Nitrate Absorption and Acetylene Reduction by Soybeans during Reproductive Development

Nodulated and unnodulated soybeans (Glycine max [L.] Merr.) were grown in N-free or N-containing nutrient solutions, respectively. Starting at the initial flowering stage, and throughout reproductive growth, the NO₃- absorption capacity of roots of intact plants from both treatments was determined in short-term uptake experiments. Acetylene reduction activity was determined for nodulated plants. Nitrate absorption rate, expressed on a root dry weight basis, was greatest at early flowering for both nodulated and unnodulated plants. At 33 days after germination, the NO₃^- absorption rate of unnodulated plants was twice as great as that of nodulated plants. During the remainder of the sampling period, NO₃^- absorption rates of both nodulated and unnodulated plants decreased progressively and similarly. Maximum nodule specific activity occurred 30 days after germination, or initial flowering. However, maximum total C₂H₂ reduction activity, oner plant basis, was observed during the early stages of pod-filling. Compared to unnodulated plants dependent on NO₃^- assimilation, nodulated plants were smaller, had less N in vegetative tissues, and produced less seed per plant. We suggest that the higher NO₃^- absorption rate of unnodulated soybean roots, particularly during early reproductive growth, may have reflected a more favorable supply of photosynthate translocated to the roots from larger, more vigorous, non-N-stressed shoots.     

Stem Anoxia Temporarily Interrupts Translocation of 11C-Photosynthate in Sunflower

Treatment of 5.0 cm of stem or petiole of sunflower with nitrogengas for 5 min or longer reduced phloem translocation of ¹¹C-photosynthateor stopped it entirely for a 15–20 min period after whichtime it slowly resumed When the N₂ was replaced by air the transportrestarted immediately and soon reached speeds higher than pre-anoxiarates. Thus anoxia, which we know inhibits export of photosynthateby preventing dark respiration in the leaf (Grodzinski, Jahnke,and Thompson, 1984), also inhibited transport in veins. It reducedthe speed of photosynthate movement eventually stopping it,probably by inhibiting the loading-unloading mechanisms alongthe sieve-tubes. These mechanisms are, apparently, themselves under control ofa metabolic respiratory mechanism (Fensom, 1981). Short-termanoxia of the stem did not appreciably affect the loading andtransport in the leaf; it probably did not destroy the integrityof membranes at the point of treatment nor cause a physicalblockage of sieve-plate pores nor change the viscosity of sieve-tubesap. A metabolic component which assists transport is indicatedin the mechanism of short-term translocation in sunflower. Key words: Sunflower, Petiole, Nitrogen gas, Phloem transport     

Incorporation of C-Photosynthate into Protein during Leaf Development in Young Populus Plants

Gas exchange and protein metabolism were studied in expanding, mature, and near-senescent leaves of young clonal Populus × euramericana cv. Wisconsin-5 plants. Dark respiration, CO₂ evolution in the light, and CO₂ compensation concentrations were highest in unexpanded leaves but declined markedly as leaves matured and aged. Net photosynthesis was highest in nearly mature leaves. Fresh weight continued to increase after leaf expansion was complete, whereas soluble protein levels declined. Changes in the distribution of photosynthetically incorporated ¹⁴C indicated that a high level of protein synthesis and rapid formation of structural components occurred only in expanding leaves. Protein turnover was slight in expanding leaves but was substantial after leaves were mature. Expanding leaves synthesized predominantly fraction I protein (ribulose diphosphate carboxylase). However, formation of this protein from photosynthate was slight once leaves matured.     

Water Potential, Translocation and Assimilate Partitioning

Lang, A. and Thorpe, M. R. 1986. Water potential, translocationand assimilate partitioning.—J. exp. Bot. 37: 495–503. The effect of water status upon translocation and assimilatepartitioning is examined both from theory and in an experimentwith young Phaseolus plants. Theory predicts that translocationis unlikely to be directly affected by water status. However,water potential differences within plants should influence translocationflow, with regions at lower potentials attracting disproportionatelylarge shares of assimilate. This prediction is supported in the experiment with Phaseolusin which the pattern of partitioning in the root changed rapidlyin response to bathing portions of it in solutions of differentosmolarity. The relevance of these findings to the growth of plants undernatural conditions is considered and evidence is presented thatwater potential gradients may be an Important factor in thecontrol of partitioning Key words: Phloem translocation, xylem transport, partitioning, water potential, control, osmotic potential     

Rapid Changes in Translocation Patterns in Soybeans following Source-Sink Alterations

The effects of source-sink alterations on the translocation patterns to soybean (“Fiskeby V”) pods were studied using a pod leakage technique. The distribution of assimilates from a source leaf using double pulse experiments was followed at the pods at the source node and the node immediately below. Alterations were made by shading, illuminating, or excising two-thirds of the leaf area of the leaf at the node below. In control experiments both pulses exhibited identical time-course patterns at both nodes. Shading the lower leaf during the first half of the experiment and illuminating during the second reduced the distribution of ¹⁴C-assimilate to the lower node's pods from the source leaf by approximately 30 to 50% while having no effect at the source node. Illuminating the lower leaf during the first half of the experiment followed by excision of two-thirds of that leaf's area and shading increased the import from the source leaf by 4- to 33-fold relative to the control while reducing the distribution to the source node by up to 40%. The change in distribution pattern took place in less than 30 minutes with no apparent change in the source leaf net photosynthesis or in the rate of movement to the pods. The results indicate that any alterations in the source-sink balance will quickly produce a change in the distribution patterns to the pods.     

Abscisic Acid Translocation and Metabolism in Soybeans following Depodding and Petiole Girdling Treatments

It was found earlier that depodding and girdling treatments which obstruct translocation, result in increased leaf AbA levels and partial stomatal closure. In the present work (±) [2-¹⁴C]abscisic acid (AbA) was introduced into leaves and the mass, and radioactivity of AbA and AbA-metabolites were analyzed following translocation obstruction to determine whether the increased AbA was due to higher rates of synthesis, or lower rates of catabolism or export. The (±) [2-¹⁴C]AbA was introduced into soybean (Merr.) leaves by injection into the petiole region. AbA and AbA-metabolites (phaseic acid [PA], dihydrophaseic acid [DPA], AbA-conjugate, and an unknown metabolite) were separated with preparative high performance liquid chromatography. Methyl esters of AbA (free and that released after hydrolysis of AbA-conjugate), PA and DPA were determined with gas chromatography using electron capture detection.     

Characteristics of Photosynthate Partitioning during Chloroplast Development in Avena Leaves

The first leaves (40 millimeters long) of 4-day-old light-grown Avena sativa L. cv Victory I seedlings contained a complete age sequence of cells from the base to the tip, and within these tissues all stages of chloroplast development could be observed. Although chloroplasts underwent progressive development, a marked increase in number of thylakoids per granum, in chloroplast volume, and in chlorophyll content occurred in the region between 20 and 30 millimeters from the base. Photosynthetic CO₂ fixation (per unit chlorophyll) increased markedly during chloroplast development and closely followed structural changes in chloroplasts. It was also found that the partitioning of photosynthates differed greatly in the segment from 30 to 40 millimeters (at the tip of the leaf) compared with the segment nearer to the leaf base, although both total ¹⁴CO₂ fixation and chlorophyll content per segment did not change significantly along the length of the leaves. As the thylakoid system reached full maturation, partitioning of photosynthates into sucrose increased but partitioning decreased into starch, lipids, and phosphorylated intermediates.     

Abscisic Acid Levels in Soybean Reproductive Structures during Development

Abscisic acid (ABA) concentrations and growth rates of developing soybean (Glycine max [L.] Merr. cv. Wye) seeds and pod walls were determined from anthesis to maturation using high pressure liquid chromatographic techniques. Developing soybean seeds contain up to 12,200 ng/g fresh weight of ABA compared to 330 ng/g fresh weight for pod walls. In the developing seeds ABA levels correlated with growth rates, being the highest during the most active growth period of seed enlargement, and then decreasing to less than 10 ng/g fresh weight at maturity. Higher levels of ABA were found to occur in the cotyledons and seed coats than the root-shoot axes at 21 days postanthesis. The time required for excised root-shoot axes to initiate growth in liquid culture decreased as seed development progressed and ABA levels of the seeds declined.     

Chilling Stress to Soybeans during Imhibition

Embryos, excised from seed coats of soybeans (Glycine max Merr. cv. `Wayne'), leak profusely during the first minutes of imbibition. A discontinuity of temperature/leakage patterns occurs between 10 and 15 C; as embryos imbibe at 10 C or lower, disproportionately more solutes leak out per unit of water imbibed. Short periods of imbibition at or below 12 to 14 C reduce embryo germination and axis elongation; injury results from imbibition at 2 C for as little as 5 minutes. Humidifying embryos to 35 to 50% moisture before imbibition reduced leakage during imbibition and imparted some resistance to imbibitional chilling injury.

The period of profuse leakage is interpreted as a time of membrane reorganization. Imposing a low temperature during this period prolongs the rapid leakage, suggesting delayed or faulty membrane reorganization. Reduced cold sensitivity of embryos with an initial 35 to 50% moisture content is presumed to be due to at least partial membrane reorganization in the embryo before imbibition. These data collectively are taken to indicate that low temperature interferes with normal membrane reorganization during imbibition, probably by modifying the physical state of membrane phospholipids, and that the consequent abnormal organization of membranes is a basic cause of low temperature injury.

     

Partitioning of 14C-Photosynthate of Leaves in Roots,Rhizome, and in Essential Oil and Curcumin in Turmeric (Curcuma longa L.)

Incorporation of photosynthetically fixed ¹⁴C was studied at different time intervals of 12, 24, and 36 h in various plant parts—leaf 1 to 4 from apex, roots, and rhizome—into primary metabolites—sugars, amino acids, and organic acids, and secondary metabolites—essential oil and curcumin—in turmeric. The youngest leaves were most active in fixing ¹⁴C at 24 h. Fixation capacity into primary metabolites decreased with leaf position and time. The primary metabolite levels in leaves were maximal in sugars and organic acids and lowest in amino acids. Roots as well as rhizome received maximum photoassimilate from leaves at 24 h; this declined with time. The maximum metabolite concentrations in the roots and rhizome were high in sugars and organic acids and least in amino acids. ¹⁴C incorporation into oil in leaf and into curcumin in rhizome was maximal at 24 h and declined with time. These studies highlight importance of time-dependent translocation of ¹⁴C-primary metabolites from leaves to roots and rhizome and their subsequent biosynthesis into secondary metabolite, curcumin, in rhizome. This might be one of factors regulating the secondary metabolite accumulation and rhizome development.     

Carbon Transfer and Partitioning between Vegetative and Reproductive Organs in Pisum sativum L

Assimilate partitioning was studied in the common pea (Pisum sativum L.) by feeding ¹⁴CO₂ to whole plants and measuring radioactivity in different organs 48 hours after labeling. Two experimental protocols were used. For the first, one reproductive node was darkened with an aluminum foil, to prevent photosynthesis during labeling. The aim was to study assimilate translocation among nodes. The second was carried out to assess any priority among sinks. Whole plants were shaded, during labeling, to reduce carbon assimilation. Various developmental stages between the onset of flowering and the final stage in seed abortion of the last pod were chosen for labeling. When all photosynthetic structures at the first reproductive node were darkened at any stage of development after the formation of the first flower, the first pod was supplied with assimilates from other nodes. In contrast, later developed pods, when photosynthetic structures at their node were darkened, received assimilates from other nodes only when they were beyond their final stage in seed abortion. Reducing illumination to 30% did not change distribution of assimilated carbon between vegetative and reproductive structures, nor among pods. It appears that the relative proportion of ¹⁴C allocated to any one pod, compared to other pods, depends on the dry weight of that pod as a proportion of the total reproductive dry weight. When the plant was growing actively, following the start of the reproductive phase until a few days before the end of flowering, the top of the plant (i.e., all the organs above the last opened flower) had a higher sink strength and a higher relative specific activity than pods, suggesting that it was a more competitive sink for assimilates. The pattern of assimilate distribution described here provides an explanation for pod and seed abortion.     

Studies on Genetic Male-Sterile Soybeans : II. Effect of Nodulation on Photosynthesis and Carbon Partitioning in Leaves

Soybean (Glycine max L. Merr.) germplasm, essentially isogenic except for loci controlling male sterility (ms₁) and nodulation (rj₁), were developed to study the effects of reproductive development and nitrogen source on certain aspects of photosynthesis. Plants were sampled from flowering (77 days after transplanting) until maturity (150 days after transplanting). With all four genotypes, net carbon exchange rates were highest at flowering and declined thereafter. Photosynthetic rates of the sterile genotypes (nodulated and non-nodulated) declined more rapidly than the fertile genotypes, and after 105 days, both sterile genotypes maintained low but relatively constant carbon exchange rates (<3 milligrams CO₂/gram fresh weight per hour). Photosynthetic rates and starch accumulation (difference between afternoon and morning levels) declined with time. The sterile genotypes attained the highest morning starch levels, which reflected reduced starch mobilization. After 92 days, the proportion of photosynthetically fixed carbon that was partitioning into starch (relative leaf starch accumulation) in the sterile genotypes increased dramatically. In contrast, relative leaf starch accumulation in the fertile genotypes remained relatively constant with time. Throughout the test period, all four genotypes maintained leaf sucrose levels between 5 and 15 micromoles glucose equivalents per gram fresh weight.

The activities of sucrose phosphate synthase (SPS) in leaf extracts of the four genotypes declined from 77 to 147 days. Nodulated genotypes tended to maintain higher activities (leaf fresh weight basis) than did the non-nodulated genotypes. In general, relative leaf starch accumulation was correlated negatively with the activity of SPS (normalized with leaf net carbon exchange rate) in leaf extracts for all four genotypes during early reproductive development, and for the fertile genotypes at all sampling dates. In contrast, leaf sucrose content was correlated positively with SPS activity during early reproductive development. These results suggested that a direct relation existed between the activity of SPS and starch/sucrose levels in soybean leaves. However, the interaction between these processes also may be influenced by other factors, particularly when leaf photosynthetic rates and plant demand for assimilates is low, as in the sterile genotypes.

     

Light and Shade Effects on Abscission and C-Photoassimilate Partitioning among Reproductive Structures in Soybean

Field experiments were conducted in 1981 and 1982 to study the effects of low-irradiance supplemental light on soybean (Glycine max [L.] Merr. cv Evans) flower and pod abscission. Cool-white and red fluorescent lights illuminated the lower part of the soybean canopy during daylight hours for 3 weeks late in flowering. At the same time, flowers and young pods on half the plants were shaded with aluminum foil. Flowers were tagged at anthesis and monitored through abscission or pod maturity.

Responses to red and white lights were similar. Supplemental light tended to reduce abscission and increase seed weight per node compared to natural light. Shading flowers and pods increased abscission and reduced seed weight per node. Number of flowers produced per node, individual seed weight, and seeds per pod were not affected by light or shade treatments.

Further studies examined the effects of shading reproductive structures on their capacity to accumulate ¹⁴C-photoassimilates. Individual leaves were pulse labeled with ¹⁴CO₂ 1, 2, and 4 weeks post anthesis. Flowers and pods in the axil of the labeled leaf were covered with aluminum foil 0, 24, 72, and 120 hours before pulsing.

Shading flowers and pods resulted in a 30% reduction in the relative amount of radiolabel accumulated from the source leaf. The reduction in ¹⁴C accumulation due to shading was evident regardless of the length of the shading period and was most pronounced when the shades were applied early in reproductive development. We conclude that light perceived by soybean flowers and young pods has a role in regulating both their abscission and their capacity to accumulate photoassimilates.

     

Studies on Genetic Male-Sterile Soybeans: I. Distribution of Plant Carbohydrate and Nitrogen during Development

Soybeans (Glycine max [L.] Merr. cv. NC 69-2774) were used to study the nonstructural carbohydrate and nitrogen content of plant tissues, and nitrogenase activity throughout the development of male-sterile and male-fertile plants. Male-sterile plants set approximately 85% fewer pods plus seed than the male-fertile siblings and retained green leaves until a killing frost at 145 days after emergence. Reduced pod set caused increased carbohydrate accumulation in the leaf and root systems of male-sterile plants. Total carbohydrate in roots of male-sterile plants increased from 1.7 to 7.6 times that in the male-fertile roots. A high proportion (60 to 70%) of the male-sterile root carbohydrate was starch. Apparently, root starch was not metabolized by the male-sterile plants. Late in plant development per cent nitrogen was higher in the male-sterile soybean tissues. However, no difference was found in the ability of the nodulated root systems from either genotype to fix nitrogen.     

Preferential Leaching of Pinitol from Soybeans during Imbibition

Sugars and cyclitols leached from soybeans (Glycine max var Sparks) during imbibition were assayed as a function of time. Pinitol leached many times faster than carbohydrates. During the initial 20 minutes of imibition, the pinitol/carbohydrate ratio was 3.4, declining to 0.29 for fully imbibed seeds. The value for dry soybeans was 0.14. Hypochlorite treatment of seeds more than doubled the rate at which carbohydrates leached out, but had little effect on pinitol. A role in development of soil microorganisms is postulated for pinitol.     

Vacuole Partitioning during Meiotic Division in Yeast

We have examined the partitioning of the yeast vacuole during meiotic division. In pulse-chase experiments, vacuoles labeled with the lumenal ade2 fluorophore or the membrane-specific dye FM 4-64 were not inherited by haploid spores. Instead, these fluorescent markers were excluded from spores and trapped between the spore cell walls and the ascus. Serial optical sections using a confocal microscope confirmed that spores did not inherit detectable amounts of fluorescently labeled vacuoles. Moreover, indirect immunofluorescence studies established that an endogenous vacuolar membrane protein, alkaline phosphatase, and a soluable vacuolar protease, carboxypeptidase Y, were also detected outside spores after meiotic division. Spores that did not inherit ade2- or FM 4-64-labeled vacuoles did generate an organelle that could be visualized by subsequent staining with vacuole-specific fluorophores. These data contrast with genetic evidence that a soluble vacuolar protease is inherited by spores. When the partitioning of both types of markers was examined in sporulating cultures, the vacuolar protease activity was inherited by spores while fluorescently labeled vacuoles were largely excluded from spores. Our results indicate that the majority of the diploid vacuole, both soluble contents and membrane-bound components, are excluded from spores formed during meiotic division.     

Patterns of Niche Partitioning and Alternative Reproductive Strategies in an East African Dung Beetle Assemblage

Sympatric species of coprophagus dung beetles rely on essentially the same resource for provisioning broods, which raises the question of how local species diversity is maintained. Interspecific competition may be mitigated to some extent by large-scale spatial (e.g., habitat type) and temporal (e.g., seasonal) variation in activity. Niche partitioning also occurs at the scale of individual dung pads. We examined the extent to which inter- and intraspecific variation in adult morphology and behavior contribute to spatial and temporal partitioning above and below the dung pad in the dung beetle community of Kibale Forest, Uganda. Excavations in the vicinity of dung pads revealed significant heterogeneity among species and guilds in the lateral distance between tunnels and dung, and in tunnel depth. Patterns of succession and diel activity demonstrated strong temporal dissociations among guilds. In one species, Onthophagus multicornis, we found a bimodal distribution of male horn sizes and a tunneling pattern consistent with alternative reproductive tactics.     

柑橘果实的光合特性、产物运输及分配在糖分积累中的作用

测定了温州蜜柑 (CitrusunshiuMarc .cv .Miyagawawase)果实发育进程中干鲜重、果皮光合速率和叶绿素含量的变化 ,并用14 CO2 示踪技术研究了果皮和叶同化生成的光合产物在果实内的运输分配特性。结果表明 :果皮光合速率与叶绿素含量有关 ,随着叶绿素含量的下降 ,果实光合速率也快速下降。在果实完熟之前 ,即使是当果皮积累的干重超过汁囊时 ,叶同化产物仍主要分配到汁囊中 ;而在完熟阶段 ,果皮光合速率接近零 ,果皮成了叶同化产物的主要库。果皮的同化产物 ,主要保留在果皮中 ,输入到汁囊的比率随果实发育而下降 ,但高峰时也有 12 %输入汁囊。与对照相比 ,果实遮光处理后降低了果皮与汁囊的干重和含糖量。上述结果表明果皮光合产物主要用于果皮自身的发育并能减少对叶光合产物的依赖 ,同时也能部分增加汁囊糖的积累     

Carbon Partitioning among Leaves, Fruits, and Seeds during Development of Phaseolus vulgaris L

Development of vegetative and floral buds was found to be a key factor in establishing the way carbon is distributed among growing leaves and fruits in Phaseolus vulgaris L. plants. Leaves emerged principally during a period 14 to 32 days after planting while flowers were produced during a 10- to 12-day period near the end of leaf emergence. Timing of anthesis established the sigmoidal time course for dry weight accumulated by the composite of all fruits on the plant. During the first 12 days following anthesis, fruit growth mainly consisted of elongation and dry weight accumulation by the pod wall. Thereafter, seed dry weight increased for about 1 week, decreased markedly for several days, and then increased again over the next 2 weeks. Accumulation of imported carbon in individual seeds, measured by steady-state labeling, confirmed the time course for dry weight accumulation observed during seed development. Seed respiration rate initially increased rapidly along with dry weight and then remained nearly steady until seed maturation. A number of developmental events described in the literature coincided with the different phases of diauxic growth. The results demonstrated the feasibility of relating current rates of carbon import in individual seeds measured with tracer ¹⁴C to the rates of conversion of imported sucrose and use of the products for specific developmental processes. The resulting data are useful for evaluating the roles of conversion and utilization of imported sucrose in regulating import by developing seeds.