R. A. Richards

Carbon Isotope Fractionation and Plant Water-Use Efficiency

In order for plants to grow, they must fix carbon. Carbon usually enters the leaves as carbon dioxide, diffusing through pores in the epidermis called stomata. Increased stomatal conductance, g, of leaves causes an increase in the partial pressure of CO2 inside the leaves, p i . This usually causes an increase in the rate of CO2 assimilation, A, but also allows a greater rate of transpirational water loss, E. Such an action by a plant is a gamble, because while it increases the likelihood of growth and reproductive success, it also increases the probability of desiccation and death (Cowan 1986).

Perfect markers for the Rht-B1b and Rht-D1b dwarfing genes in wheat

Abstract.PCR-based markers were developed to detect the point mutations responsible for the two major semi-dwarfing genes Rht-B1b (Rht1) and Rht-D1b (Rht2) in wheat. These markers were validated by testing 19 wheat varieties of known Rht genotype. They included Rht-B1b and Rht-D1b dwarfs, double-mutant varieties and tall wheats. These were correctly genotyped with the Rht-B1b and Rht-D1b-specific primers, as well as markers specific for the tall alleles Rht-B1a and Rht-D1a. Using a family of doubled-haploid lines segregating for Rht-B1b and Rht-D1b, the markers were mapped to the expected homoeologous regions of chromosomes 4B and 4D, respectively. Both markers were strongly correlated with a reduction in height, accounting for 23% (Rht-B1b) and 44% (Rht-D1b) of the phenotypic variance in the population. These markers will have utility in marker-assisted selection of the Rht-B1b and Rht-D1b genes in wheat breeding programs.

Traits and selection strategies to improve root systems and water uptake in water-limited wheat crops

Wheat yields globally will depend increasingly on good management to conserve rainfall and new varieties that use water efficiently for grain production. Here we propose an approach for developing new varieties to make better use of deep stored water. We focus on water-limited wheat production in the summer-dominant rainfall regions of India and Australia, but the approach is generally applicable to other environments and root-based constraints. Use of stored deep water is valuable because it is more predictable than variable in-season rainfall and can be measured prior to sowing. Further, this moisture is converted into grain with twice the efficiently of in-season rainfall since it is taken up later in crop growth during the grain-filling period when the roots reach deeper layers. We propose that wheat varieties with a deeper root system, a redistribution of branch root density from the surface to depth, and with greater radial hydraulic conductivity at depth would have higher yields in rainfed systems where crops rely on deep water for grain fill. Developing selection systems for mature root system traits is challenging as there are limited high-throughput phenotyping methods for roots in the field, and there is a risk that traits selected in the lab on young plants will not translate into mature root system traits in the field. We give an example of a breeding programme that combines laboratory and field phenotyping with proof of concept evaluation of the trait at the beginning of the selection programme. This would greatly enhance confidence in a high-throughput laboratory or field screen, and avoid investment in screens without yield value. This approach requires careful selection of field sites and years that allow expression of deep roots and increased yield. It also requires careful selection and crossing of germplasm to allow comparison of root expression among genotypes that are similar for other traits, especially flowering time and disease and toxicity resistances. Such a programme with field and laboratory evaluation at the outset will speed up delivery of varieties with improved root systems for higher yield.

Crop improvement for temperate Australia: Future opportunities

Abstract Temperate crops, particularly wheat, grown during winter and spring dominate crop production in southern Australia. Environmental hazards are numerous, but lack of water is the most widespread and important one limiting yields. The key ingredients for high yield are considered to be an appropriate phenology, a high water-use efficiency and a high harvest index. Ways to improve these in temperate crops by selection and breeding are discussed.

Breeding for improved water productivity in temperate cereals: phenotyping, quantitative trait loci, markers and the selection environment.

Consistent gains in grain yield in dry environments have been made by empirical breeding although there is disturbing evidence that these gains may have slowed. There are few examples where an understanding of the physiology and the genetics of putative important drought-related traits has led to improved yields. Success will first depend on identifying the most important traits in the target regions. It will then depend on accurate and fast phenotyping, which, in turn, will lead to: (1) trait-based selection being immediately transferable into breeding operations and (2) being able to identify the underlying genes or the important genomic regions (quantitative trait loci), perhaps leading to efficient marker-based selection (MBS). Genetic complexity, extent of genotypeenvironment (GE) interaction and sampling cost per line will determine value of phenotyping over MBS methods. Here, we review traits of importance in dry environments and review whether molecular or phenotypic selection methods are likely to be the most effective in crop improvement programs and where the main bottlenecks to selection are. We also consider whether selection for these traits should be made in dry environments or environments where there is no soil water limitation. The development of lines/ populations for trait validation studies and for varietal development is also described. We firstly conclude that despite the spectacular improvements in molecular technologies, fast and accurate phenotyping remains the major bottleneck to enhancing yield gains in water-limited environments. Secondly, for most traits of importance in dry environments, selection is generally conducted most effectively in favourable moisture environments.

Should selection for yield in saline regions be made on saline or non-saline soils?

SummarySaline soils are typically very patchy in their salinity. The yield of crops growing on them is similarly patchy. This paper argues that because most of the yield from such soils comes from the least saline areas, the best breeding strategy for improving the overall yield of crops growing on them is to select for high yield on non-saline soils. This conclusion derives from comparing the effects that four different breeding goals, namely: (1) a 10% increase in yield on non-saline soils, (ii) a 20% increase in the threshold salinity that first reduces yield, (iii) a doubling of yield at an electrical conductivity of the saturation extract (ECe) of 20 dS/m and (iv) a combination of (i) and (iii), would have on total yield. The effects of achieving these goals in barley, common wheat, durum wheat and triticale in fields exhibiting different salinities are predicted from actual yields of these species grown on different salinities in the field.

Importance of pre-anthesis anther sink strength for maintenance of grain number during reproductive stage water stress in wheat

Reproductive stage water stress leads to spikelet sterility in wheat. Whereas drought stress at anthesis affects mainly grain size, stress at the young microspore stage of pollen development is characterized by abortion of pollen development and reduction in grain number. We identified genetic variability for drought tolerance at the reproductive stage. Drought-tolerant wheat germplasm is able to maintain carbohydrate accumulation in the reproductive organs throughout the stress treatment. Starch depletion in the ovary of drought-sensitive wheat is reversible upon re-watering and cross-pollination experiments indicate that the ovary is more resilient than the anther. The effect on anthers and pollen fertility is irreversible, suggesting that pollen sterility is the main cause of grain loss during drought conditions in wheat. The difference in storage carbohydrate accumulation in drought-sensitive and drought-tolerant wheat is correlated with differences in sugar profiles, cell wall invertase gene expression and expression of fructan biosynthesis genes in anther and ovary (sucrose : sucrose 1-fructosyl-transferase, 1-SST; sucrose : fructan 6-fructosyl-transferase, 6-SFT). Our results indicate that the ability to control and maintain sink strength and carbohydrate supply to anthers may be the key to maintaining pollen fertility and grain number in wheat and this mechanism may also provide protection against other abiotic stresses.

'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser II.Carbohydrate and protein dynamics

Changes in carbohydrate and protein in stems, leaves, spikes, and grain between anthesis and maturity were measured in 3 dryland wheat crops whose responses to applied nitrogen (N) ranged from increases in grain yield through to decreases in grain yield. This decrease in grain yield, known as haying-off, was described in Paper I in this series. Measurements reported there showed that apparent retranslocation, defined as the decrease in weight of vegetative organs during grain filling, was generally greater for crops of high-N status than for those of low-N status. Retranslocation in this context is the process of moving compounds assimilated before anthesis to the grain. The largest source of assimilates available for retranslocation in all crops at anthesis was water-soluble carbohydrates (WSC) contained in the stems and spikes, and represented a potential contribution of 34-50% to yield for the most severely hayed-off crops. The absolute amount of WSC present in high-N crops was less than that in low-N crops, despite a greater biomass. The lack of this form of assimilate available for retranslocation was the greatest single contributor to the yield reduction of the crops of high-N status. The quantity of protein retranslocated increased with crop N status, but the amounts involved were smaller than the quantity of WSC. Virtually all of the WSC reserves were utilised in all crops, in contrast to the protein reserves which were poorly retranslocated in the hayed-off crops. Most of the WSC was contained in the stems and most of the protein in the leaves. The potential contribution of retranslocated WSC and protein from leaves was more difficult to estimate because of an apparent loss of 40-50% of leaf tissue after anthesis. The nature of the loss was estimated from the amounts of acid detergent fibre (ADF; fibre not solubilised by hot acid detergent) present at anthesis and maturity. Since ADF comprises cellulose and lignin which decompose slowly, the loss of 30-37% of ADF was applied as a correction factor in calculating potential retranslocation from leaves. There was no loss of stem ADF. Using the correction, the potential retranslocation of leaf protein and leaf WSC was equivalent to 6-15% of yield. The export of all WSC and protein failed to account for the total decrease in leaf biomass, even after correction of leaf losses. We identified hemicellulose as an additional and previously unsuspected source of carbohydrate for retranslocation. Unlike WSC, the amount of leaf and stem hemicellulose at anthesis increased with crop N status, and the increase in hemicellulose between anthesis and maturity was equal to 10-17% of yield.

Genotypic variation in water-soluble carbohydrate accumulation in wheat

The water-soluble carbohydrate (WSC) that accumulates in the stems of wheat during growth can be an important contributor to grain filling, particularly under conditions when assimilation is limited, such as during end-of-season drought. WSC concentration was measured at anthesis across a diverse set of wheat genotypes over multiple environments. Environmental differences in WSC concentration were large (means for the set ranging between 108 and 203 mg g-1 dry weight), and there were significant and repeatable differences in WSC accumulation among genotypes (means ranging from 112 to 213 mg g-1 dry weight averaged across environments), associated with large broad-sense heritability (H = 0.90 ± 0.12). These results suggest that breeding for high WSC should be possible in wheat. The composition of the WSC, examined in selected genotypes, indicated that the variation in total WSC was attributed mainly to variation in the fructan component, with the other major soluble carbohydrates, sucrose and hexose, varying less. The degree of polymerisation (DP) of fructo-oligosaccharides was up to ~13 in samples where higher levels of WSC were accumulated, owing either to genotype or environment, but the higher DP components (DP > 6) were decreased in samples of lower total WSC. The results are consistent with fructan biosynthesis occurring via a sequential mechanism that is dependent on the availability of sucrose, and differences in WSC contents of genotypes are unlikely to be due to major mechanistic differences.

Seedling vigour in wheat - sources of variation for genetic and agronomic improvement

The early growth of wheat is slow compared with that of barley and triticale. This is expected to limit the yield of wheat in environments where greater seedling vigour is advantageous. To overcome the slow growth of wheat, genetic sources of seedling vigour are required for use in breeding programs, and/or ways to increase seedling growth by manipulating seed characteristics. This study reports (i) new sources of and 3 heights at cutting (cutting was done when the grass reached 0.5, 1, and 1.5 m above the ground). The N fertiliser treatment did not yield any significant difference in DMD, ND, or IVDMD. Height at cutting had a significant (P < 0.05) effect on rumen DMD and ND and their degradability characteristics for all incubation times. There was a reduction in DMD and ND and their degradability characteristics as plant height increased at cutting. Similarly, as height at cutting increased there was a decline in IVDMD. There was a positive linear correlation between IVDMD and both DMD and ND at 48, 72, 96, and 120 h (r = 0.917, 0.923, 0.921, and 0.850 for DMD; r = 0.795, 0.814, 0.787, and 0.787 for ND). Hence, further study on intake and performance of animals is suggested to develop Napier-based diets for smallholders.