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- Author or Editor: F. S. Davies x
Field studies conducted over two growing seasons were designed to study the effects of reclaimed water on the development of 1-and 2-year old `Redblush' grapefruit trees (Citrus paradisi Macf.) on Swingle citrumelo rootstock. Experiments were conducted at two locations on Kanapaha and Arredondo fine sands and treatments were arranged in a 3×3 factorial experiment. Treatments included reclaimed water, well water plus fertigation and reclaimed water plus fertigation, which received <0.023, 0.23 and 0.23kg N/tree/yr in 1990, and <0.034, 0.34 and 0.34kg N/tree/yr in 1991, respectively. In addition irrigation was applied at 20% soil moisture depletion, 1.5 cm/wk and 2.5 cm/wk for 31 weeks in 1990 and 39 weeks in 1991. Tree growth and vigor were greatest for the reclaimed water plus fertigation based on visual ratings and trunk diameter measurements and lowest for reclaimed water alone, where leaves exhibited visual signs of N deficiency. No differences in tree growth or vigor were observed among irrigation rates. Similar results were observed at both experimental locations.
Young `Hamlin' orange trees [Citrus sinensis (L.) Osbeck] were fertilized six times/year with water-soluble N fertilizer at recommended rates (0.20, 0.34, and 0.38 kg N/tree per year) and with controlled-release fertilizer one time/year [Osmocote, IDBU, and a 44.5% urea-N fertilizer coated with a sulfonated ethylene-propylene-diene polymer (Sherritt, Inc.)] at 0.04, 0.06, and 0.08 kg N/tree per year for years 1, 2, and 3, respectively. There were no differences in trunk diameter, tree height, or tree rating among treatments in any year, although there was a slight reduction in tree rating for some trees with biuret symptoms in the Sherritt treatment in year 2. Leaf nitrogen content was acceptable for all treatments in all 3 years, except for the Osmocote treatment in year 2, which had low to deficient levels. Levels of other nutrients were all within acceptable ranges, except for low potassium levels for the Osmocote in year 2. There were no significant differences in yields of young trees in year 3, the first bearing year. Given its 44.5% N analysis, the total amount of Sherritt controlled-release fertilizer applied to young citrus trees was 4% that of the standard, water-soluble fertilizer and from 39% to 45% that of the two other controlled-release fertilizers in years 1, 2, and 3.
The design for a chamber to control high root-zone temperature is presented. Air within the insulated chamber was heated with vinyl-jacketed heating cables and cooled with 1:1 water and antifreeze (v/v) pumped through copper coils. Control of the soil temperature was based upon air temperature around the soil containers within the sealed chambers. Chamber operating temperatures could be held between 20° and 50°C ± 0.2° SE.
High root-zone temperatures can stress container-grown plants and ultimately reduce nursery productivity in the southern United States. Water relations of glasshouse-grown Berberis thunbergii DC ‘Atropurpurea’ Buxus microphylla Seibold and Zucc japonica and Pittosporum tobira, (Thunb.) Ait. ‘Wheeler’ were studied under high-temperature root-stress conditions using container-grown plants that were either colonized with vesicular arbuscular mycorrhizal fungi (VAM) or noncolonized. Predawn xylem water potential in stems (ψstem) increased initially (more positive) in response to high root-zone temperatures (40° to 45°C), and then decreased over a 5-day period. Stomatal conductance (gs) and evapotranspiration (ET) were reduced incrementally over time in response to high root-zone temperatures. Root damage occurred, as indicated by reductions in root quality and gs at 35° and 40° for B. thunbergii and P. tobira, and at 40° and 45° for the more high-temperature-resistant B. microphylla. Colonization increased gs and ET of B. microphylla at ambient (25°) and high temperatures (45°) and increased ET of B. thunbergii at 25°. Colonized plants had lower (more negative) ψshoot with initial exposure to increased root-zone temperatures; however, throughout the remainder of the study period there was little reduction in plant stress with the mycorrhizal isolates used. Root hydraulic conductivity (Lp) increased markedly in B. thunbergii compared to B. microphylla at 40° and 45°, indicating less high-temperature resistance in B. thunbergii roots. Mycorrhizal colonization did not moderate hydraulic conductivity at high root-zone temperatures of 40° and 45°. Of the two species, mycorrhizal B. thunbergii had lower Lp at 25° and B. microphylla had lower Lp at 35°.
Photosynthesis in rabbiteye blueberry (Vaccinium ashei Reade) was found to saturate at low irradiances and had a low light compensation level. In contrast to many other plants, both stomatal and residual diffusion resistance contributed equally to the total leaf diffusion to CO2. Relatively higher stomatal resistances in rabbiteye blueberry result in an efficient water-use, enabling it to withstand drought. No cultivar differences were found in photosynthesis, transpiration, or dark respiration to account for differences in yield.
‘Tifblue’ rabbiteye blueberry (Vaccinium ashei Reade) yields were greatest when expressed as kg of fruit per bush; however, ‘Woodard’ outyielded ‘Tifblue’ and ‘Bluegem’ per m3 of canopy volume. ‘Tifblue’ was subjected to the greatest daytime water stress due to its large canopy volume and limited feeder root density. Consequently, changes in feeder root density had a pronounced effect on yields in ‘Tifblue’. Yield was also affected to a lesser extent by feeder root density in ‘Bluegem’ but was independent of this factor in ‘Woodard’.
Two experiments were conducted with containerized `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 growth of plants in the nursery. Treatments consisted of the following N rates: 12, 50, 100 and 200 mg·liter-1 applied once a week through drip irrigation. In Expt. 1, fertilization at the 200 mg·liter-1 rate resulted in greater scion growth, trunk diameter and total leaf dry weight as compared to the other rates. In Expt. 2, application of 100 and 200 mg·liter-1 rates resulted in greater scion growth and trunk diameter as compared to lower rates, but no differences were seen between the two highest rates. Trees receiving the 12 and 50 mg·liter-1 rates were stunted and leaves were chlorotic. Therefore, the optimum N rate for trees on `Swingle' citrumelo rootstock is between 100 and 200 mg·liter-1, although the 200 mg·liter-1 rate may not be economically justified. Moreover, the N rate for nursery plants growing on `Swingle' citrumelo rootstock in commercial medium may be higher than for other rootstocks, where rates less than 50 mg·liter-1 produce optimum growth.
A field study was conducted on mature `Redblush' grapefruit trees (Citrus paradisi Macf.) on sour orange (Citrus aurantium L.) rootstock from 1991 to 1993 near Vero Beach, Fla. on poorly drained (flatwoods) soil to determine the effects of reclaimed water on leaf, soil and shallow well-water nutrients. Treatments consisted of a canal water applied based on soil moisture depletion, and reclaimed water applied at 23.1, 30.7 and 36.6 mm/wk. Reclaimed water treatments received supplemental fertilization in addition to the N present in the water. All treatments received about 130 kg/ha/yr N. Leaf tissue N, P, K, Ca, Mg and Na concentrations were similar for all treatments, but B concentrations were significantly higher for the reclaimed water treatments in 1991 and 1993. Soil P and Na concentrations also increased in the reclaimed water treatments. Water samples taken from shallow depth wells showed that reclaimed water treatments had lower levels of NO, compared to the control possibly due to leaching. Reclaimed water contained only trace or undetectable levels of heavy metals.
Four abscission materials were evaluated to determine their effects on ethylene production and abscission of fruit and leaves of orange (Citrus sinensis (L.) Osbeck cvs. Hamlin and Valencia): 5-chloro-3-methyl-4-nitro-1H-pyrazole (Release) glyoxal dioxime (Pik-Off), cycloheximide (3-[2-(3,5-dimethyl-2-oxocyclohexyl-2-hydroxyethyl]-glutarimide or Acti-Aid) and cycloheximide in combination with chlorothalonil (DS-27914). All chemicals enhanced internal fruit ethylene levels and subsequent fruit loosening. However, the magnitude and timing of the fruit ethylene response varied with each chemical and correlated with fruit loosening and subsequent retightening. The pattern of ethylene production may indicate the optimum fruit harvest period and the degree of leaf abscission for each chemical.
Cuttings of neem trees (Azadirachta indica) were grown for 65 days at four P levels: 0, 15, 30, and 60 mg P/kg soil. Half of the plants were inoculated with the vesicular–arbuscular mycorrhizal fungi (VAM) Glomus intraradices. VAM increased growth and net photosynthesis (A) at the lowest two soil P levels. Increased A was attributed to increased stomatal conductance (g) and greater leaf P concentration. Nonstomatal inhibition of A due to P deficiency also was observed in non-VAM plants at lower soil P levels. At higher soil P, VAM and non-VAM plants had comparable growth, A, g, and tissue concentration of P and other elements. VAM plants at 0 mg P/kg soil had similar growth and leaf P concentration when compared to non-VAM plants at 15 mg P/kg soil, yet had a 11% higher A, indicating a direct effect of VAM on gas exchange. As soil P increased, total VAM colonization and vesicle formation decreased, while the amount of extraradical hyphae increased. Arbuscule formation was highest at 0 and 15 mg P/kg soil. Apparently, arbuscules and extraradical hyphae play an important role in the enhanced growth and gas exchange of VAM plants at lower soil P levels.