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- Author or Editor: William S. Castle x
A rootstock trial with a ‘Marsh’ grapefruit (Citrus paradisi Macf.) scion was established in the flatwoods of the Florida east coast Indian River region in 1990. The trees were planted in an Alfisol of the Pineda series. The trial consisted of trees on 16 rootstocks, primarily citranges [C. sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.], citrumelos (C. paradisi × P. trifoliata), mandarins (C. reticulata Blanco) and various hybrids, in three or six replicates of three-tree plots in a randomized complete block design. Tree growth and survival, yield, and juice quality were measured annually or periodically for 10 years. In three seasons, whole-tree crops were sized in the field. Using the fruit size distribution data, crop value or income/tree was estimated. Tree height after 10 years ranged from 1.7 to 4.1 m and survival was greater than 90% with a few exceptions. Mean cumulative yield was 1202 kg/tree and ranged from 649 (Hamlin + Flying Dragon trifoliate orange) to 1615 kg/tree for a hybrid of trifoliate orange × Milam. At tree age 9 years, mean soluble solids production was 3594 kg·ha−1 with a 240% difference between the lowest and highest value. There were differences in the distribution of four commercial-sized categories based on analysis of individual sizes and using PROC FREQ to examine and compare whole-tree distributions. When the yield and fruit size data were combined for 3 years and converted to income/tree using commercial Florida Freight On Board prices in November and March, the trees on a trifoliate orange × Milam hybrid (1584) had the highest estimated income ($354 U.S., March data) followed by Calamandarin ($321) and Norton citrange ($292). The lowest income/tree was $112 (Hamlin + Flying Dragon trifoliate orange). When all data were considered, the best matches to current grower interest in smaller sized trees, and high yield and fruit quality, were the hybrid 1584 and C-35 citrange rootstocks.
Two groups of 8-year-old ‘Murcott’ [Citrus reticulata Blanco × C. sinensis (L.) Osbeck hybrid?] trees on rough lemon (C. jambhiri Lush.) rootstock were transplanted with a Vermeer tree digger in March and July, respectively. Root and shoot pruning and a 2% (v/v) spray of a pinolene-based antitranspirant (Vapor Gard) formed the treatments either individually or in combination. Canopy size at transplanting had the greatest effect on tree water-stress and subsequent tree growth and yield. The antitranspirant and root pruning tended to reduce leaf water-stress, but the effects generally were small and nonsignificant. Root pruning also seemed to stimulate new root growth. After transplanting, roots grew 2 to 3 m beyond the soil ball in one year. Four years after transplanting there were virtually no differences in tree height or canopy volume. Cumulative yield was less for trees with 30% and 85% of their foliage removed as compared to those with 50% removed. Trees pruned 50% bore fruit the year after transplanting and consistently yielded more throughout the study.
Fibrous root densities were determined for 16-year-old ‘Pineapple’ orange [Citrus sinensis (L.) Osb.] trees on rough lemon (C. jambhiri Lush.) rootstock spaced 3.0 × 4.6 m, 4.6 × 6.1 m, and 6.1 × 7.6 m and growing in a deep, central Florida sandy soil. Samples were taken from 1.9 m deep borings at 2 dripline locations and midway between trees in the row and between adjacent rows. The fibrous root systems penetrated to 1.9 m and were well-distributed. Samples from the widest spaced trees generally had lower root densities while the densities of samples from the other spacings were greater and suggested the overlapping of adjacent root systems. Root density was greatest in the surface sample for all spacings and generally decreased with depth. The extensive development and density of the fibrous root systems suggested that root competition was not likely to be a primary limiting factor in higher density plantings grown under comparable conditions.
Root samples were obtained from 9-year-old trees of 3 citrus scions propagated on ‘Milam’ (a possible Citrus jambhiri Lush, hybrid) cuttings and seedlings. The root distribution and root density of trees on cuttings compared favorably with that of trees on seedlings, the usual commercial method of rootstock propagation. Significant factors affecting their respective root systems were method of propagation, scion, and soil depth; one interaction, propagation × depth, was significant. Trees on cuttings had a significantly larger total feeder root weight and surface root density than trees on seedlings.
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.
The annual pattern of root growth of ‘Valencia’ orange [Citrus sinensis (L.) Osb.] trees on rough lemon (C. jambhiri Lush.) and Carrizo citrange [Poncirus trifoliata (L.) Raf. × C. sinensis] rootstocks was studied in relation to shoot growth, soil temperature, and water stress in Plexiglas-walled root observation chambers. The chambers were filled with a reconstituted profile of a fine sand soil and were installed below-ground at a field site in central Florida. The chambers periodically were raised above-ground to record root growth. Under nonlimiting soil water conditions, continuous root extension growth was evident from February to November. The overall seasonal trend in root growth was significantly correlated with soil temperature. The most intense root growth occurred when soil temperatures were above 27°C, and was limited at soil temperatures below 22°. Root growth was cyclic. During periods of shoot elongation, the number of growing roots and the rate of root elongation declined. Shoot growth was a major factor controlling the intensity of root growth when soil temperature and soil water content were nonlimiting. When the soil water content was decreased intentionally, root growth was checked at a soil matric potential of −0.05 MPa. After rewatering, there was a lag period of 2 days before root growth increased. Both number of growing roots and rate of root elongation were influenced by shoot growth, soil temperature, and soil water content; however, the growth response of the root system to these factors was mediated largely through the number of roots initiating growth. No rootstock differences were apparent in the pattern of root growth.
Roots of ‘Orlando’ tangelo (Citrus paradisi MacF. × C. reticulata Blanco) on rough lemon (C. jambhiri Lush.), sour orange (C aurantium L.), sweet orange (C. sinesis (L.) Osb.) and Rusk citrange (C. sinensis × Poncirus trifoliata Raf.) were field-sampled from depths extending to 2.5 m in a well-drained sandy soil and from about 3 m in an underlying sandy clay layer of undetermined thickness. Root anatomy and morphology were examined by light and scanning electron microscopy respectively. Epidermal hairs and radially elongated groups of hypodermal cells were observed on all roots. Root hairs differed in size and shape and hypo-dermal hairs seemed to occur more frequently on surface roots. Fibrous root bunches tended to be less branched and individual roots were smaller in diameter as sampling depth increased. General root anatomy was similar to previous descriptions.
The influence of 12 rootstocks on the performance of a sweet orange seedling and ‘Valencia’ orange (Citrus sinensis (L.) Osbeck) was evaluated in a study to determine their effect on eventual tree size. Trees on some rootstocks were larger than those on rough lemon, the standard for comparison, but a few were smaller and might be considered for closer spacings. The trifoliate orange selections (Poncirus trifoliata (L.) Rof.), particularly ‘English small’ merit consideration since yields were high in proportion to tree size and fruit quality was good. Results with ‘Rusk’ citrange (P. trifoliate × C. sinensis) were also quite satisfactory because the trees were semi-dwarf in size and had relatively good yields of high quality fruit of acceptable size. ‘Troyer’ and ‘Car-rizo’ citranges would be suitable candidates for ‘Valencia’ orange, based on this study, if semi-standard trees are desired. The performance of trees on some rootstocks was probably influenced by the sandy soil and the dwarfing effect could be more environmental than genetic.
Citrus rootstocks have well-known effects on tree size, crop load, fruit size, and various fruit quality factors. Fruit from trees budded on invigorating rootstocks are generally larger with lower soluble solids concentration (SSC) and titratable acidity compared to fruit from trees budded on less invigorating rootstocks. Although it is unclear how rootstocks exert their influence on juice quality of Citrus L. species, plant water relations are thought to play a central role. In addition, the larger fruit size associated with invigorating rootstocks and the inverse relationship between SSC and fruit size implies that fruit borne on trees on invigorating rootstocks have lower SSC due to dilution effects in larger fruit. To determine how rootstock type affects sugar accumulation in fruit of Citrus species, controlled water-deficit stress was applied to mature `Valencia' sweet orange [C. sinensis (L.) Osb.] trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] or rough lemon (C. jambhiri Lush.) rootstocks. Withholding water from the root zone of citrus trees during stage II of fruit development decreased midday stem water potential and increased the concentrations of primary osmotica, fructose and glucose. Sucrose concentration was not affected, suggesting that sucrose hydrolysis took place. Increased concentrations of sugars and SSC in fruit from moderately water-stressed trees occurred independently of fruit size and juice content. Thus, passive dehydration of juice sacs, and concentration of soluble solids, was not the primary cause of differences in sugar accumulation. Controlled water-deficit stress caused active osmotic adjustment in fruit of `Valencia' sweet orange. However, when water-deficit stress was applied later in fruit development (e.g., stage III) there was no increase in sugars or SSC. The evidence presented supports the hypothesis that differential sugar accumulation of citrus fruit from trees on rootstocks of contrasting vigor and, hence, plant water relations, is caused by differences in tree water status and the enhancement of sucrose hydrolysis into component hexose sugars resulting in osmotic adjustment. Therefore, inherent rootstock differences affecting plant water relations are proposed as a primary cause of differences in sugar accumulation and SSC among citrus rootstocks.