Phloem Supply Research Articles

Floral nectar production and carbohydrate composition and the structure of receptacular nectaries in the invasive plant Bunias orientalis L. (Brassicaceae)

The data relating to the nectaries and nectar secretion in invasive Brassicacean taxa are scarce. In the present paper, the nectar production and nectar carbohydrate composition as well as the morphology, anatomy and ultrastructure of the floral nectaries in Bunias orientalis were investigated. Nectary glands were examined using light, fluorescence, scanning electron and transmission electron microscopy. The quantities of nectar produced by flowers and total sugar mass in nectar were relatively low. Total nectar carbohydrate production per 10 flowers averaged 0.3 mg. Nectar contained exclusively glucose (G) and fructose (F) with overall G/F ratio greater than 1. The flowers of B. orientalis have four nectaries placed at the base of the ovary. The nectarium is intermediate between two nectary types: the lateral and median nectary type (lateral and median glands stay separated) and the annular nectary type (both nectaries are united into one). Both pairs of glands represent photosynthetic type and consist of epidermis and glandular tissue. However, they differ in their shape, size, secretory activity, dimensions of epidermal and parenchyma cells, thickness of secretory parenchyma, phloem supply, presence of modified stomata and cuticle ornamentation. The cells of nectaries contain dense cytoplasm, plastids with starch grains and numerous mitochondria. Companion cells of phloem lack cell wall ingrowths. The ultrastructure of secretory cells indicates an eccrine mechanism of secretion. Nectar is exuded throughout modified stomata.

Adenylate-Coupled Ion Movement. A Mechanism for the Control of Nodule Permeability to O2 Diffusion

In response to changes in phloem supply, adenylate demand, and oxygen status, legume nodules are known to exercise rapid (seconds to hours) physiological control over their permeability to oxygen diffusion. Diffusion models have attributed this permeability control to the reversible flow of water into or out of intercellular spaces. To test hypotheses on the mechanism of diffusion barrier control, nodulated soybean (Glycine max L. Merr.) plants were exposed to a range of treatments known to alter nodule O2 permeability (i.e. 10% O2, 30% O2, Ar:O2 exposure, and stem girdling) before the nodules were rapidly frozen, freeze dried, and dissected into cortex and central zone (CZ) fractions that were assayed for K, Mg, and Ca ion concentrations. Treatments known to decrease nodule permeability (30% O2, Ar:O2 exposure, and stem girdling) were consistently associated with an increase in the ratio of [K+] in cortex to [K+] in the CZ tissue, whereas the 10% O2 treatment, known to increase nodule permeability, was associated with a decrease in the [K+]cortex:[K+](CZ). When these findings were considered in the light of previous results, a proposed mechanism was developed for the adenylate-coupled movement of ions and water into and out of infected cells as a possible mechanism for diffusion barrier control in legume nodules.

Branch roots of young maize seedlings, their production, growth, and phloem supply from the primary root.

Branch root development on the primary root of maize (Zea mays L.) seedlings was followed for 9 d after planting. This period includes the shift from seedling heterotrophy to autotrophy. Linear density of branches in the basal region ranged from ~38 cm-1 at the base to ~10 cm-1 beyond 10 cm. Branch roots in the first ~8 cm were produced before assimilate was available. Branch length decreased from ~26 mm at 1 cm along the primary root to ~8 mm at 10 cm from the base. Without the cotyledon, branch root density in the basal region was ~10 cm-1 and roots were short (~5 mm). Beyond 8-10 cm both measurements matched those of intact seedlings. Dark-grown seedlings had basal branch root densities higher than those without cotyledons but none beyond 10 cm. There were more and smaller diameter sieve tubes in the basal region of the primary root. These decreased distally in number but had larger diameters where branches formed after assimilate was available. Proliferation of basal branch roots in very young seedlings can have major advantages for successful seedling establishment in the field and could be screened for without difficulty.

Signaling of phosphorus deficiency‐induced gene expression in white lupin requires sugar and phloem transport

Roots of phosphorus (P)-deficient white lupin exhibit striking changes in morphology and gene expression. In this report we provide further insight into genetic elements affecting transcription of P-deficiency-induced genes. Moreover, we also show that sugars and photosynthates are integrally related to P-deficiency-induced gene expression. White lupin phosphate transporter (LaPT1) and secreted acid phosphatase (LaSAP1) promoter-reporter genes when transformed into alfalfa, a heterologous legume, showed significant induction in roots specifically in response to P-deficiency. In addition, both promoters were active in nitrogen-fixing root nodules but not in ineffective nodules indicating a link between P-deficiency and factors related to nitrogen fixation/metabolism. As sugars play a role in signal transduction during nitrogen assimilation and are required for effective nitrogen fixation, we tested the relationship of sugars to P-deficiency-induced gene expression. Exogenous sucrose, glucose, and fructose stimulated LaPT1 and LaSAP1 transcript accumulation in dark-grown P-sufficient white lupin seedlings. Furthermore, in intact P-deficient white lupin plants, LaPT1 and LaSAP1 expression in cluster roots was strikingly reduced in dark-adapted plants with expression rapidly restored upon reexposure to light. Likewise, interruption of phloem supply to P-deficient roots resulted in a rapid decline in LaPT1 and LaSAP1 transcript accumulation. Similar results were also obtained with a third lupin P-deficiency-induced gene encoding a putative multidrug and toxin efflux protein (LaMATE). Taken together, our data show that the regulation of P-deficiency-induced genes is conserved across plant species and sugars/photosynthates are crucial for P-deficiency signal transduction.

Phloem Water Transport Maintains Stem Growth in a Drought-stressed Crop Cactus (Hylocereus undatus)

Hylocereus undatus [(Haworth) Britton and Rose] is a vine cactus from central America that has been established as a new fruit crop (pitaya) in many tropical and subtropical countries. In order to develop improved irrigation practices, the relationships between water parameters and growth were studied in rooted stem cuttings growing in pots with sandy soil under well-watered and drought-stressed conditions, in a controlled environment. Soil water potential rapidly decreased from -0.02 to -1.5 MPa during the first 5 days of drought. However, growth of new stems emerging from the succulent mature stems only decreased significantly after 3 weeks of drought. After 3 weeks of drought, the water content of growing stems decreased by 2% (P < 0.05) and their water potentials by 0.05 MPa (P > 0.05), as compared with the irrigated controls. At the same time, water content in drought-treated mature stems decreased by 4% (P < 0.05) and water potentials by 0.25 MPa (P < 0.05). Several lines of evidence indicated that active phloem supply of assimilates and associated water reserves from mature stems was the mechanism that allowed developing stems of H. undatus to maintain growth under well-watered and drought conditions: 1) Girdling the phloem of growing stems rapidly inhibited stem elongation. 2) Secretion of sucrose-containing nectar by growing stems was maintained during drought. 3) The water potential gradient was in the wrong direction for xylem transport from mature to young growing stems and axial hydraulic conductivity in young stems was either zero or comparatively low.

Uptake, metabolism and distribution of nitrogen in crop plants traced by enriched and natural 15N: Progress over the last 30 years

The major findings of many years of research into plant N cycling are summarised in this review, firstly as revealed by 15N-enriched methods and secondly, in relation to natural 15N abundance (δ15N) in plants and their metabolites. This work has mainly been done in an agricultural context. As many groups especially attempt to relate δ15N to N cycling, atmospheric N deposition and the interactions of N with carbon budgets, we deem it useful to synthesize these major findings. Primary assimilation and distribution of N within plants were investigated from the 15N enrichment in individual plant organs and in individual amino acids after feeding them 15N-labelled compounds. In both roots and leaves, NH4+ and NO3− were assimilated into amino acids, largely by a combination of glutamine synthetase (GS) and glutamate synthase (GOGAT). In the leaves, the transfer of glutamine (amide) N to glutamic acid was accelerated in the light, and amino N in some amino acids was deaminated to ammonia in the dark, followed by its incorporation into glutamine. The N in the growing parts such as growing leaves, filling grains and growing root parts were from two sources: re-allocation (phloem supply) of reserved N (amino acids), and currently-absorbed N. The metabolites from the mature parts may perform the roles of substrates for plant growth and signals for gene expression. δ15N values, measured for plants/soils and plant metabolites (inorganic N, amino acids, polyamines) were related with the acquisition, metabolism and distribution of N in plants. Small 15N/14N fractionation in the acquisition of N2 and NO3− and large 15N/14N fractionation in NH4+ uptake were found. The δ15N values of whole shoots or grains from field-grown crops were largely reflected major sources of N. In some legumes, 15N was enriched in their nodules and an extremely 15N-enriched compound was homospermidine. Nitrate reduction to ammonia (NR) and ammonia assimilation to glutamine (GS) showed large 15N/14N fractionations. Specific attention was paid to the δ15N values in xylem and phloem exudates compared to those of plant organs.

Physiology of the legume nodule and its response to stress

A common mechanism is sought for the effect of environmental variables (e.g. light, defoliation, temperature, N: nitrate, ammonium or organic) on nodule metabolism. A decrease in nodule cortical permeability has been suggested to enforce a primary limitation of O 2 supply to the infected zone during periods of environmental stress, but the mechanism by which cortical permeability is altered is controversial. It is proposed that the water requirement of xylem export from the nodule achieves a balance with water delivery to the nodule in the phloem, such that a change in either export or import will affect the turgor of cells surrounding the nodule vascular bundles. Cortical cell turgor may also be influenced by the balance of solutes between the apoplast and symplast. Stress events involving variations of phloem supply or decrease in rate of N 2 fixation (e.g. C 2H 2 assay) will result in changes in the soluble sugar and nitrogenous solute content of the cortex, with implication to cortical cell turgor. Change in cortical cell turgor may affect the turgor gradient driving phloem supply into the nodule in a form of turgor homeostat, and coincidentally may affect cortical cell shape or the intercellular water content and so alter the permeability of the nodule cortex to gases. It is proposed this regulation of nodule permeability is localized in the vicinity of the vascular bundles. Nodule anatomy and physiology is, however, heterogeneous. For example, nodule cortical anatomy is variable between species. Thus, the response of N 2 fixation to the effect of an environmental variable is species specific. The response will also reflect the duration and severity of stress, the plant growth history and will be complicated by effects on nodulation and anatomical adaptation in the longer term.

Oxygen‐induced recovery from short‐term nitrate inhibition of N2 fixation in white clover plants from spaced and dense stands

Nitrogenase (N2ase; EC 1.18.6.1) activity (H2 evolution) and root respiration (CO2 evolution) were measured under either N2:O2 or Ar:O2 gas mixtures in intact nodulated roots from white clover (Trifolium repens L.) plants grown either as spaced or as dense stands. The short‐term nitrate (5 mM) inhibition of N2‐fixation was promoted by competition for light between clover shoots, which reduced CO2 net assimilation rate. Oxygen‐diffusion permeability of the nodule declined during nitrate treatment but after nitrate removal from the liquid medium its recovery parallelled that of nitrogenase activity. Rhizosphere pO2 was increased from 20 to 80 kPa under N2:O2. A simple mono‐exponential model, fitted to the nodule permeability response to pO2, indicated NO−3 induced changes in minimum and maximum nodule O2‐diffusion permeability. Peak H2 production rates at 80 kPa O2 and in Ar:O2 were close to the pre‐decline rates at 20 kPa O2. At the end of the nitrate treatment, this O2‐induced recovery in nitrogenase activity reached 71 and 82%; for clover plants from spaced and dense stands, respectively. The respective roles of oxygen diffusion and phloem supply for the short‐term inhibition of nitrogenase activity in nitrate‐treated clovers are discussed.

Noninvasive Measurement of Internal Oxygen Concentration of Field‐Grown Soybean Nodules

AbstractRecent research suggests that decreased N2 fixation with drought, defoliation, or nitrate exposure is due to decreased nodule permeability to O2. Decreased O2 permeability would result in lower O2 concentration in the nodule interior and a decreased rate of bacteroid respiration in support of N2 fixation. To test this hypothesis under field conditions, a noninvasive method (based on spectrophotometry of leghemoglobin) was used to measure O2 concentration within intact, attached nodules of soybean [Glycine max (L.) Merr.]. A detopping (shoot removal) treatment, which interrupted phloem supply to nodules, resulted in lower internal O2 concentration relative to controls, suggesting that nodule respiration was directly limited by O2 rather than carbohydrate. When the gas stream around a nodule was switched from pure N2 to pure O2, internal O2 concentration increased more rapidly in control nodules than in nodules attached to detopped plants. This difference in reoxygenation rate is also consistent with reduced nodule gas permeability after detopping. The method described here, with modifications, should be useful for a variety of field studies.

Vascular transport and soybean nodule function. III: Implications of a continual phloem supply of carbon and water

Abstract. In soybean, stores of carbon within the leaf have been demonstrated to support nodule metabolism under both photosynthetic and non‐photosynthetic conditions. Indeed, a net depletion of nodule starch is observed only under conditions of suboptimal rates of nodule metabolism. Therefore, maximal rates of nodule metabolism are associated with a continual supply of phloem sap to the nodule, delivering water, carbon and other solutes. A restriction of phloem supply to the nodule may result in changes in turgor between the apoplast of the export pathway and the symplast of the nodule. This change may cause the observed decrease in the permeability to gases and to the rate of product export from nodules deprived of a phloem supply. It is suggested that nodule metabolism is homeostatically regulated in terms of internal O2 levels by the delivery of phloem water and solutes.

Distribution and Metabolism of Xylem-Borne Ureido and Amino Compounds in Developing Soybean Shoots

Pulse-chase feeding (30-120 minutes) of (14)C-labeled nitrogenous compounds to cut transpiring shoots was used to investigate the early fate of the major xylem-borne solutes in N(2)-fixing soybean (Glycine max) plants at the V(4) growth stage. By comparison with the foliar distribution of [(14)C]inulin (a xylem marker), it was determined that the phloem supply of allantoin, allantoic acid, asparagine, glutamine, aspartate, and arginine, respectively, provided about 20, 10, three, two, five, and 20 times the (14)C delivered to the developing trifoliolate in the xylem stream. Recovery of unmetabolized asparagine, aspartate, and arginine in this indicator trifoliolate, and significant declines in the percentage of (14)C from allantoic acid and allantoin recovered in the first trifoliolate, provided some support for the direct xylem-to-phloem transfer of these compounds, but did not preclude the involvement of indirect transfer. Data on stem retention and foliar distribution, expressed as a function of the relative xylem sap composition, indicated that ureides provide the major sources of nitrogen to all plant parts. There was no consistent distinction in distribution patterns between pairs of similar anionic and neutral compounds. The extent of xylem-to-phloem transfer among the ureido or the amino compounds was inversely related to its prominence in xylem sap.

Vascular transport and soybean nodule function: II. A role for phloem supply in product export*

Abstract The ureide content of soybean (Glycine max (L.) Merr.) nodules was unaffected by variations in the transpirational rate, while whole plant manipulations designed to decrease phloem supply to nodules resulted in lower rates of nitrogenase activity and an increase in the ureide content of the nodules. The rate of ureide export from the nodule was estimated from the exponential rate of decrease in the pool size of ureides in nodules, following exposure to an N2‐free atmosphere (Ar:O2). Export was greatly reduced under treatments which reduced phloem supply to the nodule. A water budget for nodules suggested that the delivery of water to the nodule via mass flow in the phloem was comparable to that required for export of ureides from the nodule in the xylem from the nodule. Therefore, we suggest that xylem export from nodules is related to the phloem supply to the nodule rather than to the transpirational flux in the parent root. This suggestion is related to the reported decreases in nodule permeability to gases under conditions of phloem deprivation.

Can a limitation in pholem supply to nodules account for the inhibitory effect of nitrate on nitrogenase activity in soybean?

Within 48 h of exposure of nodulated soybean [Glycine max (L.) Merr. cv. Harosoy 63 x Bradyrhizobium japonicum USDA 16] to 10 mM NO3, significant decreases were observed in nodule‐specific nitrogenase (EC 1.7.99.2) activity and CO2 evolution and in the proportion of [14C]‐labeled photosynthate partitioned to nodule biomass and respiration. These trends continued over the subsequent 3 days of the study period. Concomitant with these events was an 137% increase in the relative growth rate of the whole plant and a cessation in nodule growth. Although the concentration of total soluble sugar in nodules was not affected by NO3 treatment, the concentration of starch declined to 13% of the control level after 2 days exposure to NO3−. In contrast to the effects of NO3−, nodules in which nitrogenase activity was partially inhibited by a 30 min exposure to 100% O2, showed a 52% increase over control in the starch pool over a 72 h period.The results were compared with recent studies of NO3− inhibition of nitrogenase activity in legumes, and in contrast to these studies it was concluded that the inhibitory effects of NO3− could be accounted for by alterations in photosynthate partitioning to nodules. A hypothesis is proposed which attempts to account for the recent observation (J. K. Vessey, K. B. Walsh, and D. B. Layzell 1988. Physiol. Plant. 73: 113–121) that nitrogenase activity in phloem‐limited and nitrate‐inhibited nodules is limited by O2 diffusion. This hypothesis separates the concepts of photosynthate partitioning and phloem supply from that of carbohydrate deprivation and related effects on the size of the carbohydrate pools in nodules.

Oxygen limitation of N2 fixation in stem‐girdled and nitrate‐treated soybean

The effects of increasing rhizosphere pO2on nitrogenase activity and nodule resistance to O2diffusion were investigated in soybean plants [Glycine max (L.) Merr. cv. Harosoy 63] in which nitrogenase (EC 1.7.99.2) activities were inhibited by (a) removal of the phloem tissue at the base of the stem (stem girdling), (b) exposure of roots to 10 mM NO3over 5 days (NO3‐treated), or (c) partial inactivation of nitrogenase activity by an exposure of nodulated roots to 100 kPa O2(O2‐inhibitcd).In control plants and in plants which had been treated with 100 kPa O2, increasing rhizosphere O2concentrations in 10 kPa increments from 20 to 70 kPa did not alter the steady‐state nitrogenase activity. In contrast, in plants in which nitrogenase activities were depressed by stem girdling or by exposure to NO3, increasing rhizosphere pO2resulted in a recovery of 57 or 67%, respectively, of the initial, depressed rates of nitrogenase activity. This suggests that the nitrogenase activity of stem‐girdled and NO3‐treated soybeans was O2‐limited.For each treatment, theoretical resistance values for O2diffusion into nodules were estimated from measured rates of CO2exchange, assuming a respiratory quotient of 1.1 and 0 kPa of O2in the infected cells. At an external partial pressure of 20 kPa O2, the stem‐girdled and NO3‐‐treated plants displayed resistance values which were 4 to 8.6 times higher than those in the nodules of the control plants. In control and O2‐inhibited plants, increases in pO2from 20 to 70 kPa in 10 kPa increments resulted in a 2.5‐ to 3.9‐fold increase in diffusion resistance to O2, and had little effect on either respiration or nitrogenase activity. In contrast, in stem‐girdled and NO3‐‐treated plants, increases in external pO2had little effect on diffusion resistance to O2, but resulted in a 2.3‐ to 3.2‐fold increase in nodule respiration and nitrogenase activity. These results are consistent with stem‐girdling and NO3‐‐inhibition treatments limiting phloem supply to nodules causing an increase in diffusion resistance to O2at 20 kPa and an apparent insensitivity of diffusion resistance to increases in external pO2.

Anatomy and vasculature of the floral nectaries of Brassica napus (Brassicaceae)

The floral nectaries of Brassica napus L. (Argentine or Swede rape) consist of two pairs of glands which are supplied by phloem only. The lateral pair has an extensive phloem supply and produces most of the flowers' nectar, whereas the median pair is supplied by limited phloem and produces relatively little nectar. Because both lateral and median nectaries contain cells exhibiting similar structural features, the disparity in phloem supply between them could account for the observed difference in nectar production. Ultrastructural evidence suggests an energy-requiring, eccrine mechanism of nectar secretion in Brassica napus. Both apoplastic and symplastic routes for nectar movement and escape appear feasible. Stomata on the nectary surfaces may serve as exits.

NECTAR PRODUCTION AND FLORAL CHARACTERISTICS IN BIRDSFOOT TREFOIL (Lotus corniculatus L.)

Nectar production was studied in several varieties of birdsfoot trefoil in relation to flower color, aroma, and some anatomical features in order to identify characteristics which might be of use in a program of selection for attractiveness to honeybee pollinators. Color did not appear to be an important variable; no major varietal differences in light reflectance were noted at any wavelength. There was a twofold varietal range in nectar yield per umbel and a larger difference in potential yield per plant. Selection within varieties increased the difference between high- and low-nectar plants by several times. Large varietal differences in aroma strength were also noted. While nectar and aroma production were not closely correlated in all varieties, those with weak aroma were generally low nectar yielders. Nectar yield per umbel was directly related to the cross-sectional area of functional phloem in the peduncle, and the amount of functional phloem and peduncle diameter were closely correlated. The ratio of nectar yield to phloem supply was much larger in high-nectar than in low-nectar plants, indicating an important effect of an unidentified factor. Preliminary screening in selection for high nectar lines could be facilitated by discarding plants whose flowers have small peduncles and weak aroma.

Course of the Zeatin Content in Tomato Fruits and Seeds developing on Intact or partially Defoliated Plants

In purified extracts from developing fruits and seeds of tomato (Lycopersicon esculentum Mill., CV. Moneymaker), practically all the cytokinin activity coincided with 8-14C-zeatin, added for absolute determination. It is concluded that zeatin is the predominant cytokinin in these materials. Partial defoliation of the plants greatly increased the zeatin content of fruits and seeds. This is attributed to the enhanced supply of root-produced cytokinin because of decreased competition and phloem supply from the reduced foliage. During development the zeatin content increased up to tens of ppm in these fruits and, particularly, in the seeds at the pink stage of ripening. During further ripening the amount of butanol-soluble zeatin decreased over several orders of magnitude. The function of seeds in the sink activity of the fruit is discussed.

Nutrition of a Developing Legume Fruit

The economy of functioning of the developing fruit of white lupin (Lupinus albus L.) is assessed quantitatively in relation to intake and usage of carbon, nitrogen, and water. Of every 100 units of carbon imported from the parent plant, 52 are incorporated into seeds, 37 into nonmobilizable material of the pod, and the remaining 11 lost as CO(2) to the atmosphere. An illuminated fruit can make net gains of CO(2) from the atmosphere during the photoperiods of all but the last 2 weeks of its life, suggesting that it is active in assimilation of CO(2) respired from pods and seeds. This conservation activity is important to carbon economy.Phloem supplies 98% of the fruit's carbon and 89% of its nitrogen. Most of the xylem's contribution enters early in development. Xylem and phloem supply similar sets of amino compounds, amides predominating. Ninety-six per cent of the fruit's nitrogen becomes incorporated into seeds. Sixteen per cent of the seed's nitrogen is mobilized from the senescing pod.The transpiration ratio of the fruit is 22.5 ml per gram dry matter accumulated. Xylem supplies 60% of a fruit's total water requirement and the equivalent of two-thirds of its transpiration loss. Phloem becomes prominent as a water donor once the seeds start to fill.The fruit exhibits a 31% conversion by weight of organic imports into food reserves of seeds. This entails an intake through vascular channels of 1756 mg sucrose and 384 mg amino compounds and an accumulation in seeds of 412 mg protein, 132 mg oil, and 110 mg perchloric acid-soluble carbohydrate.