Anthony G. Condon

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).

Avenues for increasing salt tolerance of crops, and the role of physiologically based selection traits

Increased salt tolerance is needed for crops grown in areas at risk of salinisation. This requires new genetic sources of salt tolerance, and more efficient techniques for identifying salt-tolerant germplasm, so that new genes for tolerance can be introduced into crop cultivars. Screening a large number of genotypes for salt tolerance is not easy. Salt tolerance is achieved through the control of salt movement into and through the plant, and salt-specific effects on growth are seen only after long periods of time. Early effects on growth and metabolism are likely due to osmotic effects of the salt, that is to the salt in the soil solution. To avoid the necessity of growing plants for long periods of time to measure biomass or yield, practical selection techniques can be based on physiological traits. We illustrate this with current work on durum wheat, on selection for the trait of sodium exclusion. We have explored a wide range of genetic diversity, identified a new source of sodium exclusion, confirmed that the trait has a high heritability, checked for possible penalties associated with the trait, and are currently developing molecular markers. This illustrates the potential for marker-assisted selection based on sound physiological principles in producing salt-tolerant crop cultivars.

Raising yield potential of wheat. II. Increasing photosynthetic capacity and efficiency

Past increases in yield potential of wheat have largely resulted from improvements in harvest index rather than increased biomass. Further large increases in harvest index are unlikely, but an opportunity exists for increasing productive biomass and harvestable grain. Photosynthetic capacity and efficiency are bottlenecks to raising productivity and there is strong evidence that increasing photosynthesis will increase crop yields provided that other constraints do not become limiting. Even small increases in the rate of net photosynthesis can translate into large increases in biomass and hence yield, since carbon assimilation is integrated over the entire growing season and crop canopy. This review discusses the strategies to increase photosynthesis that are being proposed by the wheat yield consortium in order to increase wheat yields. These include: selection for photosynthetic capacity and efficiency, increasing ear photosynthesis, optimizing canopy photosynthesis, introducing chloroplast CO(2) pumps, increasing RuBP regeneration, improving the thermal stability of Rubisco activase, and replacing wheat Rubisco with that from other species with different kinetic properties.

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.

Different mechanisms of adaptation to cyclic water stress in two South Australian bread wheat cultivars

In the South Australian wheat belt, cyclic drought is a frequent event represented by intermittent periods of rainfall which can occur around anthesis and post-anthesis in wheat. Three South Australian bread wheat (Triticum aestivum L.) cultivars, Excalibur, Kukri, and RAC875, were evaluated in one greenhouse and two growth-room experiments. In the first growth-room experiment, where plants were subjected to severe cyclic water-limiting conditions, RAC875 and Excalibur (drought-tolerant) showed significantly higher grain yield under cyclic water availability compared to Kukri (drought-susceptible), producing 44% and 18% more grain compared to Kukri, respectively. In the second growth-room experiment, where plants were subjected to a milder drought stress, the differences between cultivars were less pronounced, with only RAC875 showing significantly higher grain yield under the cyclic water treatment. Grain number per spike and the percentage of aborted tillers were the major components that affected yield under cyclic water stress. Excalibur and RAC875 adopted different morpho-physiological traits and mechanisms to reduce water stress. Excalibur was most responsive to cyclic water availability and showed the highest level of osmotic adjustment (OA), high stomatal conductance, lowest ABA content, and rapid recovery from stress under cyclic water stress. RAC875 was more conservative and restrained, with moderate OA, high leaf waxiness, high chlorophyll content, and slower recovery from stress. Within this germplasm, the capacity for osmotic adjustment was the main physiological attribute associated with tolerance under cyclic water stress which enabled plants to recover from water deficit.

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.

Genetic variation in tolerance to the osmotic stress component of salinity stress in durum wheat

Salinity affects plant growth by the osmotic stress of the salt around the roots as well as by toxicity caused by excessive accumulation of salt in leaves. The aim of this study was to determine whether there is significant genetic variation in tolerance to osmotic stress that can be useful in improving the salinity tolerance of crop plants. Durum wheat is a salt-sensitive crop whose yield is reduced by moderately saline soils. Genetic variation in tolerance to osmotic stress in durum wheat was examined in 50 international durum varieties and landraces by measuring the response of stomatal conductance to salt stress before salts built up in the leaf. Stomatal conductance is a sensitive indicator of the osmotic stress because it is reduced immediately with the onset of salinity, and is the initial and most profound cause of a decline in CO2 assimilation rate. Genetic differences of 2-3-fold were found in the magnitude of the response of stomatal conductance to salt-induced osmotic stress. Higher stomatal conductance in salt related to higher CO2 assimilation rate. There was a positive relationship between stomatal conductance and relative growth rate in salt. This study shows the potential for new genetic gains in salt tolerance in durum wheat.

Effect of sodium exclusion trait on chlorophyll retention and growth of durum wheat in saline soil

Six durum wheat genotypes with contrasting extents of sodium accumulation in leaves were used to assess the effects of sodium exclusion on leaf longevity and biomass production in saline soil. Leaf chlorophyll content, ion concentration, plant height, and dry biomass were measured at 3 salinity levels (1, 75, and 150 mM NaCl, with supplemental Ca2+). Yield and yield components were measured on 2 contrasting groups of genotypes. The low Na+ genotypes showed much longer chlorophyll retention than the high Na+ genotypes, the start of leaf senescence being prolonged by a week or more in the low Na+ genotypes. The difference was greatest at 75 mM NaCl. At ear emergence, the effects of salinity on biomass were less on the low Na+ than on the high Na+ genotypes at 75 mM NaCl, but there was no difference between groups at 150 mM NaCl. At maturity, salinity had a similar effect on biomass of both genotypes, at both 75 and 150 mM NaCl. Grain yield at 150 mM NaCl was equally reduced in both genotypes, being only 12% of controls. However, at 75 mM NaCl there was a signficant yield difference between genotypes; yield of the high Na+ genotype was only 70% of controls, whereas yield of the low Na+ genotype was 88% of controls. The greater yield of the low Na+ genotype was due to enhanced grain number and grain weight in the tiller ears.

Effect of salinity on water relations and growth of wheat genotypes with contrasting sodium uptake

Four wheat genotypes with contrasting degrees of Na+ exclusion were selected to see if low Na+ uptake had an adverse effect on water relations or growth rates during exposure to saline conditions. Plants were grown in supported hydroponics with and without 150 mM NaCl, and sampled for measurements of water relations, biomass, stomatal conductance, and ion accumulation. After 4 weeks exposure to salt, biomass was reduced in all genotypes to a similar extent (about 50%), with the effect of salinity on relative growth rate confined largely to the first week. There was little difference between genotypes in the effect of salinity on water relations, as indicated by their relative water content and estimated turgor. Osmotic adjustment occurred in all genotypes, with one of the low-Na+ genotypes having the greatest adjustment. In the low-Na+ genotypes, osmotic adjustment depended on higher K+ and high organic solute accumulation. Stomatal conductance of all genotypes was reduced by saline conditions, but the reduction was greater in the low-Na+ genotypes. These genotypes also showed a larger fall in the value of carbon isotope discrimination measured in expanding leaves, indicating a greater transpiration efficiency when exposed to saline conditions. Chlorophyll fluorescence measurements failed to indicate damage to photochemical pathways in either high- or low-Na+ genotypes. These data indicate that selecting lines with low Na+ accumulation for the purpose of improving salt tolerance is unlikely to introduce limitations for osmotic adjustment.

Genomic regions for canopy temperature and their genetic association with stomatal conductance and grain yield in wheat

Stomata are the site of CO2 exchange for water in a leaf. Variation in stomatal control offers promise in genetic improvement of transpiration and photosynthetic rates to improve wheat performance. However, techniques for estimating stomatal conductance (SC) are slow, limiting potential for efficient measurement and genetic modification of this trait. Genotypic variation in canopy temperature (CT) and leaf porosity (LP), as surrogates for SC, were assessed in three wheat mapping populations grown under well-watered conditions. The range and resulting genetic variance were large but not always repeatable across days and years for CT and LP alike. Leaf-to-leaf variation was large for LP, reducing heritability to near zero on a single-leaf basis. Replication across dates and years increased line-mean heritability to ~75% for both CT and LP. Across sampling dates and populations, CT showed a large, additive genetic correlation with LP (rg=-0.67 to -0.83) as expected. Genetic increases in pre-flowering CT were associated with reduced final plant height and both increased harvest index and grain yield but were uncorrelated with aerial biomass. In contrast, post-flowering, cooler canopies were associated with greater aerial biomass and increased grain number and yield. A multi-environment QTL analysis identified up to 16 and 15 genomic regions for CT and LP, respectively, across all three populations. Several of the LP and CT QTL co-located with known QTL for plant height and phenological development and intervals for many of the CT and LP quantitative trait loci (QTL) overlapped, supporting a common genetic basis for the two traits. Notably, both Rht-B1b and Rht-D1b dwarfing alleles were paradoxically positive for LP and CT (i.e. semi-dwarfs had higher stomatal conductance but warmer canopies) highlighting the issue of translation from leaf to canopy in screening for greater transpiration. The strong requirement for repeated assessment of SC suggests the more rapid CT assessment may be of greater value for indirect screening of high or low SC among large numbers of early-generation breeding lines. However, account must be taken of variation in development and canopy architecture when interpreting performance and selecting breeding lines on the basis of CT.