Introduction. Consumers and processors value tomatoes with high fruit sugar
content; however, most breeding and cultural practices negatively impact this trait. Wild
tomato species can accumulate two- to three-fold more fruit sugar than cultivars and are
proving to be valuable both as a source of high-sugar loci to broaden the
genetic base of currently produced cultivars, and as research material to understand this
trait. Synthesis. While cutting-edge genomic approaches have taught us much
about fruit phenotypes, it is still important to assess fruit enzyme activities and
metabolic fluxes in lines with contrasting fruit sugar accumulation. These metabolic
functions are closest to the ripe fruit sugar trait. In this review, we focus our
attention on the biochemical pathways, especially starch biosynthesis, that may influence
tomato fruit sugars. We try where possible to put this information into a physiological
context because together they influence yield. We compare and contrast sugar metabolism in
cultivars and wild tomato species and identify factors that may influence differences in
their fruit size. Conclusion. Although difficult, we show that it is possible
to develop fruit with high horticultural yield and use the breeding line ‘Solara’ as an
example. In addition, we suggest avenues of further investigation to understand the
regulation and control of fruit carbohydrate content.