Young-tree Performance of Juvenile Sweet Orange Scions on Swingle Citrumelo Rootstock

Abstract

A worldwide search was conducted for sweet orange [Citrus sinensis (L.) Osb.] selections with higher yield and better juice quality than existing commercial cultivars used in Florida primarily by the processing industry. Seeds of nearly 100 selections were introduced, germinated, and used as a source of buds for propagation. The scion selections were divided among six trials established by propagating juvenile buds from ≈12-month-old scion seedlings onto Swingle citrumelo [C. paradisi Macf. × Poncirus trifoliata (L.) Raf.] rootstock plants already in place in the field. Comparison trees using buds from mature sources were produced in a commercial nursery. The trials consisted of four to five replications of one- or two-tree plots with trees planted 4.3 × 6.7 m within and between rows, respectively. The scions were early-maturing (fall to early winter), midseason (winter to early spring), and late-season (early spring to early summer) common orange, blood orange, and ‘Pera’ orange selections. Data collected routinely included seed counts, standard measurements of juice quality, and yield during an ≈13-year period of evaluation. All trees exhibited typical juvenile traits such as vigor and thorniness; however, flowering and first cropping were not substantially delayed. Many selections began fruiting within 3 years after planting, which is the common commercial experience among trees propagated with mature bud sources. Many selections were low-seeded with counts of less than 10/fruit. Mean cumulative yield (8 years) among the early- and midseason selections in the first-planted trial was 1390 kg/tree and ranged to a high of 1751 kg/tree; for the late-season types, the mean was 947 kg/tree with little variability among eight selections. The yields of the early- to late-season selections in the other trials were similar. The blood orange selections proved to be mostly midseason in maturity. They lacked the deep peel and flesh coloration of blood oranges grown in a Mediterranean-type climate, but some selections did develop an enhanced orange color of the juice and the different flavor typical of blood oranges. ‘Pera’ orange selections exhibited a bud union incompatibility and subsequent decline with Swingle citrumelo rootstock and also when another sweet orange was inserted as an interstock. Their mean cumulative yield over six seasons was 797 kg/tree with an ≈30% difference between the lowest and highest values. Juice soluble solids, acid, and color values were typical of ‘Pera’ fruit grown in Brazil. The overall collection of sweet oranges displayed considerable diversity in their traits despite their supposed origin as a monophyletic group. Several early-season selections were released for commercialization, including ‘Earlygold’ and ‘Itaborai’, because of their better juice color and flavor. ‘Vernia’, a midseason selection, was released because of its high juice quality in late winter–early spring and its cropping precocity.