A. K. Borrell

IMPROVING EFFICIENCY OF WATER USE FOR IRRIGATED RICE IN A SEMI - ARID TROPICAL ENVIRONMENT

Irrigation water accounts for almost 40% of total variable production costs for rice (Oryza sativa L.) cropping in the Burdekin River Irrigation Area, northern Australia. Increasing the efficiency of water use would improve the economic viability of growers and long-term environmental benefits would also be likely due to lower water tables and decreased salinisation in irrigation areas. The aim of these studies was to maximize grain yield by optimizing its functional components: water use, efficiency of water use for dry matter production (WUEdm) and harvest index (HI). The responses of dry matter and yield in rice (cv. Lemont) to five methods of irrigation were studied in a wet and dry season in the region. Applying a permanent flood at sowing, the 3-leaf stage (traditional) and prior to panicle initiation were compared with two unflooded methods: saturated soil culture (SSC) and intermittent irrigation at weekly intervals. Saturated soil culture consisted of growing rice on raised beds of height 0.2 m and width 1.2 m, with water maintained in the furrows (0.3 m wide) some 0.1 m below the bed surface. The results of these studies show that it is not necessary to flood rice to obtain high grain yield and quality. The trend was for yield to increase with water supply, but there was no significant difference in yield and quality between SSC and traditional flooded production, although SSC used about 32% less water in both seasons. Therefore the efficiency of water use for grain production (WUEg, g m−2 mm−1) was higher in SSC than in traditional flooded production in the wet season and a similar trend existed in the dry season. There were no differences between SSC and the traditional method of irrigation in any of the grain quality components measured, indicating that this water saving method did not lower grain quality. Weed growth was generally higher in unflooded treatments, although weed populations in SSC and traditionally flooded rice were equivalent in the dry season, suggesting that weeds can be controlled in SSC. Saturated soil culture provides a viable alternative to flooded rice production for growers in semi-arid tropical environments. Substantial reductions in variable costs of production are attainable by reducing water use without reducing yield and quality.

Developmental and physiological traits associated with high yield and stay-green phenotype in wheat

Water availability is a key limiting factor in wheat production in the northern grain belt of Australia. Varieties with improved adaptation to such conditions are actively sought. The CIMMYT wheat line SeriM82 has shown a significant yield advantage in multi-environment screening trials in this region. The objective of this study was to identify the physiological basis of the adaptive traits underpinning this advantage. Six detailed experiments were conducted to compare the growth, development, and yield of SeriM82 with that of the adapted cultivar, Hartog. The experiments were undertaken in field environments that represented the range of moisture availability conditions commonly encountered by winter crops grown on the deep Vertosol soils of this region. The yield of SeriM82 was 6-28% greater than that of Hartog, and SeriM82 exhibited a stay-green phenotype by maintaining green leaf area longer during the grain-filling period in all environments where yield was significantly greater than Hartog. However, where the availability of deep soil moisture was limited, SeriM82 failed to exhibit significantly greater yield or to express the stay-green phenotype. Thus, the stay-green phenotype was closely associated with the yield advantage of SeriM82. SeriM82 also exhibited higher mean grain mass than Hartog in all environments. It is suggested that small differences in water use before anthesis, or greater water extraction from depth after anthesis, could underlie the stay-green phenotype. The inability of SeriM82 to exhibit stay-green and higher yield where deep soil moisture was depleted indicates that extraction of deep soil moisture is important.

Drought adaptation of stay-green sorghum is associated with canopy development, leaf anatomy, root growth, and water uptake

Summary The positive effects of stay-green quantitative trait loci on grain yield of sorghum under post-anthesis drought are emergent consequences of their effects on water-use patterns, resulting from changes in pre-anthesis canopy size.

Stay-green alleles individually enhance grain yield in sorghum under drought by modifying canopy development and water uptake patterns

Stay-green is an integrated drought adaptation trait characterized by a distinct green leaf phenotype during grain filling under terminal drought. We used sorghum (Sorghum bicolor), a repository of drought adaptation mechanisms, to elucidate the physiological and genetic mechanisms underpinning stay-green. Near-isogenic sorghum lines (cv RTx7000) were characterized in a series of field and managed-environment trials (seven experiments and 14 environments) to determine the influence of four individual stay-green (Stg1-4) quantitative trait loci (QTLs) on canopy development, water use and grain yield under post-anthesis drought. The Stg QTL decreased tillering and the size of upper leaves, which reduced canopy size at anthesis. This reduction in transpirational leaf area conserved soil water before anthesis for use during grain filling. Increased water uptake during grain filling of Stg near-isogenic lines (NILs) relative to RTx7000 resulted in higher post-anthesis biomass production, grain number and yield. Importantly, there was no consistent yield penalty associated with the Stg QTL in the irrigated control. These results establish a link between the role of the Stg QTL in modifying canopy development and the subsequent impact on crop water use patterns and grain yield under terminal drought.

Stay-green quantitative trait loci’s effects on water extraction, transpiration efficiency and seed yield depend on recipient parent background

A stay-green phenotype enhances the adaptation of sorghum (Sorghum bicolor (L.) Moench) to terminal drought, although the mechanisms leading to its expression remain unclear. Differences in tillering and leaf area at anthesis, transpiration efficiency (TE), water extraction, harvest index (HI) and yield under terminal drought and fully irrigated conditions were assessed in 29 introgression lines (IL) targeting stay-green quantitative trait loci (QTLs) Stg1, Stg2, Stg3, Stg4, StgA and StgB in an S35 background, and 16 IL targeting Stg1, Stg3, Stg4 and StgB in an R16 background. TE was increased by StgB in the R16 background, whereas there was no effect in the S35 background. Water extraction was increased by Stg1 in the S35 background but not in R16. StgB modified the proportion of water extracted before and after anthesis in the S35 background. While tillering and leaf area at anthesis were decreased by Stg1 and Stg3 in S35, there was no effect in R16. Yield data under fully irrigated conditions showed higher tiller grain yield in Stg1, Stg2 and Stg3 ILs. Although yield differences were mostly explained by HI variation, the yield variation unexplained by HI was closely related to TE in S35 (R2=0.29) and R16 (R2=0.72), and was closely related to total water extracted in S35 (R2=0.41) but not in R16. These data indicate the potential for several stay-green QTLs to affect traits related to plant water use. However, these effects depend on the interaction between the genetic background and individual QTLs.

Yield, transpiration efficiency, and water-use variations and their interrelationships in the sorghum reference collection

Sorghum is well adapted to water-limited conditions, but the traits responsible for this enhanced adaptation underdroughtconditionsremainunclear.Inthisstudy,yield,transpirationefficiency(TE)andwaterextractionwereassessed in 149 germplasm entries from the sorghum reference set (plus three control cultivars) using a lysimetric system under terminal water stress and fully irrigated conditions outdoors. A 10-fold range for grain yield and harvest index (HI), 2-fold range for TE and a 1.25-fold variation for water extraction were observed under terminal water stress conditions. Transpiration efficiency and water extraction under water stress related poorly to that under fully irrigated conditions, reflectingalargegenotype-by-watertreatmentinteraction.Underdroughtstress,totalwaterextractionvariedby~3Lplant -1 among germplasm. Entries from the Durra race had highest water extraction capacity, whereas Caudatum-Bicolor and Caudatum-Durra intermediate races had poor water extraction. Durra, Caudatum and Caudatum-Guinea races had highest TE,whereastheGuinearacehadthelowest.AlthoughyieldwascloselyrelatedtoHI,atanylevelofHIthereweresubstantial yield differences that remained unexplained, and these residual yield variations were closely related to TE (R 2 =0.60). Similarly,substantialyieldvariationsthatwerestillnotexplainedbyHIorTEwerecloselyrelatedtothetotalwaterextracted underwaterstress(R 2 =0.35).Amultilinearregressionanalysisconfirmedtheseresultsandshowedtheimportanceofwater extractionduringgrain filling.Therefore,nexttoHI,theyielddifferencesunderterminaldroughtinsorghumweredrivenby TE, and then next by water extraction. The large genetic variation for TE and water extraction offer great breeding opportunities and in particular, highlight the Durra race as a critical source of variation.

Toward Sequencing the Sorghum Genome: A U.S. National Science Foundation-Sponsored Workshop Report

Members of the worldwide sorghum (Sorghum spp.) community, including private sector and international scientists as well as community representatives from closely related crops such as sugarcane (Saccharum spp.) and maize (Zea mays), met in St. Louis, Missouri, on November 9, 2004, to lay the groundwork for future advances in sorghum genomics and, in particular, to coordinate plans for sequencing of the sorghum genome. Key developments that made this workshop timely included advances in knowledge of the sorghum genome that provide for the development of a genetically anchored physical map to guide sequence assembly and annotation, the growing role of the sorghum genome as a nucleation point for comparative genomics of diverse tropical grasses including many leading crops, and the need for dramatically increased sorghum production to sustain human populations in many regions where its inherent abiotic stress tolerance makes it an essential staple. This report reviews current knowledge of the sorghum genome, a community-endorsed schema for integrating this knowledge into a finished sequence, and early plans for translating the sequence into sustained advances to benefit a worldwide group of stakeholders.

Genomic Regions Influencing Seminal Root Traits in Barley.

Water availability is a major limiting factor for crop production, making drought adaptation and its many component traits a desirable attribute of plant cultivars. Previous studies in cereal crops indicate that root traits expressed at early plant developmental stages, such as seminal root angle and root number, are associated with water extraction at different depths. Here, we conducted the first study to map seminal root traits in barley (Hordeum vulgare L.). Using a recently developed high‐throughput phenotyping method, a panel of 30 barley genotypes and a doubled‐haploid (DH) population (ND24260 × ‘Flagship’) comprising 330 lines genotyped with diversity array technology (DArT) markers were evaluated for seminal root angle (deviation from vertical) and root number under controlled environmental conditions. A high degree of phenotypic variation was observed in the panel of 30 genotypes: 13.5 to 82.2 and 3.6 to 6.9° for root angle and root number, respectively. A similar range was observed in the DH population: 16.4 to 70.5 and 3.6 to 6.5° for root angle and number, respectively. Seven quantitative trait loci (QTL) for seminal root traits (root angle, two QTL; root number, five QTL) were detected in the DH population. A major QTL influencing both root angle and root number (RAQ2/RNQ4) was positioned on chromosome 5HL. Across‐species analysis identified 10 common genes underlying root trait QTL in barley, wheat (Triticum aestivum L.), and sorghum [Sorghum bicolor (L.) Moench]. Here, we provide insight into seminal root phenotypes and provide a first look at the genetics controlling these traits in barley.

Season, nitrogen rate, and plant type affect nitrogen uptake and nitrogen use efficiency in rice

Studies were undertaken in the Burdekin River Irrigation Area of northern Australia to improve the efficiency of nitrogen (N) use for rice (Oryza sativa L.) production. The aim was to maximise grain yield by optimising its functional components: N uptake, efficiency of N use for dry matter production (NUE(dm)), and harvest index (HI). The effects of season (wet and dry), N rate (0, 70, 140, 210, and 280 kg/ha), and plant type (maturity and stature) on N uptake, NUE(dm), and HI were examined in 2 wet and 2 dry seasons. Leaf area development was closely related to N uptake. In the wet season, genotypes had similar rates of increase in leaf area index (LAI) with N uptake but differed in the level of LAI (curves were parallel). In the dry season, the relationship between N uptake and LAI was different for each genotype (curves not parallel). In both seasons cv. Newbonnet generally had a lower LAI per unit N uptake (i.e. leaf area production was not excessive) than cvv. Lemont and Starbonnet. Dry matter production and grain yield were also closely related to N uptake. At low levels of N availability (N uptake 140 kg/ha in all seasons, i.e. fertiliser N uptake efficiency declined with increasing N rate. Nitrogen was used more effectively by the rice crop to produce grain compared with non-grain parts when average daily mean temperatures were lower during the period between panicle initiation and anthesis. Genotypic variation was found in N uptake, NUE(dm), and HI. The ability to capture these components in crop improvement programs depends on the extent to which genetic linkages between N uptake and both NUE(dm) and HI can be broken. While our data suggest that N uptake is generally negatively correlated with both NUE(dm) and HI, there is some evidence that these linkages can be broken. For example, the fact that HI did not change with increasing N uptake in Lemont and, to a lesser extent, in Newbonnet suggests that HI does not always decline with increasing N uptake. The example of Newbonnet suggests that, to some extent, it is possible to increase yield by increasing each of the functional components independently within a specific genotype.

Phenotyping novel stay-green traits to capture genetic variation in senescence dynamics

Stay-green plants retain green leaves longer after anthesis and can have improved yield, particularly under water limitation. As senescence is a dynamic process, genotypes with different senescence patterns may exhibit similar final normalised difference vegetative index (NDVI). By monitoring NDVI from as early as awn emergence to maturity, we demonstrate that analysing senescence dynamics improves insight into genotypic stay-green variation. A senescence evaluation tool was developed to fit a logistic function to NDVI data and used to analyse data from three environments for a wheat (Triticum aestivum L.) population whose lines contrast for stay-green. Key stay-green traits were estimated including, maximum NDVI, senescence rate and a trait integrating NDVI variation after anthesis, as well as the timing from anthesis to onset, midpoint and conclusion of senescence. The integrative trait and the timing to onset and mid-senescence exhibited high positive correlations with yield and a high heritability in the three studied environments. Senescence rate was correlated with yield in some environments, whereas maximum NDVI was associated with yield in a drought-stressed environment. Where resources preclude frequent measurements, we found that NDVI measurements may be restricted to the period of rapid senescence, but caution is required when dealing with lines of different phenology. In contrast, regular monitoring during the whole period after flowering allows the estimation of senescence dynamics traits that may be reliably compared across genotypes and environments. We anticipate that selection for stay-green traits will enhance genetic progress towards high-yielding, stay-green germplasm.