded leaf was no. 9 at the first harvest and no. 12 at the second harvest (only in control plants). At harvest, plants were separated into roots, stem, top (apex and enclosed leaves) and the lower, middle, and upper stratum of three leaves each; at the second harvest, an additional stratum of three newly formed leaves was harvested in the controls. Because at the second harvest it was not possible to harvest all plants in a day, the plants of control were harvested on the 7th day and those of the other two treatments on the 8th day after commencing the treatments. Roots were washed free of sand with water. All plant parts were weighed (fresh wt.), dried (60 °C) and weighed again (dry wt.). Relative growth rates (RGR) of individual organs were estimated as
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Measurement of transpiration
Whole shoot transpiration was measured on a daily basis by weighing a subsample of three of each of the treatments (control, decapitated and decapitated+NAA) at the beginning and end of the light period and after the daily addition of nutrient solution and draining. Corrections were applied for the water loss from pots without plants. The partitioning of transpiration between various plant parts was determined gravimetrically at harvest. This was done by first measuring the water loss of a whole potted plant and then that of its separate, excised organs by a series of consecutive weightings over a 6 min period immediately following detachment of each organ.
Collection of xylem sap
For xylem sap collection, plants were grown in special pots (Seel and Jeschke, 1999) for the application of pressure to the root system, but treated in the same way as the plants for the harvests. Xylem sap was collected by pressurizing both the moist quartz sand substrate and the root system contained in a pressure vessel (Passioura, 1980; as described in Jeschke and Pate, 1991a). At about the middle of the leaves an incision was made into the midrib of leaves number 2, 5 and 8 from the base. The cut surface was carefully washed and a Teflon tubing attached. After slowly applying pressure, xylem sap started to exude from the midribs after reaching a balancing pressure (see Seel and Jeschke, 1999), and sap was collected at a slightly, 50 kPa higher pressure. The first exudate was discarded to avoid contamination from cut cells. Xylem sap was also collected from the stem base of the pressurized plants either from an incision into the stem (Hibberd et al., 1999) or by inserting a syringe needle into the stem until it reached the xylem vessels. A major advantage of this method of xylem sap collection is that the composition of the sap collected reflects that in an intact plant. This is because almost none of the phloem is cut and so phloem translocation of nutrient solutes from shoot to root and circulation through the root continues. Another advantage is that sap from leaves can be harvested. Independently, root pressure xylem exudate from detopped stem bases was also collected at the time of harvest. Xylem sap was kept on ice during collection and stored at -20 °C before analyses. K+ was analysed directly after appropriate dilution using ICP (Perkin Elmer 3300DV).
Estimation of the net flows of water and K+ through xylem and phloem in the whole plant
Net flows of water and K+ in plants were estimated using the method introduced by Hibberd et al. (Hibberd et al., 1999). Based on water loss by the plants and the gains in tissue water the total water uptake was calculated. Using the partitioning of transpiration and the individual gains in water the xylem water intake into each organ was obtained and these data were then used to construct the net water flows into each of the organs studied during the experimental period (see Fig. 2). Additionally, the small flows of water in the phloem were estimated using ratios of water flows in the phloem relative to the xylem, as they have been measured for Ricinus (Jeschke and Pate, 1991a).
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Based on the assumption that mass flow occurred in the xylem, the net xylem flows of K+, JK,x, toward each organ was calculated from the net water flows, JH2O,x and the measured concentration of K+ [K+]x in the xylem sap:
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The net flow of K+ in the phloem, JK,p was then calculated from the difference between the measured K+ increment 
K in each organ and the net xylem import.
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A positive difference indicated net phloem import, while a negative difference implied net phloem export from an organ. Working progressively along the plant from the root to the leaves and top, the net flows of K+ within the whole plant as depicted in Figs 3 and 4 were obtained; the differences between the quantities of K translocated in the phloem and in the xylem then allowed the estimation of the transfer processes between these translocation streams. Incidentally, the net phloem flows of K+ during the study period were independent of the above-described, rather indirectly estimated water flows in the phloem.
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Statistical treatment
Dry weight increments were obtained from six replicates of each of the treatments at the first and the second harvest. All further analyses were made with six individual samples for each organ. Where appropriate, data are presented as ±standard error (SE) of the mean.
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Plant growth
In the control plants there was substantial dry matter gain in the top and newly formed leaves, which established the principal sink for assimilates at this stage of development (Table 1
). In both decapitated and NAA-treated plants, however, dry matter increments of the three upper leaves remaining after apex excision and of the roots were much higher than in these organs in control plants, showing that without or with NAA application apex excision resulted in continued overproportional growth of these leaves and the roots (Table 1). The net increase in root dry matter of decapitated plants was up to twice that of the control plants and as a consequence shoot–root dry weight ratios of the control, decapitated and NAA-treated plants were 7.7, 5.2 and 6.4, respectively, at the second harvest. As there was no new leaf formation after decapitation, the growth of lateral buds was promoted and measurable quantities of dry matter were formed in buds of decapitated plants, although less so in NAA-treated plants. In these NAA-treated plants dry matter increments of the stem were substantially higher than in controls and decapitated plants, but those of middle leaves and roots were less than in the decapitated plants (Table 1).
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Contents and changes in K+
Figure 1 presents K+ concentrations in individual plant organs at the first and second harvest and Table 2 shows the net K+ increments during the study period. The highest increments of K+ were found in the rapidly growing organs, for example, top, new and upper leaves for control plants, and upper leaves for decapitated and NAA-treated plants (Table 2). Large increments were also found in stems and roots except in control plant roots. In control plants substantial decreases in K+ content were found, in decreasing order, in middle leaves, lower leaves, and in the root. In the treated plants, on the other hand, decreases in K+ content were significantly smaller and essentially restricted to the lower, senescing leaves (Table 2).
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Compared to first harvest, K+ concentrations in all plant organs decreased over the study period in all three treatments (Fig. 1
). However, compared with the decapitated plants, K+ concentrations were higher in all of the leaves of NAA-treated plants because of their relatively lower increase in dry weight (Table 1), and higher increase (upper and middle leaves) or lower decrease (lower leaves) in K+ content (Table 2).
Estimation of transpiration and net water flows within tobacco plants
As seen in Fig. 2, the control plants used about the same quantities of water during the study period of 7 d as the treated plants over 8 d. However, the water use in the controls was progressively higher, because new leaves (nos 10–12) were formed, and the extrapolated consumption after 8 d was substantially higher (4100 ml) compared to 3578 or 3703 ml in the treated plants. Together with the changes in dry weight (Table 1), it was clear that with additional leaf tissues more water could be deposited and transpired. Thus about 62%, 54% and 55% of total flows of water via xylem were translocated into the upper leaves of the control (including new leaves and top), decapitated and NAA-treated plants, respectively (calculated according to the values in Fig. 2). Along with the reduction in dry weight of lower leaves in all treatments (Table 1), net decreases in tissue water also occurred. The water was used for deposition in tissues or transpiration by different parts of shoot. The results showed that the amount of retranslocated water via the phloem was very small (Fig. 2).
Estimation of net flows of K+ within tobacco plants
In control plants the dominant sink for K+ was the growing tissues of the apex and new leaves, into which 94% of the K+ taken up was deposited. 28% of this was imported via phloem and 72% via xylem. Other major sinks were the upper leaves and the stem, which deposited 25% and 15%, respectively (calculated according to the values in Fig. 3A). Since the sum of depositions exceeded total uptake, there was massive mobilization of K from middle and lower leaves (some loss in K+ was even recorded in the roots) and phloem retranslocation of K+ from shoot to root contributed 62% to the xylem transport of K+, whereas only 38% were taken up by roots (Fig. 3A).
In decapitated plants, the upper leaves continued to grow and therefore constituted the principal sink for K+ (70% of the K+ uptake), almost three-times more than that in the controls. Since upper leaves had already matured, part of xylem-imported K+ (43%) was exported via phloem, however, this was less than in the upper leaves of control plants (70%). The net export of K+ from middle leaves was strongly reduced as compared with the control plants. On the whole, a major phloem sink for K+ was missing in the shoot of decapitated plants and apparently as a result, the roots replaced the top as a phloem-sink for K+ and 1.28 mmol of K+ were deposited in this organ. In the decapitated plants, the retranslocation of K+ in phloem was 62.3% of xylem transport (Fig. 3B).
Treatment with NAA on the cut surface of stem stumps stimulated partitioning of K+ into upper leaves and most dramatically into the stem, which deposited 64.5% or 27% of the K+ uptake, respectively (calculated according to the values in Fig. 3C). In the case of the NAA-treated plants, the modelling suggested that the stem constituted a major sink for phloem-translocated K+ and, similar to the decapitated plants but to a lesser degree, the roots were a phloem-sink for K+. On the whole, the modelling of K+ flows predicted lower phloem translocation than in the controls and in the decapitated plants.
The present modelling, based on the flows of water (Hibberd et al., 1999), enabled the estimation of the transfer of K+ from xylem to xylem. This process has been primarily found for nitrogen (Layzell et al., 1981; Jeschke and Pate, 1991a), but has also been shown for K+ in Ricinus (Pate and Jeschke, 1995). This transfer, however, contributed only 2.6%, 4.4% or 4.3% to increasing the ascending xylem sap concentration in controls, decapitated and NAA-treated plants, respectively.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Effect of decapitation on the sink–source relationship
The results demonstrated that shoot apices and young leaves of control plants are very strong sinks both for assimilates and mineral elements in agreement with a large body of literature. During the study period, up to 57.7% of the total net increment in dry weight occurred in these organs (Table 1
). Intriguingly, 93.8% of the currently absorbed K+ was deposited into these growing organs (Fig. 3) and almost a third of this was imported via phloem. During the experimental period, the presence and growth of shoot apices not only strongly inhibited the growth of upper leaves and roots (Table 1), but also kept ion contents in leaves, especially in upper ones relatively lower in comparison with the other two decapitated shoot treatments (Table 1). Under the present conditions, assimilates synthesized by photosynthesis and mineral nutrients (K+) taken up by roots could not meet the demands of new leaves for growth. Remobilization occurred for assimilates and K+ from middle and lower leaves (Tables 1, 2). However, in practice this would be very harmful for the leaf quality. Therefore decapitation is very important for keeping the yield and quality of tobacco leaves. Because decapitation, i.e. removal of the principal phloem sink (see Fig. 3) resulted in changes in the sink–source relationships in the plants, the root dry matter increase in decapitated plants was doubled compared to that of control plants, 56.2% of the total net increase in plant dry matter occurred in upper leaves (Table 1), and most of the K+ taken up during the study period was deposited into upper leaves and roots (Table 2; Fig. 3). The RGR of upper and middle leaves and roots was clearly higher after decapitation than in control plants (Fig. 4). Only slight remobilization of both assimilates and K+ occurred from the lower leaves (Tables 1, 2). Treatment with NAA on the cut surface of the stem stump did not cause much change in growth of leaves, but the dry weights of stem and roots increased or decreased respectively (Table 1).
Flows and partitioning of K+ in plants
K is one of the essential macroelements in plants. Its concentration in tobacco plants can be higher than 6% in dry weight (Fig. 1). As a big difference from other macroelements, K is not incorporated into plant tissues (Marschner, 1995). The observed pattern of K+ flows in control plants (Fig. 3A) agrees with the high phloem mobility of K+ (Marschner et al., 1997) and with results obtained with lupin (Jeschke et al., 1987), castor bean (Jeschke and Pate, 1991b) and barley (Wolf et al., 1991) and highlights the high degree of reutilization by retranslocation via phloem. Thus, the apex including the newly formed leaves constituted a phloem sink. They attracted 94%, together with the upper leaves 118% and together with the stem 134% of the K+ currently taken up, a situation enabled only by concomitant massive mobilization and net export of K+ from middle and lower leaves. Phloem retranslocation of K+ from the shoot and cycling through the root in the present experiment contributed 62.3% and hence more than concurrent uptake to the xylem transport of K+. This is in good agreement with other data using tobacco (69%: Hibberd et al., 1999).
The impact of excising the apex, i.e. removing the principal phloem sink for K+ was 3-fold. It resulted in (i) a 3.6-fold higher K+ deposition in upper leaves, although these already exported via phloem (Fig. 3B); (ii) an almost total halt of K+ remobilization from middle leaves; and (iii) in substantially increased K+ deposition in the stem and in the root. These latter organs replaced the apex as the principal phloem sink for K+, particularly since newly formed lateral buds were continually removed. Irrespective of this reallocation of K+ flows within the shoot, the net increase in dry weight of upper leaves exceeded K+ deposition, so that K+ concentrations relative to dry weight decreased during the study period (Fig. 1).
Effects of NAA treatment on leaf K+ concentration
The positive effect of NAA application after decapitation on improvement of K+ concentration in all of the leaves was demonstrated in Fig. 1. These results were similar with previous field experiments (CJ Li, unpublished results). In the present study, during a period of only 8 d the K+ concentration in upper, middle and lower leaves was improved from 4.7%, 5.41% and 5.6% in the decapitated plants to 5.14%, 6.06% and 6.06% by NAA treatment. Leaf K+ concentration is an important index for tobacco quality (McCants and Woltz, 1967; Leggett et al., 1977). The possible reasons for the effect of auxin on increase in K+ concentration in leaves might be as follows.
First is the effect on nutrient transport in phloem. It is well known that auxin is produced in shoot apices and young leaves and can be transported basipetally into roots (Phillips, 1975). A direct effect of auxin on K+ flows occurs in membrane transport (loading into or unloading from the transport pathways) or in transport in the phloem (Davies and Wareing, 1965). The present finding of a substantially favoured K+ deposition in the stem and leaves after NAA application (Table 2) may point in this direction. IAA in lanolin has been applied to decapitated internodes of Pisum sativum, Phaseolus vulgaris or Populus robusta and an enhanced accumulation of endogenous nitrogen, phosphorus and potassium was found in the IAA-treated areas (Phillips, 1968).
Second is the effect on the K+ uptake activity of plant roots. The net increase in K+ content in leaves and whole plants after NAA application was higher than that in the decapitated plants during the experimental period, although the increment of root dry weight of the NAA-treated plants was less (Tables 1, 2). These results implied that K+ uptake activity was stimulated after the treatment with NAA. Also the K+ uptake activity of the control plants was similar to the NAA-treated plants and much higher than that in decapitated plants (Table 3). It was shown that 90.2%, 80.7% and 86.4% of the total dry matter increment during the experimental period was located in the shoot of the control, decapitated and NAA-treated plants respectively (Table 1). It was clear that auxin either produced in shoot apices of the control plants or applied on the cut surfaces of the NAA-treated plants stimulated shoot sink strength, and higher shoot demand might stimulate root uptake activity. Here K+ recirculated from shoot to root might serve as a signal of feedback control for nutrient uptake. Evidence for such feedback control of nutrient uptake has been presented for K (Drew et al., 1990) and P (Drew and Saker, 1984).
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Acknowledgments |
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The authors thank the NKBRSF (No: G1999011707) and NNSFC (No. 30070452) for financial support, the Deutscher Akademischer Auslandsdienst (DAAD) for a travel grant to WDJ and Dr Wolfram Hartung, Würzburg, for critical evaluation and improvement of the manuscript.
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Notes |
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3 To whom correspondence should be addressed. Fax: +86 10 6289 1016. lichj{at}mail.cau.edu.cn
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References |
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