Stable radicals and biochemical compounds in embryos and endosperm of wheat grains differentiating sensitive and tolerant genotypes--EPR and Raman studies

The aim of this study was to uncover the specific species in grains that might differentiate the wheat genotypes according to their tolerance to oxidative stress. Measurements by EPR and Raman spectroscopy techniques were used to examine whole grains and their parts (embryo, endosperm, seed coat) originating from four wheat genotypes with differing tolerance to drought stress. Raman spectra showed that, in spite of the similar amounts of proteins in whole grains from tolerant and sensitive genotypes, in tolerant ones they were accumulated mainly in embryos. Moreover, in embryos from these grains, a higher content of unsaturated fatty acids was observed. Endosperm of grains from the tolerant genotype, richer with starch than that of sensitive one, exhibited higher content of amylopectin. Detailed analysis of EPR signals and simulation procedures of the spectra allowed the estimation of the nature of interactions of Fe(III) and Mn(II) with organic and inorganic structures of grains and the character of organic stable radicals. Three types of these radicals: carbohydrate, semiquinone and phenoxyl, were identified. The amounts of these radicals were higher in grains of sensitive genotypes, mostly because of differences in carbohydrate radical content in endosperm. Taking into account the level of radical concentration and greater capacity for radical formation in grains from plants of lower tolerance to stress, the content of radicals, especially of a carbohydrate nature, was considered as a marker of the plant resistance to stress conditions.

Electron paramagnetic resonance (EPR) spectroscopy characterization of wheat grains from plants of different water stress tolerance

Grains of five genotypes of wheat (four Polish and one Finnish), differing in their tolerance to drought stress were chosen for this investigation. Electron paramagnetic resonance spectroscopy allowed observation of transition metal ions (Mn, Fe, Cu) and different types of stable radicals, including semiquinone centers, present in seed coats, as well as several types of carbohydrate radicals found mainly in the inner parts of grains. The content of paramagnetic metal centers was higher in sensitive genotypes (Radunia, Raweta) than in tolerant ones (Parabola, Nawra), whereas the Finnish genotype (Manu) exhibited intermediate amounts. Similarly, the concentrations of both types of radicals, carbohydrates and semiquinone were significantly higher in the grains originating from more sensitive wheat genotypes. The nature of carbohydrate radicals and their concentrations were confronted with the kinds and amounts of sugars found by the biochemical analyses and microscopy observations. It is suggested that some long lived radicals (semiquinone and starch radicals) occurring in grains could be indicators of stress resistance of wheat plants.