Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2006
Publication Date: 5/1/2006
Citation: Delwiche, S.R., Graybosch, R.A., Hansen, L.E., Souza, E., Dowell, F. 2006. Single kernel near-infrared analysis of tetraploid (durum) wheat for classification of the waxy condition. Cereal Chemistry. 83(3):287-292.
Interpretive Summary: The endosperm of durum wheats, used in the production of pasta, consists primarily of starch, protein, and to a lesser extent, lipids and nonstarchy polysaccharides. Starch, being the largest fraction, consists of two macromolecules, amylose and amylopectin in approximate relative proportions of 1:3 in natural conditions. Amylose, being comprised of long linear chains of glucopyranosyl units, is synthesized in amyloplasts through the control of an enzyme called granule bound starch synthase (GBSS), otherwise known as the waxy protein. Without the presence of the functional alleles of the gene that encodes for GBSS, the starch is essentially devoid of amylose and thus exhibits unique processing and functional characteristics. Currently, wheat programs in North America and elsewhere are exploring the development of these types of waxy durum wheats for possible new and unique uses. Plant breeders must rely on slow chemical techniques to identify waxy seed during development of new lines. Even more difficult is the task of identifying durum seed that is neither waxy nor natural (wild type) in state, but instead contains one functional allele and one null allele for the waxy condition. The current study explored the potential of a rapid, nondestructive method for determination of the waxiness state. This method, which is optically based, examines each kernel in the near-infrared wavelength region of 1000 to 1700 nm. Two seasons of durum breeders' lines were used in the study, involving 48 kernels from each of 142 samples. Statistical classification algorithms were developed in the attempt to separate the four states of the waxy condition (wild type, waxy, and two states of partial waxy). The results indicated that identification of waxy seed could occur at greater than 95% accuracy; however, discrimination among the three other classes was not possible. Chemical measurements of amylose content confirmed that the amylose contents of the partial waxy lines were virtually indistinguishable from those of wild type lines, thus corroborating the near-infrared classification findings. Fortunately, in waxy breeding programs, the identification of the waxy seed is the most critical. Therefore, this technique stands as a demonstrated tool for use in breeding programs. Further, when waxy wheats become commercialized, this technique can have application in the marketing and processing channels, where a rapid means to identify and identity-preserve seed of this condition will be necessary.
Technical Abstract: Plant breeding programs are currently active worldwide in the development of waxy hexaploid (bread) and tetraploid (pasta) wheats. It is generally believed that by means of conventional breeding practices, waxy cultivars, adapted to their intended geographical region will confer unique processing and end use characteristics such that markets will develop around their use. Less is known of the market potential for wheat releases that would be genetically intermediate for the waxy condition, these being termed partial waxy wheats. Essential to this development, a means to rapidly and, ideally, non-destructively identify the waxy condition will need to be developed for use at the point of sale and to assist breeders in the development of waxy wheat varieties. The study described herein evaluated the effectiveness of near-infrared reflectance single-kernel spectroscopy for classification of durum wheat into its four possible waxiness genotypes, these being wild type, waxy, and the two intermediate states in which a null allele occurs at either of the two homologous genes (Wx-1A and Wx-1B) that encode for the production of the enzyme, granule bound starch synthase, that controls amylose synthesis. Two years of breeders samples, corresponding to 47 unique lines subdivided approximately equally into the four waxiness states, were scanned in reflectance (1000-1700 nm) on an individual kernel basis. Linear discriminant analysis models were developed using the best set of four wavelengths, best four wavelength differences, and best four principal components. Each model consistently demonstrated the high ability (typically greater than 95% of the time) to classify the fully waxy genotype. However, correct classification among the three other genotypes (wild type, wx-A1 null, and wx-B1 null) was generally not possible. Chemical measurements of amylose content confirmed that the amylose contents of the partial waxy lines were virtually indistinguishable from those of wild type lines, thus corroborating the near-infrared classification findings.