Physiological Traits Contributing to Water Productivity and Yield Stability of Barley on the Canadian Prairies

Even though some progress has been made in developing high yielding varieties, average barley (Hordeum vulgare L.) yield have remained flat on the Canadian Prairies over the last decade. This supports the notion that genetic gain in yield is difficult under rainfed conditions due to low heritability, complexity of yield components and significant genotype x environment interactions. To improve efficiency and increase potential yield, barley breeding must exploit new strategies to complement traditional breeding approaches. In the short growing season of the Canadian Prairies, moisture is often not limiting early in the season but may become limiting in mid or late season. Targeting physiological traits that affect yield at different stages rather than yield per se should lead to increases in yield and yield stability. This presentation describes on-going research to improve barley yield by focussing on traits related to early seedling vigour, transpiration efficiency and stem carbohydrate reserves. High early seedling vigour allows rapid canopy formation, thereby reducing soil evaporation and increasing canopy transpiration. High transpiration efficiency ensures more assimilates fixed per unit water use when water becomes limiting in the mid to late season. Improved capacity to accumulate assimilates in vegetative organs, with an increased capacity to remobilize, will enhance grain filling and reduce yield losses to terminal stresses such as drought and heat that severely inhibit photosynthesis. Media Summary Physio-breeding approaches to improve water productivity and yield stability of barley on the Canadian Prairies Key Worlds: Water-use, transpiration efficiency, early vigour, non structural carbohydrates, physio-breeding Introduction Average barley yields on the Canadian Prairies are low and unstable due to seasonal and within season variation in soil moisture availability across locations. Genetic gain in grain yield under rainfed conditions with traditional breeding technique is difficult to achieve due to low heritability of yield traits and significant genotype x environment (G x E) interactions (Ceccarelli et al., 1991). Advances in molecular biology such as quantitative trait loci (QTL) mapping and their application to directly breed for yield have also proved difficult (Reyna & Sneller 2001). For traditional or biotechnological approaches directed to yield improvement, understanding and targeting of the physiological determinants of yield at crop level rather than yield per se has been suggested (Richards et al., 2002). Several factors limit the application of physiological traits in plant breeding despite the abundant knowledge and literature on their relevance to crop yield (see Richards et al., 2002). Measurement of physiological traits can be time consuming and laborious to be applied directly in a breeding program. For physiological traits to be attractive to the breeder, they must be easy to quantify on a large scale under field conditions, have higher heritability than yield, and stable across several locations. Advances in biotechnology and in the measurement of physiological traits would enable plant breeders to deploy some physiological traits in their programs. For example, the development of molecular markers diagnostic of quantitative trait loci (QTLs) of physiological traits related to yield potential can improve selection efficiency of yield through marker assisted breeding. At the 4ICSC in Brisbane Australia, Dr Richard Richards described several examples where the application of physiological trait has resulted in improvement of yield in dry environments. He explained that physiological traits for water limited environments may not be universal, some may prove useful in one region but detrimental in another. In the short growing season of the Canadian Prairies, moisture availability is often not a limiting factor for crop growth early in the season but may become limiting in mid or late season. Therefore targeting of physiological traits that affect yield of barley to specifically match growth stage and moisture availability rather than yield per se should lead to increases in yield and yield stability. In this paper, we describe a physio-breeding approach, defined here as breeding for physiological traits, that targets early seedling vigour, transpiration efficiency and stem non structural carbohydrate (NSC) storage and remobilization capacity and how their use may lead to increased water productivity (defined here as the yield per available moisture) and yield stability of barley on the Canadian Prairie. Characteristics of Canadian Prairies and spring barley development The Canadian Prairie is characterized by short and dry growing season. Precipitation during the growing season in not sufficient to meet crop demand. Crop production therefore relies on water stored in the soil from fall and winter precipitation in addition to within season precipitation, which is often minimal. Heat stress is a common occurrence in the summer during anthesis and grain filling. Frost damage can be expected later in the season just before the crop is fully matured. The winter season is long and cold with snow cover, and frozen soil. The soils are relatively flat with poorly defined drainage, which often leads to flooding and water logging in spring. The soil moisture availability in the growing season is highly unpredictable with season to season and within season variation. The developments of spring barley can be broadly divided into three stages (Figure 1) including establishment and growth, pre-anthesis and post anthesis stages. Stages: Establishment & Growth Foundation for future yield Post-Anthesis determinant of actual yield Phase: Reproductive Vegetative Usually good moisture Moisture is limiting without in-season precipitation. Drought and heat stress common. Growth Conditions Pre-Anthesis Formation of yield potential Figure 1: Typical spring barley development in relation to soil moisture (Modified after Feekes scale) In a typical year, there is adequate moisture available for seedling establishment and growth, which is the critical stage at which the foundation yield is set. As the season progresses, the soil moisture is rapidly depleted such that at the time of anthesis to grain filling water deficit may occur if within season rainfall is low. Assuming that moisture was adequate early in the season, the final yield attained at harvest would depend on how well moisture was conserved and how much within season precipitation was received. Water productivity and physiological traits Water productivity and yield stability of spring barley can be enhanced by manipulation of traits contributing to the different stages of development to match the growing conditions (Figure 1). We have identified the following traits in our barley improvement program as those relevant to improving water productivity and maintaining yield of barley on the Canadian Prairies. 1) Early seedling vigour Spring barley is often the last crop sown by farmers on the Canadian Prairie. Early seedling vigour, used here to refer to an increase in seedling leaf area, would help plants to form a canopy more quickly, reducing water evaporation from the soil surface, increasing canopy transpiration and inhibiting the growth of weeds (Lemerle et al. 2001). To assess early seedling vigour, presence of a coleoptile tiller, leaf area and specific leaf area (SLA) of the second leaf at BBCH 13 and leaf area index (LAI) at BBCH 30 were assessed at three field locations representing different moisture zones in Alberta, Canada. The interval between germination and the appearance of the first two seedling leaves was previously identified to be responsible for the greater vigour of barley compared with wheat (Lopez-Castafied et al., 1995). Seed weight, embryo size and seed vigour were determined to see if seed characteristics also affect early seedling vigour. When data were averaged across locations, large genotypic differences in all leaf traits were observed. Leaf area of extreme genotypes differed by 8.5 cm 2 and 5.6 cm 2 for two-row and six-row genotypes, respectively (Table 1). Table 1: Mean and range of leaf-2 area, LAI and SLA of a set of 120 barley genotypes under field conditions Genotype Leaf-2 area (cm 2 ) LAI SLA (cm 2 /g) 2-row Mean 10.04 3.18 372.3 Range 6.5–15.0 2.4-4.3 267-470 6-row Mean 10.06 3.33 379.5 Range 7.9-13.5 2.5-4.2 275-626 Similarly, LAI differed by 1.9 and 1.7 while SLA differed by 203 cm 2 /g and 351 cm 2 /g for two-row and six-row genotypes, respectively. Except for SLA, genotypic performance was consistent for traits measured under field or greenhouse conditions, which may suggest high heritability and the possibility of screening of breeding lines in a greenhouse for these traits. 2) Transpiration efficiency (TE) Stem elongation (when formation of yield potential begins) to head emergence often take place when the soil available moisture is gradually becoming a limiting factor to crop growth and development. This is the stage when TE begins to become an important trait. Maintaining high transpiration efficiency will allow plants to set a good foundation for yield and ensure that some water will be available to fill the grains later in the season. Theory suggests that plants assimilate stable isotopes of carbon at different rate because of discrimination against 13 C relative to 12 C (Richards et al., 2002). Results from the wheat breeding program in Australia have shown that selection for TE by way of low leaf carbon isotope discrimination (CID) can enhance yield in drier years (Rebetzke et al., 2002). We validated the relationship between WUE and CID of barley in greenhouse and field experiments (Anyia et al., 2007). y = -0.5242x + 15.211 R = 0.8943