Relationship between carbon isotope discrimination, grain yield and water use efficiency in bread wheat under well-watered conditions
Hamid Khazaei1 , Philippe Monneveux2, Shahram Mohammady1 and Zheng-bin Zhang3
1Faculty of Agriculture, University of Shahrekord, P.O. Box 115, Iran
2Generation Challenge Programme, c/o CIMMYT, A.P. 6-641, 06600 Mexico D.F., Mexico
3Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050021, China
Corresponding author: Hamid Khazaei
E-mail: firstname.lastname@example.org, Phone: +989125253792
The relationship between carbon isotope discrimination (Δ), grain yield, and water use efficiency for biomass (WUEB) and grain yield (WUEG) were analyzed in a set of bread wheat (Triticum aestivum L. subsp. aestivum) genotypes including seven landraces as well as the Iranian cultivar Zakros. No significant relationship was found between Δ and grain yield. A significant negative correlation was noted between Δ, WUEG and WUEB. The Iranian landraces and the modern cultivar Zakros mainly differed for grain yield, harvest index and WUEG. These results indicate that under well-watered conditions, Δ does not represent an accurate secondary trait for yield but can be used to estimate water use efficiency. They also suggest that WUEG under well-watered conditions has been improved by modern selection through an increase of harvest index rather than of transpiration efficiency.
The use of carbon isotope discrimination (Δ) has been proposed by several authors to select C3 crops for grain yield and water use efficiency (WUE). In bread wheat (Triticum aestivum L. subsp. aestivum), Ehdaie and Waines (1993) reported a negative correlation between Δ and WUE while the relationship between Δ and grain yield was found highly dependent on the analyzed organ or tissue, the stage of sampling, and the environment and water regime (Monneveux et al. 2005).
The relationship between Δ, grain yield and WUE has been investigated in a set of Iranian bread wheat including seven landraces as well as the modern Iranian cultivar Zakros. Landraces still represent an important part of the 6.3 million hectares wheat area in Iran (Moghaddam et al. 1997). Little is known, however, about their efficiency to use available water.
Material and Methods
Plants were grown in a greenhouse, in plastic pots (one seedling per pot) filled with 1 kg of soil composed of 42% sand, 36% silt and 22% clay and placed in a randomized complete block design with three replications. Each pot was brought to water holding capacity. Pots were weighted every 2 or 3 day and amounts of water equal to the loss in weight were added until flag leaf yellowing. The total amount of water used was calculated as the difference between final and initial weight of the pot plus the total amount of water supplied to each pot and included both transpired and evaporated water (Ehdaie et al. 2003). Carbon isotope composition (δ13C) of grain samples was determined using an elemental analyzer isotope ratio mass spectrometer (University of Winnipeg Isotope Laboratory, Canada). δ13C was calculated by comparing 13C to 12C composition of the samples (Rsample) relative to the composition of the Pee Dee Belemnite standard (RPDB): δ13C (permil)= [(Rsample / RPDB −1]*1000. Δ value of the samples was obtained according to the formula: Δ (permil) = (δa−δp)/(1+δp), where δp is the δ13C of the plant sample and δa is the δ13C of the atmospheric CO2. δa was assumed to be −8 permil(Farquhar et al. 1989). At physiological maturity, aerial parts and roots were collected and dried separately. Grain yield per plant and total (aerial and root) biomass, were determined. Water use efficiency for biomass production (WUEB) and for grain production (WUEG) were calculated according to Ehdaie and Waines (1993) as total biomass per plant kg-1 water used and grain yield per plant kg-1 water used, respectively.
Results and Discussion
A wide variation was observed among accessions for Δ, which was in the range 18.60-21.20 permil. No significant relationship was found between Δ, grain yield and total biomass (Fig. 1). A significant negative correlation was noted between Δ, WUEG and WUEB (Fig. 2). The Iranian landraces and the modern cultivar Zakros mainly differed for grain yield, harvest index and WUEG (Table 1).
The lack of correlation between Δ and grain yield in bread wheat grown well-watered conditions has been already reported by Monneveux et al. (2004 and 2005), Misra et al. (2006) and Xu et al. (2007). Under well-watered conditions, stomatal conductance is likely to be high in all cultivars resulting in increased Ci/Ca, the ratio of intercellular to atmospheric CO2, and Δ values (Morgan et al. 1993), while increased photosynthetic capacity potentially decreases Ci/Ca. The decrease in Ci/Ca associated with increased photosynthetic capacity is consequently offset by the Ci/Ca increase resulting from stomatal aperture, hence reducing the possibility of association between Δ and grain yield.
The significant negative correlation noted between Δ, WUEG and WUEB is in good agreement with Knight et al. (1994) who reported negative associations between Δ and WUE in six field-grown crops, including bread and durum wheat, and with Ehdaie and Waines (1993) who found a negative correlation between Δ and WUE among bread cultivars from China, Iran and CIMMYT. According to Passioura (1977), higher WUEG can be obtained by moving more of the available water through the crop (higher ratio of transpired to evaporated water), by acquiring more carbon in exchange for the water transpired by the crop (higher transpiration efficiency), or by partitioning more of the achieved biomass into the grain (higher harvest index). While the ratio of transpired to evaporated water is mainly under the dependence on crop and water management, transpiration efficiency and harvest index variations are mainly driven by genetic factors (Gregory et al. 1997). Transpiration efficiency was shown to be negatively associated to Δ (Farquhar et al. 1982). In the present study, significant differences were found between Iranian landraces and the modern cultivar Zakros for harvest index and not for Δ, suggesting that the difference observed for WUEG between the two types of germplasm was due to carbon partitioning rather than to transpiration efficiency differences. Similar results have been obtained by Khazaei et al. (2008) in a set of Iranian diploid, tetraploid and hexaploid wheats. The results of this study all together indicate that under well-watered conditions, Δ does not represent an accurate predictor of yield but can be used for estimating WUE. They also suggest that WUEG under well-watered conditions has been improved by modern selection mainly through an increase of harvest index.
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