Lemna minor L. grown in Hoagland solution containing zinc (10 ppm) or copper (5 ppm) for 4 d accumulated high levels of these metals. Zinc and Cu accumulation in the test plants was accompanied by a ...specific pattern of change in proline content. The latter showed a steep rise during early stages (peak at 12-24 h) followed by a gradual decline until 96 h of treatment. In a dose-response study, lower metal concentrations induced a sharp rise in proline level with a maximum value at 5 ppm, which declined when the concentration was further enhanced. There was a correspondence between the level of proline and total free amino acids in metal treated plants. The possibility of proline involvement in tolerance mechanisms to heavy metals has been discussed.
The purpose of these experiments was to study the effects of various experimental procedures and conditions on bioassay sensitivity in allelopathic studies. The following factors were considered: ...bioassay methods, choice of test species, the effect of osmotic potential on germination and growth of the test species, growth in dark or in light and the effect of concentrating the extracts in vacuo. Extracts from rape and rye were used in the studies to act as allelopathic agents. Ryegrass appeared as the most sensitive test species when grown horizontally on quartz sand, while radish was the most sensitive test species when grown on filter paper in transparent boxes at a 45 degrees angle. Root length was the most sensitive growth variable measured. Percent germination appeared a more sensitive measure than the speed of germination index, "S", when germinated seeds were counted after one and two days for radish and ryegrass, respectively. A rise in the osmotic potential affected germination more than root length, and radish appeared more sensitive to a rise in osmotic potential than ryegrass. A confounding of germination and root length inhibition may therefore, give misleading results
The effects of tomato and bean rhizospheres on hyphal spreading of the arbuscular mycorrhizal (AM) fungus Glomus mosseae were studied using a soil compartment system in combination with hydrophobic ...polytetrafluorethylene (PTFE) membranes. Both the nylon screen and the PTFE membrane were freely permeable to hyphae but not to roots. Furthermore, the hydrophobic PTFE membrane seemed to be a barrier to the flux of soil solutions containing root exudates. The results show that water soluble exudates of tomato and bean roots greatly stimulate hyphal growth in the soil compartment system used. Moreover, water soluble root exudates of bean exert a clear attractional effect on AM hyphal growth.
Maize (Zea mays L.) breeders have successfully exploited heterosis by crossing inbred lines to develop hybrids. Epistatic effects can contribute to the expression of heterosis for specific hybrids. ...The hybrid B73 x Mo17 was a widely grown hybrid with exceptional performance in the central U.S. Corn Belt during the late 1970s and early 1980s. The objective of this study was to determine if epistatic effects contribute significantly to the performance of B73 x Mo17. With use of the triple testcross design, 100 random F2 plants from B73 x Mo17 were testcrossed to B73, Mo17, and their F2. The 300 testcrosses were evaluated at three locations in 1992 and two locations in 1993. Triple testcross analysis suggested that epistatic effects were important for ear length, number of kernel rows, ear height, and flowering traits. In the analysis of variance, additive by additive effects were not significant for grain yield, whereas additive by dominance and dominance by dominance effects were significant. The additive by additive by environment interaction was more important than additive by additive effects per se for grain yield. Epistatic deviations from the comparison of testcross means suggest that B73 had favorable additive by additive effects for grain yield, barren plants, kernel-row number, ear height, and silk delay. Inbred Mo17 had favorable additive by additive effects for ear length. The presence of significant positive epistatic effects may have contributed to the expression of heterosis and could explain why B73 x Mo17 was an exceptional and widely grown hybrid
Many prime wetlands in North America have been degraded following encroachment by the exotic plant purple loosestrife. Conventional methods are unsuccessful in providing long-term control. Host ...specificity studies demonstrated the suitability of two leaf beetles, Galerucella calmariensis and G. pusilla, as biological weed control agents. Adults oviposited only on plants within the genus Lythrum. The only species other than purple loosestrife where adult feeding and oviposition occurred and that supported successful larval development was winged lythrum. Swamp loosestrife and winged lythrum may be vulnerable to limited attack by newly emerged teneral adults. Evaluation of the potential environmental impact of the two leaf beetles showed that benefits of an introduction outweigh potential risks to winged lythrum or swamp loosestrife. Their field release was approved in 1992.
Populations of rhizosphere microflora of plants which have demonstrated an antagonism toward phytopathogenic nematodes, including velvet bean (Mucuna deeringiand), castor bean (Ricinus communis), ...sword bean (Cannavalia ensiformis), and Abruzzi rye (Secale cereale), were compared to the rhizosphere microflora of soybean. Population densities of total bacteria were significantly lower for young Abruzzi rye, mature velvet bean, and mature castor bean, and fungi from mature velvet bean than for soybean. Population densities of spore-forming bacilli were significantly higher for Abruzzi rye than for soybean. Population densities of coryneform bacteria for mature sword bean and velvet bean were significantly higher than for soybean. All seedling test plants supported significantly higher population densities of chitinolytic fungi than soybean. On mature plants, chitinolytic bacteria were significantly higher on all test plants except velvet bean. Populations of endophytic root bacteria for three of the four test plants were significantly higher than for soybean. Fifty randomly selected bacterial strains from seedlings and mature plants of soybean and each test plant were characterized for various physiological traits associated with rhizosphere competence, including chitinolytic activity, gelatin hydrolysis, production of hydrogen cyanide, starch hydrolysis, phenol oxidation, siderophore production, and production of antifungal compounds (inhibition of Pythium ultimum and/or Rhizoctonia solani). There was a strong trend to increased frequency in each of the physiological tests with bacteria from test plants in comparison to those from soybean. The frequency of starch hydrolysis was up to 24 times greater for strains from test plants than for soybean strains, and siderophore production was up to 22 times more frequent for test plants. These results demonstrate that, compared to soybean, plants with properties antagonistic to phytopathogenic nematodes have a distinct rhizosphere microflora.
Coefficient of parentage (CP) and restriction fragment length polymorphism-based genetic similarity estimates (RFLP-GS) have been proposed as measures of genetic distance in crop species. If these ...measures are to have application in practical breeding, it is important to validate their utility in predicting genetic traits of interest such as heterosis. The objectives of this paper were to (i) estimate heterosis for yield in soybean Glycine max (L.) Merr. adapted to the southern USA, and (ii) predict heterosis by means of CP and RFLP-GS as genetic distance measures. Twenty-four F2 populations were developed by crossing three testers ('Young', 'Centennial', and 'Tracy') to eight contrasting parents, representing a wide range of CP and RFLP-GS. The experimental material was divided into three sets representing the testers above, and was evaluated in eight replications at Clayton and Plymouth, NC, in 1994. Midparent heterosis for yield was 7.9, 4.5, and 7.9% for Sets 1, 2, and 3, respectively. Heterosis was 3.5, 1.6, and 3.0% for 100-seed weight, and 4.1, 5.4, and 13.2% for plant height. The CP and RFLP-GS were highly correlated (r = 0.80, 0.92 and 0.95 for Sets 1, 2, and 3, respectively, P = 0.01), but neither predicted heterosis well for yield averaged across locations because of a large genotype x environment (G x E) interaction. In contrast, CP and RFLP-GS predicted heterosis well for 100-seed weight and plant height in two of the three sets averaged over locations. Our estimates of high parent heterosis for yield (as high as 11% over locations), may justify soybean hybrids as a breeding objective. However, the limited predictive value of CP and RFLP-GS in our study indicates that the identification of favorable heterotic combinations may require extensive field testing
Chrysanthemum yellows phytoplasma (CY), subgroup 16SrI-B, was experimentally transmitted to seedlings of grapevine (Vitis vinifera), cv. 'Barbera' and 'White Muscat' with four leafhopper species: ...Euscelidius variegatus, Euscelis incisus, Macrosteles quadripunctulatus and Scaphoideus titanus. Acquisition was accomplished in three days by 3rd instar nymphs on CY-infected marguerites (Chrysanthemum carinatum). After three weeks, insects were moved to new test plants of the same species and allowed to feed for seven days. They were then transferred to grapevine seedlings for another seven-day-period, and so on as long as they survived. The four leafhopper species transmitted CY to both marguerite and grapevine with the following respective rates: M. quadripunctulatus, 100 (25/25) and 33.3 (9/27); E. variegatus, 59.6 (31/52) and 47.5 (28/59); E. incisus, 95.6 (22/23) and 33.3 (6/18); S. titanus 12.5 (7/56) and 17.0% (8/47). Symptoms on C carinatum were typical of those already described, while on V. vinifera they consisted of growth reduction and abnormal production of axillary shoots bearing small chlorotic leaves. Downward rolling and reddening of basal leaves was occasionally observed on cv. 'Barbera'. RFLP analysis of ribosomal DNA amplified in direct and/or nested PCRs driven by universal or 16SrI group-specific primers confirmed that the phytoplasma present in symptomatic plants, either used as inoculum or obtained by transmission, and in inoculative S. titanus did belong to subgroup 16SrI-B and was identical to the CY reference isolate.
A suction trap and soybean plants (Glicine max (L.) Merr.) were used during the summer season 1988 and 1989 to study the population of flying aphids and their natural infectivity for soybean mosaic ...potyvirus (SoyMV) in North-east Italy. In total 1,419 flying aphids were caught alive, used individually for experimental transmissions on soybean test plants and identified. Catches with suction trap were compared in 1989 with winged aphids directly collected on soybean plants. Great differences in the composition of populations and in their infectivity were obtained using the two catching methods. With the suction trap a great number of aphid species were found, while 99% of aphids collected from the soybean plants belonged to the Aphis genus and, in particular, about 73% were Aphis craccivora Koch. Only one positive transmission with Therioaphis trifolii Monell caught in a 1.7 m suction trap was obtained in 1988; none in 1989, while 10 infected A. craccivora were collected on soybean plants in the same year. At least 40 aphid genera were assayed but no transmission was obtained. Only one of the 5 Aphis species identified among 569 flying aphids collected on field plants in 1989 may be considered actually important as a vector of SoyMV in the Friuli-Venezia Giulia region.
A multi-year programme was performed to assess the effects of atmospheric ethylene on potato (Solanum tuberosum L.) in the vicinity of polyethylene manufacturing plants. There was a strong temporal ...variation of the hourly ethylene concentrations measured close to the sources from 1982 through 1991. Growing seasonal means exceeded 12 mu g m super(-3) ethylene, the threshold for phytotoxic effects under laboratory conditions. Young test plants of potato showed an epinastic response to enhanced levels of ethylene. This response was reversible and did not occur when atmospheric ethylene was not detected. Based on hourly observations for the growing seasons of 1984 through 1991, epinasty occurred on average during circa 5% of the growing season and varied from circa 1% in 1985 to circa 18% in 1991. At night, ethylene concentrations were higher and epinasty was more frequent than during daylight hours. The intermittent exposures to ethylene did not affect tuber yield for the growing seasons of 1982 through 1990. The occurrence of epinasty indicated that ethylene exposure levels in the vicinity of the industrial sources might be sufficiently high to affect sensitive plants.
