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L. Gene Albrigo

The recent infestation of Florida citrus by the Asian citrus leafminer required that more information be obtained about the time interval for a flush to expand and the leaf area contributed by flushes in seasons when leafminer populations are likely to increase and cause leaf area loss. Time for leaf and shoot expansion was determined for spring and summer flush. Leaf area contribution from previous-year and current flushes was determined by seasonal tagging and measuring leaf area for flush in frame areas of 1/4 m2 surface projected to the center of the tree. Flush of 1/3 m length required 30 days to expand from first leaf feathers to full expansion. Summer flush in 1994 was 40% to 45% of total leaf area. Spring and previous year's flush averaged 20% each. Fall flush contributed 5% to 12% to leaf area, more on young, low-bearing trees. Summer flush resulted in more canopy leaf area and previous year's flushes less leaf area than expected by the end of the growing season.

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L. Gene Albrigo

Three hurricanes in Florida starting in late Summer 2004 caused severe leaf loss, which stimulated many fall shoots. Flush occurred after each hurricane and by December, shoots were 6- to 12-weeks-old when cool temperatures capable of causing flower bud induction started. To evaluate the potential for these flushes to mature buds that could be induced to flower, flushes that were stimulated on potted trees in a greenhouse were allowed to mature 4, 6, 8, or 10 weeks before moving trees to flower-inducing conditions for 6 weeks (15 °C day/10 °C night). Plants were then returned to the greenhouse, which was kept at 20 °C or higher (ambient), until buds sprouted. Only 1% of sprouting buds on shoots that matured for 4 weeks had flowers. In shoots that matured for 6 weeks, 18% of sprouting buds had flowers. After 8 weeks of growth, 57% of the buds that sprouted were flower buds, while after allowing 10 weeks for shoots to mature, induction resulted in 76% of the sprouting buds producing flowers. Consequently, 8 weeks of development were necessary for citrus shoots to develop mostly mature buds that responded to flower inductive conditions. This is about the same amount of time required for new citrus leaves to fully mature.

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Jacqueline K. Burns and L. Gene Albrigo

Temporal studies were conducted from mid- to late-harvest season of `Ruby Red' grapefruit (Citrus paradisi Macf.) to evaluate the effect of on- and off-tree storage, fruit size, and juice vesicle position on the development of granulation. Juice vesicle fresh and dry masses were highest at the stem and stylar positions of the fruit section and were not affected significantly by time of harvest or by storage. Juice vesicles isolated from each position were subjectively evaluated for the presence of granulation. Granulation was highest in stylar juice vesicles obtained from large fruit (≈600 g) that were harvested late in the season (March and May) and stored in air at 21 °C for 60 days. Large fruit harvested in March and May and examined immediately, and fruit harvested in January and stored for 60 days had low granulation scores. Thus, fruit remaining on the tree until May are less susceptible to the disorder than those harvested in March and held in storage until May. Levels of alcohol-insoluble solids (AIS), largely composed of pectins and other cell wall materials, were significantly higher in juice vesicles that were granulated. The results suggest that storage itself was not responsible for the marked accumulation of AIS in granulated juice vesicles. Rather, some interaction of fruit size with maturation, as well as other factors such as tree age and rootstock, likely contributed to the development of granulation.

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L. Gene Albrigo and Jude W. Grosser

In Florida, pesticides, nutritional and growth regulators are often sprayed in tank mixes to reduce sprayer use. Many individual spray components are phytotoxic and result in spray burns in combination or if applied with adjuvants. The toxicity level of standard spray materials is not known and new product testing for phytotoxicity is not routine. Three tests were developed to allow testing of cellular and whole fruit susceptibility to spray chemicals. Cell suspension cultures initiated from `nucellar derived' embryonic callus of `Hamlin' sweet orange were grown in log phase for 2 weeks with various levels of test chemicals. Fresh weight increase was measured. Peel disks of orange or grapefruit were grown for 4 weeks on solid media. Color changes and callus growth were used to evaluate phytotoxicity. Dilute sprays and droplet applications to on-tree-fruit were used to evaluate individual and combinations of chemicals with and without spray adjuvants. The 3 tests combined effectively demonstrated levels of phytotoxicity and are useful for testing new citrus production chemicals.

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L. Gene Albrigo and James P. Syvertsen

In order to evaluate possible reduced nitrate leaching while maintaining yield, `Hamlin' orange and `Flame' grapefruit trees on `Carrizo' or `Swingle Citrumelo' rootstocks were grown from planting using only foliar urea or soil-applied nitrate or ammonium N. An intermediate treatment of foliar and ground N was included also. From the 4th year, yields were recorded for 3 years. As previously reported, canopy growth was greater for the foliar urea treatment for the first 3 years. For 2 of the next 3 bearing years, the grapefruit trees in the foliar urea N treatment produced significantly less yield than the soil-applied treatment and the intermediate treatment was intermediate. The orange trees in the foliar urea treatment produced significantly less fruit than the soil N treatment in only 1 of 3 years, but the yields were numerically less every year. Results for fruit quality and nitrate leaching will be reported also. Foliar urea application alone was more costly and less productive than a soil N program.

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Juan I. Valiente and L. Gene Albrigo

Citrus flowering is a complex phenological process influenced by a number of interacting factors. Low winter temperatures are recognized as an important factor, but the flowering response has not been quantified under Variable natural conditions. A study was conducted to monitor the flower bud induction response of `Valencia' and `Hamlin' sweet orange trees [Citrus sinensis (L.) Osbeck] to naturally occurring winter weather conditions during the 1999 and 2000 seasons. The flowering response was quantified and related to shoot age, bud position along the shoot, local weather information, and crop load status. Results indicate that buds on previous summer shoots developed 2.52 and 3.59 to 1 flower on spring shoots, for `Hamlin' and `Valencia', respectively. In addition, buds at apical positions produced more flowers than buds located far from the apex. These basal positions buds required higher induction levels. Under Florida conditions, greater accumulation of hours of temperatures 11 to 15 °C increased floral intensity by the combined effect on the number of sprouting buds with reproductive growth and the number of flowers per flowering bud. Some statistical analyses indicated that high winter temperatures reduced flowering in `Valencia' and `Hamlin' oranges. The presence of fruit consistently reduced reproductive response for both cultivars. Crop load reduced flowering by an average of 41.5% compared to no crop and varied by cultivar. A discussion on the different induction requirements as well as on the differential effect of crop load on flowering by cultivar is presented.

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Eduardo J. Chica and L. Gene Albrigo

Cool ambient temperatures (5 to 20 °C) and water deficit are the only factors known to induce flowering in sweet orange (Citrus sinensis). Whereas the effects of cool ambient temperatures on flowering have been described extensively, reports on the mechanisms underlying floral induction by water deficit in sweet orange (and other tropical and subtropical species) are scarce. We report changes in the accumulation of transcripts of four flower-promoting genes, CsFT, CsSL1, CsAP1, and CsLFY, in sweet orange trees in response to water deficit or a combination of water deficit and cool temperatures under controlled conditions. Exposure to water deficit increased the accumulation of CsFT transcripts, whereas transcripts of CsSL1, CsAP1, and CsLFY were reduced. However, when water deficit was interrupted by irrigation, accumulation of CsFT transcripts returned rapidly to pre-treatment levels and accumulation of CsSL1, CsAP1, and CsLFY increased. The accumulation of CsFT transcripts in trees during the combined water deficit and cool temperatures treatment was higher than in trees exposed to either factor separately, and accumulation of CsAP1 and CsLFY transcripts after the combined treatment was also higher. These results suggest that water deficit induces flowering through the upregulation of CsFT and that CsFT is the leaf integrator of flower-inducing signals generated by the exposure to water deficit and cool temperatures in sweet orange.

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Eduardo J. Chica and L. Gene Albrigo

As in arabidopsis (Arabidopsis thaliana), putative citrus (Citrus) Flowering locus T (FT) homologs are strong promoters of flowering and apparently are key components of the molecular mechanism controlling floral induction in these species. An abundance of citrus FT gene transcripts during floral induction is consistent with the role of their products as floral-promoting signals. However, specific details about how the floral induction process is initiated and sustained remain largely unknown. We report changes in transcript abundance of a FT gene (CsFT) from sweet orange (Citrus sinensis) at the onset of floral induction by low temperatures and at different times of the day. Using a combination of field and growth room experiments, we determined that the abundance of CsFT transcripts increased within 1 day after initial exposure to cool floral-inductive temperatures, and that CsFT transcript abundance was higher in the afternoon than in the morning and evening. The presence of photoperiod cycles seemed to be required to sustain the increasing CsFT transcript abundance, because exposure to floral inductive conditions under continuous light or darkness did not increase the abundance of CsFT transcripts after 3 days. Our results suggest that the regulation of CsFT expression responds rapidly (overnight) to the onset of floral-inductive cool temperatures, is sensitive to changes in temperature, and requires alternation of light and dark cycles to sustain transcript accumulation during induction.

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Juan Carlos Melgar, Jill M. Dunlop, L. Gene Albrigo, and James P. Syvertsen

We determined if winter drought stress could delay flowering and fruit development of immature ‘Valencia’ sweet oranges to avoid young fruit loss during late-season mechanical harvesting. Beginning in December over three consecutive seasons (2007–2009), Tyvek® water-resistive barrier material was used as a rain shield groundcover under 13- to 15-year-old trees. There were three treatments: 1) drought = no irrigation and covered soil; 2) rain only = no irrigation, no cover; and 3) normal irrigation with rain and no cover. Covers were removed in February or March and normal irrigation and fertilization were resumed. The drought stress did not affect fruit yield, size, percentage juice, or juice quality of the current crop harvested in May and June relative to continuously irrigated trees. Drought stress delayed flowering by 2 to 4 weeks so that the immature fruit for next season's crop were smaller than on continuously irrigated trees during June but fruit growth caught up by September. During mechanical harvesting, previously drought-stressed trees lost fewer young fruit than continuously irrigated trees. Thus, winter drought stress effectively delayed flowering and avoided young fruit loss during late-season mechanical harvesting without negative impacts on yield or fruit quality of ‘Valencia’ orange trees.

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James J. Salvatore, Mark A. Ritenour, Brian T. Scully, and L. Gene Albrigo

Up to three hurricanes (Charley, Frances, and Jeanne) passed over the same citrus-producing areas of Florida in August and September 2004. In October 2005, hurricane Wilma also passed over South Florida. We began evaluating citrus tree recovery in four commercial groves (red and white grapefruit, and `Murcott' tangerine) following the 2004 hurricanes to determine how quickly commercial groves recover following such catastrophic events. We previously reported that, among other things, even branches formed after the last 2004 hurricane matured sufficiently to flower the following spring, but to a lesser extent than older shoots. Here, we report hurricane effects on tree yield, fruit quality, and shelf life. Fruit loss was dramatic following the 2004 hurricanes (>90%). Fruit loss was also substantial following hurricane Wilma, with `Murcott' yields reduced 18% and grapefruit yields reduced 58%-65%. However, in comparison to 2003 pre-hurricane yields, yields following hurricane Wilma declined only 9% for `Murcott,' and 26%-40% for grapefruit. These yield reductions are less than the fruit lost due to the present year's hurricane. Therefore, the citrus trees studied demonstrated tremendous resilience and, if not for another hurricane the following year, would have likely exceeded pre-hurricane yields only 1 year after the devastating 2004 hurricanes. Effects of the hurricanes on harvested fruit quality and shelf life will also be discussed.