Sink Demand Research Articles

Alteration of 14C-Assimilate Partitioning in Leaves of Soybeans Having Increased Reproductive Loads at One Node

The objectives of this study were to determine if the partitioning of recently fixed carbon between starch and water-soluble compounds could be altered by increasing the pod load in the leaf axil, and if the presence of source leaves acropetal to such a node would influence the partitioning of carbon within the subtending leaf. Soybeans (Glycine max L. Merr. cv Hodgson 78) were grown to full-bloom in a controlled environment chamber, and then deflowered at all nodes except the eighth. This treatment resulted in an 83% increase in the number of pods at the eighth node. At 24 days after flowering, one-half of the treated plants were girdled above the untreated node. Forty-two hours later, the eighth trifoliolate was pulsed with (14)CO(2) and sampled for radiolabeled starch and water-soluble compounds (WSC) at 0.5, 2, 4, 8, 12, and 24th after labeling.When no girdling was applied above the increased pod load at the eighth node more label was accumulated by the pod walls (+6.9%) and seeds (+6.3%) when compared to the controls. Starch accumulation was not altered in the labeled leaf of the nongirdled plants. When the stem was girdled above the eighth node, significantly less starch was retained in the labeled leaf. Girdling also resulted in an increase in label accumulation by the pod walls (+5.4%) and seeds (+6.6%). These data suggest that the plant will change the distribution patterns of assimilate to supply added sink demand before altering the partitioning of recently fixed carbon in the subtending leaf.

Conversion of mature leaves into sinks on the dicotyledonous C4 plants Amaranthus caudatus and Gomphrena globosa

The effect of an artificially created sink leaf on translocation of 14C-labeled assimilates from a source leaf of the C4 plants Amaranthus caudatus L. and Gomphrena globosa L. was studied with autoradiography. Anatomical studies showed that the vasculature of two successive leaves within a given orthostichy is directly connected by stem bundles. Therefore, in an intact plant, much of the acropetally translocated 14C-labeled assimilates from a mature source leaf moved into the next younger leaf directly above. When such a young leaf on an otherwise illuminated plant was kept under sink conditions (dark, CO2-free air), its increased sink demand resulted in a clearly higher 14C-import, compared with a control. Even a mature leaf of Gomphrena subjected to the similar sink conditions developed sufficient sink demand to import a certain amount of basipetally transported 14C-labeled assimilates from a younger source leaf. In Amaranthus, there was no basipetal translocation from a younger source leaf and, consequently, no 14C-import into a mature leaf under sink conditions occurred. A complete conversion of a mature leaf into a sink was attained only with a pruned source–sink transport system of both plants under source and sink limitation. This conversion from the source to the sink status caused a change in the direction of assimilate flow within the petiolar and stem bundles. The data indicate that a mature leaf under sink conditions can be characterized as a sink with regard to assimilate translocation, although no growth or storage processes are involved in controlling this sink activity.

Foliar Application of Urea to Olive: Translocation of Urea Nitrogen as Influenced by Sink Demand and Nitrogen Deficiency

Abstract Export of urea N was quantified after 15N-enriched urea was applied to ‘Manzanillo’ olive leaves (Olea europaea L.). Labeled N derived from foliar urea applications to mature trees in October 1982 and March 1983 constituted 4.3% and 23.0%, respectively, of the nitrogen composition of olive flowers at anthesis (May 1983). Labeled N applied to leaves following anthesis was translocated to fruit within 3 days. Export of N from treated leaves was greatly reduced when developing fruit and shoot tips were removed. Preloading of leaves with standard urea (before foliar application of labeled urea) increased leaf N by 30% and increased slightly the translocation of labeled urea N to the fruit 2 to 4 weeks later. The nitrogen status of small, potted ‘Manzanillo’ plants did not influence subsequent absorption of foliage-applied urea; however, nitrogen deficiency reduced translocation of labeled urea N from the treated leaves to new shoots and, to a lesser extent, roots. Olive leaves represent storage organs for N and release N in response to the metabolic demands of developing reproductive and vegetative organs.

Role of Nitrogen Assimilation in Seed Development of Soybean

A nondestructive acetylene reduction assay for nitrogenase activity of soybean (Glycine max L. Merr) field plots is presented. Plots consisted of 120 x 150 x 30 centimeter boxes containing 65 plants. The plants were grown in a medium grade sand under controlled nutrient, moisture, and root temperature conditions. Acetylene at a concentration of 10 milliliters per liter was circulated through manifolds in the chambers; equilibration required 5 minutes, and activity was linear with time. Optimum growth and assay environments resulted in activity of 70 micromoles ethylene per plant per hour. Plant development and yield were comparable to soil-grown companion plots.The well accepted hypothesis that developing seeds deprive the nodules of carbohydrate was not substantiated. The nondestructive acetylene reduction profile did not decline until 30 days after the onset of seed development (R-5). This result was consistent with reports from the literature which indicated that 60% of seasonal nitrogen was fixed after R-5. Further, a high correlation shown between integrated seasonal acetylene reduction and yield (r = 0.999) suggested a cooperative relationship between the roots and shoot. A reduction in source:sink ratio (60% defoliation) after R-5 had no effect on acetylene reduction. This showed that neither an increase in sink demand by the pods nor a carbon shortage during podfill decreased dinitrogen fixation. A conceptual model relating seed growth with carbon and nitrogen assimilation is proposed.

Comparative Physiology and Water Relations of Two Corn Hybrids During Water Stress1

The responses of several physiologlcal processes to water stress imposed at three stages of growth were compared in two corn (Zea mays L.) hybrids. One hybrid (XL62AA) was a good drought tolerant commercial hybrid, while the other was an experimental hybrid (LH) derived from a cross of two Latente parents. The hybrids were compared to assess their relative responses to water stress. The LH hybrid exhibited higher rates of leaf photosynthesis than the XL62AA hybrid, but only at high leaf water potentials. Photosynthetic rates of both hybrids decreased as leaves aged. The LH hybrid maintained higher levels of soluble sugars and starch in the leaves as water potentials declined, perhaps owing to higher photosynthetic rates and/or reduced sink demand for photosynthate.The concentration of abscisic acid was also higher in the LH hybrid particularly when plants became waters tressed. Comparisons of the internal water relations between the hybrids indicated that differences in the leaf turgor‐leaf water potentlal relationship existed only during the time tassels were emerging. During this period, the LH hybrid maintained higher turgo pressures at all leaf water potentials compared with the XL62AA hybrid. When stress was imposed during the time of rapid leaf expansion and the told‐grain filling stages of development the leaf turgor‐leaf water potential relatlonships were similar in the two hybrids. The LH hybrid exhibited certain physiological characteristics that may allow greater drought tolerance or resistance when compared with a good drought tolerant commercial hybrid. As such, the LH hybrid may represent an important source of germplasm for developing more drought tolerant hybrids.

Control of Import and Export of Photosynthate in Leaves

The flow of photosynthetic assimilate in leaves was traced using radioactive 11CO2. In younger leaves the direction of flow can change in response to changes in assimilate potential in the rest of the plant whereas inolder leaves it cannot. The irreversibility of flow in older leaves is apparently a result of their having a loading process able to maintain a sieve tube assimilate potential higher than elsewhere in the plant. Systematic shifts in the responses of leaves to a succession of short term changes in sink demand are best understood in terms of radial exchanges between the sieve tubes and storage pools of limited capacity in the surrounding tissue. These take place throughout the length of the sieve tubes and may give rise to flows in opposite directions, at the same time, at different points in the same sieve tube. Behaviour indicates that flow between different pools of assimilate in the plant can be thought of as being a functionof differences in their assimilate potentials, with changes in potential eliciting changes in flow.

Effect of number and configuration of fruits, photon flux and age on the growth and dry matter distribution of fruits of Pisum sativum L.

Abstract. In experiments with Pisum sativum cv. Sleaford Orbiter in a controlled environment, the effect of fruit number and position, photon flux density and developmental stage on fruit growth was studied. During early development (up to 22 d from anthesis) growth of the first fruit was unaffected by the presence of one or two additional fruits irrespective of their position. When grown to maturity in competition with fruits at the same node a small decrease in weight of this fruit was observed. Where plants retained a full complement (20‐30) of fruits the growth of the first fruit was markedly decreased at all stages of development (6‐40 d). In all instances where competition was observed, the pericarp was more affected than the seeds. This was particularly so when photon flux was decreased 18‐22 d from anthesis compared with a decrease at an earlier stage. Partition of dry matter between fruits showed a progressively increasing allocation to the later‐formed fruits with time for all treatments. The actual proportions allocated to different fruits were not changed by the number of competing fruits. Decreasing photon flux by more than two‐thirds decreased fruit growth rates but had little effect on dry matter partitioning in most cases, although where all fruits were retained, there was a tendency for fruits at the lower reproductive nodes to be less affected. These findings are discussed in relation to known sources of assimilate for fruits, assimilate transport and sink demand. It is suggested that partition of dry matter between fruits can be estimated on the basis of fruit size and the developmental trend in relative growth rate of fruits grown in the absence of competition for assimilate from other fruits.

Water Stress Recovery in Soybeans as Affected by Photoperiod During Seed Development1

Since long‐day photoperiods during seed filling can reduce reproductive sink demand of soybeans [Glycine max (L.) Merr.], a study was conducted to determine whether long‐day treatment during seed filling would also enhance the capacity of the plants to recover from a temporary drought stress. Plants of ‘Ransom’ and ‘D72‐8126’, a high protein line, were grown under long or short‐day photoperiods and subjected to a temporary drought stress during seed‐filling stage. Carbon dioxide exchange rates and leaf water potentials were measured during and immediately after the stress treatment. Dry matter distribution and leaf area also were measured throughout the reproductive growth period. Water‐stressed Ransom and D72‐8126 plants grown under long days maintained 45 and 25% of their maximum leaf areas, respectively, vs. 16 and 0% under short‐day conditions. Carbon exchange rates during the period of water deficit were comparable between plants exposed to long and short days, but recovery was 73 vs 38% for Ransom plants grown under long and short days, respectively, and 76 and 0% for D72‐8126 plants grown under long and short days, respectively. In addition, dry matter assimilation efficiency (mg dry matter cm‐2 day‐1) showed no recovery for plants of either genotype under short days, while recovering approximately 77% for Ransom and 100% for D72‐8126 under long days. For Ransom under short days, water stress shortened the seed‐filling period by 9 days and reduced yield by 33%; but under long days, the duration of seed filling was unaffected by the stress and yield was reduced 19%. for D72‐8126, the water deficit occurred late in the seed‐filling period, and no photoperiod differences were observed in the effect of the stress on seed‐filling duration (2 days abbreviation for each treatment) or on yield (a 19 and 17% reduction under short days and long days, respectively). These results demonstrated that drought sensitivity during reproductive growth of soybeans may be reduced, and yield losses may be minimized, through manipulation of photoperiodic responses.

The effect of SO 2 on 14C translocation in Agropyron smithii RYDB

Translocation of photoassimilated 14CO 2 by individual leaves of Agropyron smithii was stimulated by exposure of tillers to 200 μg m −3 SO 2. In response to increased sink demand of expanding leaves, labeled leaves exposed to SO 2 exported an average of 12% more carbon than control leaves. The relative concentration and relative partitioning of 14C in developing leaves was 177 and 153% greater, respectively, in tillers on the SO 2 treatment than in control tillers. Below-ground 14C concentrations were greater in SO 2-exposed tillers only on the early-growing-season sampling date. The concurrent increase in translocation to above-ground sinks in tillers exposed to SO 2 and of relative rhizome 14C concentration indicates a stimulation in reproductive and leaf growth with 200 μgm −3 SO 2. Productivity, photosynthetic, and 14C-partitioning responses were compared in monitoring the subtle effects of low-level SO 2 exposure.

Response of Phloem Loading and Export to Rapid Changes in Sink Demand

AbstractSink demand was abruptly changed for an illuminated sugar beet source leaf by shading the six to ten other source leaves. Export of recently assimilated, labeled material underwent a transient increase and then returned to a steady rate approximately equal to the pretreatment rate. Uncovering the darkened leaves caused a transient decrease in export of 14C; following recovery there was a gradual decline. It remains to be established whether export of unlabeled reserves occurs in response to increased sink demand. The possibility that phloem loading increases in response to decreased sieve tube turgor was tested. Phloem loading of exogenous 14C‐sucrose increased when turgor in leaf cells was decreased by floating leaf discs on solutions with up to 1 M mannitol osmoticum. However, the increase appeared to be the result of plasmolysis of mesophyll cells possibly resulting from easier access to minor veins via the free space. Phloem loading in leaf discs continued undiminished even though sieve tube‐companion cell sucrose concentration exceeded a calculated value of 1 M. Regulation of export to meet sink demand by a direct response of phloem loading to a turgor or concentration set point does not appear to occur. Phloem loading may be promoted by the influx of water which drives mass flow, increasing phloem loading in response to increased velocity of transport.

Phloem turgor and the regulation of sucrose loading in Ricinus communis L.

The influence of plant water relations on phloem loading was studied in Ricinus communis L. Phloem transport was maintained in response to bark incisions even at severe water deficits. Water stress was associated with a net increase in the solute content of the sieve tubes, which resulted in maintenance of a positive phloem turgor pressure Ψp. There was a significant increase in solute flux through the phloem with decreasing xylem water potential (Ψ). In addition, sugar uptake by leaf discs was examined in media adjusted to different water potentials with either sorbitol (a relatively impermeant solute) or ethylene glycol (a relatively permeant solute). The limitations in this experimental system are discussed. The results nevertheless indicated that sucrose uptake can be stimulated by a reduction in cell Ψp, but that it is little affected by cell Ψ or solute potential Ψs. On the basis of these data we suggest that sucrose loading is turgor-pressure dependent. This may provide the mechanism by which transport responds to changes in sink demand in the whole plant.

Effects of Fertilization and Irrigation on the Seasonal Changes of Carbohydrate Reserves in Different Age‐Classes of Needle on 20‐Year‐Old Scots Pine Trees (Pinus silvestris)

AbstractThe effects of fertilization, irrigation or both on the seasonal changes of starch and soluble carbohydrates (glucose, fructose, myo‐inositol, pinitol and sucrose) in needles of 20‐year‐old Scots pine trees (Pinus silvestris L.) were studied during three consecutive years. The starch content of the mature needles increased during spring and early summer to about 25% of dry weight. Neither fertilization nor irrigation affected the general pattern of starch accumulation during the spring. The starch reserves were mobilized when the shoot started to grow. Starch content decreased more rapidly in needles from fertilized than in those from unfertilized trees. The current needles from the control trees accumulated starch while they were still growing. The current needles of the fertilized trees did so to a lesser extent. The amount of starch was closely correlated to the air temperature and to the growth rate. Large amounts were found at low temperatures and low growth rates.The concentrations of soluble carbohydrates showed the well‐known seasonal variation, with the highest value during the winter. The levels of sugars were nearly similar, irrespective of fertilization. An exception was sucrose, which was found in small quantities in needles from fertilized plots. Small amounts of sucrose were also found in growing current needles.The results are discussed in relation to growth limitation by assimilate availability and indicate that the ‘sink demand’ is the limiting factor.

Utilization of Photosynthates for Growth, Respiration, and Storage in Tops and Roots of Lolium multiflorum

Abstract Lolium multiflorum L. was grown in pots in controlled environments. CO2‐exchange rates were continuously measured on two pots during 46 and 52 days, respectively, separating between tops and roots. After 20 days, the plants were entirely defoliated and the plants were then followed during the regrowth period. During the experiment, alternating 2–3 day periods of high and low irradiance were applied. Analogously treated plants were frequently harvested to obtain the distribution of assimilates between tops and roots. From integration of CO2‐exchange rates, diurnal photosynthesis and respiration were obtained, and utilization of assimilates was analysed.The respiration associated with the synthesis of new structural material (growth respiration) was dependent on assimilates originating from both the current and the preceding 24 h diurnal cycles. The amount of new structural material synthesized during the current 24 h diurnal cycle was estimated from the relative contribution of assimilates accumulated from the preceding and the current 24 h and diurnal cycles to growth respiration of the current 24 h. From this approximation, the respiratory components connected to synthesis of new structural material and to maintenance of already established material were found.Growth and maintenance respirations of the tops were alike during the predefoliation and the regrowth periods. For the roots, however, growth respiration was higher and maintenance respiration lower in the regrowth period. The difference between daily integrated CO2‐exchange and the amount converted into new structural material was assumed to be the daily change in assimilates stored. On the first day of a period of high irradiance, the assimilation per unit leaf weight was higher than on the following day of high irradiance, and an accumulation of storage material took place. On the first day of a period of low irradiance, the assimilation per unit leaf weight was lower than on the following day of low irradiance, and there was a depletion of assimilates stored. These effects were most pronounced during the regrowth period, indicating a change in the metabolic sink demand. This indicates a strong feedback mechanism between sources and sinks, in the sense that accumulation of products will inhibit assimilation.

Effect of Ammonium and Nitrate Nitrogen upon Photosynthate Supply and Nitrogen Fixation by Soybeans1

The N required by soybeans [Glycine max L.) Merr.] is furnished by soil and by symbiotically fixed N. The latter is associated with available photosynthate. A determinate soybean cultivar, ‘Bragg’, was grown in sand in the greenhouse in 1974 and outdoors in 1975 to further evaluate the effect of growth stages and NH4+‐N and NO3‐‐N on 14C translocation, N fixation as measured by acetylene reduction by intact nodules, and accumulation of dry matter and nitrogen.Plants exported very little 14C to the nodules from photosynthesis of 14CO2, and acetylene reduction was very low when significant sink demand by pods became evident. Inorganic N supplied throughout the season or 10 days prior to sampling reduced 14C in nodules and acetylene reduction by nodules. NO3‐‐N generally decreased acetylene reduction more than NH4+‐N. 14C transport to plant parts other than nodules was not influenced by inorganic N. There is indication that in the determinate cultivar of soybeans used, more photosynthate was transferred to nodules during the vegetative and early reproductive stages than that reported previously for indeterminate cultivars. The data further emphasize the high demand for N at the pod‐fill stage where 333 mg N per plant (23% of total) was required during a 20‐day period, mid‐pod to late pod fill stage, and at the same time N fixation was declining.

Effects of translocation and assimilate demand on photosynthesis

Net carbon dioxide uptake by a photosynthesizing primary leaf of bean plants, Phaseolus vulgaris cv. Black Valentine, was measured during treatments designed to alter export from the leaf. Removal of shoot apices lessened sink demand while removal of all source leaves except the one being observed increased sink demand. Export from the leaf under study was lessened by chilling the primary leaf petiole and node to 2 °C. No adjustments in the rate of net photosynthesis were observed during the 33-h period after any of the treatments.The results of this study are in general agreement with previous reports in the literature. After modification of sink demand or of export by experimental manipulation of plants, a period of 2 or 3 days is usually required for adjustment of net photosynthesis rate. Rapid changes in net photosynthesis rate are generally the result of concomitant changes in morphology or metabolism during the course of plant development. The results of this work, and of others in the literature, indicate an indirect mechanism possibly involving hormonal control in some instances rather than direct feedback control of photosynthesis by product inhibition.

Effects of Light Intensity and Oxygen on Photosynthesis and Translocation in Sugar Beet

The mass transfer rate of (14)C-sucrose translocation from sugar beet (Beta vulgaris, L.) leaves was measured over a range of net photosynthesis rates from 0 to 60 milligrams of CO(2) decimeters(-2) hour(-1) under varying conditions of light intensity, CO(2) concentration, and O(2) concentration. The resulting rate of translocation of labeled photosynthate into total sink tissue was a linear function (slope = 0.18) of the net photosynthesis rate of the source leaf regardless of light intensity (2000, 3700, or 7200 foot-candles), O(2) concentration (21% or 1% O(2)), or CO(2) concentration (900 microliters/liter of CO(2) to compensation concentration). These data support the theory that the mass transfer rate of translocation under conditions of sufficient sink demand is limited by the net photosynthesis rate or more specifically by sucrose synthesis and this limitation is independent of light intensity per se. The rate of translocation was not saturated even at net photosynthesis rates four times greater than the rate occurring at 300 microliters/liter of CO(2), 21% O(2), and saturating light intensity.

Physiology of Grain Filling in Barley

Walpole and Morgan1 have suggested that differences in absolute growth rates between grains at different spikelet nodes in barley result from differences in quantity of assimilate supplied by each subtending awn, and that this quantity varies in proportion to awn size. They assume implicitly that awns affect grain growth rates by functioning solely as additional assimilate sources. If this is so, then differences in awn size between different nodal positions could determine these growth rate differences only if the assimilate requirements for grain growth were in excess of the total potential pool available for grain growth. Results from other work2,3 suggest that in barley (as in wheat4,5) the converse situation may occur. From studies of temperature effects on the relationship between grain growth and leaf area duration6,7 and on CO2 exchange rates8 in barley, it is a reasonable supposition that under the ‘cool greenhouse’ conditions used by Walpole and Morgan (with presumably a high light intensity and adequate water supply) the potential assimilate supply may have exceeded the grain growth demand in their plants. Support for this hypothesis is provided by their data as these show no evidence of any competition for assimilate between grains such as might be expected if demand had exceeded supply. If grain growth rates were not limited by assimilate supply, then either awns did not affect them or they did so primarily by affecting potential growth rates independently of assimilate supply, that is, by influencing the ‘sink demand’ (or ‘sink capacity’) of each grain.

Translocation of Photosynthetic Assimilate From Grass Leaves, as Influenced by Environment and Species

The export of 14C-labelled photosynthate from leaves of two grasses with Calvin cycle (C*3) photosynthesis, and five grasses with the C4 dicarboxylic acid pathway, was followed by continuous monitoring through the hours in light immediately after assimilation of 14CO2 and also through the subsequent dark period. The proportion of activity exported in the first few hours was much greater in all C4 grasses than in the C3 species, being greatest in the C4 grasses best adapted to growth at cool temperatures. Large differences between species were also apparent in their rates of assimilate export per unit leaf area, but there was no consistent relation between these rates and the cross-sectional area of phloem serving a given catchment leaf area. The rate of assimilate transfer per unit phloem area in Lolium temulentum was comparable to rates previously recorded for other C3 plants, but the rates for all C4 grasses except Digitaria sanguinalis were considerably faster. The rate of export in Paspalum dilatatum increased progressively up to the highest light intensity used, giving no evidence of saturation of translocation capacity. It was greater in plants grown under higher light intensities, and increased with increasing sink demand, but was not increased by darkening or removing the leaf blade distal to the area exposed to 14CO2 in Lolium temulentum. Most of the photosynthate not exported within 6 h was stored as polysaccharide in Panicum maximum, and was subsequently mobilized over a brief period during the following night. 14C assimilated at the end of the light period was mobilized first, while that assimilated at the beginning of the day was mobilized only at the end of the following night. However, the rate of mobilization could be increased by partial defoliation or by other treatments which increased the demand for assimilates from the leaf. The time to mobilization was unaffected by night temperature in Lolium temulentum, but increased markedly at temperatures below 15°C in the C4 grasses Panicum maximum and Paspalum dilatatum. The adaptive value and some possible explanations of the differences between C3 and C4 grasses in their translocation of assimilates are considered.

A Comparison of 14C‐Labeled Photosynthate Export from Two Leaf Positions in a Corn (Zea mays L.) Canopy1

The time course and direction of 14C‐labeled photosynthate export from upper and lower leaf positions in a corn (Zea mays L.) canopy were compared at five different stages from silking to kernel maturity. Export of 14C‐assimilates was slower from the lower than from the upper position. Some of the difference was due to more accumulation of 14C in leaf starch at the lower position. The direction of export from a given leaf position changed as development progressed, indicating that sink demand strongly influenced the direction photosynthate moved. Transport of 14C from upper leaves was predominantly downward, whereas direction of export from lower leaves changed from downward to upward as the ear became the dominant sink.