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Frederick S. Davies and Glenn Zalman

The objective of this study was to determine the effects of various levels of nitrogen (N) on growth of ‘Hamlin’ orange (Citrus sinensis) trees on Carrizo citrange (C. sinensis × Poncirus trifoliata) rootstock in a field nursery. Newly budded liners were obtained from commercial nurseries and received from 0 to 3976 kg N per treated hectare annually (8N–0P–6.6K) in 14 applications per season. Tree trunk diameter, height, and dry weight were measured in two separate experiments. Total dry weight and trunk diameter were greatest for trees receiving 794 kg·ha−1 N annually during both seasons. However, annual N rates more than 1589 kg·ha−1 reduced trunk diameters and dry weight compared with the optimum N rate during both seasons. Leaf N concentration and N rate were positively correlated in both seasons, but leaf N concentration was poorly correlated with tree trunk diameter and dry weight. Therefore, very high rates of N fertilization may actually reduce ‘Hamlin’ orange tree growth in field nurseries when growing in an Arredondo fine sand.

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Frederick S. Davies and Glenn Zalman

Our objectives were to determine if gibberellic acid (GA3) application at color break in the fall affected the juice content, soluble solids content (SSC), titratable acid (TA), and ratio of SSC: TA of `Hamlin' orange (Citrus sinensis) fruit following moderate to severe freezes. We also wanted to know if GA3 affected the post-freeze rate of decrease in juice content, fruit and tree cold hardiness, and the amount of fruit drop following a freeze. GA3 (18 floz/acre) was applied at color break in the fall of 2002, 2003, and 2004 to `Hamlin' orange trees on Swingle citrumelo (C. sinensis × Poncirus trifoliata) rootstock planted in 1995 at Gainesville, Fla. Moderate to severe freezes occurred in all three seasons. Fruit were harvested at about 2-week intervals following freezes in each season and the internal fruit quality was determined. GA3-treated fruit generally had higher juice content compared with nontreated fruit for 8 weeks after moderate to severe freezes in all three seasons, which may be economically important to citrus processors and growers since Florida growers are paid based on fruit pounds-solids (juice content × SSC). The rate of decrease in juice content over time was similar for both treatments in seasons one and two, but was less for GA3-treated fruit than nontreated fruit in season three. In addition, SSC was equal to or slightly greater for fruit treated with GA3 than for nontreated fruit. Fruit drop rate and magnitude were also significantly less for the GA-treated compared with nontreated trees in two of three seasons. GA3 did not affect fruit, leaf, or tree cold hardiness in any season.

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Frederick S. Davies and Glenn Zalman

The authors’ objectives were to determine whether gibberellic acid (GA3) initially increases juice content of ‘Rohde Red’ valencia oranges and prevents or delays decreases in juice content after a freeze, and to determine whether there is an interaction between GA3, rootstock, and juice content. The experiment consisted of a 2 (+, –GA3) × 3 (rootstock) factorial using a completely randomized design with 10 replications per treatment. Gibberellic acid was applied to mature ‘Rohde Red’ valencia [Citrus sinensis (L.) Osb.] orange trees on three rootstocks—Citrus volkameriana Ten. & Pasq.(Volk), Swingle citumelo [C. paradisi Macf. ×Poncirus trifoliata (L.) Raf.], and Carrizo citrange [C. sinensis ×P. trifoliata]—at color break in Fall 2002, 2003, and 2004. Juice content, soluble solids content (SSC), titratable acidity (TA), ratio of SSC to TA, and kilogram solids per box were determined at about 2-week intervals after several freezes. In 2002–03 and 2004–05, juice content in the fall was greater and the rate of decrease in juice content lower for GA3-treated fruit than nontreated fruit for about 8 weeks after a freeze. In contrast, in 2003–04, juice content and rate of decrease in juice content were not different between treatments. Juice content was lower for fruit from ‘Rohde Red’ trees on Volk compared with those on Carrizo and Swingle, and more important, the rate of decrease in juice content after a freeze was greatest for trees on Volk in all three seasons. Soluble solids content, TA, SSC-to-TA ratio, and kilogram solids generally were not effected by GA3 treatment. Therefore, GA3 application at color break in the fall generally increased juice content and slowed the rate of decrease in juice content after a freeze compared with nontreated fruit. In addition, juice content differed significantly with rootstock, but there was no GA3 × rootstock interaction.

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Maritza Ojeda, Bruce Schaffer, and Frederick S. Davies

Root ferric chelate reductase (FCR) activity in Annona glabra L. (pond apple), native to subtropical wetland habitats and Annona muricata L. (soursop), native to non-wetland tropical habitats, was determined under Fe-sufficient and Fe-deficient conditions. Four-month-old seedlings of each species were grown hydroponically in a complete nutrient solution containing 90 μm Fe or no Fe. The degree of tolerance of Fe deficiency was evaluated by measuring root FCR activity, chlorophyll and Fe concentration in recently matured leaves and plant growth. Root FCR activity was higher in soursop than in pond apple in the nutrient solution with Fe. However, there were no differences in root FCR activity between species under Fe-deficient conditions. Root FCR activity in pond apple and soursop was not induced in the absence of Fe. Leaf chlorophyll index and Fe concentration, and dry weights of pond apple were lower when plants were grown without Fe compared to plants grown with Fe. Leaves of pond apple grown without Fe became chlorotic within 3 weeks. Lack of Fe decreased the chlorophyll index and Fe concentration in young leaves less in soursop than in pond apple. In contrast, the Fe level in the nutrient solution had no effect on dry weights of soursop. The rapid development of leaf chlorosis and low FCR activity of pond apple may be due to its native origin in wetland areas where there is sufficient soluble Fe for plant growth and development.

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Michael A. Maurer and Frederick S. Davies

Two field studies conducted from 1990 to 1991 evaluated the effects of reclaimed water on growth and development of 1- and 2-year-old `Redblush' grapefruit (Citrus paradisi Macf.) trees on Swingle citrumelo [Citrus paradisi (L.) Osb. ×Poncirus trifoliata (L.) Raf.] rootstock. Treatments were arranged as a3 (water sources) x 3 (irrigation levels) factorial at two locations on an Arredondo (well drained) and Kanapaha (poorly drained) fine sand near Gainesville, Fla. Irrigation treatments included 1) reclaimed water, 2) reclaimed water plus fertigation, and 3) well water plus fertigation. The reclaimed water was formulated to simulate that of a sewage treatment plant at Vero Beach, Fla. Irrigation was applied at 20% soil moisture depletion, or at 19 or 25 mm·week regardless of rainfall. In both experiments, visual ratings of tree vigor, and measured tree height and trunk diameter, were significantly lower for trees watered with reclaimed water without fertilizer than for the others in both years. Moreover, there was no fourth leaf flush in 1991 with reclaimed water. There was a significant increase in leaf Na, Cl, and B concentrations for the reclaimed water and reclaimed water plus fertigation treatments in 1990; however, in 1991 only leaf B concentrations showed a similar trend. In 1991, there were no significant differences in leaf Cl concentrations. Visual symptoms of N deficiency were observed by the end of the first season in trees grown with reclaimed water. Irrigation levels generallv did not affect tree growth.

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Matthew W. Fidelibus and Frederick S. Davies

In Florida, gibberellic acid (GA3) is applied to citrus in the late summer or early fall to reduce senescence-related peel disorders of fresh fruit and to increase juice yield of processing oranges. Heavy rainfall may occur daily during this time that could reduce the efficacy of GA3 sprays. Experiments were conducted in 1998-99 and 1999-2000 to test the effect of timed “wash off” treatments on the peel color and peel puncture resistance (PPR) of `Hamlin' orange (Citrus sinensis [L.] Osb.) fruit that were previously treated with GA3. In Oct. 1998 and 1999, the canopy of 14- or 15-year-old trees were sprayed to runoff (≈10 L) with GA3 (45 g a.i./ha) and a non-ionic surfactant (Silwet, 0.05%). For the next 4 (1998-99) or 5 (1999-2000) h, three different GA3-treated trees each hour were then sprayed with ≈20 L of tap water to simulate rainfall that might remove or dilute the GA3. An additional three trees did not receive a GA3 or a washoff treatment. Fruit were harvested in Nov. 1998 and Jan. 1999 and Dec. 1999 and Jan. 2000 and evaluated for PPR and color. Data were subjected to regression analysis to determine the relationship between peel variables and time until washoff. In 1998-99, PPR and peel hue (level of green color) increased linearly with time until washoff, indicating that some GA3 uptake was still occurring after 4 h. In 1999-2000, PPR and hue increased linearly until about 3 h before washoff. Therefore, heavy rainfall within 3 to 4 h of application may reduce GA3 effectiveness, even when a surfactant is used.

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Rebecca L. Darnell and Frederick S. Davies

Potted `Tifblue', Woodard', and `Climax' rabbiteye blueberry plants (Vaccinium ashei Reade) were exposed to artificial or natural chilling regimes (< 7C) ranging from 100 to 1000 hours during the dormant season to determine the effects on budbreak and fruit set. Insufficient chilling increased the days to 50% vegetative and floral budbreak in all three cultivars. The amount of floral budbreak increased in `Tifblue' and `Woodard', but decreased in `Climax' as chilling increased. Insufficient chilling did not decrease percent fruit set of hand-pollinated flowers in any cultivar, indicating that the fruit-setting potential of these cultivars is unrelated to chilling.

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Frederick S. Davies and Glenn R. Zalman

Several studies suggest that optimum N rate and application frequency differ among citrus rootstocks. `Rhode Red' valencia orange trees [Citrus sinensis (L.) Osb.] on three rootstocks, C. volkameriana Ten. and Pasq., `Carrizo' citrange [C. sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.], and `Swingle' citrumelo [C. paradisi Macf. × P. trifoliata (L.) Raf.], were used to determine if N rate and application frequency should be adjusted, based on rootstock, during the first 3 years in the field. Treatments were arranged in a 3×3×3 (rootstock, N rates, N application frequency, respectively) factorial experiment. Annual N application rates ranged from 68 to 272 g/tree depending on tree age, and N was applied biweekly, weekly or monthly. Application frequency had no effect on trunk diameter or leaf N concentration in any year. Rootstock had a significant effect on growth in all 3 years, with trees on C. volkameriana being largest and having the greatest yields, followed by those on `Carrizo' and `Swingle', respectively. Trees on C. volkameriana were larger than those on the other rootstocks because they were larger at planting, grew over a longer period during the year, and often grew at a faster rate. Nitrogen rate had no effect on growth during the first 2 years in the field, but the highest N rate increased yields in year 3 for trees on C. volkameriana and `Swingle' rootstocks. Interaction between rootstock and N rate was nonsignificant for trunk diameter, but it was significant for yield, suggesting that trees on C. volkameriana responded more to increased N than did those on the other rootstocks.

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Thomas E. Marler and Frederick S. Davies

Growth responses of young `Hamlin' orange [Citrus sinensis (L.) Osbeck] on sour orange (C. aurantium L.) trees to microsprinkler irrigation were studied under field conditions from 1985 to 1987 to determine the most-efficient irrigation rates and duration. Trees were irrigated when available soil water depletion (SWD) reached 20% (high frequency), 45% (moderate frequency), and 65% (low frequency). Trees at the moderate and low levels received 49% and 13%, respectively, as much irrigation water as the high treatment. Canopy volume, trunk cross-sectional area, dry weight, shoot length, leaf area, total root dry weight and volume, and new root dry weight were similar for the high and moderate levels in 2 of 3 years, but were significantly reduced at the low level. Summer and fall growth flushes were delayed or did not occur at the moderate and low levels. More than 90% of root dry weight was within 80 cm of the trunk at the end of the first growing season.