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- Author or Editor: Frederick S. Davies x
Abstract
‘Orlando’ tangelo (Citrus reticulata Blanco × Citrus paradisi Macf.) trees not irrigated in the fall, but protected by under-tree sprinkling during a frost, sustained the lowest percentage of leaf and fruit damage as determined 6 weeks after the frost. Trees irrigated both in the fall and during a frost, or those receiving no fall irrigation or under-tree sprinkling, were intermediate in fruit damage. Fall irrigation without sprinkling the night of a frost contributed to the most severe damage to leaves and fruit. Soil moisture content of irrigated blocks was significantly greater than for non-irrigated blocks during the fall, yet afternoon leaf xylem water potential and stem water content were comparable. Leaf freezing point of detached leaves of ‘Orlando’ and navel orange (Citrus sinensis (L.) Osbeck) was poorly correlated with leaf xylem water potential, abaxial diffusion resistance, and relative water content. Leaf freezing and killing temperature was unaffected by fall irrigation and ranged from -5.8 to -6.8°C from October until December in 1978 and 1979.
Abstract
Hydraulic conductivity decreased logarithmically over an 8-day period in detached flowering peach stems [Prunus persica (L.) Batsch]. Decreased hydraulic conductivity was negatively correlated (r = - 0.96) with an increase in the number of plugged vessels and positively correlated with a decrease in the percentage of functional vessels (r = 0.97). Ethylene production from flowering shoots peaked after 7 days in the holding solution but decreased rapidly thereafter. Addition of 1% ethanol to the holding solution increased xylem hydraulic conductivity and delayed the surge in ethylene production.
It is desirable to mix gibberellic acid (GA3) with other commonly applied materials to reduce application cost. However, applying GA3 with some compounds can reduce its efficacy or cause phytotoxicity. We conducted experiments in 1997-98 and 1998-99 to determine if GA3 (ProGibb) can be tank-mixed with fosetyl-Al (Aliette), or avermectin (Agri-Mek) and oil, without reducing GA3 efficacy. In addition, we compared Silwet and Kinetic adjuvants for enhancement of GA3 efficacy. Five tank mixes were tested along with a nonsprayed control. These included 1) GA3; 2) GA3 and Silwet; 3) GA3 and Kinetic; 4) GA3 Silwet, and fosetyl-Al; and 5) GA3, Silwet, avermectin, and oil. All compounds were applied at recommended concentrations. In September 1997 or October 1998, about 2.5 gal (9.5 L) of each tank mix was applied with a hand sprayer to 14- or 15-year-old `Hamlin' orange (Citrus sinensis) trees on sour orange (Citrus aurantium) rootstock. Peel puncture resistance (PPR), color, and juice yield (% juice weight) were evaluated monthly between December 1997 and March 1998, and December 1998 and January 1999. In both years, fruit of treated trees usually had higher PPR and were less yellow in color than fruit from control trees. There were tank mix effects on juice yield in January of both seasons and February 1998. Gibberellic acid was most effective at enhancing juice yield when applied singly or with avermectin and oil. In both seasons there were dates when GA3 applied singly was superior at enhancing juice yield than a tank mix of GA3, Silwet and fosetyl-Al, indicating that GA3 was incompatible with fosetyl-Al. Neither Kinetic nor Silwet adjuvants consistently enhanced GA3 effects on peel quality or juice yield over GA3 alone.
Abstract
Fruit drop of navel orange [Citrus sinensis (L.) Osbeck], which occurs after June drop, was studied from 1980 through 1982 in 2 groves in north-central Florida. Summer drop, which occurs from June to August, and summer-fall drop, which occurs from late August through October, were the major periods of fruit drop. Fruit drop was significant in 1981, ranging from 63 to 200 fruit per tree. A dilute spray of 10 or 20 ppm 2,4-dichlorophenoxyacetic acid (2,4-D), alone or in combination with 20 ppm gibberellic acid (GA) and applied 5 to 9 weeks after midbloom, reduced summer fruit drop in all seasons. The same materials applied 13 weeks after midbloom were ineffective. An application of 2,4-D at 5 weeks after midbloom increased yields in 1981, but not in 1982, because summer drop was minor in that year. Midbloom application of GA + 2,4-D increased splitting and summer-fall drop in 1981, but generally these materials had no effect. Sprays of 2,4-D, GA, or the combination did not adversely affect internal fruit quality, although GA or GA + 2,4-D increased rind firmness when applied 13 weeks after midbloom.
Abstract
Secondary-fruit yellowing (SFY) of navel orange [Citrus sinensis (L.) Osbeck], a major cause of summer fruit drop, was induced artificially by girdling the stem 5 to 10 cm from the fruit. Girdling from 26 to 28 May 1982 increased ethylene levels at the stylar end and induced SFY in 90% of treated fruit. Girdling at other times, or at 30 to 45 cm from the fruit, however, caused minor SFY, particularly if leaves were present between the fruit and girdled area, indicating some material is produced by the leaves that reduces the incidence of SFY. Increase of ethylene levels and cellulase activity in the abscission zone of the secondary fruit prior to increases in the abscission zone of the primary fruit indicate that secondary-fruit abscission leads to primary fruit abscission. Water stress or changes in nonstructural carbohydrate levels within the leaves induced by branch sawing did not cause SFY. Application of 2,4-dichlorophenoxyacetic acid (2,4-D) prior to or during induction lessened the severity of SFY and significantly decreased ethylene levels at the stylar end of the fruit.
We investigated the effect of gibberellic acid (GA3) application before color break on peel color, fruit respiration, and soluble sugars in different tissues of ‘Hamlin’ sweet orange [Citrus sinensis (L.) Osb.] fruit to test the hypothesis that GA3 influence on peel color might be mediated by sugars. Fruit were sprayed with GA3 (45 g·ha−1 a.i.) in early October of 2 consecutive years. Peel color, whole-fruit respiration, and fructose, glucose, and sucrose levels were quantified in flavedo and albedo tissues when nontreated fruit were still green, at precolor break, color break, and when peels were fully yellow. Fruit treated with GA3 remained more green-colored than nontreated fruit, and differences between them were detectable by 12 or 21 days after treatment (Years 1 and 2, respectively). Fruit respiration rates were similar in both groups regardless of peel color. Effects of GA3 on color transition were evident only after significant differences emerged in flavedo glucose (both years) and fructose (second year) levels. Moreover, there was a linear, inverse relationship between green peel color and flavedo fructose (r 2 = 0.68, first year; 0.72, second year) and glucose levels (r 2 = 0.60, first year; 0.50, second year). In contrast, sucrose levels in the flavedo showed a less consistent relationship with peel color. The GA3 treatment maintained a descending sucrose gradient from the albedo to the flavedo that was typical of young, photosynthetically active fruit. This gradient dissipated during peel color change of nontreated fruit. These data support the hypothesis that soluble sugars could be contributing effectors of the GA3-mediated delay in chloroplast-to-chromoplast conversions by the orange flavedo.
Sources of variation in juice quality of `Valencia'sweet orange [Citrus sinensis(L.) Osb.] were quantified and their relative contributions to variability in juice quality were determined, from which sample sizes were estimated. Commercial orchards of `Valencia' sweet orange trees on Carrizo citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] rootstock were selected at four geographic locations representing the major citrus-producing regions in Florida. Within- and between-tree variation in soluble solids concentration (SSC) and titratable acidity (TA) were estimated in two experiments over two or three seasons, respectively. Variance components for all treatment effects were estimated to partition total variation into all possible component sources of variation. Seasonal variation in SSC and TA was relatively small, but larger for TA than SSC. Variation in SSC among blocks within a location was intermediate to low, and was less than variation among locations. In contrast, tree-to-tree variation in SSC and TA was large, in spite of sampling from trees of similar vigor and crop load, and variation in SSC and TA among fruit was relatively large. Based on results of this study, samples consisting of 35 fruit are required to detect differences (P ≤ 0.05) of 0.3% SSC and 0.06% TA, whereas 20-fruit samples can be used to detect differences of 0.4% SSC and 0.08% TA. Seven replications are required to detect differences of 0.5% SSC and 0.1% TA, with small gains in precision when tree numbers exceed 10.
Variability in fruit quality of citrus occurs among and within trees due to an interaction of several factors, e.g., fruit position, leaf: fruit ratio, and fruit size. By determining variability in fruit quality among i) fruit, ii) trees, iii) orchards, and iv) geographic locations where citrus is produced in Florida, optimal sample size for fruit quality experiments can be estimated. To estimate within-tree variability, five trees were randomly selected from each of three `Valencia' orange orchards in four geographic locations in Florida. Six fruit were harvested from each of two tree canopy positions, southwest top and northeast bottom; fruit were not selected or graded according to fruit size. °Brix and titratable acidity of juice samples were determined, and the °Brix: acid ratio was calculated. Statistical analysis of fruit quality variables was done using a crossed-nested design. The number of trees to sample and the number of fruit per sample were calculated. To estimate between-tree variability, 10 trees were randomly selected from each of three `Valencia' orange orchards from four geographic locations in Florida. Fifty-fruit composite samples were picked from around the tree canopy (0.9 to 1.8 m). Juice content, SSC, acid content, and ratio were determined. Using a nested design, the number of orchards and number of trees to sample were determined. There was greater variability in fruit quality among trees than within trees for a given canopy position; the optimal sample size when taking individual fruit samples from a given location and canopy position is four fruit from 20 trees. There was less variability in fruit quality when 50-fruit composite samples were used, resulting in an optimal sample size of five samples from three orchards within each location.
Two experiments were conducted with container-grown `Hamlin' orange trees [Citrus sinensis (L.) Osb.] on `Swingle' citrumelo [C. paradisi Macf. × Poncirus trifoliata (L.) Raf.] rootstock to study the effects of N rate on plant growth in the nursery. Treatments consisted of 12, 50, 100, or 200 mg N/liter per tree applied once a week by drip irrigation. Commercial media was used and soil water content was maintained at container capacity. In Expt. 1, fertilization at 200 mg·liter−1 resulted in greater scion growth, trunk diameter, and total leaf dry weight compared to the other rates. In Expt. 2, fertilization at 100 and 200 mg·liter−1 resulted in greater scion growth,” trunk diameter, and leaf and stem dry weights compared to lower rates, but no differences were observed between the two highest rates. Trees that received 12 and 50 mg·liter−1 were stunted and leaves were chlorotic. Therefore, the optimum calculated N rate for `Hamlin' nursery trees on `Swingle' citrumelo rootstock, based on critical level analysis, is 155 to 165 mg·liter-1.
Poncirus trifoliata (L.) Raf. seeds were germinated in perlite under intermittent mist at about 25 °C and natural daylight in a greenhouse. Two-week-old seedlings were then transferred into a growth chamber at 25 °C and 16-hour daylength for 1 week. Tissue samples were collected at 0, 6, 24, 168, and 504 hours after temperature equilibration at 10 °C. Freezing tolerance at –6.7 °C, as determined by electrolyte leakage, and stem (leaves attached) water potential (ψx), measured using a pressure chamber, was recorded for a subset of seedlings for each time interval. Red coloration (apparently anthocyanin) developed at the petiole leaflet junction and buds after 48 hours at 10 °C and gradually occurred throughout the leaves during further exposure. Complementary DNA clones for phenylalanine ammonia lyase (PAL), 4-coumarate: coA ligase (4CL), and chalcone synthase (CHS) were used to probe RNA isolated from the leaves. No increase in steady-state messenger RNA level was detected. Increases in freeze hardiness occurred within 6 hours in the leaves, and continued for up to 1 week. Water potential initially decreased from –0.6 to –2.0 MPa after 6 hours, then returned to –0.6 MPa after 1 week. Thus, Poncirus trifoliata seedlings freeze-acclimate significantly after only 6 hours at 10 °C.