Alfalfa physiological response to potato leafhopper injury depends on leafhopper and alfalfa developmental stage

We examined the effects of potato leafhopper (Empoasca fabae) developmental stage and alfalfa (Medicago sativa) developmental stage on the physiological response of the plant to injury. We used radioactive carbon dioxide to label the photoassimilate stream and evaluate the phloem health of alfalfa. In one experiment, six first instar, four fourth instar, and three adult leafhoppers were caged by stage on single alfalfa stems for approximately one day. Only fourth instar nymphs significantly reduced the amount of label transported to injured tissues above the source of the labeled assimilate. First instar nymphs had no effect and adults reduced assimilate transport to stem tips, but this trend was not significant possibly because of confounding variables. However, injury by both first instar nymphs and adults resulted in greater concentration of labeled assimilate in portions of the stem below the feeding site. In another experiment, the developmental stage of alfalfa stems was central to the physiological response of alfalfa to leafhopper injury. A 20 h exposure to three adult leafhoppers significantly reduced the amount of label translocated to the tip and crown tissues of early vegetative plants, and to the crown tissue only of late vegetative plants. In reproductive plants, assimilate translocation was not affected by leafhopper injury. In a final experiment, we found no evidence of an effect on the photosynthesis of leaves of similar age and position to those used as source leaves in our translocation studies. Our findings contribute to our understanding of the physiological response of plants to injury by sap-feeding insects, and suggest the need for greater refinement of economic injury levels based on leafhopper and plant developmental stage.     

Physiological response of glandular-haired alfalfa to potato leafhopper (Hemiptera: Cicadellidae) injury

Plant tolerance to herbivory is a key approach for managing pests. In alfalfa, Medicago sativa, the potato leafhopper, Empoasca fabae, is a major pest as a result of the cascade of plant responses to piercing-sucking injury. To identify tolerance to its injury based on alfalfa physiology, experiments were conducted in the field and greenhouse. In our comparison of the response of field-grown alfalfa cultivars to standardized leafhopper densities, net photosynthesis and transpiration rates of 'Geneva' leaves were reduced by 18 and 21%, respectively, by leafhopper presence compared with a rate change of <1% of resistant 'EverGreen' leaves. Under greenhouse conditions, alfalfa clones varied in their level of gas exchange (net photosynthesis and transpiration) and stem elongation responses to leafhopper injury. For example, in the comparison of seven clones, net photosynthesis declined an average of 40.7% with leafhopper injury, although individual clones varied from 26.6 to 74.3% reduction. Internode elongation after 2 d was 60.3% less on injured stems compared with healthy stems, but again, the individual clones varied from 17.3 to 91.9%. In a time-course study of selected clones, clones varied in their level of injury just after and 3 d after insect removal. Gas exchange responses of all clones recovered by 7 d after cessation of injury. In a choice test, leafhoppers spent similar amounts of time on the susceptible clone and the most tolerant clone; however, their precise feeding behaviors were not measured. Thus, the variable response of clones to injury may be either true physiological tolerance or antixenosis from a change in feeding behavior. This study showed putative tolerance to leafhopper injury among alfalfa genotypes, suggesting that tolerance could be the basis for crop protection in alfalfa from potato leafhopper injury.     

Mechanical and salivary aspects of potato leafhopper probing in alfalfa stems

Studies were conducted to separate the salivary and mechanical aspects of probing damage by the potato leafhopper,Empoasca fabae (Harris) (Homoptera: Cicadellidae), to stem vascular tissue of alfalfa,Medicago sativa L. Implantation of salivary gland tissue or fed-upon artificial diet under the stem epidermis yielded no evidence, three days later, of hopperburn-associated, anatomical changes. Mechanical puncturing of stems with implements approximating the size and shape of leafhopper stylets caused some anatomical changes, three days later, similar to those underlying hopperburn, i.e. tracks of necrosis, chlorosis, cell enlargement, and cell division. These changes, however, were much less severe than those observed in tissues three days after potato leafhopper probing. In contrast, puncturing through salivary gland or Malpighian tubule tissues produced extreme hyperplasia and other symptoms of wounding in cells near the puncture. This was similar to but more severe than effects from leafhopper probing, and was probably caused by leafhopper structural tissues or larger amounts of saliva being conveyed into the plant than normally occurs during leafhopper probing. We conclude that both salivation and mechanical wounding by leafhopper stylets are probably necessary to cause hopperburn-associated anatomical changes to vascular tissue in stems of alfalfa. This conclusion supports our hypothesis that hopperburn is a saliva-enhanced wound response.     

Plant diversity increases herbivore movement and vulnerability to predation

Understanding how changes in plant diversity affect agroecosystem functioning remains a key challenge. We examined how intercropping alfalfa, Medicago sativa, with orchardgrass, Dactylis glomerata, affects the potato leafhopper, Empoasca fabae, its host plant (alfalfa), and the efficiency of a leafhopper predator, Nabis americoferus. In a field experiment, intercropping reduced the reproductive efficiency of the leafhopper. Nabis was more effective at reducing leafhopper abundance, and protecting alfalfa from hopperburn, in the polyculture than in the monoculture of alfalfa. In a series of laboratory experiments, we investigated mechanisms by which intercropping could enhance the efficiency of Nabis. Intercropping resulted in changes in vegetation structure and the spatial distribution of leafhoppers, but there was little evidence that these factors influenced the efficiency of Nabis. Instead, orchardgrass, a nonhost for leafhoppers, increased leafhopper movement, and Nabis captured leafhoppers more efficiently when the herbivores were more mobile. These results indicate that intercropping with nonhost plants promotes leafhopper movement and vulnerability to predation, and reveal a novel mechanism by which plant diversity can reduce herbivory.     

Influence of plant ontogeny and abiotic factors on resistance of glandular-haired alfalfa to potato leafhopper (Hemiptera: Cicadellidae)

Laboratory experiments were conducted to characterize the trichome-based defense of glandular-haired alfalfa, Medicago sativa L., against the potato leafhopper, Empoasca fabae (Harris). Within-plant variability in leafhopper resistance was examined by caging adult leafhoppers to either basal or apical stem internodes of the leafhopper-resistant, glandular-haired M. sativa genotype G98A or the susceptible, nonglandular-haired M. sativa 'Ranger'. Young, actively secreting glandular trichomes are located on apical internodes of G98A, whereas senesced gland heads are found on older, basal internodes of G98A. After 96 h, the highest cumulative leafhopper mortality and lowest number of excretory droplets were associated with apical internodes of G98A. No difference was detected in mortality and feeding levels among insects caged to basal internodes of G98A and basal and apical internodes of Ranger. The influence of abiotic factors on leafhopper resistance was evaluated by caging adult leafhoppers to either G98A or Ranger under four combinations of low and high light (250 and 1,000 micromol s(-1) m(-2)) and temperature regimes (17 and 30 degrees C). After 96 h, the highest cumulative mortality was associated with leafhoppers confined to G98A under high light and high temperature conditions. Temperature level and plant type also had an effect on the production of excretory droplets, resulting in the highest number of excretory droplets being associated with Ranger under the high temperature regime. These results indicate that certain regions of M. sativa G98A are better protected against the potato leafhopper than others and that temperature influences resistance levels of glandular-haired alfalfa.     

Glandular-haired alfalfa resistance to potato leafhopper (Homoptera: Cicadellidae) and hopperburn: development of resistance indices

Eight proprietary genotypes of glandular-haired alfalfa, Medicago sativa L., supplied by two different companies, were compared for the degree and types of resistance to the potato leafhopper, Ernpoasca fabae (Harris), and hopperburn. A tube cage no-choice bioassay was developed to test leafhopper mortality, feeding, settling preferences, severity of hopperburn symptoms (in this case, defined as both yellowing and stem growth reduction), and trichome density and type on feeding sites. Leafhopper mortality was both strongly and significantly associated with feeding and leaf trichome density; decreased hopperburn symptom severity was weakly, although significantly, associated with increased mortality. To quantify hopperburn in terms of both yellowing and stem growth reduction, we developed a ranking system that reduces overall hopperburn expression to a single number that considers the varying responses to both types of symptoms. Great variability in leafhopper settling, leafhopper mortality, and stem glandular trichome density was detected among alfalfa genotypes, suggesting that genotypic differences may be based on the concentration and/or chemical constituency of the trichome exudates. We postulate that, among variably resistant genotypes of glandular-haired alfalfa, differences among leafhopper responses and hopperburn severity are linked to forced movement from the stems to the leaves as refuge feeding sites. Principal component analysis was performed to reduce the 10 variables down to five biologically significant factors. Scores for these factors were then used to develop resistance indices for potato leafhopper resistance, hopperburn resistance, and an overall glandular-haired alfalfa resistance index.     

THE ABSORPTION AND TRANSLOCATION OF C14-LABELED PROTEINS IN YOUNG TOMATO PLANTS

The uptake of C¹⁴-labeled proteins (lysozyme, hemoglobin, lactoglobulin, and ovalbumin) from solution by tomato plants with sterile roots was studied. It was found that C¹⁴-compounds (proteins and/or protein-degradation products) were translocated to the foliage if the roots had undergone minor mechanical injury or if the plants were subjected to temporary wilting, i.e., physiological damage. C¹⁴-lysozyme was not transported to foliar tissue in healthy plants; C¹⁴-hemoglobin showed radioactivity in leaves of both healthy and injured plants, but there was evidence of a breakdown of the molecule; C¹⁴-ovalbumin gave a faint labeling of foliar tissues of some plants in which wilting or mechanical damage was below the threshold of detection. It is concluded, however, that translocation of proteins from roots in nutrient solution to tomato leaves does not occur in significant amounts in healthy plants in spite of the large uptake of proteins by root cortex, as found in earlier studies.     

Studies on Genetic Male-Sterile Soybeans : IV. Effect of Male Sterility and Source of Nitrogen Nutrition on Accumulation, Partitioning, and Transport of Nitrogen

Soybean (Glycine max [L.] Merr.) germplasm, isogenic except for loci controlling male sterility (ms₁) and nodulation (rj₁), was used to investigate the effects of reproductive tissue development and source of nitrogen nutrition on accumulation, transport, and partitioning of nitrogen in a greenhouse experiment. Nodulated plants were supplied nitrogen-free nutrient solution, and nonnodulated plants were supplied nutrient solution containing 20 millimolar KNO₃. Plants were sampled from flowering until maturity (77 to 147 days after transplanting).

Accumulation rates of nitrogen in whole plants during reproductive growth were not significantly different among the four plant types. Nitrogen accumulation in the sterile, nonnodulated plants, however, ceased 2 weeks earlier than in fertile, nonnodulated or fertile and sterile, nodulated plants. This early cessation in nitrogen accumulation resulted in sterile, nonnodulated plants accumulating significantly less whole plant nitrogen by 133 days after transplanting (DAT) than fertile, nonnodulated plants. Thus, changing the site of nitrogen assimilation from nodules (N₂-fixing plants) to roots and leaves (NO₃-fed plants) resulted in similar whole-plant nitrogen accumulation rates in fertile and sterile plants, despite the absence of seed in the latter.

Leaflet and stem plus petiole tissues of both types of sterile plants had significantly higher nitrogen concentrations after 119 DAT than both types of fertile plants. Significantly higher concentrations and exudation rates of nonureide, reduced-nitrogen in xylem sap of sterile than of fertile plants after 105 DAT were observed. These latter results indicated possible cycling of nonureide, reduced-nitrogen from the downward phloem translocation stream to the upward xylem translocation stream in roots of sterile plants. Collectively, these results suggest a lack of sinks for nitrogen utilization in the shoots of sterile plants. Hence, comparison of nitrogen accumulation rates for sterile and fertile plants does not provide a definitive test of the hypothesis that reproductive tissue development limits photosynthate availability for support of N₂ fixation and nitrate assimilation in determinate soybeans.

Nitrogen assimilation during reproductive growth met a larger proportion of the reproductive-tissue nitrogen requirement of nitrate-dependent plants (73%) than of N₂-fixing plants (63%). Hence, vegetative-tissue nitrogen mobilization to reproductive tissue was a more prominent process in N₂-fixing than in nitrate-dependent plants. N₂-fixing plants partitioned nitrogen to reproductive tissue more efficiently than nitrate-dependent plants as the reproductive tissues of the former and latter contained 65 and 55%, respectively, of the whole-plant nitrogen at the time that nitrogen accumulation in reproductive parts had ceased (133 DAT).

     

Host suitability and gas exchange response of grapevines to potato leafhopper (Hemiptera: Cicadellidae)

Although potato leafhopper, Empoasca fabae (Harris) (Hemiptera: Cicadellidae), is highly polyphagous, classic host studies do not recognize grapevines (Vitis spp.), as suitable hosts. Recently, injury has been reported and reproduction documented within grape vineyards, suggesting a host expansion for the leafhopper. To document this apparent expansion in host use, we determined whether grape plants were suitable hosts for potato leafhopper reproduction, measured the consequence of feeding injury on gas exchange rates of grape leaves, and compared the susceptibility to feeding injury among cultivars. We found that potato leafhopper adults survived equally well on grape (Vitis vinifera L.), alfalfa (Medicago sativa L.), and fava bean (Vicia faba L.). The total number of offspring was greater on fava bean but did not differ between alfalfa and grape. Injury to grapevines was assessed by measuring gas exchange responses of leaves in field cages and in greenhouse tests. We found marginally significant declines in photosynthesis and transpiration rates in the field (9.6 and 13.2%, respectively), and much stronger effects in greenhouse tests (ranging between 22 and 52%). Our results verify that Vitis is a suitable host, and that potato leafhopper is capable of injuring its gas exchange physiology. We discuss possible explanations for the host expansion, and its potential to damage commercial grapevines.     

Host effects of glandular-haired alfalfa on alfalfa weevil (Coleoptera: Curculionidae) and potato leafhopper (Homoptera: Cicadellidae) populations in Virginia

Cultivars of glandular-haired alfalfa, Medicago sativa L., such as '54H69', are currently available and marketed as being resistant to potato leafhopper, Empoasca fabae (Harris). Between 2000 and 2002, studies were conducted to compare the effects of '54H69' and a standard, nonglandular-haired alfalfa cultivar, 'Choice', on alfalfa weevil, Hypera postica (Gyllenhal), and potato leafhopper populations at Campbell and Montgomery counties, Virginia. '54H69' had no effect on alfalfa weevil populations. At each location, densities of alfalfa weevil in '54H69' and 'Choice' were similar, but pest pressure was higher at Campbell Co. than at Montgomery Co. and always exceeded the economic threshold before insecticide was applied. Densities of potato leafhopper also did not differ between '54H69' and 'Choice' in any year at the two locations. Insecticide treatment effectively reduced potato leafhopper densities in the two cultivars, although populations were below the economic threshold at both locations when the insecticides were applied. Overall, postinsecticide treatment comparisons showed that the densities of alfalfa weevil and potato leafhoppers were similar or higher in untreated '54H69' compared with insecticide-treated 'Choice'. In addition, there were no differences in seasonal dry yields between '54H69' and 'Choice' in any year at either location. Our results indicate that the glandular-haired alfalfa '54H69' does not provide acceptable resistance to potato leafhopper and also does not offer a yield advantage to growers in Virginia.     

Glandular Trichomes on Alfalfa Impede Searching Behavior of the Potato Leafhopper Parasitoid

New cultivars of alfalfa, Medicago sativa L., have been released with glandular trichomes for resistance to potato leafhopper, Empoasca fabae (Harris). Yet, the impact of the glandular trichomes on the primary natural enemy of the leafhopper, Anagrus nigriventris Girault, is unknown. We compared the host searching behavior of the egg parasitoid on four alfalfa clones varying in trichome characters. Female wasps were videotaped on Ranger, a susceptible clone with relatively sparse trichomes, B14, a resistant clone with dense but nonglandular trichomes, and FG12 and FG18, two resistant clones with glandular trichomes. Although the number of leafhopper eggs per stem exposed to wasps did not significantly differ among the four clones, the frequency of foraging and total foraging time were less on the two clones with glandular trichomes than on the two clones with nonglandular trichomes. In addition, an analysis of covariance demonstrated that, although the number of ovipositional probes increased with egg density on a stem, the number of probes on stems with glandular trichomes was significantly less than that on stems without glandular trichomes. The allocation of time by wasps among drumming, probing, and grooming behaviors was similar among the clones. Wasps tended to fly off of clones with glandular trichomes more often than off of clones with nonglandular trichomes. This study suggests that cultivars with glandular trichomes may interfere with host searching by A. nigriventris.     

Yield and forage quality of glandular-haired alfalfa under alfalfa weevil (Coleoptera: Curculionidae) and potato leafhopper (Hemiptera: Cicadellidae) pest pressure in Virginia

Cultivars of glandular-haired alfalfa, Medicago sativa L., such as '54H69', are currently available and marketed as being resistant to potato leafhopper, Empoasca fabae (Harris) (Hemiptera: Cicadellidae). 54H69 and a standard, nonglandular-haired alfalfa 'Choice' were evaluated at two locations in Virginia over a 3-yr period. Dry matter yields and concentrations of crude protein and acid detergent fiber were compared at the first, second, and third harvests. Overall, the two cultivars produced similar dry matter yields of comparable forage quality in the absence of insecticides at both locations in each year. Untreated 54H69 did not produce greater dry matter yields than untreated Choice under either light or heavier potato leafhopper pest pressure. Concentrations of crude protein did not vary between the two cultivars at any harvest. Some differences in concentrations of acid detergent fiber were detected between cultivars, but these differences were not consistent among years, harvests, or between locations. Further comparisons between untreated 54H69 and treated Choice were made, but few significant differences were detected in dry matter yields or forage quality. An economic analysis for the study indicated that a grower planting 54H69 would realize less net revenue than a grower planting Choice, largely because of the seed premium for the glandular-haired cultivar and the evident need to treat 54H69 with insecticide for control of alfalfa weevil, Hypera postica (Gyllenhal) (Coleoptera: Curculionidae), and potato leafhopper.     

Mass spectral characterization of fatty acid amides from alfalfa trichomes and their deterrence against the potato leafhopper

A homologous series of N-(3-methylbutyl)amides of normal saturated C14, C15, C16, C17 and C18 fatty acids were identified as major components of glandular trichome extracts from Medicago sativa G98A, an alfalfa genotype resistant to the potato leafhopper, Empoasca fabae. A second homologous series of N-(2-methylpropyl)amides of C14 through C18 normal fatty acids were minor components. Saturated free fatty acids C12, C13, C14, C15, C16, C17 and C18 were present in trace amounts, as was the N-(3-methylbutyl)amide of linoleic acid (C18:2). N-(3-methylbutyl)amides and N-(2-methylpropyl)amides of C14 through C18 fatty acids, along with the N-(3-methylbutyl)amide of linoleic acid, were synthesized and bioassayed for leafhopper deterrence by applying the compounds to the surface of a sachet containing an artificial diet. Leafhoppers were then offered a two-way choice between diet surfaces treated with the synthetic amides or an untreated control. N-(3-methylbutyl)amides and N-(2-methylpropyl)amides of C14 through C18 fatty acids did not deter leafhopper settling in a dose-dependent fashion. In contrast, when tested singly, N-(3-methylbutyl)amide of linoleic acid exhibited dose-dependent deterrence against leafhopper settling. Fatty acid amides localized in alfalfa glandular trichomes likely contribute to leafhopper resistance.     

Morphogenesis in selaginella: auxin transport in the stem

Selaginella willdenovii Baker is a prostrate vascular cryptogam with a dorsiventral stem. At each major branching of the stem apex a dorsal and a ventral angle meristem is formed. The ventral meristem becomes determined as a root, and the dorsal meristem as a shoot. The present investigation examined the distribution and transport of ¹⁴C-indoleacetic acid through stem tissues as a basis for the pattern of meristem determination. Externally applied indoleacetic acid is transported into receiver blocks with a velocity of 12 millimeters per hour. Much of the auxin becomes immobilized in the tissue and is not transported. The polar ratio of auxin transport is approximately 2. Auxin is transported equally on the dorsal and the ventral sides of the stem axis, and the auxin flux in vascular tissue is twice that in the cortex. In the branch junctions twice as much auxin is transported on the dorsal side as on the ventral side, and this is held to be the consequence of the lateral branch vascular tissue connecting with the dorsal and median, but not with the ventral vascular strand of the stem axis.     

Role of the glandular trichomes in resistance of perennial alfalfa to the potato leafhopper (Homoptera: Cicadellidae)

Experiments were performed to elucidate resistance of glandular-haired alfalfa, Medicago sativa L., to the potato leafhopper, Empoasca fabae (Harris). The primary objective was to examine the potential role of the glandular trichomes and stem lignification for imparting resistance to this pest. During free-choice foliar discoloration experiments, the resistant alfalfa clone FGplh13 expressed lower levels of injury than the susceptible P5373 after 13 d. No-choice tests performed using nymphal potato leafhoppers showed higher levels of mortality associated with the resistant glandular-haired clone FGplh13, and no nymphs survived after 48 h. Additional no-choice experiments using FGplh13 and P5373 alfalfa with the glandular and nonglandular trichomes intact or removed show that mortality of nymphs and adults decreased after removal of the glandular trichomes from FGplh13. Nymphal and adult mortality and nymphal development time did not differ on FGplh13 with the trichomes removed and P5373 with the trichomes intact or removed. Firstinstar, and possibly second-instar, potato leafhoppers were entrapped by an exudate produced by the glandular trichomes on the FGplh13 alfalfa. Stem lignification and the number of vascular bundles did not differ between FGplh13 and P5373 alfalfas. The glandular trichomes on FGplh13 alfalfa appeared to provide the major host resistance factor, with resistance to adults being chemically based and resistance to nymphs being chemically and mechanically based. To maintain levels of potato leafhopper resistance, breeders would appear to benefit by continuing to select for the expression of the glandular trichome phenotype.     

Translocation of Assimilates Within and Between Potato Stems

Three aspects of translocation in potato were examined: (i)translocation within stems (ii) translocation between individualstems of a plant (iii) translocation between tubers followinginjection of ¹⁴C sucrose into a single daughter tuber. Assimilatesexported from single leaves of evenly illuminated potato stemsremained confined to the same side of the stem as the sourceleaf in a pattern consistent with the internal arrangement ofvascular bundles in the stem, and tubers borne on stolons verticallybelow the source leaf contained higher concentrations of ¹⁴Cthan those on the opposite side. Consequently ¹⁴C import intothe tubers bore little relationship to tuber growth rates. However,alteration of source/sink relations by pruning stems to a singlesouce leaf resulted in an even distribution of ¹⁴C throughoutthe vascular bundles of the stem and ¹⁴C import into the tubersbore a stronger relationship to tuber growth rates than to thephyllotactic relationship of the tubers with the source leaf. Labelling one stem of a potato plant resulted in little or nomovement of ¹⁴C into tubers on other unlabelled stems. However,removal of the unlabelled stems at ground level induced a significantmovement of ¹⁴C from the labelled stem to the tubers on unlabelledstems, this movement occurring via the mother tuber. Shadingthe unlabelled stems had less effect than stem removal. ¹⁴C sucrose injected into single daughter tubers was translocatedto other tubers on the same stem and also to tubers on a secondstem at the opposite end of the mother tuber. The sucrose wasconverted to starch in these tubers. The results favour the view that each potato stem functionsas an independent unit with potential for assimilate redistributionwithin a stem but with little or no carbon exchange occurringbetween stems, unless under severely altered source/sink patterns. Assimilates, ¹⁴C, autoradiography, potato (Solanum tuberosum L.), tuber growth     

Photosynthetic assimilation of 14C into amino acids in potato (Solanum tuberosum) and asparagine in the tubers

Asparagine is the predominant free amino acid in potato tubers and the present study aimed to establish whether it is imported from the leaves or synthesised in situ. Free amino acid concentrations are important quality determinants for potato tubers because they react with reducing sugars at high temperatures in the Maillard reaction. This reaction produces melanoidin pigments and a host of aroma and flavour volatiles, but if free asparagine participates in the final stages, it results in the production of acrylamide, an undesirable contaminant. ¹⁴CO₂ was supplied to a leaf or leaves of potato plants (cv. Saturna) in the light and radioactivity incorporated into amino acids was determined in the leaves, stems, stolons and tubers. Radioactivity was found in free amino acids, including asparagine, in all tissues, but the amount incorporated in asparagine transported to the tubers and stolons was much less than that in glutamate, glutamine, serine and alanine. The study showed that free asparagine does not play an important role in the transport of nitrogen from leaf to tuber in potato, and that the high concentrations of free asparagine that accumulate in potato tubers arise from synthesis in situ. This indicates that genetic interventions to reduce free asparagine concentration in potato tubers will have to target asparagine metabolism in the tuber.     

Elimination of Mycoplasma-like Organisms from Witches' Broom Infected Sweet Potato

The witches' broom disease of sweet potato has so far been reported only from Asia and the South Pacific. To insure safe distribution of germplasm it is important that vegetative material introduced from regions in which this disease is present has to be free of this pathogen. Meristem tip culture was found to be effective in eliminating MLO from infected sweet potato tissue. On the other hand, all plantlets originating from 1 cm long stem tips immediately below the meristems developed typical witches' broom symptoms as early as 5 weeks after excision of the stem tips. A high percentage of stem tips originating from cuttings previou.sly treated with 25, 50 and 100 mg tetracycline for 3 days developed into healthy plants. Similarly, more than 90 % of plants derived from stem tips excised from cuttings which had received a one-month heat treatment at 37°C remained without symptoms for 9 months.     

The effect of altered sink-source relations on photosynthetic traits and matter transport during the phase of reproductive growth in the annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis>

Annual plants transport a large portion of carbohydrates and nitrogenous compounds from leaves to seeds during the phase of reproductive growth. This study aimed to clarify how reproductive growth affects photosynthetic traits in leaves and matter transport within the plant in the annual herb Chenopodium album L. Plants were grown in pots and either reproductive tissues or axillary leaves were removed at anthesis. Matter transport was evaluated as temporal changes in dry mass (as a substitute of carbohydrates) and nitrogen content of aboveground organs: leaves, axillary leaves, stems and reproductive tissues. Photosynthetic capacity (light-saturated photosynthetic rate under ambient CO₂ concentration), nitrogen, chlorophyll and soluble protein content were followed in the 20^th leaf that was mature at the start of the experiment. Removal of reproductive tissues resulted in accumulation of dry mass in leaves and axillary leaves, and accumulation of nitrogen in stem as nitrogen resorption from leaves and axillary leaves proceeded with time. Removal of axillary leaves proportionally reduced dry mass and nitrogen allocation to reproductive tissues, thus affecting the quantity but not quality of seeds. Removal treatments did not alter the time course of photosynthetic capacity, nitrogen, chlorophyll or soluble protein content during senescence in the 20^th leaf, but changed the photosynthetic capacity per unit of leaf nitrogen according to demand from reproductive tissues. Together, the results indicate that reproductive tissues affected carbon and nitrogen economy separately. The amount of carbon was adjusted in leaves through photosynthetic capacity and carbohydrate export from them, and the amount of nitrogen was adjusted by transport from stem to reproductive tissues. The plant’s ability to independently regulate carbon and nitrogen economy should be important in natural habitats where the plant carbon-nitrogen balance can easily be disturbed by external factors.     

Fluorene and Phenanthrene Uptake and Accumulation by Wheat,Alfalfa and Sunflower from the Contaminated Soil

Polycyclic Aromatic Hydrocarbons (PAHs) are diverse organic contaminants released into the environment by both natural and anthropogenic activities. These compounds have negative impacts on plants growth and development. Although there are many reports on their existence in different parts of plant, their uptake and translocation pathways and mechanisms are not well understood yet. This paper highlights the uptake, translocation and accumulation of PAHs by wheat, sunflower and alfalfa through an experimental study under controlled conditions. Seeds were cultivated in a soil containing 50 mg/kg of phenanthrene and fluorene and their concentrations in plants roots and shoots were determined using a gas chromatograph after 7 and 14 days. The results showed that phenanthrene and fluorene concentrations in the treated plants were increased over the time. PAHs bioavailability was time and species dependent and generally, phenanthrene uptake and translocation was faster than that of fluorene, probably due to their higher K_ow. Fluorene tended to accumulate in roots, but phenanthrene was transported to aerial parts of plants.