Greenhouse Screening Research Articles

Improving crop competitive ability using allelopathy — an example from rice

AbstractThe objective of this paper is to review the possibilities for using allelopathy to improve overall crop competitive ability against weeds, using rice, Oryza sativa, as an example. Laboratory, greenhouse and field screenings for allelopathy and overall weed suppression in rice have been made and allelopathic rice germplasm has been identified in laboratory and greenhouse screening. Field experiments revealed that allelopathy accounted for 34% of overall competitive ability in rice. For strongly allelopathic cultivars, allelopathy was the dominant factor determining competitive ability. Based on the results of the screenings, recombinant inbred line populations were developed for identification of quantitative trait loci (QTL) controlling allelopathy. Populations of recombinant inbred lines (RILs) were derived through single‐seed descent from crosses between varieties with contrasting behaviour and QTL controlling allelopathy were identified. For rice and most probably also for other cereal crops, the findings presented can explain the limited success in previous breeding programmes for weed competition, as allelopathy has never before been acknowledged as an important factor. The findings in allelopathy indicate that it is possible to improve allelopathy in rice using marker‐assisted selection. Optimizing allelopathy in combination with breeding for competitive plant types could result in crop cultivars with superior weed‐suppressive ability.

In vitro, greenhouse and field assessments of cassava lines for resistance to anthracnose disease caused by Colletotrichum gloeosporioides f.sp. manihotis.

Fifty-three cassava lines were selected from breeding populations at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria and screened in vitro for resistance to cassava anthracnose disease (CAD). The in vitro inoculation of stem cuttings with the fungus Colletotrichum gloeosporioides f.sp. manihotis showed significant differences (p +/- 0.05) in acervuli production and in the sensitivity of the cassava lines to the fungal infection after 7 days of incubation at 25 degrees C. Cassava lines 88/01084, 91/00595, 91/00475, 91/00344, 91/00684, 91/00313, 91/00422, and 91/00344 were highly resistant, with necrotic lesion sizes less than 7 mm. In contrast pedigree lines 88/02549, 89/0008, 91/00390 and 91/00402 were highly susceptible with the largest necrotic lesion size being greater than 20 mm. Ten cassava lines from the in vitro screening that showed varying levels of resistance to CAD were selected, based on their flowering abilities for diallel hydridization trials, and were further screened in greenhouse and field trials for CAD resistance. The greenhouse and field screening showed significant varietal differences (p +/- 0.05) in sensitivity to the fungus. In all cases, the progeny lines showed correlated levels of resistance irrespective of the type of screening or assessments. Correlation analysis of the in vitro, greenhouse and field assessments showed that there was a good correspondence among all three methods of evaluating for CAD.

Reliability of field, greenhouse and cut‐shoot screening procedures for evaluating susceptibility of Scots pine to Melampsora pinitorqua

The reliability of field, greenhouse and cut‐shoot screening procedures for the assessment of the susceptibility of Scots pine (Pinus sylvestris) genotypes to Melampsora pinitorqua, the causal agent of twisting rust, was evaluated. Fourteen genotypes evaluated on half‐sib progenies after natural infection of 2‐year‐old plants showed comparable rank for rust susceptibility with those evaluated after controlled inoculation of 1‐year‐old seedlings in a greenhouse. The dynamic of pycnia production was assessed at 2‐day intervals in cut‐shoot assays under controlled conditions, giving reliable rankings of Scots pine genotypes between years and being significantly correlated with rust susceptibility assessed after natural infection of 2‐year‐old progenies. These results underlined the importance of some critical factors such as inoculum load and host phenology in the reliability of susceptibility evaluations assessed under different experimental conditions. Artificial inoculation of 1‐year‐old seedlings in greenhouse experiments could provide a useful early test for the management of Scots pine breeding programmes and study of inheritance of twisting rust susceptibility. However, in specific investigations the cut‐shoot assay would constitute a reliable laboratory test for studying host–pathogen interactions and the variability in pathogenicity of Melampsora pinitorqua populations.

Greenhouse screening of the saprophytic resident microflora for control of leaf spots of wheat (Triticum aestivum)

Ten microorganisms of the epiphytic microflora of wheat leaves in Buenos Aires Province, Argentina, were evaluated under greenhouse conditions as potential biocontrol agents of the pathogensAlternaria triticimaculans, Bipolaris sorokiniana, Drechslera tritici-repentis andSeptoria tritici in two application sequences (prior to or together with the pathogens). The antagonists significantly reduced the expression of the diseases on wheat plants compared with control plants not inoculated with the antagonists. Maximum percentage of reduction of the necrotic lesion area (NLA) (40–55%) ofS. tritici resulted whenCryptococcus sp.,Rhodotorula rubra andPenicillium lilacinwn were sprayed on leaves prior to inoculations with the pathogen.Bacillus sp.,Cryptococcus sp.,Fusarium moniliforme var.anthophylium,P. lilacinum andR. rubra reduced significantly (34–52%) the NLA ofB. sorokiniana in both of the application sequences. The best antagonistic effect againstA. triticimaculans was shown byAspergillus niger, Bacillus sp.,Chaetomium globosum, F. moniliforme var.anthophylium andNigrospora sphaerica, with a NLA reduction from 21% to 35% in the co-inoculation or in the sequential application. All microorganisms exceptN. sphaerica performed better than the control againstD. tritici-repentis. The area under disease progress curve (AUDPC) of the pathogens appeared to progress similarly, but at lower values, in treated plants than in untreated controls. The two yeasts and the bacteria decreased AUDPC to 50–55% ofS. tritici andB. sorokiniana compared with the control in both application sequences, whereas the maximum efficacy againstA. triticimaculans was reached byN. sphaerica andA. niger for the sequential application and byF. moniliforme var.anthophylium for the co-inoculation. If the parasitism occurs also in nature, application of antagonists for biological control might provide the opportunity to compete with the pathogens and regulate their colonization in wheat leaves.

Economic evaluation of insect-proof screens for preventing tomato yellow leaf curl virus of tomatoes in Israel

The major cause of crop loss of greenhouse produce in Israel is a virus disease vectored by the tobacco whitefly, Bemisia tabaci. An exclusion screen developed to minimise B. tabaci entry to greenhouses has been adopted nationwide. To examine the cost effectiveness of greenhouse screening, a model was developed relating the aerial density of Bemisia tabaci, the volume of tomato production, the price of tomatoes, and the cost of screening greenhouses. Using this model, the costs and benefits of insect-exclusion screens are examined from grower and consumer perspectives. The introduction of screening has proved cost-effective for both consumers and growers. As a result of the smaller tomato crops from 1980 to 1990, Israeli consumers spent on average between $r;15 M and $32 M per year more on tomatoes than they would have if the greenhouses had been screened at that time. The introduction of screening resulted in windfall profits for early adopters of the technology who benefited from the price inflation with their larger yields. Because the cost of harvesting declined with yield and the demand for tomatoes is inelastic, the average regional revenue varied very little from 1980 to 1990, except for 1986. From 1990 to 98, the average regional value continued to remain stable as the proportion of screened winter crop increased to 100% compensating for the declining productivity of the unscreened crop.

Evaluation of Glyphosate Resistance in Transgenic Lettuce

Six transgenic `South Bay' lettuce lines (Lactuca sativa L.) with elevated levels of 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS) were evaluated for tolerance to the herbicide glyphosate. The six lines were selected from ≈150 independent transformation events using an Agrobacterium tumefaciens system. Three assay methods were used to identify gene expression with regard to glyphosate resistance. Leaf disks of the transgenic lines were cultured on media containing 0 to 1280 μm glyphosate. Leaf disks of the control had lower dry weight (DW) at 40 μm and greater glyphosate than all the transgenic lines. The transgenic lines continued to grow even at 1280 μm. Plants 21 days old were sprayed in the greenhouse with rates of glyphosate at 0 to 35.84 kg·ha-1. DW of all the lines were similar to the control, with a few exceptions, at glyphosate concentrations from 0 to 0.56 kg·ha-1. At 2.24 to 8.96 kg·ha-1 all of the transgenic lines had DW greater than the control, while at 17.92 and 35.84 kg·ha-1 only B-32, B-33, C-3, and C-14 had DW greater than the control. The resistant line from the greenhouse experiment, B-32, grew normally in field trials at the highest glyphosate rate, 17.92 kg·ha-1, while control plants died at 0.56 kg·ha-1 glyphosate. Lines A-11 and C-3 had lower DW than B-32 at 2.24 kg·ha-1 glyphosate and greater. While leaf disk assays can identify potential transformed lines expressing the EPSPS and glyphosate oxidase (GOX) gene, and greenhouse screening can evaluate seedling vigor after glyphosate application, field trials are necessary to evaluate plant growth and yield through the growing season. Chemical name used: N-(phosphono-methyl) glycine (glyphosate).

Zinc toxicity thresholds for important reclamation grass species of the western united states

AbstractHeavy metal toxicity thresholds for plant species that are used in reclamation activities in Western North America have not been established. As a result, ecological risk assessments must rely on toxicity thresholds established for agronomic species, which may differ from those of species used in reclamation. As a result, risk assessors have the potential for classifying sites as phytotoxic to perennial, nonagronomic species and calling for intensive remediation activities that may not be necessary. The objective of this study was to provide a better estimate of Zn toxicity thresholds for five grass species that are commonly used in reclamation efforts in the western United States. We used a greenhouse screening study where seedlings of redtop (Agrostis albas Roth), slender wheatgrass (Elymus trachycaulus [Link] Gould ex Shinners var. Pryor), tufted hairgrass (Deschampsia caespitosa (L.) Beauv.), big bluegrass (Poa ampla J. Presl var. Sherman), and Great Basin wildrye (Leymus cinereus [Scribn.&Merr.] A. Löve var. Magnar) were grown in sand culture and exposed to concentrations of soluble Zn ranging from 0.6 (control) to 500 mg/L. We determined six measures of toxicity: the 50‐d LC50, 50‐d EC50‐plant, 50‐d EC50‐shoot, 50‐d EC50‐root, PT50‐shoot, and PT50‐root. Results suggest that these grass species have higher Zn phytotoxicity thresholds (PT50s) than agronomic species reported in the past. Roots appeared to be more susceptible to the toxic effects of zinc (as indicated by reduced growth) than shoots. This observation suggests that measures of EC50‐shoot alone may be misleading for these species where growth reduction is manifested primarily in roots. Values of EC50‐plant for these reclamation grasses were between 84 and 222 mg/L. Measured PT50‐shoot values were between 2,449 and 5,026 mg/L. These thresholds should be more useful for risk assessors than those based on agronomic crops that are currently used.

Screening Plant Species for Growth on Weathered, Petroleum Hydrocarbon-Contaminated Sediments

ABSTRACT Rapid and cost-effective techniques are needed to select plant species and genotypes for use in phytoremediation, vegetative capping, or revegetation at hazardous waste sites. A greenhouse screening procedure to aid the selection of plant materials would help increase success and decrease the cost. Twenty-nine vascular plant species were compared for growth in weathered sediments contaminated with petroleum hydrocarbons. An uncontaminated reference soil was used to estimate relative seedling growth in stressed and unstressed conditions. Plants were grown in a greenhouse and harvested at 60 and 180 days after planting to estimate variation in seedling growth and full-season growth. Plant growth characteristics measured included height, aboveground biomass, root biomass, root diameter, root-length density, and root surface area density. Concentration of total petroleum hydrocarbons (TPH) was estimated at the final harvest. Considerable variation existed among species for all characteristics except TPH concentration. Under the conditions and length of this trial, no variations in rates of TPH degradation were detected. In general, plant growth was stunted in the contaminated soil compared with the uncontaminated soil; however, differences among plant species for relative seedling growth indicated that they varied in their tolerance to the petroleum hydrocarbon-contaminated soil. For example, tall fescue, Festuca arundinacea, seemed tolerant to the contaminated soil, whereas barley, Hordeum vulgare, seemed sensitive. Comparison of results from the 60- and 180-day harvests suggested that a short-season greenhouse screening could aid selection of species for planting in contaminated soil, if plant growth results are interpreted along with information on the life history characteristics of the species under consideration.

A Greenhouse Screening Protocol for Fusarium Root Rot in Bean

Root rot, caused by Fusarium solani f.sp. phaseoli, is a serious disease of bean for which successful control has been elusive. Genetic resistance to the pathogen is considered quantitative and is strongly influenced by environmental factors. To reduce environmental variation and facilitate selection in earlier generations, an accurate, consistent, and nondestructive greenhouse screen was developed for the evaluation of Fusarium root rot resistance in bean. We describe a protocol that involves the germination of seedlings in perlite, inoculation of roots and hypocotyls 10 days after planting and evaluation within 4 weeks. The accuracy of this greenhouse screen was confirmed by demonstrating significant correlations between greenhouse and field ratings. Two experiments that included 24 and 21 diverse bean genotypes, respectively, were performed in the greenhouse and the ratings were correlated with field ratings over two growing seasons. Correlation coefficients between the greenhouse and field ratings were significant and as high as 0.99. Numerous genotypes can be evaluated within a short time for relatively minimal costs and labor. Furthermore, once roots have been rated and dipped in fungicide, plants can be transplanted for production of seed. This simple, rapid, and inexpensive protocol reduces environmental variation inherent to field ratings, thereby more accurately representing physiological resistance while maintaining a close association with observed field ratings.

Marker‐assisted selection of common beans for resistance to common bacterial blight: efficacy and economics

AbstractThe possibility of using random amplified polymorphic DNA (RAPD) markers previously mapped in the common bean PC50/XANI59 population to select for resistance to common bacterial blight (CBB) in different populations was examined. Two out of 02 selected RAPD markers were polymorphic in HR56 and W0633d, the parental lines used in this experiment. Cosegregation analysis of the two polymorphic markers and disease reaction in a recombinant inbred (RI) population derived from HR67/W1744d confirmed that one of the two RAPD markers, BC420900, was significantly associated with a major quantitative trait locus‐conditioning resistance to CBB in HR67. This locus accounted for approximately 51) of the phenotypic variation. The RAPD marker was transformed into a sequence characterized amplified region (SCAR) marker and used for selection in a different population derived from ‘Envoy’/HR67. Prediction for resistance to CBB with the BC420.990 SCAR marker was 94.2% accurate in this population. A comparison between marker‐assisted selection (MAS) and conventional greenhouse screening showed that the cost of MAS is about one‐third less than that of the greenhouse test.

A Greenhouse Method for Screening Grapevine Seedlings for Resistance to Anthracnose

Screening for resistance to Elsinoe ampelina (de Bary) Shear, causal agent of grape anthracnose, in grapevine seedlings is commonly conducted by natural infection over 3 to 4 years in the vineyard. The objective of this research was to develop a greenhouse screening method for selecting grapevine seedlings with resistance to anthracnose. Spores of E. ampelina were obtained from 3- to 4-week-old cultures on potato dextrose agar. Inoculum concentrations ranging from 1.3 × 103 to 1.3 × 107 E. ampelina conidia per mL were evaluated and 106 conidia/mL was optimum. The time of incubation of seedlings in a moist chamber after inoculation varied from 24 to 120 hours with 24 to 72 hours resulting in good symptom development. Temperatures in the moist chamber from 16 to 32 °C were evaluated and the most consistent results were obtained at 20 to 28 °C. The most effective method for selecting anthracnose-resistant grape seedlings in the two-to-three true-leaf stage was misting the seedlings with a suspension containing 106 conidia/mL in water and placing the inoculated seedlings in a moist chamber at 24 °C for 48 hours, followed by 8 days on a greenhouse bench. Resistant seedlings from the greenhouse screening (those with <10 foliar lesions) were transplanted into the vineyard and found to be resistant to anthracnose infection under rainy, humid conditions. This greenhouse procedure for selecting grapevine cultivars and breeding lines with resistance to anthracnose is accurate, economical, and labor-saving.

ZINC TOXICITY THRESHOLDS FOR IMPORTANT RECLAMATION GRASS SPECIES OF THE WESTERN UNITED STATES

Heavy metal toxicity thresholds for plant species that are used in reclamation activities in Western North America have not been established. As a result, ecological risk assessments must rely on toxicity thresholds established for agronomic species, which may differ from those of species used in reclamation. As a result, risk assessors have the potential for classifying sites as phytotoxic to perennial, nonagronomic species and calling for intensive remediation activities that may not be necessary. The objective of this study was to provide a better estimate of Zn toxicity thresholds for five grass species that are commonly used in reclamation efforts in the western United States. We used a greenhouse screening study where seedlings of redtop (Agrostis albas Roth), slender wheatgrass (Elymus trachycaulus [Link] Gould ex Shinners var. Pryor), tufted hairgrass (Deschampsia caespitosa (L.) Beauv.), big bluegrass (Poa ampla J. Presl var. Sherman), and Great Basin wildrye (Leymus cinereus [Scribn.&Merr.] A. Löve var. Magnar) were grown in sand culture and exposed to concentrations of soluble Zn ranging from 0.6 (control) to 500 mg/L. We determined six measures of toxicity: the 50-d LC50, 50-d EC50-plant, 50-d EC50-shoot, 50-d EC50-root, PT50-shoot, and PT50-root. Results suggest that these grass species have higher Zn phytotoxicity thresholds (PT50s) than agronomic species reported in the past. Roots appeared to be more susceptible to the toxic effects of zinc (as indicated by reduced growth) than shoots. This observation suggests that measures of EC50-shoot alone may be misleading for these species where growth reduction is manifested primarily in roots. Values of EC50-plant for these reclamation grasses were between 84 and 222 mg/L. Measured PT50-shoot values were between 2,449 and 5,026 mg/L. These thresholds should be more useful for risk assessors than those based on agronomic crops that are currently used.

Comparison of the reaction of tomato lines to infection by tomato yellow leaf curl begomovirus in Lebanon

Tomato yellow leaf curl begomovirus (TYLCV), transmitted by the whitefly Bemisia tabaci, is epidemic in Africa, the Middle East and South‐East Asia. It is also reported in some European countries and the American continent. In Lebanon, it is the major limiting factor for summer and autumn production of tomato. Comparison of the nucleotide sequence in the intergenic region with other reported leaf curl viruses showed the Lebanese TYLCV isolate to be closely related to Egyptian, Israeli and Jamaican isolates (94–96% identity). However, it is not closely related to isolates from Sardinia, Spain and Thailand, or to tomato leaf curl isolates from India, Taiwan and Australia. In field and greenhouse screening tests conducted for 5 years on 67 tomato lines, several were identified as promising. TY‐Carla, PSR and RS lines were among the most promising with determinate growth, while S&G 143 and the DR lines were the most promising with semi‐determinate and indeterminate growth, respectively. Virus concentrations in most, but not all, tolerant tomato lines were significantly lower than in the susceptible lines. None of the lines tested was immune to the virus. A survey of TYLCV alternative hosts on at least 58 plant species, using nucleic acid hybridization, showed that Amaranthus sp., Malva sp., Sonchus oleraceus, Plantago sp., Solanum melongena, Phaseolus vulgaris and Mercurialis annua may play an important role in the epidemiology of TYLCV in Lebanon. Mercurialis annua is a newly reported host for TYLCV.

Septoria Leaf Spot of Stevia rebaudiana in Canada and Methods for Screening for Resistance

The herb Stevia rebaudiana is a potential source of low‐calorie sweeteners. In 1995, a severe leaf spot and blight was observed in stevia production fields and research plots in the provinces of Ontario and British Columbia, Canada. The disease was characterized by angular, shiny, olive‐grey lesions that rapidly coalesced and were often surrounded by a chlorotic halo. Leaves quickly became necrotic and often dropped off the plant. The disease progressed upwards in the foliage during the growing season. A Septoria sp. was isolated from diseased leaves. Ten isolates (five from each of the two provinces) of the Septoria sp. were compared with respect to conidial size. Across isolates, conidia lengths and widths overlapped (grand means for length and width were 71.4 μm and 1.4 μm, respectively). Conidiogenesis was holoblastic. Morphological characteristics and disease symptoms were similar to those of Septoria steviae, previously reported only from Japan. It was concluded that the Canadian isolates belonged to S. steviae. Isolates from Canada did not differ significantly from one another with respect to effects of temperature on colony growth or germination of conidia. Optimum temperatures for these parameters were between 20 and 25°C. In field trials, the pathogen was shown to successfully over‐winter in diseased leaf tissue. In order to develop procedures for identification of resistant germplasm and greenhouse screening of candidate fungicides, effects of leaf wetness period, inoculum concentration, and plant age on disease development were determined. Thirty‐six hours of leaf wetness were required for consistent development of leaf spots. Inoculum concentrations of 5 × 105 conidia/ml or more were required to produce high disease severities; 6‐week‐old plants were more susceptible than older plants. In the growth chamber, greenhouse, and field trials, germplasm selections with high levels of resistance to S. steviae were identified. This is the first report of resistance to this disease in S. rebaudiana.

Comparison of Leaf Disk, Greenhouse, and Field Screening Procedures for Evaluation of Grape Seedlings for Downy Mildew Resistance

This research was conducted to compare an in vitro leaf disk technique with greenhouse and field evaluations for screening large populations of grape (Vitis sp.) seedlings for downy mildew (Plasmopara viticola Berk. & Curt. Berl. & de Toni) resistance. Seedlings produced by crossing resistant × resistant, resistant × susceptible, and susceptible × susceptible parents were rated for sporulation, chlorosis, and necrosis. Leaf disk sporulation ratings at the first and second rating were highly correlated with the second sporulation rating in the field. Necrosis ratings from the leaf disk evaluations were significantly correlated with field necrosis ratings, but leaf disk chlorosis ratings were not correlated with field ratings. Some correlations, including evaluations of chlorosis, between the greenhouse and field ratings were highly significant. Seedling ratings of 0 or 1 for sporulation, chlorosis, and necrosis in the leaf disk assay agreed with field evaluations 85.6% of the time vs. 80.3% agreement between greenhouse and field ratings. Sporulation was the parameter most highly correlated between leaf disk or greenhouse and field evaluation of resistance. The leaf disk procedure appeared to be a good predictor of field resistance, and is more practical than the greenhouse method for screening large populations.

First source of resistance in durum wheat to Hessian fly (Diptera: Cecidomyiidae) in Morocco

Hessian fly, Mayetiola destructor (Say), is the major pest of wheat in North Africa. In Morocco, durum wheat (Triticum turgidum L. subsp. durum (Desf). Husn.) losses due to this pest have been estimated at 32%. Genetic resistance is the only economical and practical means of controlling this insect. Field and greenhouse screening of durum wheat genotypes resulted in the identification of one source of resistance to Hessian fly in Morocco. This is the first source of durum wheat Hessian fly-resistance identified in Morocco. This source of resistance expresses a medium level of antibiosis against first-instar Hessian fly larvae; about 25% of the larvae survive on resistant plants. The deployment of varieties that allow for larval survival on resistant plants should reduce selection for biotype development. This source of resistance is being used by CIMMYT/ICARDA and Moroccan breeders to develop resistant durum wheat varieties.

`Duplin' and `Sampson' Southern Highbush Blueberries

`Duplin' (NC 1852) and `Sampson' (NC 2675) are southern highbush blueberry genotypes that bloom with `Croatan', the predominant highbush cultivar in eastern North Carolina, and ripen with or slightly later than `Croatan'. `Duplin' appears to have flower bud hardiness similar to standard highbush cultivars when grown at intermediate elevations in the mountains of western North Carolina. `Sampson' appears to be bud hardy primarily in the piedmont and coastal plain of North Carolina. Yield overall has been equal to or better than `Croatan' and `O'Neal'. Fruit size, color, picking scar, firmness and flavor are superior to `Croatan'. Both `Duplin' and `Sampson' were found to be tolerant to stem blight caused by Botryosphaeria dothidea in greenhouse screening in North Carolina. `Sampson' also appears resistant to Botrytis blossom blight in Oregon. `Duplin' and `Sampson' are recommended for trial as potential replacements for `Croatan' in the commercial blueberry region of eastern North Carolina.

Tissue Culture Methods for the Screening and Analysis of Putative Virus-Resistant Transgenic Potato Plants

ABSTRACT Following regeneration, putative virus-resistant transgenic plants are usually transferred from tissue culture to a greenhouse or growth chamber to screen for resistance to infection and disease development using mechanical, graft, or insect vector inoculation methods. To reduce initial screening costs and time, we developed mechanical and graft inoculation methods suitable for tissue culture use. The in vitro methods were validated by comparing them with similar greenhouse screens using putative potato virus Y strain o (PVY degrees ) replicase-mediated resistant regenerants of the potato cultivar Atlantic. Five transgenic lines were tested, with similar results obtained from in vitro and greenhouse experiments. Two of the transgenic lines, A1 and A3, showed the greatest resistance to PVY degrees infection, as indicated by low enzyme-linked immunosorbent assay values and infection rates. In vitro mechanical inoculation methods were also used to infect wild-type tomato and tobacco plants with cucumber mosaic virus and potato virus Y. Potato plants were also infected with the phloem-restricted potato leafroll virus, a low-titer virus, using in vitro graft inoculation methods. These results suggest the potential usefulness of these simple, effective, and economical techniques for screening large numbers of putative virus-resistant plants.

Discovery of a Primisulfuron-Resistant Shattercane (Sorghum bicolor) Biotype

Shattercane seeds were collected in 1992 from 16 fields in southeastern and south-central Nebraska that had been treated for 3 consecutive yr with primisulfuron or nicosulfuron. Approximately 11,200 plants, from a total of 16,000 seeds (1,000 per field) planted, were evaluated for primisulfuron resistance in greenhouse screening. A single resistant plant was found. No other primisulfuron resistance was found in evaluating an additional 1,718 plants from the remaining seeds collected in 1992 from the field where the original resistant plant was found. Shattercane seeds were collected in 1993 and 1994 from that field. All 17,038 plants from these collections were killed by primisulfuron. A field assessment of 118 descendents of the original resistant plant (S1progeny) produced three responses to primisulfuron at an approximate 1:2:1 ratio: (1) death; (2) some interveinal chlorosis; or (3) no visible injury or symptoms. Discovery of this resistant shattercane biotype indicates the potential for selection of primisulfuron-resistant shattercane populations under field conditions.

MODELLING OF TRANSPORT PHENOMENA NEAR GREENHOUSE SCREENS

Experiments were conducted with the aim to examine the airflow characteristics (permeability and porosity) of some currently employed greenhouse screens, and to obtain equation coefficients for estimation of free convection heat transfer between heating pipes and the air, and horizontal screen and the air. Experiments demonstrated that thermal and shading screens have permeabilities of approximately 10-11m2 and their porosities are less than 10EFree convective heat transfer coefficient () for the upper and lower screen surface is given, respectively by =3.24 ΔT0.30 and =2.16 Δ0.31.