Carbohydrates fulfill several roles in plants; as building stones, as a source of energy, and also, as recently demonstrated, as modulators of gene expression. Primary, photosynthetic production of carbohydrates (as well as their release from tree reserves) is linked with the carbohydrate-consuming processes through complex feedback and feedforward regulatory loops. With horticultural productivity as the goal, maximum resources must be diverted toward reproductive processes. Persistence of viable vegetative structures must be secured, however, to enable the function of tree systems and ensure the tree's potential for future years. Thus, in the carbohydrate resource allocation of fruit trees, a delicate balance must be maintained between the vegetative and reproductive needs. Flowering, fruit set, and fruit enlargement have been identified as three distinct, critical stages within the annual course of yield formation in citrus. While each of these stages has its own, salient developmental features, all require considerable amounts of energy and their occurrence is dependent, at least to some extent, on the availability of carbohydrates. Whereas flower bud differentiation may require only a threshold level of carbohydrates, floral development and anthesis consume large amounts of carbohydrates due to the very large number of flowers per tree and their high rates of respiration. Fruit set is more closely linked to carbohydrate levels. Fruitlet abscission acts as a self-thinning, feedback mechanism to adjust the tree's fruit population to the carbohydrate supply. This mechanism does not operate properly in certain mandarin cultivars, resulting in alternate bearing. The acquisition of fruit size appears to be most directly correlated with the availability of photosynthate. Elimination of alternative sinks by extreme thinning and girdling brings about tremendous increases in fruit size. The fruit's vascular system also expands in response to the increase in photosynthate availability. A dynamic model has been constructed to provide a quantitative analysis of citrus trees' carbon balance during the annual productivity cycle.
Eliezer E. Goldschmidt
Noboru Muramatsu, Toshio Takahara, Tatsushi Ogata, and Kiyohide Kojima
Changes in rind firmness and cell wall polysaccharide composition were measured in fruit with a) a soft rind, (`Satsuma' mandarin, Citrus unshiu Marc., cv. Aoshima), and b) a firm rind (hassaku, C. hassaku Hort. ex Tanaka), from August to January of the following year. Rind firmness was similar in both species in August, but hassaku had significantly firmer rind than did mandarin from September to January. Both flavedo and albedo tissues were extracted, and the extracts were hydrolyzed and fractionated to yield four fractions: (hot water, EDTA, hemicellulose, and cellulose). In flavedo tissue, sugar concentration was highest in the cellulose fraction, and lowest in the hemicellulose fraction. The concentration in all fractions decreased as the fruit developed and matured. Although the sugar concentration in the cellulose and EDTA fractions of both species was similar in August, it was significantly higher in both fractions in hassaku than in mandarin in January. The sugar concentration of each fraction from albedo tissue was in the order: cellulose > hemicellulose > hot water > EDTA. The range of variation in cell wall sugars in albedo tissue was smaller than that in flavedo tissue. Chemical name used: ethylenediaminetetraacetic acid (EDTA).
Laurie G. Houck, Joel F. Jenner, and Bruce E. Mackey
Commercially packed lemons (Citrus limon (L.) Burm.), grapefruit (C. paradisi Macfayden) and oranges (C. sinensis (L.) Osbeck) from CA and AZ were fumigated in corrugated fiberboard shipping boxes with methyl bromide (MB) at doses efficacious for controlling various postharvest insect pests. Fruit developed no rind injury when fumigated at 24 or 32 g MB/m3 for 2 hr at 21C. At 40 g MB fruit developed slight to moderate peel injury, and sometimes there were more decayed fruit. More rind injury developed at 48 gm MB, the injury was more severe, and there were more decays. Curing fruit for 3-4 days at 15-20C before fumigation, and extending the aeration period after fumigation from a few hours to 1 or 3 days reduced fruit injury. Early-season fruit were not injured as severely as late-season fruit. Lemons picked with green-colored peel but fumigated after they turned yellow (by holding at 13C for 4-10 weeks to degreen) were not injured as much as silver or yellow lemons.
E. Cohen, B. Shapiro, Y. Shalom, and J.D. Klein
Water loss was found to be a nondestructive indicator before visible symptoms of chilling injury (CI) in cold-stored grapefruit (Citrus paradisi Macf.) and lemon (C. limon L. Burm. f.). The water-loss rate increased significantly after removing the fruit from cold storage and holding at 20C. Scanning electron microscopy revealed large cracks around the stomata. Changes in electrical conductivity of the flavedo tissues, total electrolyte leakage, and K+ or Ca2+ leakage were all inadequate predictors of CI, appearing only after CI was evident.
Larry R. Parsons* and Ed Etxeberria
Earlier work has shown that moderate water deficits imposed on citrus trees can increase fruit Brix without adversely affecting yield. Increased water restrictions have been imposed on citrus growers as Florida's population continues to increase. The objectives of this study were to determine: 1) the effects of no irrigation in the fall and winter on orange fruit quality parameters in Florida; and 2) amount of potential irrigation water savings. Trees were irrigated identically in the spring and summer. A non-irrigation treatment was started on Hamlin and Valencia oranges in September and October, respectively, while controls continued to be irrigated following established irrigation practices. An additional non-irrigated treatment was started at the same time on the Valencias that consisted of a soil covering with a water barrier (Tyvek) to exclude rainfall. Stem water potential was monitored during the fall and winter to estimate differences in water stress among the treatments. Brix and organic acids increased in fruit from non-irrigated treatments when compared to fruit from irrigated trees. Results also demonstrate that reduced irrigation did not affect yield greatly. Amount of irrigation savings was determined for both cultivars that differ in maturity dates.
Eliezer E. Goldschmidt
G. K. Rasmussen and J. W. Jones
Cellulase activity in separation zones of ‘Orlando’ tangelo and ‘Valencia’ orange fruits was associated with the ethylene concn in their internal atm. The pull force required to separate the fruit from its stem decreased when ethylene increased cellulase activity.
J. Bussel and S. S. Kamburov
Grapefruit, orange, and lemon fruits were fumigated with ethylene dibromide (EDB) at 12 and 14 g/m3 for 2 hr exposures against eggs, larvae, and pupae of the Mediterranean fruit fly. Grapefruit also was fumigated at 16 g/m3 for 2 and 2.5 hr exposures. EDB sorption was determined in an empty chamber and when the chamber was loaded with fruit. At EDB dosages of 12 and 14 g/m3, a complete kill of eggs, larvae, and pupae was obtained by fumigating artificially infested oranges and lemons. With grapefruit, complete kill of eggs and pupae and high mortality of larvae were obtained at those dosages. When the dosage was increased to 16 g/m3 and exposure prolonged to 2.5 hr a 100% kill of larvae was obtained. All citrus fruits were tolerant to dosages used and no peel injury occurred during subsequent 1 month storage and 2 weeks under shelf-life conditions. The rate of EDB residue desorption from various citrus fruits after fumigation was determined. The amounts of inorganic bromide residue resulting from fumigation were below safety limits.
K. E. Koch
Translocation of 14C-photosynthetic products from source leaves to fruit segments in direct vertical alignment to them was examined in ‘Hamlin’ sweet orange [Citrus sinensis (L.) Osbeck] and ‘Marsh’ grapefruit (Citrus paradisi Macf.). A single source leaf nearest (or next nearest) to a fruit was exposed for 1 hour to 14CO2 and then flushed with ambient air for 5 hours. Source, path, and sink tissues were separated and fruit was longitudinally quartered with respect to source-leaf attachment position. About 20% of the total 14C-photosynthate was translocated into the fruit within 6 hours. The quarter aligned directly to the source leaf was allocated 77% and 85% of these assimilates in sweet oranges and grapefruit, respectively.
S. Ben-Yehoshua, E. Barak, and B. Shapiro
Curing of sealed lemons of normal and decay-prone types [Citrus limon (L.) Burm.f] and of sealed Goliath pomelo [Citrus grandis (L.) Osbeck] inhibited postharvest decay without deleterious effects on fruit quality and prevented the development of Penicillium digitatum on inoculated fruit. Curing of nonsealed fruit was less effective in reducing decay than curing sealed fruit and caused prohibitive weight loss, shrinkage, and softening. Curing of sealed and waxed ‘Shamouti’ and ‘Valencia’ oranges (C. sinensis), in comparison to only sealed fruit, resulted in some CO2 injury of the peel and off-flavor.