COMMERCIAL UTILISATION OF THE OKRA LEAF MUTANT OF COTTON – THE AUSTRALIAN EXPERIENCE

Abstract                                                                         Back to Table of contents

Effects of the okra leaf (OL) mutant of cotton on plant and agronomic traits, and its interaction with insect pests are reviewed.  While a number of favourable effects e.g. reduction of bollrot, earliness of maturity have often been noted, only in Australia has there been widespread use (circa 50% of total Australian cotton plantings over the last eight years) of OL cultivars.  This commercial acceptance and success of OL varieties in Australia is ascribed to a number of factors.  The Australian-bred OL varieties (known as Siokras) have a high yield and quality potential and are resistant to bacterial blight.  OL provides some resistance to bollworm and mite pests and finally Australian farmers are not prejudiced against OL’s unconventional appearance.

A questionnaire survey of Australian cotton consultants provided insights into commercial experiences and attitudes to OL varieties and their advantages and disadvantages.  The survey revealed OL is well regarded commercially with many consultants citing better pest control, often accompanied by saving of insecticides.  Other favourable features included earliness, ability of OL to recover from setbacks such as hailstorms and high yielding ability.  Disadvantages included susceptibility to Verticillium wilt and high trash content at harvest resulting in reduction of grade.

It is concluded that in Australia at least, OL is a beneficial plant characteristic.  Susceptibility of the Siokra varieties to Verticillium wilt is being overcome by breeding while a gene for glabrousness of leaves and stems is being incorporated to decrease trash levels at harvest and to enhance their resistance to bollworms and mites.

Conclusions

I believe that, viewed as a grand experiment, the Australian experience has conclusively shown that OL cottons can be bred that are commercially at least the equal for yield and intrinsic quality of NL cottons.  Further, under Australian conditions, OL cottons have a HPR advantage over NL while their earlier maturity is also an advantage.  However against these positive features there is an offsetting trend for lower grades associated with mechanically picked OL crops.

On balance I believe that OL cottons have “paid their way” in Australia where there is considerable concern about environmental contamination from the application of insecticides to cotton crops.  Although OL cottons have only enabled modest reductions in insecticide usage and hence have had relatively small economic benefits they have  helped the industry present a “greener” image to the wider community by showing that it is exploring all avenues of reducing spraying.

The response by consultants to the question of what proportion of area they would sow to OL whereby most consultants stated they would prefer a greater proportion of OL than NL crops (Table 3) if there were no yield or quality difference between leaf shapes shows that OL is indeed valued in  commercial cotton circles.

We are also seeking to develop glabrous (ultra-smooth) OL cottons. This combination enhances HPR effects against both Heliothis and mites (Thomson et al., 1987; G. Fitt, L. Wilson, pers. comm.), and we expect (from experience with glabrous, frego bract cottons (Thomson, 1987)) will also help reduce trash content and thereby enhance the grade of OL cottons.

Finally the successes achieved with OL in Australia should encourage renewed attempts by other countries to develop their own adapted, productive OL cottons.  For example boll rot is presumably still as serious a problem in Louisiana and other nearby states in the USA as it was when Jack Jones first started in the sixties on his quest to reduce its severity by the development of OL cottons.  Again white flies have become a major threat in many countries.  Surely this is another situation where OL can be used to advantage.  But, breeders beware! For OL cottons to be acceptable to cotton producers they must be at least the equal of their NL competitors in yield and quality even in those seasons and locations when the environmental stress favouring OL, be it boll rot or white flies, or mites on whatever, is absent.  Indeed to break down prejudices and to make up any grade loss associated with the mechanical picking of OL, such cultivars, at least initially, probably need to be discernibly better than their NL competitors.

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PERFORMANCE OF F1 AND F2 HYBRIDS BETWEEN AUSTRALIAN AND USA COMMERCIAL COTTON CULTIVARS

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Research on F1 cotton hybrids in Australia has produced some promising examples of heterosis but results have been inconsistent.  Given the difficulties of F1 seed production, little research has been conducted in recent years.  However recent research and commercial development in the USA have generated interest in F2 hybrids, particularly with the advent of chemical sterilants.

Two high performing cultivars from the USA  and four from Australia were crossed and the six parents and 15 F2’s were grown under irrigation at two sites in northern NSW in the 1991/92 and 1992/93 seasons and at three sites in 1993/94.  At one of the sites in the first two years the 15 F1’s were also included.

Over the two environments the F1’s averaged 10% more than the parents with the largest advantage (15% higher yield) coming in a lower yielding Verticillium wilt affected trial.  Over the seven environments the F2’s averaged only 3% greater yield than the parents and the best F2 did not significantly outyield the best commercial cultivar.  In the three highest yielding trials the F2’s did not outyield the parents but in the four lower yielding, Verticillium or rain-affected trials the F2’s averaged 6% more.  No commercially important effects were found for fibre quality for the F1’s or F2’s.

The low level of heterosis at the high yielding sites is in accord with previous Australian results and, given the difficulties of F1 hybrid seed production, suggests that at present the only possible uses for hybrids may be with F2 hybrids in lower yielding disease prone or raingrown situations.

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THE AUSTRALIAN WILD SPECIES OF GOSSYPIUM

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Of the 50 species of Gossypium, 17 are endemic to Australia.  These plants are classified in three sections, Sturtia, Hibiscoidea and Grandicalyx, in the endemic subgenus Sturtia.  The first two sections are relatively well‑known through the species G. sturtianum and G. australe, respectively.  Section Grandicalyx, although more restricted in distribution than the other two, contains the greatest taxic diversity.  Historically, this section is poorly known.  Of its 12 species, six were first recognised and described only within the last 11 years.  The twelfth species was collected and recognised for the first time in 1993.  The wild Australian species share a feature unique in the genus, i.e. a terpenoid‑glanded plant but functionally glandless embryo.

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RISK OF ESCAPE OF TRANSGENES FROM COTTON INTO NATURAL POPULATIONS OF AUSTRALIAN GOSSYPIUM SPECIES

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Two endemic species of wild cotton, Gossypium sturtianum and G. australe, occur near the eastern Australian cotton growing regions. These species can be crossed artificially with cotton (G. hirsutum) and experimental doubling the chromosome number of the hybrid overcomes its sterility. It is therefore remotely possible that the same processes might occur in nature, and transgenes from genetically engineered cultivars might leak into natural populations. To evaluate the risk further, we compared the distribution of the wild species with the Australian cotton growing area, and examined their natural breeding system. A survey of herbarium records showed that only a small portion of the distribution of these wild species approaches the cropping area. Isozyme variation in samples from two populations each of G. sturtianum and G. australe gave estimates of outcrossing of about 4% and 40% respectively. The major barriers to gene escape into the wild species via pollen is the remoteness of most of their populations and the unlikelihood and the sterility of interspecific hybridization. Predominant self-pollination in G. sturtianum thus forms an additional barrier, and reduces still further the risk of their escape from the crop into wild populations.

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VOLATILE TERPENES AND TERPENOID ALDEHYDES IN AUSTRALIAN-GROWN GOSSYPIUM HIRSUTUM L. CULTIVARS AND LINES

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Volatile terpenes were assayed in sixteen Gossypium hirsutum L. genotypes, seven of which were also analysed for terpenoid aldehydes. Consistent with previous studies, the major volatile terpenes detected were trans-b-caryophyllene, b-bisabolol, a-humulene, myrcene, g-bisabolene, trans-b-ocimene, a-pinene and b-pinene. Across genotypes, relative percentages of major individual terpenes were similar within, but differed widely between the leaf, square and boll coat. The boll coat was relatively rich in trans-b-ocimene and heliocides H1 and H4. These heliocides together accounted for approximately 50-80% of total terpenoid aldehydes in the boll coat. This finding may have significance for the development of cultivars resistant to Helicoverpa  spp. in Australia.

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