Insoluble Solids

Tomato fruit insoluble solids contribute to fruit viscosity and firmness. Insoluble solids are comprised of water insoluble solids (WIS) and alcohol insoluble solids (AIS), the former being slightly larger. A strong relationship exists between AIS and viscos ity, with fruit pericarp accounting for high correlation coefficients (Janoria and Rhodes 1974). The inheritance of AIS in a cross of high and low AIS cultivars demonstrated high heritability (0.68 and 0.75), additive genetic variation, and that less than three genes influence AIS levels (Stevens 1976). Fractionation of AIS into polyuronide and polysaccharide fractions demonstrated that water soluble polyuronides and water insoluble polyuronides accounted for approximately 90% of the differences in viscosity between ahigh and a low viscosity cultivar (Stevens 1976). Water soluble polyuronides are comprised of short and intermediate length chains as found in the fruit serum fraction. Water insoluble polyuronides represent the protopectin fraction. An increase in the water insoluble polysaccharides had the greatest potential for increasing fruit viscosity.

Genetic variation for texture and factors that contribute to AIS in tomato results from the interaction of numerous interacting QTLs. QTLs associated with AIS and its constituents have been described (Fulton et al. 2000; Causse et al. 2002; Frary et al. 2003b; Yates et al. 2004). These studies have shown that a few chromosomal regions on chromosomes 2 and 4 have a large influence on fruit biochemical composition and organoleptic quality as determined by both physical and sensory measures (Causse et al. 2002; Yates etal. 2004) (See Sect. 1.15.1).

Early work on the molecular genetics of fruit ripening and corresponding changes in fruit softening focused on polygalacturonase and its effects on ripening fruit. Antisense suppression of polygalacturonase accumulation demonstrated that the enzyme has only a minor effect on fruit softening, but has substantial effects on increasing viscosity of processed products and the integrity of stored fruit (Schuch etal. 1991; Kramer et al. 1992;Langleyet al. 1994). Related efforts directed towards suppression of pectin methylesterase activity likewise had little influence on fruit firmness, but increased soluble solids of juice, serum viscosity, paste viscosity, and serum separation of processed juice (Tieman et al. 1992; Thakur et al. 1996).

Atleast seven tomato P-galactosidase genes are expressed during tomato fruit development, six of which are expressed during ripening and may influence fruit textural properties (Smith and Gross 2000). Antisense suppression of the tomato P-galactosidase 3 gene did not improve fruit firmness but resulted in fruit that processed into pastes with an increased proportion of insoluble solids and slightly increased viscosity (de Silva and Verhoeyen 1998). Similar studies examin ing the tomato P-galactosidase 4 gene produced fruit from antisense lines that were 40% firmer than controls. Ongoing studies of a number of tomato ripening mutants (rin, nor, Nr, Cnr) that exhibit altered fruit textural properties offer promise for further elucidation of fruit AIS constituents (Seymour 2002).

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