Mapping genes for resistance to net form of net blotch and strip rust in barley (Hordeum vulgare L.)

A dihaploid mapping population comprising 65 lines was developed between barley parent varieties Tallon and Kaputar and used to construct a genetic linkage map. This map, comprising 195 amplified fragment length polymorphism and 38 simple sequence repeat markers, was used to identify markers linked to the net form of net blotch (Pyrenophora teres f.sp. teres) and to stripe rust (Puccinia striiformis f.sp. hordei) in barley. The population was screened with five pathotypes of net blotch at the seedling stage in the glasshouse and subjected to a natural inoculation in Hermitage, Queensland. Stripe rust screening was conducted at the adult plant stage in Toluca, Mexico. Analyses of the markers were performed using Mapmanager and Qgene software. One region on chromosome 6H was highly significantly associated with resistance to the net blotch (R2 = 79%). This association was consistent for all pathotypes studied. One region on chromosome 5H was found to be highly significantly associated with resistance to stripe rust (R2= 65%). There are a number of very closely linked markers showing strong associations in these regions, and these markers present an opportunity for marker assisted selection of these traits in barley breeding programs.

Yield response of rice (Oryza sativa L.) genotypes to different types of drought under rainfed lowlands - Part 3. Plant factors contributing to drought resistance

A series of experiments were conducted in drought-prone northeast Thailand to examine the magnitude of yield responses of diverse genotypes to drought stress environments and to identify traits that may confer drought resistance to rainfed lowland rice. One hundred and twenty eight genotypes were grown under non-stress and four different types of drought stress conditions. Under severe drought conditions, the maintenance of PWP of genotypes played a significant role in determining final grain yield. Because of their smaller plant size (lower total dry matter at anthesis) genotypes that extracted less soil water during the early stages of the drought period, tended to maintain higher PWP and had a higher fertile panicle percentage, filled grain percentage and final grain yield than other genotypes. PWP was correlated with delay in flowering (r = -0.387) indicating that the latter could be used as a measure of water potential under stress. Genotypes with well-developed root systems extracted water too rapidly and experienced severe water stress at flowering. RPR which showed smaller coefficient of variation was more useful than root mass density in identifying genotypes with large root system. Under less severe and prolonged drought conditions, genotypes that could achieve higher plant dry matter at anthesis were desirable. They had less delay in flowering, higher grain yield and higher drought response index, indicating the importance of ability to grow during the prolonged stress period. Other shoot characters (osmotic potential, leaf temperature, leaf rolling, leaf death) had little effect on grain yield under different drought conditions. This was associated with a lack of genetic variation and difficulty in estimating trait values precisely. Under mild stress conditions (yield loss less than 50%), there was no significant relationship between the measured drought characters and grain yield. Under these mild drought conditions, yield is determined more by yield potential and phenotype than by drought resistant mechanisms per se. (C) 2002 Elsevier Science B.V. All rights reserved.

A foliar rating system for comparing the resistance of banana cultivars grown as tissue-cultured plantlets in the laboratory to Fusarium wilt

A foliar rating system was developed to assess the progress of Fusarium wilt ( Panama disease) caused by Fusarium oxysporum f. sp. cubense in seven banana cultivars differing in their resistance to race 1 of the pathogen. Plantlets were transplanted into unamended soil naturally infested with the pathogen, soil amended with urea and soil amended with aged chicken manure. A corm invasion score was also developed to assess the accuracy of the foliar symptom score as an indicator of cultivar resistance. On the basis of foliar symptom scores alone, the response of five of the seven cultivars in the chicken manure treatment corresponded to their known field response. However, the response of the other two cultivars, both susceptible to the pathogen in the field, fell into two categories. One had a high foliar symptom score and a correspondingly high corm invasion score, whereas the other had a low foliar symptom score and a high corm invasion score. Breeders need to be aware of the two categories of susceptible response, if inferior breeding material is to be rejected early on in a breeding program.

Facilitating soil-and plant-water research through the use of TDR

The development of TDR for measurement of soil water content and electrical conductivity has resulted in a large shift in measurement methods for a breadth of soil and hydrological characterization efforts. TDR has also opened new possibilities for soil and plant research. Five examples show how TDR has enhanced our ability to conduct our soil- and plant-water research. (i) Oxygen is necessary for healthy root growth and plant development but quantitative evaluation of the factors controlling oxygen supply in soil depends on knowledge of the soil water content by TDR. With water content information we have modeled successfully some impact of tillage methods on oxygen supply to roots and their growth response. (ii) For field assessment of soil mechanical properties influencing crop growth, water content capability was added to two portable soil strength measuring devices; (a) A TDT (Time Domain Transmittivity)-equipped soil cone penetrometer was used to evaluate seasonal soil strengthwater content relationships. In conventional tillage systems the relationships are dynamic and achieve the more stable no-tillage relationships only relatively late in each growing season; (b) A small TDR transmission line was added to a modified sheargraph that allowed shear strength and water content to be measured simultaneously on the same sample. In addition, the conventional graphing procedure for data acquisition was converted to datalogging using strain gauges. Data acquisition rate was improved by more than a factor of three with improved data quality. (iii) How do drought tolerant plants maintain leaf water content? Non-destructive measurement of TDR water content using a flat serpentine triple wire transmission line replaces more lengthy procedures of measuring relative water content. Two challenges remain: drought-stressed leaves alter salt content, changing electrical conductivity, and drought induced changes in leaf morphology affect TDR measurements. (iv) Remote radar signals are reflected from within the first 2 cm of soil. Appropriate calibration of radar imaging for soil water content can be achieved by a parallel pair of blades separated by 8 cm, reaching 1.7 cm into soil and forming a 20 cm TDR transmission line. The correlation between apparent relative permittivity from TDR and synthetic aperture radar (SAR) backscatter coefficient was 0.57 from an airborne flyover. These five examples highlight the diversity in the application of TDR in soil and plant research.

Changes in the properties of a Vertisol and responses of wheat after compaction with harvester traffic

Soil compaction has been recognised as the greatest problem in terms of damage to Australia's soil resource. Compaction by tractor and harvester tyres, related to trafficking of wet soil, is one source of the problem. In this paper an array of soil properties was measured before and immediately after the application of a known compaction force to a wet Vertisol, A local grain harvester was used on soil that was just trafficable; a common scenario at harvest. The primary aim was to determine the changes in various soil properties in order to provide a benchmark against which the effectiveness of future remedial treatments could be evaluated. A secondary aim was a comparison of the measurements' efficiency to assess a soil's structural degradation status. Also assessed was the subsequent effect of the applied compaction on wheat growth and yield in the following cropping season. Nine of the soil properties measured gave statistically significant differences as a result of the soil compaction. Differences were mostly restricted to the top 0.2 m of the soil. The greatest measured depth of effect was decreased soil porosity to 0.4 m measured from intact soil clods. There was 72% emergence of the wheat crop planted into the compact soil and 93% in the uncompact soil. Wheat yield, however, was not affected by the compaction. This may demonstrate that wheat, growing on a full profile of stored soil water as did the current crop, may be little affected by compaction, Also, wheat may have potential to facilitate rapid repair of the damage in a Vertisol such as the current soil by drying the topsoil between rainfall events so increasing shrinking and swelling cycles. If this is true, then sowing a suitable crop species in a Vertisol may be a better option than tillage for repairing compaction damage by agricultural traffic. (C) 2000 Elsevier Science B.V. All rights reserved.