Increased Sink Demand Research Articles

Influence of vesicular–arbuscular mycorrhizal fungi and phosphorus on growth, carbohydrate partitioning and mineral nutrition of greenhouse cucumber (Cucumis sativus L.) plants during establishment

The growth response of greenhouse cucumber (Cucumis sativus L.) to infection by vesicular–arbuscular mycorrhizal (VAM) fungi and to 4, 12 and 20 mg L−1 of phosphorus (P) nutrition was characterized over 38 d of plant establishment. Although maximum growth rates were not reached with the P levels studied, foliar concentrations of P were consistent with those published for healthy cucumber plants. Plants were highly receptive to colonization by Glomus mosseae, G. dimorphicum and G. intraradices. Infection by all species decreased as P nutrition increased; however, growth rates of primary yield components (e.g., stem and leaf dry weights, leaf area) of VAM-infected plants were greater than those of noninfected plants at all levels of P nutrition. The VAM-enhanced growth was similar to that induced by increases in P nutrition. The VAM-infected plants had lower concentrations of soluble nitrogen (N) than noninfected plants early in development; however, N concentrations were equal by 38 d after planting. Total soluble carbohydrate concentration of leaves of noninfected plants increased 62% faster than that of leaves of VAM-infected plants, possibly due to decreased export and a higher degree of P stress in the non-infected plants. Relative to noninfected plants, higher concentration of soluble carbohydrates in roots of VAM-infected plants may indicate increased sink demand to support the symbiosis. Since VAM stimulated growth at levels of P nutrition considered nondeficient by tissue analysis, use of VAM to accelerate the early development of transplants may be advantageous to the greenhouse industry. Key words:Cucumis sativus L., phosphorus, vesicular–arbuscular mycorrhizal fungi, mineral nutrition

Effects of cross-pollination on dry matter accumulation, nutrient partitioning and grain yield of maize hybrids grown under different levels of N fertility

This study was initiated to investigate the relationship between increased kernel sink capacity and fertiliser N use efficiency of maize (Zea mays L). Two hybrids differing in yield potential, Pioneer brand 3732 (P3732) and B73 × Mo17, were grown on field plots with four levels of N fertiliser. The kernel sink of P3732 was modified by cross-pollinating P3732 with pollen from B73 × Mo17. Vegetation and kernal samples collected from the N treatments at mid-silk and at 30, 45 and 60 days post-pollination (DPP) were analysed for accumulation and partitioning of dry matter and N content. Grain yield was also determined. Modification of the P3732 endosperm genotype through cross-pollination significantly increased kernel weight, kernel protein content and grain yield at each level of N treatment, indicating an improvement in fertiliser N use efficiency due to an increased sink demand. All three levels of N application produced similar effects on the accumulation and partitioning of dry matter and N in plant parts for both endosperm genotypes. Since the harvest index observed at 30, 45 and 60 DPP was similar between sib- and cross-pollinated plants, the additional nutrients translocated into developing kernels of P3732 cross-pollinated plants were mainly derived from increases in duration of dry matter production and N uptake by vegetative tissues rather than from improved partitioning efficiency.

Proliferation of maize (Zea mays L.) roots in response to localized supply of nitrate.

Maize (Zea mays L.) plants with two primary nodal root axes were grown for 8 d in flowing nutrient culture with each axis independently supplied with NO3-. Dry matter accumulation by roots was similar whether 1.0 mol m-3 NO3- was supplied to one or both axes. When NO3- was supplied to only one axis, however, accumulation of dry matter within the root system was significantly greater in the axis supplied with NO3-. The increased dry matter accumulation by the +N-treated axis was attributable entirely to increased density and growth of lateral branches and not to a difference in growth of the primary axis. Proliferation of lateral branches for the +N axis was associated with the capacity for in situ reduction and utilization of a portion of the absorbed NO3-, especially in the apical region where lateral primordia are initiated. Although reduced nitrogen was translocated to the -N axis, concentrations in the -N axis remained significantly lower than in the +N axis. The concentration of reduced nitrogen, as well as in vitro NO3- reductase activity, was greater in apical than in more basal regions of the +N axis. The enhanced proliferation of lateral branches in the +N axis was accompanied by an increase in total respiration rate of the axis. Part of the increased respiration was attributable to increased mass of roots. The specific respiration rate (micromoles CO2 evolved per hour per gram root dry weight) was also greater for the +N than for the -N axis. If respiration rate is taken as representative of sink demand, stimulation of initiation and growth of laterals by in situ utilization of a localized exogenous supply of NO3- establishes an increased sink demand through enhanced metabolic activity and the increased partitioning of assimilates to the +N axis responds to the difference in sink demand between +N and -N axes.

Ontogenetic Patterns of CO2 Exchange of Quercus rubra L. Leaves During Three Flushes of Shoot Growth I. Median Flush Leaves

Abstract Oak (Quercus) seedlings exhibit a pattern of shoot growth known to place demands on carbohydrate and nutrient reserves. This study was designed to determine ontogenetic patterns in CO2 exchange properties of red oak leaves, and to determine if individual leaf CO2 exchange rates (CER) increase in response to the assimilate demand placed on a seedling during flushing. Northern red oak (Q. rubra L.) seedlings were grown in environments favorable for multiple flushes of shoot growth. Measurements of CER on single, attached, median leaves from each flush were made over a range of photosynthetic photon flux densities on plants at nine stages of seedling development through three flushes of growth. Carbon dioxide exchange rate of red oak leaves increased during leaf development up to and beyond full leaf expansion before decreasing —an unusual pattern of photosynthesis during leaf ontogeny. Furthermore, first- and second-flush leaf CER initially decreased and then increased in conjunction with the third flush of shoot growth. These patterns indicate that red oak leaves have a capacity for CER adjustment in response to increased sink demand. For. Sci. 34(1):55-68.

Short‐term effects of aphid feeding on photosynthetic CO2exchange and dark respiration in legume leaves

Net CO2exchange rates and dark respiration rates were determined for single attached legume leaves (leaflets) after 6 to 9 days of aphid infestation. Plant‐aphid combinations used were broad bean (Vicia fabaL. cv. Aquadulce) and cowpea [Vigna unguiculata(L.) Walp. cv. Caloona)] infested with cowpea aphids (Aphis craccivoraKoch) and broad bean and garden pea (Pisum sativumL. cv. Victory Freezer) infested with pea aphids [Acyrthosiphon pisum(Harris)]. Leaves from all aphid‐infested plants had significantly greater net CO2exchange rates in the light than their respective controls and rates of dark respiration of leaves from infested cowpea and garden pea were also significantly greater than those of controls. Dark respiration, as a percentage of net CO2exchange rates in the light, was greater in aphid‐infested than in control plants. When the mean net daily carbon gain was calculated for the leaves of each plant‐aphid combination, leaves from aphid‐infested plants had the greatest gain. It is proposed that net CO2, exchange rates increased due to increased sink demand and dark respiration rates increased to meet the increased energy requirements of phloem loading and cellular maintenance associated with aphid feeding. The apparent compensatory carbon gain of infested leaves was consumed by the aphids.

Effect of the Reproductive Sink on 14C‐Assimilate Partitioning between Starch and Water‐Soluble Compounds in Soybean Leaves1

The objective of this study was to determine if changes in reproductive sink demand influence the partitioning of carbon within the source leaf. Temporary (Late R2‐R4) and continuous (Late R2‐R8) light and CO2 enrichment treatments were used increase the pod number and weight on field.grown soybeans (Glycine max L. Merr. ‘Hodgson 78’). During the last week of the temporary treatment, and at 1, 2, and 4 weeks thereafter, plants were pulse‐labeled with 144CO2 at the tenth trifoliolate leaf. Leaf discs were removed at 0.5, 2, 4, 8 and 24 h after labeling and subsequently extracted for determination of 14C in the starch, water‐soluble compounds (WSC), and residual fractions. Temporary and continuous CO2 and light enrichment increased the reproductive sink load of field.grown soybeans at all sampling dates. On the average, total number of pods was increased 35% by light enrichment, and 38% by CO2 enrichment. During this same period, light enrichment res~resulted in at least a 38% increase in total pod wall and seed dry weight while CO2 enrichment resulted in a 28% increase. Light and CO2 enrichment increased plant size during the sampling period. Leaf dry weight was increased 30% by light enrichment, and 39% by CO2 enrichment. Stem plus petiole dry weight was increased 26% by light enrichment and 51% by CO2 enrichment. Hence, light enrichment had a greater effect on reproductive than on vegetative growth, while CO2 enrichment had a greater effect on vegetative than on reproductive growth. On the first sampling date, 11 August, significantly less label was incorporated and retained as starch in the light‐enriched plants. No differences were observed in the levels of 14C‐labeled WSC or starch for the light‐enriched plants on 18 and 25 August, and during the entire experiment for the CO2‐enriched plants. On 7 September, 4 weeks after removal of the temporary treatment, significantly less label was retained as WSC, starch, and residual material in the light‐enriched plants. The hypothesis that increased sink demand may influence assimilate partitioning is supported by these data.

Effect of Shortened Photosynthetic Period on 14C-Assimilate Translocation and Partitioning in Reproductive Soyeans

Starch accumulation rate in leaves of vegetative soybeans is inversely related to the length of the daily photosynthetic period. However, it is not known whether a similar response would be observed during reproductive growth. Soybeans (Glycine max L. Merr. cv Amsoy 71) were grown to three stages of reproductive growth (beginning seed, mid seed-fill, and late seed-fill) under 12-hour daylengths, and then shifted to 6-hour photosynthetic periods (12-hour photoperiods) for 4 days. One and 4 days after treatment, a mid-canopy leaf was pulsed with (14)CO(2), and sampled for radiolabeled starch and water-soluble compounds at 0.5, 1, 3, 9, and 21 hours after labeling.Plants exposed to the 6-hour photosynthetic periods at the beginning seed stage retained and incorporated significantly more label as starch than did those given 12-hour photosynthetic periods. However, plants exposed to the shortened photosynthetic periods at the late seed-fill stage partitioned less label into starch. Plants exposed at mid seed-fill gave a variable response.Shortened photosynthetic periods resulted in preferential partitioning of recently fixed carbon to the seed at the expense of the pod wall. The results of these experiments suggest that the increased sink demand present during late reproductive growth may be of greater importance in control of leaf starch accumulation than is the length of the daily photosynthetic period.

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.

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.

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.

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.