Inheritance of Immature Tomato Fruit Shoulder Color Phenotypes and Their Effect on Yellow Shoulder Disorder and Soluble Solids Content of Ripe Tomatoes

A recent Science article reported increased soluble solids in tomatoes (Solanum lycopersicum L.) with dark green shoulders (U+ gene) compared to an isogenic line with uniform green shoulder  (u gene) .  Tomato breeders have generally been developing cultivars with uniform shoulders because they are less likely to be susceptible to yellow shoulder disorder (YSD) than cultivars with dark green shoulders. YSD is a hard, yellow or green area on the shoulder of tomato fruit exposed to the sun that does not turn red at maturity.  Lines with dark green (U+), uniform green (u), uniform gray-green (ug), apple green, medium green, and pale green immature fruit shoulder colors were crossed in all combinations to produce F₁ and F₂ populations. Parents, F₁ , and F₂s were planted in the field in completely randomized block designs over three seasons  to compare YSD incidence and soluble solids content for each phenotype.    Plants were visually phenotyped for shoulder color then shoulders were measured with a colorimeter. The u and ug genes were confirmed to be separate loci and recessive to U+. Apple green, pale green, and uniform green (u) are alleles at the same locus where apple green is dominant to both uniform and pale green and uniform green is dominant to pale green. Plants with dark green shoulders had higher incidence of YSD than plants of the other phenotypes which were not different from each other.  Apple green fruit showed significantly higher soluble solids in two of the three seasons. There was no significant differences in soluble solids between any of the other phenotypes.