Study On The Application Of Carbon Isotope Discrimination In Breeding For Spring Wheat Water-Saving Varieties

Carbon isotope discrimination (Δ¹³C) has been reported to correlate negatively with transpiration efficiency (TE) and yield in C3 plants and been proposed as indirect selection criterion for transpiration efficiency and grain yield in wheat. Considering the high cost forΔ¹³C analysis, attempts have been made to identify alternative screening criteria. In order to study the relationships between carbon isotope discrimination (Δ), yield and morphophysiological traits, to evaluate the value ofΔ¹³C in selecting high WUE (water use efficiency) wheat varieties and to screen the surrogates ofΔ¹³C, field experiments were conducted in three wheat growing regions with contrasting ecological conditions, i.e. Yinchuan (limited irrigation conditions), Huinong (limited irrigation conditions + salinity) and Guyuan (rain-fed condition) of the Ningxia region (North-East of China) in years, and outdoor pot experiment, with three water treatments (45%±5% FC, 55%±5%FC and 75%±5%FC), was carried out. 20 bread wheat cultivars, including landraces, released cultivars and advanced lines, were used in field experiments, while 10 wheat genotypes were used in pot experiment. Leaf, grain and basal stemΔ¹³C were analyzed, some morphophysiological traits such as, ash content (ma), SLDW, gas exchange parameters, relative water content (RWC), were measured. The genetic variation in these measured traits was studied among different experimental sites in one year and among different years in one experimental site. Relationships betweenΔ¹³C, grain yield (GY), and the morphophysiological traits measured were analyzed. Correlations betweenΔ¹³C, harvest index (HI) and above-ground biomass (AGB) were also analyzed. In 2008, specific stem dry weight (SSDW) and carbohydrate in stem after anthesis were determined. The accumulation efficiency and remobilization efficiency of carbohydrate contents were also calculated. In 2008, F6 population derived from three crossing combinations were planted in Yinchuan and Guyuan. Randomized complete block experiment was carried out. The yield, carbon isotope discrimination in flag leaf and basal stem were measured, respectively.The results showed that: i) Differences for the measured traits between different locations were highly related to the variation in water availability. GrainΔ¹³C (ΔGm) was found to correlated positively to grain yield in Yinchuan (r=0.737, P<0.001), and to HI in Yinchuan and Huinong(r=0.637, P<0.01; r=0.472, P<0.05). Flag leafΔ¹³C (ΔLa) correlated positive with HI in Huinong and Guyuan (r=0.605; P<0.01; r=0.509; P<0.05). Flag leafΔ¹³C positively correlated to RWC at anthesis in Guyuan(r=0.474, P<0.05), and negatively to SLDW at grain-filling in Huinong(r=-0.528, P<0.05). In Huinong, flag leafΔ¹³C was negatively related to dry matter weight per plant at anthesis(r=-0.475, P<0.05)which was found to associated negatively with flag leafΔ¹³C and grainΔ¹³C in Guyuan (r=-0.626; P<0.01; r=-0.606; P<0.01). There were significant negative correlations between biomass at maturity, flag leafΔ¹³C and grainΔ¹³C in Yinchuan (r=-0.531; P<0.05; r=-0.539; P<0.05). Flag leafΔ¹³C was also noted to biomass at maturity in Huinong r=-0.479, P<0.05). Flag leaf temperature (LT) at anthesis was found to be negatively correlated to flag leafΔ¹³C and grainΔ¹³C in Yinchuan (r=-0.485; P<0.05; r=-0.521; P<0.05), to flag leafΔ¹³C in Huinong (r=-0.475, P<0.05), and to grainΔ¹³C in Guyuan (r=-0.498, P<0.05). Flag leaf temperature was also negatively correlated to stomata (gs) in all the three sites (r=-0.856; P<0.01; r=-0.638; P<0.01; r=-0.517; P<0.05).ii) In the outdoor pot experiment carried out in Yinchuan, the traits measured showed significant differences among the three water treatments. There were variations inΔLa between the genotypes derived from contrasting environments. The improved varieties or advanced lines bred in irrigated areas displayed higherΔ¹³C values, while the improved and local varieties bred in rain-fed areas exhibited lowerΔ¹³C values. There were negative correlations between flag leafΔ¹³C and transpiration efficiency at anthesis(r=-0.746, P<0.05; r=-0.696, P<0.05), while grain ? significantly and negatively correlated with transpiration efficiency under severe drought (T1) and well watered (T3) treatments(r=-0.852, P<0.001; r=-0.857, P<0.001). Significant positive correlations were found betweenΔ¹³C and ma in seedlings and second fully developed leaves at elongation stage and in flag leaves at anthesis stage in severe drought treatment (T1) (r=0.790, P<0.01; r=0.788, P<0.01; r=0.656, P<0.05, respectively). GrainΔ¹³C was correlated negatively with grain ash content under moderate drought (T2) and T3 treatments(r=-0.695, P<0.05; r=0.721, P<0.05). Flag leafΔ¹³C was negatively associated with potassium (K) content in flag leaves in T2 (r=0.813, P<0.01) and positively with magnesium (Mg) content in flag leaves in T2 and T3 (r=0.725, P<0.05; r=0.826, P<0.01, respectively) at anthesis. Significant positive correlations betweenΔ¹³C and calcium (Ca) content in flag leaves at anthesis (r=0.708, P<0.05), and betweenΔ¹³C and calcium (Ca) content in grain at maturity (r=-0.649, P<0.05) in T3 were also found in this study. GrainΔ¹³C was negatively associated with Mg content in flag leaf at maturity under all the three water treatments(r=-0.753, P<0.01; r=-0.668, P<0.05; r=-0.695, P<0.05). Harvest index was associated with ma and Mg content in mature leaf under severe drought condition and Ca content in grain under moderate drought condition. Correlation was also found between K content in grain and GY.iii) In the multi-year experiments in Yinchuan, yield was positively correlated with grainΔ¹³C (r=0.601, P<0.01; r=0.611, P<0.01), basal stemΔ¹³C (ΔSm) (r=0.781, P<0.001; r=0.561, P<0.01 ) were noted in 2007 and 2008, and with flag leafΔ¹³C in 2008 (r=0.503, P<0.05) when stress occurred after anthesis, but not in 2006 when soil water condition was nearly optimal. Ash content in mature flag leaf (maLm) correlated positively to grainΔ¹³C in 2006 and 2007(r=0.542, P<0.05; r=0.600, P<0.01), to basal stemΔ¹³C in 2007 (r=0.540, P<0.05), and to harvest index (r=0.538, P<0.05; r=0.568, P<0.01) in 2006 and 2007. In 2008, stem water-soluble carbohydrate content (WSC) at grain-filling was found to associated positively with grainΔ¹³C and basal stemΔ¹³C (r=0.553, P<0.05; r=0.446, P<0.01). Specific stem dry weight 7 d after anthesis was positively associated with flag leafΔ¹³C and basal stemΔ¹³C (r=0.567, P<0.01; r=0.453, P<0.05). Significant and negative correlation was recorded between stem WSC at maturity and yield (r=-0.475, P<0.05). There were positive correlations between remobilization efficiency of stem water-soluble carbohydrate, yield, flag leafΔ¹³C, grainΔ¹³C, and basal stemΔ¹³C(r=0.562, P<0.01; r=0.463, P<0.05; r=0.454, P<0.05; r=0.490, P<0.05). The remobilization efficiency of total carbohydrate (the sum of water-soluble carbohydrate and starch) was also found to be significantly and positively correlated to grainΔ¹³C and basal stemΔ¹³C (r=-0.473, P<0.05; r=-0.445, P<0.05). There were negative relationships betweenΔGm,ΔSm, and SSDW at maturity (r=-0.473, P<0.05; r=-0.445, P<0.05).iv) In Guyuan site, theΔ¹³C values of F6 population originated from highΔ¹³C×lowΔ¹³C crossing combination were between that of highΔ¹³C parent and lowΔ¹³C parent, theΔ¹³C values of F6 population originated from lowΔ¹³C×lowΔ¹³C were higher than their parents. The yield of F6 population originated from highΔ×lowΔcombination was higher than that from lowΔ×lowΔcombination. There was no F6 population derived from lowΔ¹³C×lowΔ¹³C, whose yield exceeded their parents, while a lot of elite lines were found in highΔ¹³C×lowΔ¹³C, with yield exceeding their high-yielding parent. The deviation of yield in the latter was larger than the former.To sum up, 1) HighΔ¹³C is the selection target in irrigated region, while the application ofΔ¹³C in breeding program depends the soil water content in rain-fed region. In extremely dry year, theΔ¹³C is not supposed to be used. Instead, in the year when the soil water stress is not very severe, theΔ¹³C, integrated with other traits, could be used. 2) Among theΔ¹³Cs in the three organs, basal stemΔ¹³C which showed strongest association with transportation of carobydrates to grain, harvest index and phenology, seems to be the best indirect indicator of yield. 3) The K, Mg contents in flag leaf at anthesis, specific leaf dry weight at grain-filling stage, ash content, K and Mg contents in flag leaf at maturity were found to have close association withΔ¹³C, and are suggested to be used in breeding program. 4) HighΔ¹³C×highΔ¹³C seems to be optimal crossing combination and the selection of progenies with highΔvalue would lead to yield gain in limited irrigation system (Yinchuan), while the genotypes with contrastingΔ¹³C values should be adopted as parents in breeding program in rain-fed system (Guyuan).