The basis of improved water use efficiency and adaptation in hexaploid wheat

Abstract

Drought is the major environmental stress impacting the wheat industry and improving crop productivity in drought prone environments is a global challenge. Water use efficiency (WUE) is considered an important determinant of yield under stress and is a key component of drought resistance. There is considerable interest in increasing WUE to conserve soil moisture both in water-limited and irrigated conditions to improve productivity. This study aimed to identify sources of genetic diversity for WUE and grain yield and the trait constellations that contribute to improved WUE and increased productivity. For this purpose genetically diverse wheat genotypes (15 & 20) were evaluated for water use, water use efficiency and yield for three consecutive years in the contrasting weather conditions at Narrabri in northwestern NSW. A number of above and below ground traits were recorded and their relationship with WUE and grain yield studied. An attempt was also made to study marker-trait associations in an applied wheat breeding program to identify genomic regions related to improved grain yield, key wheat diseases and grain quality traits. Overall, the current study examined the physiological and genetic basis of genetic variation leading to improved adaptation in northwestern NSW. Significant genotype x environment interaction was observed for most traits. No genotypic differences were observed for soil water extraction in the dry environment of 2009, whereas in other environments the differences were significant due to sufficient soil moisture. In all environments soil water was extracted rapidly from the top 30 cm, whereas water content at 50 cm depth decreased gradually. The maximum water use was observed in 2010 due to the higher in season rainfall but this did not result in more biomass and grain yield. More water was used in the post-anthesis period at the dough stage and later flowering genotypes tended to use more water. Water use efficiency was higher in the environments with lower rainfall (2009 and 2011). The superior WUE genotypes were all synthetic derived whereas the least efficient were released cultivars of Australian origin. Higher water use was not associated with higher WUE and grain yield indicating that higher yielding cultivars have the potential to improve WUE thus resulting in water saving. Trait means for number of agronomic traits were higher with higher precipitation resulting in more vegetative growth, whereas harvest index was greater in less precipitation seasons (E6 and E5). Synthetic derived genotypes and the cultivar Crusader produced higher grain yield in high and low moisture environments during 2009. Overall, higher water use was not significantly associated with higher agronomic trait values, thus indicating the possibility of developing improved wheat germplasm that requires less water. Heading time was significantly associated with water use efficiency for dry matter production at maturity (WUEDM-Maturity), water use efficiency for grain yield (WUEGrain) and grain yield in both dry and some wet environments. Clearly this attribute contributes to ‘drought escape’ thus ensuring water is used for grain filling before the onset of stress. An association of superior grain yield with higher biomass and harvest index was found in this study. The synthetic genotypes produced higher biomass at maturity with lower tiller numbers which resulted in greater yield. Normalized difference vegetation index (NDVI) at grainfilling was strongly associated with biomass. Genotypes with higher chlorophyll content at anthesis tended to be the highest yielding. The synthetic genotypes used soil water more efficiently resulting in cooler canopies and higher grain yield. The high heritability of Canopy temperature depression (CTD) and strong associations with WUE and grain yield, suggests that this trait can be used as an indirect selection tool for yield improvement in northwestern NSW. Ground cover recorded at an early stage was significantly associated with improved WUE and grain yield. The significant association between earliness, NDVI and canopy cover indicates that NDVI can be used as an easy to measure indirect selection tool to screen for vigorous genotypes during early growth to improve WUE and grain yield. Leaf area was reduced significantly in the dry environment and leaf traits showed some association with WUE and grain yield. Glaucousness was strongly associated with WUE and grain yield. Significant genetic variability was identified for micro-elements and some were linked with differences in crop phenology (late maturity) and water use. Significant genotypic variation was observed for all gas exchange parameters studied during 2009 and 2011. Higher conductance was associated with lower leaf intrinsic WUE. The genotypes with higher gas exchange parameters were positively associated with biomass at maturity, WUE and grain yield. Leaf intrinsic WUE was positively correlated with WUEGrain. In a subset of genotypes tested in 2011, stomatal conductance decreased more than the photosynthetic rate with the progress of the season resulting in higher leaf intrinsic WUE. Genotypes with maximum productivity were those with the highest leaf intrinsic WUE, reduced stomatal conductance and lower ratio of internal CO2 concentration to ambient CO2 concentration (Ci/Ca). In water-deficit environments low stomatal conductance and high leaf intrinsic WUE can result in improved grain yield. In the current study the higher intrinsic WUE was due to the lower stomatal conductance in the low grain delta (ΔG) genotypes. Several synthetic genotypes and the cultivar Ventura with significantly lower ∆G (high WUE) produced the highest grain yield. The higher delta observed in some environments could be attributed to the availability of sufficient water for conductance, thus resulting in higher WUE. Nevertheless, higher delta could be targeted for selection in high moisture environments and low delta in water deficient environments. The lowest thousand grain weight was observed in the driest environment and a few spike traits (spike length, kernels per spikelet and number of grains per spike) showed a weak association with grain yield. Some synthetic genotypes had low tiller number and better spike traits and grain yield. The root traits were studied in high and low moisture environments and significant genotypic variation was observed. Overall root length, root diameter and root length density were reduced in response to water stress. Maximum root values were observed near the surface at 0-15cm and tended to decrease with the depth. Several synthetic genotypes identified as drought tolerant were also superior for root traits and resulted in improved WUE and grain yield. The grain protein of the tested wheat genotypes was in the hard and prime hard grades. All grain quality traits had high heritability. Grain protein and hardness increased whereas grain moisture and test weight decreased in the dry environment. Higher grain protein correlated with lower grain hardness. The lack of relationship between grain protein and grain yield in this study indicates no genetic limitation in increasing grain protein under these environmental conditions. Cluster analysis classified genotypes into four sub-clusters on the basis of phenotypic and molecular data. Synthetic genotypes showed substantial similarity when clustered based on both data types and tended to have superior agronomic and physiological traits, WUE, grain yield and drought tolerance. Plant breeders can use the considerable genetic variability identified in the current study for both above and below ground traits to plan crosses between genotypes from different clusters coupled with superior trait variability. Based on the results of the present study a water use efficient wheat ideotype for northwestern NSW would have higher NDVI, greater leaf length and width, cooler canopies, better biomass at anthesis and maturity, greater plant height, superior harvest index, higher spike length, greater number of kernels per spikelet and thousand grain weight, better grain yield, superior WUEDM-Maturity and WUEGrain. The population structure in the tested material was identified from the pattern analysis of the phenotypic and genotypic data. Significant marker-trait associations (MTAs) for grain yield were identified on all wheat chromosomes whereas for other traits the MTAs were found on specific chromosomes. Some MTAs were identified in genomic regions reported previously and many new regions were identified for grain yield, stripe rust, leaf rust and crown rot. It was observed that each trait is affected by many markers and each MTA affects multiple traits. The outcomes of this study are valuable to the wheat industry in number of ways. The high WUE genotypes identified can be used to develop more efficient cultivars that increase yield per unit of water used, thus improving farmer income in both dry and wet years. This study also concluded that genotypes vary in their responses to water stress and can be exploited for developing drought tolerant wheat cultivars. The MTAs identified for traits responsible for improved productivity and adaptation could be used to pyramid favorable alleles in modern cultivars.