HIGH SPEED MEASUREMENTS OF STRENGTH AND ELONGATION

Abstract                                                                         Back to Table of contents

High volume instruments used to class cotton for market are rapid and objective.  Accurate measurements combined with premiums and discounts for specific quality factors provides a good vehicle for cotton improvement. Many textile mills now use instrument results to select bales which match a desired product quality.  Since 1990, the USDA has been providing instrument results to all producers requesting cotton classification.  Herein we describe the fundamental mechanics of high speed testing and compare recent results from two new instrument designs with laboratory measurements.  Correlations with yarn quality from ring and rotor spinning systems are included.

Conclusion

Modern high speed instruments now measure cotton fibre strength with a high degree of precision and accuracy.  Their results compare well with the best available laboratory method to estimate manufactured yarn strength.  These new instruments do not require any special operator skills while laboratory measurements (Stelometer) demand highly skilled technicians for precision and accuracy.

Strength differences between instruments for some cotton varieties has been well documented in the literature.  These differences are traced to a combination of a mass sensing errors (not properly adjusted by micronaire) and a difference in the amount of residual crimp after brushing.  Development programs are underway to reduce or eliminate instrument strength differences.

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POTENTIAL FOR CROP IMPROVEMENT WITH EXOTIC GERMPLASM AND GENETIC ENGINEERING

Introduction                                                                Back to Table of contents

Of all management strategies applied to crop production, selection of a cultivar with enhanced genetics is the most significant.  The extent of genetic expression contributing to the performance of a crop in its environment is often overlooked or taken for granted because of its effectiveness.  For a producer, susceptibility of the crop to a pest is like a thorn in a finger:  the wounded member receives the attention and little thought is given to how well the other fingers function.  Consider, however, the fact that all plant-feeding or infesting organisms not pests on the crop, are not pests because the crop is genetically resistant to them.  An organism is a pest only when it has developed mechanisms to overcome the defensive systems of the crop genotype.  The genetic constituency of the cultivar defines the baseline for stress and pest resistance, and for quality and quantity of the harvested components.  The effectiveness of other management strategies is ultimately confined within the boundaries of this genetic potential.  Thus, the higher the genetically deter­mined base­line, the lower will be the managerial and material input required in the production system…

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USE OF MOLECULAR MARKERS IN COTTON BREEDING

Abstract                                                                         Back to Table of contents

Molecular markers (MM) are a new genetic tool which have the potential to enhance selection efficiency in cotton (Gossypium species) breeding.  There are about 80 morphological traits that have been used to identify 17 linkage groups in G. hirsutum L.  Besides not producing a complete linkage map, most of these morphological markers have major effects on other important quantitative traits and have very limited usefulness in selection.  In many cases the heterozygous condition is not identifiable.  In contrast, MM have the potential of being available in very large numbers; they generally have no effect in themselves on the phenotype, and the heterozygote of MM can be identified.  The three most used MM are RAPDs, isozymes, and RFLPs.  Each MM has its place as determined by the following factors:  research objective, laboratory accessibility, detectable genetic variability, repeatability, co-dominance expression and cost.

The association of RFLPs with 11 quantitative traits in a population of 124 F2/F3 progenies from the cross of ‘Prema” X MD5678ne was evaluated at Stoneville, MS.  Prema is a Western USA cultivar that has low yield and high fibre strength and fineness when grown in the Mississippi Delta.  In contrast, MD5678ne is a near isoline of ‘DES56’, a Delta cultivar, which has high yield and low fiber strength and fineness when grown in the Delta.  The progenies were grown at two Delta locations with two replications each in 1991.  A total of 113 RFLPs exhibiting polymorphism for 118 loci associated with yield, yield components, and fiber properties.  Significant associations, P£0.05, were detected for all traits.  There were 13 significant RFLP effects for yield, which individually accounted for 4.2 to 14.2% of the yield variation.  Significant over-dominance and dominance effects for yield were detected for one and four RFLPs, respectively.  Eight significant positive effects for yield were associated with bands from the Delta parent and five from Prema, indicating some useful yield genes could be obtained from Prema to increase yield in Delta cottons.  About 38 RFLPs were associated significantly with fiber strength and fineness, and 90% of the positive effects were from the Prema parent.

Selection procedures for breeding improvement range from simple backcrossing to multivariate and selection indices.  Once chromosome regions are identified fro important quantitative traits, MM should be a tremendous aid in selecting parents.  Also, MM can be useful in “fingerprinting” cultivars and detecting variability within and among cultivars and genetic populations.

Conclusions

Molecular markers are a valuable new tool which holds much promise of being used in future cotton genetic-breeding investigations.  It offers a relatively simple method to trace genetic sources of useful variability.  Specific chromosome regions with important QTL can be identified and appropriate selection strategies developed.  MM have been used to describe the genomic relationships of Gossypium species and describe the genetic structure of USA cultivars.  MM were not a good tool to select good combiners in hybrid studies.  MM have been shown to be closely associated with QTL with major effects.  Limitations to MM use are cost, availability of quality MM, a G. hirsutum x G. hirsutum map, appropriate software, and the polyploid nature of cotton.  Use of MM in cotton is in its infancy and as more research is conducted many of the limitations will be reduced.

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PROGRESS IN GENETIC ENGINEERING OF COTTON FOR FIBER MODIFICATIONS

Abstract                                                                         Back to Table of contents

Modification of cotton fiber qualities through genetic engineering is the primary focus of the cotton biotechnology program at Agracetus, and the methodologies necessary to accomplish this task have been developed. A cultivar independent (Accell®)* transformation method based on particle bombardment has been refined, and transgenic Delta and Pine 50/90, Pima, Sea Island, and Acala varieties have been generated.  Stable integration of the transgene is shown in the parent and in subsequent progenies.  A transient assay to detect promoter functions using particle bombardment and the marker gene ß-glucuronidase (GUS) has been developed.  A number of cotton gene promoters have been isolated and tested in transgenic plants for their ability to direct fiber-specific gene expression.  Genes from a variety of sources are currently being screened for their ability to modify fiber properties such as strength and length.  In addition, we are assessing various genes for their ability enhance properties such as superior dye binding, absorbency, and thermal properties in cotton fiber.  The resulting high-value transgenic fibers are expected to have a diverse impact on the fiber industry.

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HOST PLANT RESISTANCE TO INSECTS IN COTTON

Introduction                                                               Back to Table of contents

Successful crop management components are built on the cultivar foundation.  Insects are one of the major problems in cotton production and their abundance and species mix influence crop and pest management practices.  Heliothines, mirids, aphids, spider mites, and thrips are common insect pests in most production areas.  Boll weevil is a major pest in humid regions of North, Central, and some parts of South America.  Pink bollworms are pests in certain areas.  Spider mites are generally pests in arid production areas…

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BREEDING FOR RESISTANCE TO BACTERIAL BLIGHT OF COTTON IN RELATION TO RACES OF THE PATHOGEN

Abstract                                                                         Back to Table of contents

Bacterial blight of cotton incited by Xanthomonas campestris pv malvacearum (Xcm) occurs in most cotton producing regions of the world.  Bacterial blight causes a 1% yield loss annually in the USA, and more than 50% loss during epidemics in Africa.  Currently 19 races of Xcm are recognized in the USA.  A shift in races of the pathogen has recently occurred in Texas, from USA races 1, 2 and 7 to the most virulent race 18.  In Africa, new virulent isolates (HVS) of the pathogen have evolved, with HV1 being highly virulent.  At least 22 major genes (B) for resistance to Xcm have been reported.  The majority of these genes are partially to completely dominant for resistance.  Additive and digenic interactions were also reported.  Single resistance genes confer resistance to a few races (vertical resistance), but are vulnerable to the other races of the pathogen.  Thus, different combinations of single B genes and modifiers are important to obtain a stable source of horizontal resistance.  Immunity to all USA races of the pathogen conferred by the B2B3BSm gene combination has been stable for 22 years in the USA. Resistance to the HV1 isolate has been obtained in Africa.   Selection must be made utilizing a compatible race mixture of the pathogen, including virulent races to identify gene combinations that give broad spectrum resistance to many races.  Recurrent selection and the backcross methods have been used to develop horizontal resistance to all races of the pathogen.  Resistant cultivars will reduce disease incidence and severity, and control bacterial blight.

Conclusions

Cotton pathologists and breeders have made considerable advances in breeding for resistance to the bacterial blight pathogen.  High resistance is available in several cultivars throughout the world.  For example, Siokras in Australia; Barakat, Bar, and Baras in the Sudan; Allen, Uk74, UK71, Albar, Reba B50 in East and Central Africa; BJR-734 in India, and the Tamcot cultivars and MAR germplasm in the USA have been developed for resistance to the bacterial blight pathogen.

Continued focus and progress should be made worldwide  to develop cotton cultivars with high and stable resistance to the virulent races of Xanthomonas campestris pv malvacearum.  We agree with Painter’s (1951) view that resistant cultivars are not a panacea for all pest problems. The use of resistant cultivars alone should not be expected to control pests under all conditions or in all locations where the crop may be grown.  Resistant cultivars should be used in concert in integrated pest management  (IPM) programs, which may also include minimal use of pesticides, cultural management, and biological control.  Cotton cultivars with resistance to insects and pathogens, high yielding ability, earliness and improved fiber quality provide the cornerstone for a successful and profitable production system.

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INDUCTION OF SOMATIC EMBRYOGENESIS IN A RANGE OF COTTON CULTIVARS

Abstract                                                                         Back to Table of contents

Diverse genotypes of cotton Gossypium hirsutum L. were evaluated for their capacity to form callus formation and for induction of somatic embryogenesis.  Callus cultures were evaluated from hypocotyl, mesocotyl and cotyledonary leaf explants, harvested from in-vitro germinated seedlings.  For callus induction eight different initiation media were tested.  Maintenance medium for all the cultures was the same, used initially for callusing.

After a series of sub-cultures, calli were transferred into embryo initiation medium which consist of Murashige and Skoog (1962) basal medium, Gamborg B5 vitamins, glucose (3%) and agar (0.6%).  After several weeks, calli were screened for the presence of embryonic units.  The embryogenic tissue were isolated and sub-cultured on embryo-proliferation medium (Finer, 1988).  After 4-6 weekly sub-culture, three different cell lines were obtained from the hypocotyl derived callus in cv PKV 081, which was previously identified as being highly embryogenic.  Microscopic preparation showed the presence of somatic embryos in various stages of development.  In one cell line derived from Coker 417-68, differentiation of a shoot was observed.

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MECHANISMS OF DISEASE RESISTANCE IN GOSSYPIUM SPECIES AND VARIATION IN VERTICILLIUM DAHLIAE

Introduction                                                                Back to Table of contents

Studies on Verticillium wilt of cotton have concentrated on host resistance to the pathogen and on genetic variation and mechanisms of virulence in Verticillium dahliae.  The resistance of cotton to V. dahliae depends on a number of anatomical and chemical characteristics that occur both constitutively and as active defense responses.  Terpenoid phytoalexins and condensed tannins appear to be especially important in defense reactions. Resistance in cultivars correlates with the concentration of tannin in leaves, the rate of phytoalexin synthesis in xylem vessels in response to infection, and the toxicity of the phytoalexins and tannins to V. dahliae.  Enzymes that appear to have critical roles in regulating terpenoid and tannin synthesis have been characterized, and partial genes coding for these enzymes have been cloned.  Strategies for manipulation of these genes and for introduction of foreign genes from other malvaceous plants to improve cotton resistance to wilt are being developed.  Vegetative compatibility (V-C) tests and isozyme analyses have been used to show that there are four genetically isolated populations (V-C groups) within the species V. dahliae, and that each V-C group has at least two subgroups.  The V-C groups and subgroups vary in geographical occurrence, virulence to cotton and other crops, and sensitivity to fungicides.  The severe defoliating form of Verticillium wilt found in the southwestern USA and in Peru is caused by isolates in the V-C 1A subgroup, whereas wilt in Europe and Asia is caused by isolates in the V-C 2 group.  Wilt in Australia is incited by isolates in the V-C 4B subgroup.  Isolates in the V-C 1A group show unique resistance to the antibiotic sanguinarine and have the ability to induce large accumulations of ammonium ions in cotton leaf tissues.  Identifying the specific V-C groups responsible for wilts in a given field and geographical area should allow better recommendations for disease control measures…

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