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- Author or Editor: Frederick S. Davies x
Poncirus trifoliata is a comparatively hardy, cross compatible, and graft compliant relative of Citrus. The citrus industry in Florida has suffered immense economic losses due to freezes. Although much research has been done in citrus freeze hardiness, little work has been on the early induction of freeze tolerance by low temperature. Poncirus trifoliata `Rubidoux' seedlings were germinated in perlite under intermittent mist at about 25°C and natural daylight conditions in a greenhouse and grown 2 weeks. See dlings were then transferred into a growth chamber at 25°C and 16 hour daylength for 1 week. Temperature was lowered to 10°C and tissue samples were collected at 0, 6, 24, and 168 hours. Freezing tolerance, at –6.7°C as determined by electrolyte leakage, and stem (leaves attached) water potential, measured using a pressure bomb, were also recorded for a subset of seedlings for the above intervals. After exposure to low temperature for 48 hours a red coloration became visible at the petiole leaflet junction an d at the buds, with subsequent exposure to low temperature the coloration spread to the leaves. Clones for phenylalanine ammonia lyase (PAL), 4-coumarate:CoA ligase (4CL), and chlorophyll ab binding protein (CAB), and chalcone synthase (CHS) were used to probe RNA isolated from P. trifoliata. PAL and 4CL transcripts increased in response to the low temperature. Significant increases in freeze hardiness occurred within 6 hours in the leaves, and increases continued for up to one week. Water potential increased from –0.6 to –2.0 MPa after 6 hours, then returned to –0.6 MPa after 1 week. These data indicate that increases in freezing tolerance and changes in water potential and gene expression can be detected shortly after low temperature treatments are imposed on P. trifoliata seedlings.
The University of Florida is one of the 10 largest universities in the United States, with more than 35,000 students, 20 colleges andaschools, and 86 interdisciplinary research and education centers, bureaus, and institutes. Fifty-two undergraduate degree programs offer 114 majors, and there are 123 master's and 76 doctoral programs in 87 different departments. Along with Ohio State Univ. and the Univ. of Minnesota, the Univ. of Florida offers more academic programs on a single campus than any of the nation's other universities. The Carnegie Commission on Higher Education ranks it as one of the nation's leading research universities. Moreover, the Univ. of Florida is a member of the prestigious Association of American Universities, which is composed of the 56 leading public and private institutions in North America.
Phenological cycles were determined for carambola (Averrhoa carambola) trees in a gravelly loam soil at four different soil water depletion (SWD) levels in containers and in an orchard in southern Florida. The phenological cycles of young trees grown in containers were not as well defined as those of mature trees in an orchard. Shoot extension of trees in the orchard and containers was observed from the first week of March until the third week of December. Two peak flowering periods occurred during the first week of March, and from mid-September to mid-October. The major fruit harvest periods were August and December. Shoot flushing, extension shoot growth, flowering, and fruiting showed little response to irrigation at four SWD levels. This lack of response was likely caused by sufficient soil water due to precipitation and capillary rise from the high water table located about 1–2 m below the soil surface. Regardless of the lack of SWD effects on phenological cycles of carambola, the periodicity of shoot flushing, extension shoot growth, flowering, and fruiting and the intensity of these phenological events elucidated in this study should provide useful guidelines for carambola orchard management in southern Florida.
A dynamic heat transfer model was developed for simulation of freeze protection of young citrus trees by tree wraps and microsprinkler irrigation. Heat exchange at the surface of the tree wrap was a function of heat input from irrigation and heat losses to evaporation, radiation, and convection. A finite difference form of a transient heat conduction equation was used to calculate rates of trunk temperature change as a result of heat exchange at the wrap surface. Predicted trunk temperatures were generally within 1 SE of observed means when simulating the effects of tree wraps without irrigation, with high correlation (r = 0.99) between observed and predicted minimum temperatures. When simulating the effects of microsprinkler irrigation combined with tree wraps, however, predicted trunk temperatures were generally 1° to 3°C lower than observed means (r = 0.81). Under-prediction of trunk temperature was attributed to underestimation of sensible heat transfer from the irrigation water and/or inaccuracies in parameters associated with irrigation. The behavior of the real and model systems was qualitatively similar in a majority of validation trials. Thus, the simulation model could be used to analyze factors affecting freeze protection.
Computer simulation was used to study factors affecting freeze protection of young citrus trees using tree wraps and microsprinkler irrigation. Simulation results suggest tree wraps provide 1° to 2°C more freeze protection when air temperature decreased linearly rather than exponentially with time toward a minimum value. Tree wraps provided greater freeze protection when air temperature decreased rapidly (0.75°/hr) than slowly (0.5° or 0.25°/hr). Thus, variation in rate and pattern of air temperature decrease among freeze nights may be responsible for inconsistent levels of freeze protection observed with tree wraps. Changing irrigation water temperature from 12° to 7° or 17° produced changes in trunk temperature under radiative, but not advective, freeze conditions. Simulated positioning of the microsprinklers to maximize coverage of the wrap surface with water provided better freeze protection than positioning that resulted in increased water interception. Increasing windspeed from 0 to 10 m·s−1 reduced temperature of an irrigated wrapped trunk by 5°. Without irrigation, temperature of a wrapped trunk was largely unaffected by windspeed. Reducing humidity from 80% to 20% had a negligible effect on trunk temperature of irrigated wrapped trees under radiative and advective freeze conditions.
Species of Alternaria and Gloeosporium were most often isolated from fruit with blossom-end yellowing (BEY), a disorder associated with summer fruit drop of navel orange [Citrus sinensis (L.) Osbeck]. Fruit inoculated with pure cultures of these fungi did not develop BEY; however, wounded fruit which were inoculated with fungi produced higher levels of ethylene and more extensive BEY than wounded, noninoculated fruit. Fruit with BEY produced higher amounts of ethylene than symptomless fruit. The methoxy analog of rhizobitoxine (methoxyvinylglycine) did not reduce ethylene levels, and silver nitrate increased ethylene production from fruit with BEY. Ethylene and fungi are associated with BEY of navel orange but do not appear to be causal factors.
Two distinct periods of fruit drop, summer (mid-June to mid-August) and summer-fall (late August to late October), of navel sweet orange [Citrus sinensis (L.) Osbeck] were observed during the period between the end of the June drop (mid-June) and legal maturity (late October) in the 1978-79 and 1979-80 seasons. Summer drop reduced fruit number in 1979 by as much as 16.5% and was mostly caused by a yellowing and subsequent decay of the secondary fruit. A separation zone was found between the central axis and the secondary fruit of those with yellow navels. Summer-fall drop was responsible for an additional 14.5% reduction in fruit number based on catching frame counts. It was caused primarily by stylar-end decay, dieback of branches, splitting and brown rot (Phytophthora spp.). Causes of stylar-end decay, branch dieback and splitting were not identified. Incidence of stylar-end decay and splitting was greatest on fruit with a large (> 12 mm) stylar-end aperture (navel) of irregular shape, and adjacent thinner than usual peel. Stylar-end decay was associated with protrusions of rind-like tissue from the secondary fruit into segments of the primary fruit.
Three rabbiteye blueberry (Vaccinium ashei Reade) plantations of different ages were surveyed in north Florida to determine the type and extent of mycorrhizal colonization. Ascocarps of an ectendomycorrhizal fungus, Elaphomyces persoonii Vitt., were found attached to the root of V. fuscatum Ait., a common wild blueberry. This fungus was identical morphologically to that isolated from roots of rabbiteye blueberry. Mycorrhizal colonization was greatest at the 8- and 18-year-old plantations, where at least 50% of the roots were colonized, and least at the 4-year-old planting and on 2-year-old bushes in the nursery. The greatest number of ascocarps was found in the wild near the most heavily colonized plantation. Soil P was highest at the 4-year-old and lowest at the 18-year-old planting. No monthly variation in the amount of mycorrhizal colonization was observed thoughout the year at the oldest site. Inoculum potential of ectendomycorrhizal fungi in the surrounding land, age of the planting, and P content of the soil appear to influence the extent of colonization of cultivated rabbiteye blueberry.