Effects of salt and nitrogen on physiological indices and carbon isotope discrimination of wheat cultivars in the northeast of Iran

In order to study the effects of different levels of salt stress and nitrogen(N) on physiological mechanisms,carbon isotope discrimination(△13C),and yield of two wheat cultivars(cv.),a two-year field experiment was carried out during 2013-2015.The treatments included three levels of salt stress(1.3,5.2,and 10.5 dS m^-1),three levels of N(50,100,and 150 kg N ha-1),and two wheat cultivars,Bam and Toos.Under salt stress,N application(100 and 150 kg N ha^-1) produced a significant effect on both cultivars with respect to physiological traits,i.e.,net photosynthetic rate(Pn),stomatal conductance(gs),chlorophyll index(Cl),Na^+/K^+ ratio as well as the grain yield(GY).The salt-tolerant and-sensitive cultivars exhibited the maximum values of physio-biochemical and yield attributes at 100 and 150 kg N ha-1,respectively.The results of △13C showed a significant difference(P<0.001) between wheat cultivars under the control and salt stress.According to our result,salt-tolerant cultivar Bam seems to be more efficient in terms of higher GY,Pn,gs,Cl,and lower Na^+/K^+ ratio as well as higher △13C as compared with salt-sensitive cultivar Toos,under salt stress.Therefore,a significant positive correlation that observed between △13C and GY,indicated that △13C may be an effective index for indirect selection of yield potential in wheat under irrigation regimes with saline water.

Genetic Analysis of Carbon Isotope Discrimination and its Relation to Yield in a Wheat Doubled Haploid Population

Carbon isotope discrimination（△13C） is considered a useful indicator for indirect selection of grain yield（GY） in cereals.Therefore,it is important to evaluate the genetic variation in △13C and its relationship with GY.A doubled haploid（DH） population derived from a cross of two common wheat varieties,Hanxuan 10（H10） and Lumai 14（L14）,was phenotyped for △13C in the flag leaf,GY and yield associated traits in two trials contrasted by water availability,specifically,rain-fed and irrigated.Quantitative trait loci（QTLs） were identified by single locus and two locus QTL analyses.QTLs for △13C were located on chromosomes 1A,2B,3B,5A,7A and 7B,and QTLs for other traits on all chromosomes except 1A,4D,5A,5B and 6D.The population selected for high △13C had an increased frequency of QTL for high △13C,GY and number of spikes per plant（NSP） when grown under rain-fed conditions and only for high △13C and NSP when grown under irrigated conditions,which was consistent with agronomic performance of the corresponding trait values in the high △13C progeny;that is,significantly greater than that in the low △13C.Therefore,selection for △13C was beneficial in increasing grain yield in rain-fed environments.

Relationship between Carbon Isotope Discrimination and Grain Yield in Spring Wheat Cultivated under Different Water Regimes

In C3 plants, carbon isotope discrimination （A） has been proposed as an indirect selection criterion for grain yield. Reported correlations between A and grain yield however, differ highly according to the analyzed organ or tissue, the stage of sampling, and the environment and water regime. In a first experiment carried out in spring wheat during two consecutive seasons in the dry conditions of northwest Mexico （Ciudad Obregon, Sonora）, different water treatments were applied, corresponding to the main water regimes available to spring wheat worldwide, and the relationships between A values of different organs and grain yield were examined. Under terminal （post-anthesis） water stress, grain yield was positively associated with A in grain at maturity and in leaf at anthesis, confirming results previously obtained under Mediterranean environments. Under early （pre-anthesis） water stress and residual moisture stress, the association between grain A and yield was weaker and highly depended on the quantity of water stored in the soil at sowing. No correlation was found between A and grain yield under optimal irrigation. The relationship between A and grain yield was also studied during two consecutive seasons in 20 bread wheat cultivars in the Ningxia region （Northern China）, characterized by winter drought （pre-anthesis water stress）. Wheat was grown under rainfed conditions in two locations （Guyuan and Pengyang） and under irrigated conditions in another two （Yinchuan and Huinong）. In Huinong, the crop was also exposed to salt stress. Highly significant positive associations were found between leaf and grain A and grain yields across the environments. The relationship between A and yield within environments highly depended on the quantity of water stored in the soil at sowing, the quantity and distribution of rainfall during the growth cycle, the presence of salt in the soil, and the occurrence of irrigation before anthesis. These two experiments confirmed the value of A as an indirect selection criterion for yield and a phenotyping tool under post-anthesis water stress （including limited irrigation）.

Drought Tolerant Criterion of Wheat Genotypes Using Carbon Isotopes Discrimination Technique

Drought tolerant analysis using carbon isotopes discrimination could be useful tool to decide a genotype with high adaptation to drought. A pot culture experiment was conducted in glass house at the IAEA （International Atomic Energy Agency） laboratories, Seibersdorf, Austria to analyze drought tolerant of wheat genotype using carbon isotopes discrimination technique. Four wheat genotypes viz., LU-26s, Bhittai, Roshan, Taifu, were tested. There were two treatments： Control （normal irrigation） and Drought （@ 30% field capacity of the control treatment）. Drought treatment was imposed after two weeks of germination. The experiment was continued up to 10 weeks after germination and was terminated at flowering stage. The data were recorded in terms of plant height, number of tillers, shoot dry weight and carbon isotopic ratio （δ）. Results showed that genotype LU-26s was found to have best performance under drought condition, with minimum decrease in the growth parameters i.e., plant height, number of tillers and shoot dry weight. High CID （carbon isotopes discrimination） values were also observed in genotype LU-26s, showing close positive correlation between SDW （shoot dry weight） and CID. It is therefore concluded that carbon isotopes discrimination can be an important criterion for the selection of wheat genotypes for drought prone areas.

Contribution of the ear and the flag leaf to grainfilling in durum wheat inferred from the carbon isotope signature: Genotypic and growing conditions effects

The ear, together with the flag leaf, is believed to play a major role as a source of assimilates during grain filling in C3cereals. However, the intrusive nature of most of the available methodologies prevents reaching conclusive results in this regard. This study compares the carbon isotope composition（d13C） in its natural abundance in the water‐soluble fractions of the flag leaf blade and the ear with the d13C of mature kernels to assess the relative contribution of both organs to grain filling in durum wheat（Triticum turgidum L. var.durum）. The relative contribution of the ear was higher in landraces compared to modern cultivars, as well as in response to nitrogen fertilization and water stress. Such genotypic and environmentally driven differences were associated with changes in harvest index（HI）, with the relative contribution of the ear being negatively associated with HI. In the case of the genotypic differences, the lower relative contribution of the ear in modern cultivars compared with landraces is probably associated with the appearance in the former of a certain amount of source limitation driven by a higher HI. In fact, the relative contribution of the ear was far more responsive to changes in HI in modern cultivars compared with landraces.

Some Physiological Processes Related to Water Use Efficiency of Higher Plants

Water use efficiency （WUE） of higher plants is of vital importance in the dry-land agricultural ecosystem in terms of the development of water-saving agriculture. Of all the approaches used to improve WUE, the intrinsic water use efficiency （WUET, the ratio of CO2 assimilation rate to transpiration rate） can be a right index, as the variation of WUET is correlated with the physiological and biochemical processes of higher plants. The measurements of leaf gas exchange and carbon isotope discrimination （D^13C） are the two ways to detect the variation in WUEr. This article reviewed some physiological processes related to WUEv, including the relationship between CO2 assimilation and stomatal conductance and WUET and water absorption. The relationship between WUE and aquaporin and the yield are discussed as well.