Paclobutrazol Is Phloem Mobile in Castor Oil Plant (Ricinus communis L.)

It is commonly believed that the synthetic triazole growth regulator paclobutrazol (PAC) is exclusively xylem mobile within plants. By contrast, the triazole amitrole and many natural growth regulators are phloem mobile. This raises some doubt as to whether PAC must necessarily be exclusively xylem mobile. PAC was introduced into castor oil plants (Ricinus communis L.) through their hollow petioles. PAC was detected in xylem and phloem sap collected above the point of introduction but not in xylem sap below this point. This finding shows that PAC is not exclusively xylem mobile as believed previously. These results also raise the possibility of introducing PAC into plants in a different way so that it is carried by both the xylem and phloem and thus optimizing its effectiveness. Received February 25, 1997; accepted July 18, 1997     

Amino acid uptake by Ricinus communis roots: characterization and physiological significance

Abstract Roots of sterile-grown, intact 6-day-old seedlings of Ricinus communis possess at least two independent active amino acid uptake systems, one for neutral and one for basic amino acids. The kinetics of uptake of L-proline and L-arginine, which were taken as representative substrates for the two systems, are biphasic. At low concentrations (0.01–0.5 mol m^?3) Michaelis -Menten kinetics prevail, changing to a linear concentration dependence at higher substrate concentrations (1–50 mol m^?3). L-glutamate uptake velocity is linear over the whole substrate concentration range. For comparison the uptake kinetics of nitrate and ammonium were determined as well as interactions among the different nitrogen sources. The K_m value for nitrate uptake was 0.4 mol m^?3, and for ammonium 0.1 mol m^?3. The uptake capacity for nitrate or ammonium was approximately the same as for amino acids. The interaction between the uptake systems for organic and inorganic nitrogen is small. Two hypotheses for the physiological significance of amino acid uptake by roots were considered: (i) Uptake of amino acids from the soil-determination of amino acids in soil and in soil water indicates that they might contribute 15–25% to the nitrogen nutrition of the plant. (ii) Amino acid uptake systems of root cells serve primarily as retrieval of amino acids delivered from the phloem- it was found that ¹⁴C L-glutamine, which was delivered to the cotyledon and transported to the root via the phloem, was not lost by the roots, whereas it appeared in the bathing medium if L-glutamine was applied externally to the root to compete for the uptake sites; this suggests that an apoplastic pool of amino acids in the root exists due to their efflux from the phloem.     

蓖麻提取物对南方根结线虫的防治作用

通过毒力测定及盆栽试验,研究了蓖麻提取物对南方根结线虫的杀线活性及防治效果.结果表明:蓖麻碱及蓖麻水提液均具有较强毒杀线虫活性,蓖麻碱浓度为2g·L-1、处理48 h杀线虫活性最强,线虫校正死亡率达91.5％,LC50为0.6 g·L-1;蓖麻水提液浓度为100g·L-1、处理48 h杀线虫活性最强,线虫校正死亡率达83.5％,LC50为18.3 g·L-1;蓖麻碱、蓖麻水提液和蓖麻叶植物粉处理接种南方根结线虫的番茄苗后,植株平均根结数分别为(17.6±1.7)、(20.6±1.5)和(22.8±3.7),均显著低于对照(37.4±2.3),根长分别比对照提高46.8％、34.5％和33.8％,株高分别比对照提高33.5％、22.6％和15.8％,植株鲜质量分别比对照增加41.4％、18.9％和10.1％.蓖麻提取物能减轻线虫危害,对盆栽番茄南方根结线虫病控制效果明显.     

Effects of drought on nutrient and ABA transport in Ricinus communis

We studied the effects of variations of water flux through the plant, of diurnal variation of water flux, and of variation of vapour pressure deficit at the leaf on compensation pressure in the Passioura-type pressure chamber, the composition of the xylem sap and leaf conductance in Ricinus communis. The diurnal pattern of compensation pressure showed stress relaxation during the night hours, while stress increased during the day, when water limitation increased. Thus compensation pressure was a good measure of the momentary water status of the root throughout the day and during drought. The bulk soil water content at which predawn compensation pressure and abscisic acid concentration in the xylem sap increased and leaf conductance decreased, was high when the water usage of the plant was high. For all xylem sap constituents analysed, variations in concentrations during the day were larger than changes in mean concentrations with drought. Mean concentrations of phosphate and the pH of the xylem sap declined with drought, while nitrate concentration remained constant. When the measurement leaf was exposed to a different VPD from the rest of the plant, leaf conductance declined by 400mmol m^?2 s^?1 when compensation pressure increased by 1 MPa in all treatments. The compensation pressure needed to keep the shoot turgid, leaf conductance and the abscisic acid concentration in the xylem were linearly related. This was also the case when the highly dynamic development of stress was taken into account.     

Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.     

Effects of leaf position and nitrogen supply on the expansion of leaves of field grown sunflower (Helianthus annuus L.)

Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under field conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position significantly (p=0.001) affected maximum leaf area (LA_max) of target leaves through variations in leaf expansion rate (LER); effects on duration of expansion were small. Specific leaf nitrogen (SLN, g N m^-2) fell quite rapidly during the initial leaf expansion phase (LA < 35% LA_max) but leveled off during the final 65% increase of leaf area. This pattern held across leaf positions and N supply levels. Leaf nitrogen accumulation after 35% LA_max continued up to achievement of LA_max; reductions in the higher SLN characteristic of the initial phase were insufficient to cover the nitrogen requirements for expansion during the final phase. LER in the quasi-linear expansion phase (35 to 100% of LA_max) was strongly associated with SLN above a threshold that varied with leaf position (mean 1.79±0.225 g N m^-2). This contrasts with the response of photosynthesis at high irradiance to SLN, which has previously been shown to have a threshold of 0.3 g N m^-2; in the present work saturation of photosynthetic rate was evident when SLN reached 1.97 g N m^-2. Thus, once the area of a leaf exceeds 35% of LA_max, expansion proceeds provided SLN values are close to the levels required for maximum photosynthesis. However, growth of leaves during the initial expansion phase ensures a minimum production of leaf area even at low N supply levels.     

Leaf expansion and cell division are affected by reducing absorbed light before but not after the decline in cell division rate in the sunflower leaf

The effect of absorbed photosynthetic photon flux density (PPFD) on leaf expansion is a key issue for analysing the phenotypic variability between plants and for modelling feedback loops. Expansion and epidermal cell division in leaf 8 of sunflower were analysed in a series of five experiments where absorbed photosynthetic photon flux density (PPFD) was reduced either by shading or by covering part of the leaf area. These treatments were imposed at different times during leaf development. Expansion and cell division were affected by a reduction in absorbed PPFD only in the first part of leaf development, while the leaf area was less than 2% of its final value and while absolute expansion rate was slow. In contrast, it was not affected if imposed later when the leaf was visible and absolute expansion rate was at maximum. A reduction in absorbed PPFD caused the same reduction in expansion and in cell division whether it was due to a reduction in incident PPFD or to a reduction in photosynthetic leaf area, suggesting that carbon metabolism was involved. Relative expansion rate recovered to control levels when relative division rate began to decline, in all experiments and in all zones of a leaf. This was probably linked to the source–sink transition, after which the leaf had such a high priority in carbon allocation that it was largely insensitive to changes in absorbed PPFD. The final leaf area was therefore closely related to the cumulated PPFD absorbed by the plant from leaf initiation to the end of exponential cell division.     

Effects of air pollutants on proton and sucrose transport at the plasma membrane of Ricinus communis

The effects of the air pollutants O₃, SO₂ and NO₂ on aspects of sucrose/proton cotransport across the plasma membrane of Ricinus communis plants have been investigated. The H⁺-ATPase hydrolytic activity in cotyledon plasma membrane vesicles purified by phase partitioning showed small stimulations by Na₂SO₃ or NaNO₃ added separately or together to the assay medium. ATPase activity from plants pretreated by fumigation with SO₂ or O₃ also showed an increase, the effect of O₃ being quite marked. Plasma membrane H⁺-pumping in KI-treated microsomal fractions and medium acidification by intact cotyledons both showed small decreases in the presence of Na₂SO₃ or NaNO₂. Both Na₂SO₃ and NaNO₂ at high concentrations (2 mol m^–3) had significant effects on sucrose uptake by intact cotyledons, although sucrose efflux was unaffected. No significant effects on sucrose uptake or efflux by intact cotyledons were observed in plants pretreated by fumigation with SO₂ or O₃. Proton-coupled sucrose transport in isolated plasma membrane vesicles was inhibited in the presence of Na₂SO₃ or NaNO₂. However, both pollutants also significantly inhibited the uptake of acetate by the vesicles, indicating a dissipation of the pH gradient across the membrane. It was concluded that no specific aspect of the sucrose/proton cotransport mechanism was damaged by these air pollutants, and that the effects of these pollutants on carbohydrate partitioning are more likely to be due to general effects on membrane integrity or on other aspects such as leaf carbohydrate metabolism.     

Sink-stimulated photosynthesis and sink-dependent increase in nitrate uptake: nitrogen and carbon relations of the parasitic association Cuscuta reflexa–Ricinus communis

Ricinus communis L. was grown under limiting N supply in quartz sand culture, fed with 0.2, 1 or 5 mol m^?3 NO₃^?, or in liquid culture with 0.022, 0.05 or 0.5 mol m^?3 NO₃^?. Some of the plants were infected with Cuscuta reflexa Roxb. As occurred for the host, dry matter production and growth of C. reflexa were severely depressed with decreasing N supply to the host. When parasitized by C. reflexa, the shoot and root dry weight of Ricinus was diminished at all levels of N nutrition, but the total dry weight of host plus parasite was almost the same as that of uninfected Ricinus. In contrast to the situation in Lupinus albus (Jeschke et al. 1994b), infection by Cuscuta resulted in increased tissue N levels in the host and the N content of the system Ricinus plus C. reflexa was the same or even somewhat larger than that of uninfected plants. This indicated a sink-dependent stimulation of nitrate uptake. As a result of decreased root weights, nitrate uptake g^?1 FW was stimulated by 80, 60 or only 40% at 0.2, 1 or 5 mol m^?3 nitrate supply. Increased nitrate uptake was reflected, particularly at low N supply, in xylem transport; xylem sap nitrate concentrations were substantially elevated, while those of amino acids were decreased in parasitized plants. This indicated an inhibition of nitrate assimilation in roots of parasitized plants under limiting N supply. Besides these effects on N relations, C. reflexa induced a substantial sink-dependent stimulation of net photosynthesis in host leaves and a concomitant increase in stomatal opening and transpiration. This stimulation depended on the relative sink size induced by Cuscuta, on nitrogen nutrition and on leaf age, indicating that delayed senescence of leaves contributes to the overall effects of Cuscuta on its host. The Cuscuta-induced inhibition of nitrate assimilation in the roots and the increase in nitrate uptake suggest that nitrate reduction was shifted towards the leaves in the presence of C. reflexa. The stimulating effects of C. reflexa in the Ricinus-Cuscuta association are compared with the strongly inhibitory effects occurring in the tripartite association L. albus–Rhizobium–Cuscuta reflexa.     

Use of Energy Crop (Ricinus communis L.) for Phytoextraction of Heavy Metals Assisted with Citric Acid

Ricinus communis L. is a bioenergetic crop with high-biomass production and tolerance to cadmium (Cd) and lead (Pb), thus, the plant is a candidate crop for phytoremediation. Pot experiments were performed to study the effects of citric acid in enhancing phytoextraction of Cd/Pb by Ricinus communis L. Citric acid increased Cd and Pb contents in plant shoots in all treatments by about 78% and 18–45%, respectively, at the dosage of 10 mM kg^?1 soil without affecting aboveground biomass production. Addition of citric acid reduced CEC, weakened soil adsorption of heavy metals and activated Cd and Pb in soil solutions. The acid-exchangeable fraction (BCR-1) of Pb remained lower than 7% and significantly increased with citric acid amendment. Respective increases in soil evaluation index induces by 14% and 19% under the Cd1Pb50 and Cd1Pb250 treatments upon addition of citric acid resulted in soil quality improvement. Ricinus communis L. has great potential in citric acid-assisted phytoextraction for Cd and Pb remediation.     

Inorganic ions modulate the path of phloem loading of sucrose in Ricinus communis L. seedlings

The impact of inorganic ions on sucrose fluxes in the cotyledons and on the pathway of phloem loading was studied in Ricinus communis L. seedlings. The cotyledons were incubated in defined solutions which contained either potassium, sodium, magnesium or calcium as chloride salts, or the sodium salts of sulphate or phosphate. Sucrose uptake from the medium into the cotyledons was only slightly affected by the salts. Sucrose efflux to the medium was increased by phosphate and sulphate and to a lesser extent by sodium and potassium. Phloem loading from the apoplasm and the symplasm was analysed by addition of labelled sucrose to the medium, determination of the specific radioactivity of sucrose in sieve-tube exudate and quantification of export into the seedling axis. Potassium and sodium stimulated the apoplasmic route of phloem loading of sucrose, mostly at the expense of loading from cotyledon sucrose pools. In contrast, sulphate and phosphate strongly inhibited the apoplasmic route whereas the (small) symplasmic flux from the cotyledon sucrose pools was less affected. Magnesium ions inhibited phloem loading by both pathways. The potential of ions in modulating the pathways of sucrose export in day to day operation of plants is discussed.     

蓖麻提取物和淡紫拟青霉对南方根结线虫的防治作用

通过杀线活性测定及盆栽试验,研究了蓖麻提取物和淡紫拟青霉(Paecilomyces lilacinus)对南方根结线虫(Meloidogyne incognita)的杀线活性及防治效果.结果表明:蓖麻碱不影响淡紫拟青霉孢子的萌发.蓖麻碱和淡紫拟青霉均具有较强杀线活性,蓖麻碱处理对南方根结线虫的卵孵化抑制率和二龄幼虫死亡率分别达61.7%和59.2%,显著高于对照处理;蓖麻碱和孢子液复合处理接种南方根结线虫的番茄苗后,植株平均根结数为15±3,显著低于对照的平均根结数37±2,株高、鲜重和根长增长率分别比对照提高38.5%、44.0%和57.0%.说明蓖麻提取物和淡紫拟青霉能减轻线虫危害,对番茄南方根结线虫病控制效果明显.     

Phloem sap exudation in Ricinus communis: elastic responses and anatomical implications

Abstract. Ricinus communis plants have an unusually high capacity to exude considerable quantities of phloem sap from bark incisions. We have used Ricinus as an experimental system to study different aspects of sap exudation. Dimensional changes in the bark, monitored by a displacement transducer, showed that pressure release in the sieve tubes was accompanied by elastic shrinkage. The rate of exudation was also controlled by the degree of pressurization and elastic properties of the sieve tubes. A displacement transducer was used to measure the elastic modulus (ɛ) of phloem samples by immersing them in a range of different osmotica. The cells had a low elastic modulus (ɛ= 1.62 ± 0.41 MPa at full turgor). ɛ of phloem tissue in massage pretreated bark, from which exudation was enhanced, was not significantly different from that of unmassaged bark in contrast with the suggestion of Lee (1981). However, anatomical studies showed that massage pretreatment has a stimulating influence on the cambial cell division, which increased the phloem tissue cross-section up to 160%. The newly-formed sieve tubes were 'spliced' into existing ones in the unmassaged zone to re-establish vascular continuity. Plants with a greater capacity to exude phloem sap from a given stem location had a greater cross-sectional area of sieve tubes in the vicinity.
The significance of observations with respect to other sap exudation phenomenon is discussed. The importance of the present work in understanding the technique of palm tapping, on which the palm sugar industry depends, is also considered.     

Foliar application of nitrate or ammonium as sole nitrogen supply in Ricinus communis. II. The flows of cations, chloride and abscisic acid

Following a precultivation with pedospheric nitrogen nutrition, Ricinus plants were supplied with nitrogen solely by spraying nitrate or ammonium solution onto the leaves during the experimental period. The chemical composition of tissues, xylem and phloem exudates was determined and on the basis of the previously determined nitrogen flows (Peuke et al., New Phytologist (1998), 138 , 657–687) the flows of potassium, sodium, magnesium, calcium, chloride and ABA were modelled. These data, which permit quantification of net-uptake, transport in xylem and phloem, and utilization in shoot and root, were compared with results obtained in plants with pedospherically-supplied nitrate or ammonium and data in the literature. Although the overall effects on the chemical composition of supplying ammonium to the leaves were not as pronounced as in pedospherically supplied plants, there were some typical responses of plants fed with ammonium (ammonium syndrome). In particular, in ammonium-sprayed plants uptake and transport of magnesium decreased and chloride uptake was increased compared with nitrate-sprayed plants. Furthermore, acropetal ABA transport in the xylem in ammonium-sprayed Ricinus was threefold higher than in nitrate-sprayed plants. Additionally, concentrations of anions were more or less increased in tissues, particularly in the roots, and transport fluids. The overall signal from ammonium-sprayed leaves without a direct effect of ammonium ions on uptake and transport systems in the root is discussed.     

Long-distance translocation of polyamines in phloem and xylem of Ricinus communis L. plants

Polyamine content and enzyme activities in the biosynthetic and degradative pathways of polyamine metabolism were investigated in sieve-tube sap, xylem sap and tissues of seedlings and adult plants of Ricinus communis L. Polyamines were present in tissues and translocation fluids of both seedlings and adult plants in relatively high amounts. Only free polyamines were translocated through the plant, as indicated by the finding that only the free form was detected in the phloem and the xylem sap. Removal of the endosperm increased the polyamine content in the sieve-tube exudate of seedlings. The level and pattern of polyamines in tissue of adult leaves changed during leaf age, but not, however, in the sieve-tube sap. Xylem sap was relatively poor in polyamines. Polyamine loading in the phloem was demonstrated by incubating cotyledons with [¹⁴O]putrescine and several unlabelled polyamines. Feeding cotyledons with cadaverine and spermidine led to a decrease in the level of putrescine in sieve-tube sap, indicating a competitive effect. Comparison of polyamine content in the tissue and export rate showed that the export would deplete the leaves of polyamines within 1–3 d, if they were not replenished by biosynthesis. Polyamine biosynthesis in Ricinus proceeds mostly via arginine decarboxylase, which in vitro is 100-fold more active than ornithine decarboxylase. The highest arginine decarboxylase, ornithine decarboxylase and diamine oxidase activities were detected in cotyledons, while in sieve-tube sap only a slight arginine decarboxylase activity was found. Received: 18 March 1997 / Accepted: 20 August 1997     

Measurements of pH, sucrose and potassium ions in the phloem sap of castor bean (Ricinus communis) plants

The composition of phloem sap, sampled at different heights along, the stem of castor bean ( Ricinus communis L. cv. Gibsonii) plants, was determined. A gradient in pH was observed; the highest pH values occurred near the shoot apex, decreasing towards the base of the stem. The sucrose content of the exudate exhibited a similar gradient. The concentration of potassium ions was highest near the uppermost, full-grown leaves, decreasing towards the apex and the base of the stem. The importance of these findings for the understanding of phloem translocation and unloading is discussed.     

Kinematic evidence that atmospheric nitrogen dioxide increases the rates of cell proliferation and enlargement to stimulate leaf expansion in Arabidopsis

To elucidate the stimulation of leaf growth by atmospheric nitrogen dioxide (NO₂), we performed a kinematic analysis of the eighth leaves of Arabidopsis thaliana (accession C24) plants grown for 17–35 days after sowing in the presence or absence of 50 ppb NO₂ (designated +NO₂ plants and –NO₂ plants, respectively). We found that the peak and mean values of the relative rates of leaf expansion, cell division and cell expansion were always greater in +NO₂ plants than in –NO₂ plants. No evidence for prolonged duration was obtained. Thus, NO₂ treatment increased the rates of both cell proliferation and enlargement to increase leaf size. Furthermore, a fold-change analysis showed that cell proliferation and enlargement differentially regulated NO₂-induced leaf expansion.