district sets its standards according to what they do best [42]. European citrus districts, South Australia,
California, and other districts with Mediterranean-type climates (cool winter nights, bright days, and low
rainfall) can rely almost entirely on external standards to sell their oranges. Florida, with its blossom-pe-
riod winds and humid, subtropical climate, cannot compete on appearance and so relies principally on
standards based on the high sugar content of its oranges. These maturity standards are based not only on
sugar content but also on the ratio of total soluble solids (TSS, mainly sugars) to acids (titratable as citric
acid), with a sliding scale throughout the season [43,44] (Figure 3). At the beginning of the season, Florida
oranges must have 8.0% TSS with a TSS/acid ratio of 10.5:1 (Figure 3). By the end of the season, this ra-
tio may exceed 20:1, but with the proviso that (for fresh fruit sale) acid cannot be below 0.4% lest the or-
anges taste too insipid.
Regardless of growing district, consistent gradients occur within a citrus fruit, particularly in terms
of sugar content. The vascular system extends down the central axis of the fruit, reaching the blossom
(stylar, distal) end first, them ramifies back up the carpels to the stem (calyx, proximal) end of the fruit.
Apparently as a consequence of this distribution of photosynthates, sugars are higher in the blossom end.
A very thorough study reported that the proximal halves of mature California Valencia oranges averaged
7.2 g of sugar per liter of juice as compared with 9.5 g/L for the distal (blossom, stylar) halves, a differ-FRUIT DEVELOPMENT, MATURATION, AND RIPENING 151Figure 3 Respiration during fruit development of Valencia orange. (A) Expressed as CO 2 evolution per unit
fresh weight; (B) expressed as CO 2 evolution per fruit, a form that more clearly defines the stages of fruit
development. (From Ref. 34.)