Yadav, A. K., Singh, S., Dhyani, D. and Ahuja, P. S. 2011. A review on the improvement of Stevia [Stevia rebaudiana (Bertoni)]. Can. J. Plant Sci. 91: 1-27. Stevia rebaudiana (Bertoni) is a herbaceous perennial plant (2n=22) of genus Stevia Cav., which consists of approximately 230 species of herbaceous, shrub and sub-shrub plants. Leaves of stevia produce diterpene glycosides (stevioside and rebaudiosides), non-nutritive, non-toxic, high-potency sweeteners and may substitute sucrose as well as other synthetic sweetners, being 300 times sweeter than sucrose. In addition to its sweetening property, it has medicinal values and uses. Stevia is self-incompatible plant and the pollination behaviour is entomophilous. Rebaudioside-A is of particular interest among the glycosides produced in the leaves of stevia because of the most desirable flavour profile, while, stevioside is responsible for aftertaste bitterness. Development of new varieties of S. rebaudiana with a higher content of rebaudioside-A and a reduced content of stevioside is the primary aim of plant breeders concerned with the improvement and utilization of this source of natural sweeteners. The proportions of rebaudioside-A and -C are controlled by a single additive gene known to be co-segregating suggesting synthesis by the same enzyme. Stevioside and rebaudioside-A are negatively correlated, while rebaudioside-A and -C are positively correlated. Conventional plant breeding approaches such as selection and intercrossing among various desirable genotypes is the best method for improving quality traits in a highly cross-pollinated crop like stevia. Various plant types with larger amounts of specific glycoside have already been patented, such as RSIT 94-1306, RSIT 94-75, RSIT 95-166-1 through selection and intercrossing. Composites and synthetics can be used to capture part of the available heterosis because of the high degree of natural out-crossing and the absence of an efficient system of pollination control. Synthetics and composites like “AC Black Bird“ and “PTA-444“ have already been developed. Polyploidy results in better adaptability of individuals and increased organ and cell sizes. Tetraploids have larger leaf size, thickness and have potential use in increasing biomass and yield in comparison with diploid strains. Characters of interest with low variability in the population may be improved through mutation breeding. Use of biotechnological approaches, such as tissue culture for the mass propagation of elite genotypes, anther culture for development of pure homozygous doubled haploid and molecular marker technology for identification of marker loci linked to rebaudioside-A trait, can create new opportunities for plant breeders. Understanding the mechanism and pathway of biosynthesis of steviol glycosides can help to improve the glycoside profile by up-regulation and down-regulation of genes.
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