Variation in tissue culture
In a historically significant review, Larkin and Scowcroft proposed the 'somaclones' to the regenerants coming out of tissue culture, irrespective of the explant used. Variation displayed by such regenerants from tissue culture would then be 'somaclonal variation'. In perennial crops that are asexually propagated, somaclonal variation offers excellent opportunity to add new genotypes to the gene pool. Variation induced by physiological factors can be identified quite early on and often without the aid of any tools. Classic examples of such variation are those induced by habituation to plant growth regulators (PGRs) in culture and culture conditions. Genetic variation occurs among tissue culture regenerants as a result of alterations at the chromosomal level. Several biochemical variations have been identified in various crop plants in tissue culture. Examples of such biochemical variation include alterations in carbon metabolism leading to lack of photosynthetic ability, starch biosynthesis, carotenoid pathway, nitrogen metabolism, and antibiotic resistance.