Citrus Scion and Rootstock, Topping Height, and Tree Spacing Affect Tree Size, Yield, Fruit Quality, and Economic Return

A factorial experiment begun in 1980 included ‘Hamlin’ and ‘Valencia’ sweet-orange scions [Citrus sinensis (L.) Osb.], and Milam lemon (C. jambhiri Lush) and Rusk citrange [C. sinensis x Poncirus trifoliata (L.) Raf.] rootstocks, tree topping heights of 3.7 and 5.5 m, between-row spacings of 4.5 and 6.0 m, and in-row spacings of 2.5 and 4.5 m. The spacing combinations provided tree densities of 370, 494, 667, and 889 trees ha. Yield increased with increasing tree density during the early years of production. For tree ages 9 to 13 years, however, there was no consistent relationship between yield and tree density. Rusk citrange, a rootstock of moderate vigor, produced smaller trees and better yield, fruit quality, and economic returns than Milam lemon, a vigorous rootstock. After filling their allocated space, yield and fruit quality of trees on Milam rootstock declined with increasing tree density at the lower topping height. Cumulative economic returns at year 13 were not related to tree density. Table 1. Experimental citrus plantings include a wide range of tree densities. Citrus and other tree fruits yield more during the early years of production when planted at higher densities (Cary, 1981; Jackson, 1985; Patil, 1987). However, the advantage of higher densities at tree maturity is less certain. For citrus, we proposed that yield at maturity is independent of tree density over a range of densities (Wheaton et al., 1978). Production of modern mature apple orchards increased with tree density, however, possibly due to improved genetic material with a higher harvest index, rather than from higher tree density per se (Jackson, 198.5). Production of citrus approaching 100 t·ha annually is a realistic goal and has been observed over a range of tree densities in several citrus production areas (Tablel; see Cary, 1981 for a review of earlier experiments). Production per ha while the plantings were young increased with increasing tree density in each of these experiments. Except for the California studies, production of 60 to 80 t·ha was obtained at the highest density in 4 to 6 years. The experiments in Japan and Florida also provide evidence that yield at maturity is independent of tree density over a range of densities. In Japan, maximum annual production of 70 to 80 t·ha was similar for tree densities ranging from 1250 to 10,000 trees/ ha. Highest cumulative yield after 2 1 years was attained at 2500 trees/ha because yield at the highest densities declined over time. In Florida, maximum annual production of 100 t·ha was similar for trees at 359 and 7 18 trees/ha, but was lower for trees at the lowest density of 2 15 trees/ha. Production was generally low in the California experiments and was not consistently related to tree density. The Australian report did not include data on production of the orchard at maturity. However, yield leveled or decreased very early at the higher densities due to intertree competition. Vigorous trees rapidly fill their allocated space, but may be difficult to contain and may perform poorly in higher density plantings at maturity. A tree that grows rapidly to containment size r publication 14 Nov. 1994. Accepted for publication 13 Apr. 1995. We reciate the cooperation and support of Coca Cola Foods Division, Fla. We also thank Jim Baldwin, Marjorie Cody, and David Noxel for cal support. Florida Agricultural Experiment Station Journal Series no. ost of publishing this paper was defrayed in part by the payment of page nder postal regulations, this paper therefore must be hereby marked ent solely to indicate this fact. OC. HORT. SCI. 120(5):861-870. 1995. and then becomes less vigorous is desired. The range of dwarfing rootstocks used by apple growers to manipulate tree size and fruiting characteristics is not currently available to citrus growers. Although no satisfactory dwarfing rootstock for citrus is commercially available, citrus rootstocks do provide a modest range of tree vigor and final tree size (Castle et al., 1989). Another method of reducing vigor and tree size at maturity is the introduction of dwarfing citrus viroids into trees on susceptiblerootstocks (Hutton, 1986; Patil, 1987). Citrus fruit quality is important for both fresh and processing markets. Reported effects of tree density on fruit quality are conflicting. Higher densities had little effect on fruit quality in the Japanese and Australian experiments, but was a major factor in the California studies. Only minor effects of tree density on fruit quality were observed in previous Florida experiments. Financial evaluation of plantings for fruit production at various tree densities is essential. The relative costs of land, capital, and labor vary among citrus growing areas. Establishment expenses of a new planting and annual production costs vary with tree density. Discounted cash flow and the internal rate of return (IRR) were used to demonstrate the benefits of higher density plantings for apples (Jackson, 1985). Not all previous citrus spacing experiments included financial analyses. Where economic analyses were included, a relation between tree density and financial returns was present in some experiments but not in others (Boswell et al., 1975, 1982; Koo and Muraro, 1982; Wheaton et al., 1990). The principal objective of our long-term experiment was to determine the combination of cultivar, rootstock, tree height, and