Showing papers on "Shoot published in 1992"

Carbohydrate sources and sinks in woody plants

Abstract: Each perennial woody plant is a highly integrated system of competing carbohydrate sinks (utilization sites). Internal competition for carbohydrates is shown by changes in rates of carbohydrate movement from sources to sinks and reversals in direction of carbohydrate transport as the relative sink strengths of various organs change. Most carbohydrates are produced in foliage leaves but some are synthesized in cotyledons, hypocotyls, buds, twigs, stems, flowers, fruits, and strobili. Although the bulk of the carbohydrate pool moves to sinks through the phloem, some carbohydrates are obtained by sinks from the xylem sap. Sugars are actively accumulated in the phloem and move passively to sinks along a concentration gradient. The dry weight of a mature woody plant represents only a small proportion of the photosynthate it produced. This discrepancy results not only from consumption of plant tissues by herbivores and shedding of plant parts, but also from depletion of carbohydrates by respiration, leaching, exudation, secretion, translocation to other plants through root grafts and mycorrhizae and losses to parasites. Large spatial and temporal variations occur in the use of reserve- and currently produced carbohydrates in metabolism and growth of shoots, stems, roots, and reproductive structures. A portion of the carbohydrate pool is diverted for production of chemicals involved in defense against fungi, herbivores, and competing plants. Woody plants accumulate carbohydrates during periods of excess production and deplete carbohydrates when the rate of utilization exceeds the rate of production. Stored carbohydrates play an important role in metabolism, growth, defense, cold hardiness, and postponement or prevention of plant mortality.

Seedling growth in relation to seed size among species of arid Australia

Abstract: It has been reported that species from environments where seedlings are prone to drought during establishment tend to have larger seeds. One proposed reason for this is that larger-seeded species might be able to allocate a larger proportion of mass to roots rather than shoots during early growth. Seedlings of 32 species from arid central Australia were grown in coarse sand under standard conditions in a glasshouse, and harvested 10 days after germination. Seedlings from heavier-seeded species did not allocate relatively more resources to roots than lighter-seeded species. Complementary experiments confirmed this result for plants grown on sandy loam, in drying as compared to well-watered soil, and under shaded conditions (...)

Regeneration in Phaseolus vulgaris L.: High-frequency induction of direct shoot formation in intact seedlings by N(6)-benzylaminopurine and thidiazuron.

Abstract: Induction of prolific shoot formation in Phaseolus vulgaris L. cv. Kinghorn Wax was achieved by germinating mature seeds and growing seedlings on a medium supplemented with 10 μM thidiazuron (TDZ), a substituted phenylurea, or 80 μM N6-benzylaminopurine (BAP). Culture for 7 d in the presence of 10 μM TDZ was sufficient to induce maximal shoot formation, whereas a continuous presence of BAP was required for the induction and development of shoots. The differentiation of adventitious shoots occurred within four weeks of seed culture, from tissues in the regions of axillary buds on the cotyledonary node and also areas surrounding the shoot apex of the intact seedling. The number of shoots regenerated from intact seedlings was significantly higher than that obtained with expiants. Regenerated shoots developed into flowering plants. Similar results were obtained in six other bean cultivars.

Overcompensation in Response to Mammalian Herbivory: From Mutulastic to Antagonistic Interactions

Abstract: Studies of natural and simulated herbivory were conducted to assess the effects of secondary herbivory and plant association on the reproductive success of Ipomopsis aggregata. Over the 5—yr period of this study 77% of all plants were browsed by ungulate herbivores at some time during the flowering season. Of these, 33% were subsequently browsed. Removal of the single inflorescence stimulated the production of, on average, five new flowering stalks from dormant lateral buds along the remaining portion of the plant's stem. Although regrowth shoots were initially avoided by ungulates following the removal of scarlets gilia's single inflorenscence, plant types were secondarily browsed following stem elongation and flower bud formation. Secondary herbivory had no effect on the compensatory outcome. Plants that were naturally browsed produced significantly higher numbers of flowers and fruits than plants that were not eaten, even when plants were secondarily browsed. Because there were no significant differences in numbers of seeds produced per fruit or in seed mass, an increase in total fruits produced by browsed plants resulted in an increase in fitness through seed production. Observational and experimental results indicate that I. aggregata switches from a "mutualistic" to an "antagonistic" interaction with its ungulate herbivores in order to achieve its greatest fitness. Results of experimental clipping showed that high levels of secondary herbivory on I. aggregata would be detrimental, decreased fitness by °70%. An apparent change in plant quality following the initial bout of herbivory, however, deters high levels of subsequent herbivory, restricting tissue removal to the tips of the plant. When plants were found in close association with either pine or grasses (to add in the potential negative effects of competition), browsed plants still outperformed control plants, producing significantly more flowers and fruits than uneaten control plants. As in a previous study, these results support the contention that mammalian herbivores can benefit plants enhancing plant fitness.

The Effect of Nutrients and Enriched CO2 Environments on Production of Carbon-Based Allelochemicals in Plantago: A Test of the Carbon/Nutrient Balance Hypothesis

Abstract: In a test of the carbon/nutrient (C/N) balance hypothesis, we grew the perennial herb Plantago lanceolata in different CO2 and nutrient environments and then (1) measured the total allocation to shoots, roots, and reproductive parts and (2) quantified aucubin, catalpol, and verbascoside contents of replicate plants of six genotypes. Plants grown under low-nutrient conditions do have higher concentrations of carbon-based allelochemicals than plants grown under high-nutrient conditions. However, in contrast to the C/N balance hypothesis, plants grown in elevated (700 μL·L-1) CO2 conditions had similar, or lower, concentrations of carbon-based allelochemicals than plants grown in ambient (350 μL·L-1 CO2 conditions. We suggest that augmented substrate concentrations (i.e., excess carbohydrates) are a necessary but insufficient trigger for increased secondary metabolism; instead, hormonal and/or direct physical cues (such as light) may be essential to synthesize or activate the appropriate enzyme systems. More...

Thidiazuron induces high-frequency shoot regeneration in intact seedlings of pea (Pisum sativum), chickpea (Cicer arietinum) and lentil (Lens culinaris)

Abstract: Axenic seedling cultures of chickpea (Cicer arietinum L.), lentil (Lens culinaris Medik.) and garden pea (Pisum sativum L.) were established by culturing mature seeds on Murashige and Skoog medium (MS) supplemented with thidiazuron (TDZ). Of various cytokinins or compounds with cytokinin-like activity (Kinetin, TDZ, Zeatin) tested for inducing shoot formation in pea seeds cultures, TDZ was found to be most effective. Pea seedlings exhibited a unique pattern of shoot formation which was accomplished in two distinct phases. Multiple shoots developed within a week, from the nodal and basal regions of the primary epicotyl in a medium that contained 5-50 μM TDZ. When these seedlings were exposed for a prolonged time period (3-4 weeks), to the same medium, numerous shoot buds emerged de novo from the base and/or from the upper part of multiple shoots. These shoots had no apparent vascular connection with parent tissues. The inductive capability of TDZ was then tested in several other genotypes of Pisum sativum and two other large-seeded grain legumes, Cicer arietinum, and Lens culinaris. In Cicer arietinum, and Lens culinaris, multiple shoots developed after 1 week of seed culture on media that contained 1-50 μM TDZ. However, de novo differentiation of shoot buds occurred in cultures exposed to TDZ for 4-6 weeks, only from nodal and subjacent areas. Secondary shoot formation occurred frequently in all of the species tested. Developing shoots were able to form roots and eventually whole plants on a modified MS medium containing 2.5 μM NAA. No genotypic difference for morphogenesis was observed.

EMF, an Arabidopsis Gene Required for Vegetative Shoot Development

Abstract: In higher plants, the transition from the vegetative to the; reproductive state in the shoot meristem initiates flowering. To study this floral transition, Constitutively flowering mutant of Arabidopsis thaliana ecotype columbia, embryonic flower (emf), was characterized. No vegetative shoots were produced from emf embryos or calli; the shoot apical meristems (SAMs) in the emf embryos were altered compared to wild-type SAMs. The mutant SAMs enlarged precociously and produced inflorescence meritems upon germination. These results suggest that the dominant, wild-type allele EMF is required for the vegetative state of the SAM. In the absence of EMF function, the mutant embryo assumes the reproductive state.

Cell fate in the shoot apical meristem of Arabidopsis thaliana

Abstract: Seeds of Arabidopsis thaliana, heterozygous for the alb1 mutation were treated with X-rays to generate sectors of albino tissue in the mature plants. Sectors were observed in tissues derived from L2 and L3 layers of the shoot meristem. Altogether 324 sectors were obtained affecting 512 leaves or the inflorescence. The majority of sectors affected only one or other of the first leaf pair. In later leaves, sectors were less frequent, and often affected more than one leaf. Sectors seen in the flowers almost invariably included some of the cauline leaves. Sectors in any region of the plant were of variable length and width. The axillary meristems of Arabidopsis were found to be clonally related to two or more cells near the centre of the subtending leaf. Overall the data are compatible with the idea that there are few, if any, restrictions on cell fate within the cell layers of the dry seed meristem. As in other higher plants, developmental fate could only be predicted in a general and probabilistic way. Such a pattern might be generated if the acquisition of cell fate occurred continuously as the plant grows, in a position-dependent, lineage-independent fashion. A general model of the meristem has been produced to accommodate the observations concerning the great majority of the sectors.

Effects of root temperature on growth and photosynthesis in conifer seedlings during shoot elongation.

Abstract: Growth and gas exchange characteristics were studied in pine (Pinus sylvestris L.) and spruce (Picea abies Karst.) seedlings grown in hydroponic culture in the presence of N (50 mg l(-1)) and transferred at the start of their second growing season to tap water at 5, 8, 12, 16 or 20 degrees C (air temperature between 18-20 degrees C) for 3 weeks (pine) or 5 weeks (spruce). Root growth of both species was completely inhibited at root temperatures of 5 and 8 degrees C, but increased almost exponentially as root temperature increased. Shoot growth was maximal at 12 degrees C in both pine and spruce and decreased at low root temperatures. In both species, CO(2) uptake was decreased at low root temperatures and appeared to be influenced by the pattern of nitrogen retranslocation. In pine seedlings, as root temperature increased, an increasing proportion of the total nitrogen pool was retranslocated to the new shoot, whereas in spruce seedlings nitrogen was retranslocated to the roots. Differences in the retranslocation of nitrogen in the two species were reflected in the amount of soluble protein in needles, which at the end of the experiment increased with increasing root temperature in pine, but decreased in spruce. Our data suggest that in spruce, but not pine, CO(2) uptake was limited by the amount of Rubisco.

Proximate Limits to Seed Production in a Herbaceous Perennial Legume, Lathyrus Vernus

Abstract: Resources, pollen, and predation limit fruit and seed set in Lathyrus vernus. Removal of flowers increased fruit and seed set among remaining ovaries and ovules, but did not affect seed mass. The distributions of the increased fruit and seed set among remaining ovaries indicated specific and variable patterns of translocation. The importance of pollen relative to resource deficiency varied between two experi- mental seasons, among individuals, and among flowers within shoots. Pollen supplemen- tation increased fruit and seed set in one of two experimental seasons. When shoots were supplied with extra pollen a greater proportion produced fruits, but the number of fruits per fruit-producing shoot did not increase. Differences in fruit set among flowers with different phonology and different position within inflorescences persisted also among pollen- supplemented flowers. Furthermore, while pollen availability sometimes may limit fruit initiation, maturation seems to be limited primarily by resources. Genets that received supplementary pollen in 1988 produced fewer flowers in 1989, indicating that long-term fecundity is not necessarily pollen limited. Lastly, exclusion of the main ovary predators increased fruit set.

1-Aminocyclopropane-1-Carboxylic Acid Transported from Roots to Shoots Promotes Leaf Abscission in Cleopatra Mandarin (Citrus reshni Hort. ex Tan.) Seedlings Rehydrated after Water Stress

Abstract: The effect of water stress and subsequent rehydration on 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase activity, ethylene production, and leaf abscission was studied in Cleopatra mandarin (Citrus reshni Hort. ex Tan.) seedlings. Leaf abscission occurred when drought-stressed plants were allowed to rehydrate, whereas no abscission was observed in plants under water stress conditions. In roots of water-stressed plants, a high ACC accumulation and an increase in ACC synthase activity were observed. Neither increase in ACC content nor significant ethylene production were detected in leaves of water-stressed plants. After rehydration, a sharp rise in ACC content and ethylene production was observed in leaves of water-stressed plants. Content of ACC in xylem fluid was 10-fold higher in plants rehydrated for 2 h after water stress than in nonstressed plants. Leaf abscission induced by rehydration after drought stress was inhibited when roots or shoots were treated before water stress with aminooxyacetic acid (AOA, inhibitor of ACC synthase) or cobalt ion (inhibitor of ethylene-forming enzyme), respectively. However, AOA treatments to shoots did not suppress leaf abscission. The data indicate that water stress promotes ACC synthesis in roots of Cleopatra mandarin seedlings. Rehydration of plants results in ACC transport to the shoots, where it is oxidized to ethylene. Subsequently, this ethylene induces leaf abscission.

Cryopreservation of in vitro -grown shoot tips of apple and pear by vitrification

Abstract: In vitro-grown shoot tips of apples (Malus domestica Borkh. cv. Fuji) were successfully cryopreserved by vitrification. Three-week-old in vitro apple plantlets were cold-hardened at 5°C for 3 weeks. Excised shoot tips from hardened plantlets were precultured on a solidified Murashige & Skoog agar medium (MS) supplemented with 0.7 M sucrose for 1 day at 5°C. Following preculture shoot tips were transferred to a 2 ml plastic cryotube and a highly concentrated cryoprotective solution (designated PVS2) was then added at 25°C. The PVS2 contains (W/V) 30% glycerol, 15% ethylene glycol and 15% dimethylsulfoxide in medium containing 0.4 M sucrose. After dehydration at 25°C for 80 min, the shoot tips were directly plunged into liquid nitrogen. After rapid warming, the shoot tips were expelled into 2 ml of MS medium containing 1.2 M sucrose and then plated on agar MS medium. Direct shoot elongation was observed in approximately 3 weeks. The average rate of shoot formation was about 80%. This vitrification method was successfully applied to five apple species or cultivars and eight pear cultivars. This method appears to be a promising technique for cryopreserving shoot tips from in vitro-grown plantlets of fruit trees.

Storage and internal cycling of nitrogen in relation to seasonal growth of Sitka spruce.

Abstract: Three-year-old clonal cuttings of Picea sitchensis (Bong.) Carr. were grown for two years (1988-1989) in sand irrigated with a nutrient solution containing either 1.0 mol N m(-3) (low N) or 6.0 mol N m(-3) (high N) NH(4)NO(3). In 1988, all the N provided was enriched with (15)N to 4.95 atom % (labeled N). In 1989, N was supplied with (15)N at natural abundance (unlabeled N). The recovery of unlabeled and labeled N in new foliage was used to quantify the internal cycling of N. In the high-N treatment, trees had two flushes of shoot growth and a period of rapid root growth, which coincided with the second flush of shoot growth in August. The timing of root growth and the first flush of shoot growth was similar in the low-N treatment, but there was no second flush of shoot growth and a greater proportion of biomass was recovered in roots. By November 1989, the root/needle dry matter ratio was 1.95 for the low-N trees and 1.36 for the high-N trees. Nitrogen was stored overwinter in roots and current-year needles. During the first six weeks of growth in the spring of 1989, stored N was remobilized for new foliage growth. Subsequent growth depended on root uptake of N. Remobilization of stored N was apparently not affected by the current N supply, because the amount of unlabeled N recovered in foliage produced in 1988 was the same for both N treatments. During 1989, the proportion of (15)N remobilized from roots relative to that from leaves produced in 1988 was greater in low-N trees than in high-N trees. In the autumn of both years, there was rapid uptake of N into roots and current-year needles. The effects of N supply on tree growth and nitrogen use efficiency are discussed in terms of the capacity for both N storage and internal cycling.

Rhizome dynamics and resource storage in Phragmites australis

Abstract: Seasonal changes in rhizome concentrations of total nonstructural carbohydrates (TNC), water soluble carbohydrates (WSC), and mineral nutrients (N, P and K) were monitored in two Phragmites australis stands in southern Sweden. Rhizome biomass, rhizome length per unit ground area, and specific weight (weight/ length ratio) of the rhizomes were monitored in one of the stands. Rhizome biomass decreased during spring, increased during summer and decreased during winter. However, changes in spring and summer were small ( 1000 g m−2). Concentrations and standing stocks of mineral nutrients decreased during spring/ early summer and increased during summer/ fall. Only N, however, showed a pattern consistent with a spring depletion caused by translocation to shoots. This pattern indicates sufficient root uptake of P and K to support spring growth, and supports other evidence that N is generally the limiting mineral nutrient for Phragmites. The biomass data, as well as increased rhizome specific weight and TNC concentrations, clearly suggests that “reloading” of rhizomes with energy reserves starts in June, not towards the end of the growing season as has been suggested previously. This resource allocation strategy of Phragmites has consequences for vegetation management. Our data indicate that carbohydrate reserves are much larger than needed to support spring growth. We propose that large stores are needed to ensure establishment of spring shoots when deep water or stochastic environmental events, such as high rhizome mortality in winter or loss of spring shoots due to late season frost, increase the demand for reserves.

Salt tolerance of the coastal salt marsh grass, Sporobolus virginicus (L.) kunth

Abstract: summary Growth responses, leaf water and osmotic relations, and tissue ionic and organic solute contents of Sporobolus virginicus (L.) Kunth, grown under solution culture in salinities of up to 450 mM NaCl, were studied. Shoot growth was stimulated by intermediate salt levels, concurrent with both an accumulation of Na+ and Cl− in shoots and a slight increase in shoot succulence. Root growth was stimulated at salinities of up to 450 mM NaCl, Osmotic adjustment of shoots was predominantly due to Na+, Cl−, and soluble carbohydrate accumulation, though a slight reduction in shoot succulence may have played a minor role at high salinity. Shoot Na+ and Cl− accumulation was tightly controlled, not exceeding levels required for osmotic adjustment, due in part to ion secretion by leaf salt glands. Shoots were selective for K+ over Na+, maintaining fairly constant K+ concentrations with increasing salinity, resulting in relatively high K+/Na+ ratios at high salinity. Increasing NaCl stimulated the accumulation of K+ in roots, which may have acted as a reservoir of K+ for shoots at high salinity. Glycinebetaine, and to a lesser extent proline and trigonelline, accumulated in shoot tissues with increasing salinity, though trigonelline concentrations were insufficient to be osmotically significant. Accumulation was closely associated with increases in shoot sap osmolality. It is proposed that glycinebetaine may act as a compatible solute in S. virginicus, as levels were sufficiently high to effect total osmotic adjustment of the cytoplasm at high salinity.

Supplemental manganese improves the relative growth, net assimilation and photosynthetic rates of salt-stressed barley

Abstract: Previous results in our laboratory indicated that a reduced Mn concentration in the leaves of barley was highly correlated with the reduced relative growth and net assimilation rates of salt-stressed plants. If Mn deficiency limits the growth of salt-stressed barley, then increasing leaf Mn concentrations should increase growth. In the present study, the effect of supplemental Mn on the growth of salt-stressed barley (Hordeum vulgare L. cv. CM 72) was tested to determine if a salinity-induced Mn deficiency was limiting growth. Plants were salinized with 125 mol m−3 NaCl and 9.6 mol m−3 CaCl2. Supplemental Mn was applied in 2 ways: 1) by increasing the Mn concentration in the solution culture and 2) by spraying Mn solutions directly onto the leaves. Growth was markedly inhibited at this salinity level. Dry matter production was increased 100% in salt-stressed plants treated with supplemental Mn to about 32% of the level of nonsalinized controls. The optimum solution culture concentration was 2.0 mmol m−3, and the optimum concentration applied to the leaves was 5.0 mol m−3. Supplemental Mn did not affect the growth of control plants. Further experiments showed that supplemental Mn increased Mn concentrations and uptake to the shoot. Supplemental Mn increased the relative growth rate of salt-stressed plants and this increase was attributed to an increase in the net assimilation rate; there were no significant effects on the leaf area ratio. Supplemental Mn also increased the net photosynthetic rate of salt-stressed plants. The data support the hypothesis that salinity induced a Mn deficiency in the shoot, which partially reduced photosynthetic rates and growth.

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability.

Abstract: Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.

Seed bacterization with a fluorescent Pseudomonas for enhanced plant growth, yield and disease control

Abstract: Bacterization of chick-pea and soybean seeds with a siderophore-producing fluorescent pseudomonad RBT 13, isolated from tomato rhizoplane, resulted in increased seed germination, growth (in terms of length and weight of root and shoot) and yield of the plants. A streptomycin-resistant mutant of the bacterium, RBT 13 str+ , used to monitor root colonization confirmed the colonization by the organism. Seed bacterization reduced the number of chick-pea wilted plants in wilt-sick soil by 52% making the organism a potential bio-control agent against chick-pea wilt.

Anther culture of tropical japonica × indica hybrids of rice (Oryza sativa L.)

Abstract: Nine japonica × indica F1 hybrids of rice involving 6 indica and 3 japonica tropical varieties, were large scale anther cultured. The frequency of callusing anthers averaged 18.7%. The microspore-derived calli produced green plants with a mean frequency of 8.7%. Albino plants represented 61% of the shoot forming calli. Monitoring of the green and albino plant regenerating capabilities of calli arising between week 4 and week 8 of incubation of the anthers showed no increase of the albino/green ratio and a slow decrease of the shoot forming ability of the transferred calli after the sixth week of culture. Spontaneous doubled haploids (SDH) represented 46% of the regenerated green plants in 4 hybrids. However, a high frequency of partially sterile regenerants was noticed among 132 SDH plants generated from a hybrid.

Rejuvenation of Sequoia sempervirens by Repeated Grafting of Shoot Tips onto Juvenile Rootstocks in Vitro: Model for Phase Reversal of Trees

Abstract: Repeated grafting of 1.5-centimeter long shoot tips from an adult Sequoia sempervirens tree onto fresh, rooted juvenile stem cuttings in vitro resulted in progressive restoration of juvenile traits. After four successive grafts, stem cuttings of previously adult shoots rooted as well, branched as profusely, and grew with as much or more vigor as those of seedling shoots. Reassays disclosed retention for 3 years of rooting competence at similar levels as originally restored. Adventitious shoot formation was remanifested and callus development was depressed in stem segments from the repeatedly grafted adult. The reversion was associated with appearance and disappearance of distinctive leaf proteins. Neither gibberellic acid nor N(6)-beneyladenine as nutrient supplements duplicated the graft effects.

Propagation of banana through encapsulated shoot tips.

Abstract: Plants were regenerated from encapsulated shoot tips of banana. Shoot tips (ca 4 mm) isolated from multiple shoot cultures of banana cv. Basrai were encapsulated in 3% sodium alginate containing different gel matrices. The encapsulated shoot tips regenerated in vitro on different substrates. Use of White's medium resulted in 100% conversion of encapsulated shoot tips into plantlets. The plantlets were successfully established in soil.

Contrasting Selenate-Sulfate Interactions in Selenium-Accumulating and Nonaccumulating Plant Species

Abstract: The shoots of primary Se-accumulating plant species can accrue Se to several thousand micrograms per gram of dry weight, even when growing in gypsiferous soils, yet no detailed studies of the mutual antagonism between SeO4 and SO4 during plant uptake have been conducted with these species. In a comparative study, we grew a nonaccumulator (alfalfa, [Medicago sativa L.]) and a primary Se accumulator [Astragalus bisulcatus (Hook.) A. Gray] in identical nutrient solutions with varied SeO4 (2-80 and SO4 (0.5-15.5 mM) concentrations for 21 (alfalfa) or 32 to 35 d (A. bisulcatus). Shoot S concentrations in alfalfa were increased by increases in solution SeO4, but only when shoot Se was above 20 g, suggesting that SeO4induced stimulation of S uptake may be a result of incipient Se toxicity. Similar stimulations of S uptake were less apparent in A. bisulcatus. Alfalfa shoot Se concentrations ranged from 4 to 154 g, while the same treatments resulted in shoot levels of 175 to 1200 g 1 in A. bisulcatus. Uptake of SeO4 by alfalfa was profoundly inhibited by increases in solution SO4, while Se uptake by A. bisulcatus was much less, although still significantly, affected. Comparison of molar Se/S ratios in plants and nutrient solutions indicated discrimination against Se by alfalfa, but preferential accumulation of Se by A. bisulcatus. A reevaluation of previously published results was in general agreement with our findings, and suggested that primary Se accumulators have a unique ability to accumulate SeO4 in the face of competition from SO4. These species could thus prove useful in efforts to remediate Se-contaminated soils or sediments that are also enriched in SO4. Overall plant discrimination between SeO4 and SO4 may be related to (i) discrimination between the two analogues during initial absorption, or (ii) differential retranslocation of these elements to the root with subsequent afflux to the external solution. View complete article To view this complete article, insert Disc 5 then click button8

Size hierarchies of shoots and clones in clonal herb monocultures: do clonal and non-clonal plants compete differently?

Abstract: In order to test whether predictions of mortality and size hierarchy development, as derived from studies of non-clonal plants, are valid for clonal plants, an experiment was carried out with Brachypodium pinnatum and Carex flacca, two rhizomatous herbs, planted at three densities. Competition was studied at two levels of organization, the shoot (ramet) and the clone (the individual plant consisting of all interconnected ramets). Size inequalities (measured as Gini coefficients), size-dependent growth and mortality of both shoots and clones were analysed over a period of two years. The species were grown in monocultures in three treatments that differed in clone density. At the end of both the first and the second year of the experiment there were not significant differences in Gini coefficients of shoot height or weight between treatments (...)

Effect of Elevated CO2 on the Photosynthesis, Respiration and Growth of Perennial Ryegrass

Abstract: Single, seed-grown plants of ryegrass (Lolium perenne L. cv. Melle) were grown for 49 d from the early seedling stage in growth cabinets at a day/night temperature of 20/15 °C, with a 12 h photoperiod, and a C02 concentration of either 340 or 680 (A T1 C02. Following complete acclimation to the environmental regimes, leaf and whole plant C02 effluxes and influxes were measured using infra-red gas analysis techniques. Elevated C02 increased rates of photosynthesis of young, fully expanded leaves by 35-46% and of whole plants by more than 50%. For both leaves and whole plants acclimation to 680 iA 1_1 CO2 reduced rates of photosynthesis in both C02 regimes, compared with plants acclimated to 340 /d 1 "1. There was no significant effect of C02 regime on respiration rates of either leaves or whole plants, although leaves developed in elevated C02 exhibited generally lower rates than those developed in 340iA l"1 C02. Initially the seedling plants in elevated C02 grew faster than their counterparts in 340 iil 1 ~1 C02, but this effect quickly petered out and final plant weights differed by only c. 10%. Since the total area of expanded and unexpanded laminae was unaffected by C02 regime, specific leaf area was persistently 13-40% lower in elevated C02 while, similarly, root/shoot ratio was also reduced throughout the experiment. Elevated C02 reduced tissue nitrogen contents of expanded leaves, but had no effect on the nitrogen contents of unexpanded leaves, sheaths or roots. The lack of a pronounced effect of elevated C02 on plant growth was primarily due to the fact that C02 concentration did not influence tiller (branch) numbers. In the absence of an effect on tiller numbers, any possible weight increment was restricted to the c. 2-5 leaves of each tiller. The reason for the lack of an effect on tillering is not known.

Field performance of transgenic potato plants compared with controls regenerated from tuber discs and shoot cuttings.

Abstract: The objective of this study was to separate and determine effects on the field performance of transgenic potatoes that originate from the tissue culture process of transformation and from the genes inserted. The constructs introduced contained the reporter gene for betaglucuronidase (GUS) under the control of the patatin promoter (four different constructs) and the neomycin phosphotransferase gene under the control of the nopaline synthase promoter. Both genes might be expected to have a neutral effect on plant phenotype. The field performance of transgenic plants (70 independent transformants) was compared with non-transgenic plants regenerated from tuber discs by adventitious shoot formation and from shoot cultures established from tuber nodal cuttings. Plants from all three treatments were grown in a field trial from previously field-grown tubers, and plant performance was measured in terms of plant height at flowering, weight of tubers, number of tubers, weight of large tubers and number of large tubers. There was evidence of somaclonal variation among the transgenic plants; mean values for all characters were significantly lower and variances generally higher than from plants derived from nodal shoot cultures. A similar change in means and variances was observed for the non-transgenic tuber-disc regenerants when compared with shoot culture plants. Plant height, tuber weight and tuber number were, however, significantly lower in transgenic plants than in tuber-disc regenerants, suggesting an effect on plant performance either of the tissue culture process used for transformation or of the genes inserted. There were significant differences between constructs for all five plant characters. The construct with the smallest segment of patatin promoter and the lowest level of tuber specificity for GUS expression had the lowest values for all five characters. It is proposed that the nature of GUS expression is influencing plant performance. There was no indication that the NPTII gene, used widely in plant transformation, has any substantial effect on plant performance in the field.

Shading and the capture of localized soil nutrients: nutrient contents, carbohydrates, and root uptake kinetics of a perennial tussock grass.

Abstract: The ability to exploit spatial and temporal heterogeneity in soil resources can be one factor important to the competitive balance of plants. Competition above-ground may limit selective plant responses to below-ground heterogeneity, since mechanisms such as root proliferation and alterations in uptake kinetics are energy-dependent processes. We studied the effect of shading on the ability of the perennial tussock grassAgropyron desertorum to take up nutrients from enriched soil microsites in two consecutive growing seasons. Roots of unshaded plants selectively increased phosphate uptake capacity in enriched soil microsites (mean increases of up to 73%), but shading eliminated this response. There were no changes in ammonium uptake capacity for roots in control and enriched patches for either shaded or unshaded plants. The 9-day shade treatments significantly reduced total nonstructural carbohydrate (TNC) concentrations for roots in 1990, but had no apparent effect on root carbohydrates in 1991 despite dramatic reductions in shoot TNC and fructan concentrations. Enrichment of the soil patches resulted in significantly greater phosphate concentrations in roots of both shaded and unshaded plants, with less dramatic differences for nitrogen and no changes in potassium concentrations. In many respects the shaded plants did surprisingly well, at least in terms of apparent nutrient acquisition. The effects of aboveground competition on nutrient demand, energy requirements, and belowground processes are discussed for plants exploiting soil resource heterogeneity.