Jaquie Mitchell

The use of seedling leaf death score for evaluation of drought resistance of rice

Abstract Leaf death (drought score) of seedlings has been extensively used in rice breeding programs as a selection index for drought resistance. This paper investigates the factors that affect the drought score of rice cultivars exposed to water stress during the vegetative stage and the relationship between drought score and yield. Two upland experiments consisting of 21 and 8 cultivars, respectively, were subjected to water deficit during vegetative growth. In the first experiment, the plants were grown under three stress conditions and pre-dawn leaf water potential, epidermal conductance, canopy light interception and drought score were determined during the stress period. In the second experiment leaf area, water use, dry matter growth and grain yield were also examined. Large genotypic variation in plant size existed in both experiments and this affected development of water stress and drought score. The cultivars with large plant size tended to have high drought score. A significant positive correlation ( r = 0.74∗∗ , n = 63) existed between drought score and light interception. However, analysis of covariance indicated that genotypic variation in drought score existed after the effect of plant size was taken into account. Significant genotypic variation in leaf water potential existed and this was negatively correlated with drought score ( r = 0.75∗∗ , n = 42). Cultivars with large leaf area tended to lose more green leaves during the stress period, but this did not result in reduced dry matter growth during or after the stress period. Green leaf area remaining after the stress period, and not amount of dead leaf (drought score) determined subsequent dry matter growth ( r = 0.45∗∗ , n = 32), and there was no correlation between drought score of seedlings and grain yield. Thus for use in a vegetative screening program for drought resistance, estimates of drought score should be accompanied by estimates of plant size.

Evaluation of a reduced-tillering (tin) gene in wheat lines grown across different production environments

Abstract. Post-anthesis water deficit and increasing vapour pressure deficit are common and can result in reduced grain yield and the development of small or shrivelled wheat kernels (screenings) that reduce grain value. Previous studies suggest incorporation of a tiller inhibition (tin) gene to restrict tiller number and thereby slow water use and promote the development of larger, fertile spikes to increase kernel weight. This paper reports on the influence of the tin gene on grain yield and screenings in multiple wheat genetic backgrounds assessed in field experiments in 2005 and 2006. Across environments, grain yield ranged from 0.90 to 5.50 t/ha and screenings from 4 to 20%. The effect of tin on grain yield and screenings varied with environment and genetic background. Grain yield was unchanged in tin lines derived from varieties Brookton, Chara, and Wyalkatchem assessed in southern Australian environments. However, there was a 31 and 10% advantage of free-tillering over tin-containing Silverstar lines for the 2005 western and 2006 northern experiments, respectively, resulting in an average 12% reduction in grain yield of Silverstar tin lines. In northern experiments, where screenings ranged from 4 to 12%, Silverstar-based tin lines produced significantly fewer screenings than free-tillering sister lines. Reduction in screenings was associated with a higher kernel weight (+10%) and a tendency for lower grain yield, although individual Silverstar tin progeny with grain yield equivalent to the parent were readily identified. The incorporation of the tin gene has considerable potential to reduce the incidence of screenings in commercial wheat crops. Variation in grain yield associated with the tin gene was dependent on genetic background, with potential for selection of higher yielding tin progeny for commercial line development.

Understanding the Jasmine phenotype of rice through metabolite profiling and sensory evaluation

IntroductionAromatic rices are culturally and economically important for many countries in Asia. Investigation of the volatile compounds emitted by rice during cooking is the key to understanding the flavour of elite aromatic rice varieties.ObjectivesThe objectives of this study were to compare Jasmine-type aromatic rices from the Greater Mekong Subregion and Australia in terms of their metabolomics and sensory profiles and to draw out associations between the volatile organic compounds and human sensory perception of rice aroma.MethodsA set of aromatic rice varieties from South East Asia and Australia, along with non-aromatic controls, was grown in tropical and temperate areas of Australia. Untargeted metabolite profiling of volatile compounds, from the heated rice flour, by static headspace extraction and separation by two dimensional gas chromatography time-of-flight mass spectrometry was performed. Volatile compounds were also assayed in the standard references used in the sensory evaluation and compared to the compounds detected in the headspace of rice.ResultsWhile 2-acetyl-1-pyrroline (2-AP) was a discriminating compound, we identified several of its structural homologues, and a number of other metabolites that were consistently detected in fragrant Jasmine rice. 2-AP producing rice varieties have different sensory properties and these variations were defined by the discriminating compounds identified in each rice type.ConclusionsThe results of this study are valuable in understanding the aspects of aromatic rice that are important to consumers, and in the identification of compounds that breeding programs can use to select for pleasant aromas, enabling breeding programs to target markets with greater accuracy.

Evaluation of reduced-tillering (tin) wheat lines in managed, terminal water deficit environments

Small or shrivelled wheat kernels (screenings) that reduce crop value are commonly produced in terminal drought environments. The aim of this study was to establish whether the incorporation of the tiller inhibition (tin) gene would contribute to maintenance of kernel weight and reductions in screenings under terminal water deficit. Five Silverstar near-isogenic lines contrasting in high and low tiller potential and their recurrent Silverstar parent were established at two plant densities under managed terminal water deficit (mild and severe) and irrigated conditions. With irrigation (grain yield of 5.6 t ha–1), kernels of all lines weighed ~31mg, with restricted-tillering (R-tin) lines producing an average 15% lower grain yield. Under both mild and severe terminal water deficit (4.1 t ha–1 and 2.8 t ha–1), free-tillering lines had relatively high screenings ranging from 11.9% to 16.2%. Compared with free-tillering lines, R-tin lines maintained large kernel weight (~29mg kernel–1) and had 29% and 51% fewer screenings under the two stresses, and a significantly greater (+11%) grain yield under mild stress. Higher kernel weights in tin lines were realized even with the greater kernel number per spike. The higher kernel weight of the R-tin lines under stress conditions was associated with greater anthesis biomass and increased stem water-soluble carbohydrates, ensuring more assimilate for later translocation to filling grain. The incorporation of the tin gene into genetic material adapted to the target environments provides scope for improvement in both grain yield and kernel weight, and a reduction in screenings in terminal water deficit environments.

Influence of phenology on grain yield variation among barley cultivars grown under terminal drought

We investigated the influence of sowing time and genotypic variation for phenology on grain yield of barley in south-eastern Queensland. Over 3 seasons, 8 trials with 10 cultivars and 1 trial with 4 cultivars were conducted under either irrigated or terminal drought conditions at 2 locations. Rainout shelters ensured the development of severe terminal water stress. Trials were either sown on a common date, as conducted in traditional multi-environment trials, or over 3 weeks to synchronise anthesis among cultivars of different phenologies. Within the common sowing date trials, variation (P < 0.01) existed among cultivars for grain yield. From the 6 common sowing trials there was a negative correlation (P < 0.05) between grain yield and days to anthesis; that is, the shorter duration cultivars expressed the highest grain yield. Variation in days to anthesis accounted for 48-72% of the variation for grain yield. In the staggered sowing trials, where anthesis of all cultivars occurred within 4 or 2 days of the mean anthesis date, variation for grain yield was small or non-significant, and there was no association between grain yield and days to anthesis. The staggered sowing experiment with 10 cultivars indicated that duration of the vegetative phase was important in determining total dry matter production at maturity when cultivars were grown under terminal drought. Long-duration cultivars sown earlier had greater total dry matter at maturity than short-duration cultivars. This was associated with a greater water extraction by the long-duration cultivars, especially at depth, which remained inaccessible to later sown, short-duration cultivars. However, due to the low harvest index of the long-duration cultivars, grain yield of long- and short-duration cultivars was comparable when anthesis of cultivars was synchronised. When sown at the same time, a short-duration cultivar is advantageous because of a high chance of escaping water stress that develops during the critical development stage of anthesis. The results from the staggered sowing date experiments, however, indicated that the long-duration cultivars, when sown earlier in the season, had no yield disadvantage in comparison with the short-duration cultivars sown later in the season. Therefore, there is scope to develop barley cultivars of later phenology than is currently available to provide Queensland farmers with the option of utilising early rainfall events which are sometimes the only planting opportunity.

Farmer participatory variety selection conducted in high- and low-toposequence multi-location trials for improving rainfed lowland rice in Lao PDR

Abstract. Lowland rice in Lao PDR is predominantly grown under drought-prone, rainfed conditions in the wet season. We utilised a farmer participatory variety selection (PVS) approach in combination with multi-location yield trials (MLTs) conducted in high- and low-toposequence positions to test advanced breeding lines with the aim of improving the efficiency of the rice-breeding program and encouraging rapid adoption of improved lines. Upper position fields were utilised to screen for traits for unfavourable environments, including drought resistance, while lower fields were used to target yield potential. Yield was, on average, 13% lower in upper than lower (2.85 t/ha) field positions, and varieties adapted to high-toposequence position were identified. Farmer preference was not associated well with grain yield performance, with a significant positive relationship (r = 0.34*, n = 23) identified only in the Vientiane (VTN) low-toposequence trial; rather, the famers tended to choose lines they believed were best adapted to their own farm. Although a significant relationship existed for both farmer preference (r = 0.42*, n = 23) and grain yield (r = 0.50*, n = 23) in high and low toposequences across all provinces in 2010, this relationship was not significant in VTN, where the high position was low-yielding (1.2 t/ha). By utilising farmer preference information in combination with traditional MLT data, only lines agronomically acceptable to farmers were progressed into a seed-multiplication system for country-wide, farmer yield testing. Thus, the PVS-MLT approach has provided efficient delivery of highly acceptable lines to farmers, which directly contributes to improved efficiency of the rice-breeding program.

Rice-cold tolerance across reproductive stages

Abstract. Cold temperature stress at the reproductive stage, particularly at booting and flowering stages can cause significant reductions in rice (Oryza sativa L.) yield particularly at high latitudes and elevation. Although genotypic variation for cold tolerance is known to exist, the tolerance mechanisms and genotypic consistency across the stages are yet to be understood for segregating populations. Three experiments were conducted under controlled temperature glasshouse conditions to determine floral characteristics that were associated with cold tolerance at the flowering stage and to determine genotypic consistency at the booting and flowering stages. Twenty F5 Reiziq × Lijiangheigu lines from two extreme phenotypic bulks selected for cold tolerance at booting stage in the F2 generation were utilised. Spikelet sterility under cold stress at booting was significantly correlated with spikelet sterility under cold stress at flowering (r = 0.62**) with five lines identified as cold tolerant across reproductive stages. There was also a positive correlation (r = 0.47*) between spikelet sterility under cold stress at booting at the F5 and at the F2 generation. The quantitative trait loci (QTL; qLTSPKST10.1) previously identified on chromosome 10 contributing to spikelet sterility within the F2 generation, was also identified in the F5 generation. Additionally, genomic regions displaying significant segregation between the progenies contrasting for their cold tolerance response phenotype were identified on chromosomes 5 and 7 with Lijiangheigu as allelic donor and an estimated reduction in spikelet sterility of 25% and 27%, respectively. Although genotypic variation in spikelet sterility at the booting stage was not related to the development rate for heading or flowering, those cold-tolerant genotypes at the flowering stage were the quickest to complete flowering. Cold-tolerant genotypes at the flowering stage had larger numbers of dehisced anthers and subsequently pollen number on stigma, which contributed to reduced spikelet sterility. It is concluded that enhanced anther dehiscence plays a significant role in improved cold tolerance at the flowering stage.

Fissured grain and head rice yield of crops harvested manually or by combine at different ripening stages in Cambodia

ABSTRACT Combine has been well adopted by smallholders in lowlands of Cambodia and is contributing to the development of mechanized rice production for commercial purposes. Broken rice is a major issue for the commercial rice product, and fissured grain at harvest leads to broken rice and lowers head rice yield (HRY) during milling. Factors that determine grain fissures and broken rice were obtained from three studies: an on-farm study of fissured grain, a research station experiment of fissured grain and broken rice harvested at different ripening stages and a mill study of fissured and broken rice during drying and milling processes in Cambodia. There was significant variation in fissured rice percentage among 20 farms sampled, and the fissured grain was negatively correlated with grain moisture content at harvest. Time of harvesting was crucial, as delay in harvesting after 25 days after flowering (DAF) often resulted in lower grain moisture content and higher fissured grain, which subsequently reduced HRY. However, the optimum time of harvesting varied across four seasons for crops harvested manually or by combine. In some years, crops harvested at 35 DAF had rather low per cent fissured grain with subsequently high HRY, this may have been associated with slightly lower temperatures. The mill study showed that fissured grain developed during the drying, storage and milling processes. It is concluded that while grain moisture content at the time of harvest may be used as an indication of subsequent HRY, the latter was more strongly related to fissured grain at harvest.