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- Author or Editor: Eliezer E. Goldschmidt x
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.
An overview of the conceptual and methodological changes in citrus developmental research during the past half century is provided. Beginning with the hormonal era, the extensive use of plant growth regulators (PGRs) is described and the presumed role of gibberellins in various stages of the reproductive processes is emphasized. Introduction of the source-sink concept opened the way for alternative, carbohydrate-mediated explanations of productivity and alternate bearing. The potential role of ethylene in citrus abscission and fruit ripening is reviewed. The recent adoption of genetic–molecular approaches paves the way for deeper understanding of citrus developmental enigmas.
The citron (Citrus medica L.) is one of the forefathers of the citrus tribe and was the first Citrus to reach the Mediterranean, according to Theophrastus’ testimony (). Citron’s biology is inseparable from its cultural and traditional legacy, from the Classic period until now. The rapidly evolving molecular tools and the increasing availability of germplasm enable reconstruction of the genomic pathway of citrus evolution and speciation. The recent progress in citron genomics is reviewed, paying special attention to the fingered citron. Among the unique biological traits of citrons, the persistent style (Pitam) played a special role. The biology of style persistence and its regulation by the synthetic auxin, Picloram, are described. Analysis of Jewish scripts and archaeological evidence regarding the requirement of style persistence for the use of citrons in the Feast of Tabernacles does not provide an unequivocal answer. However, citrons with a persistent style have been in high preference in recent centuries’ citron trade.
Studies of growth and development in plants have been imbued with attempts to characterize and identify the endogenous growth substances of plant tissues and to relate changes in their quantities to processes in which they are assumed to participate. Studies of endogenous growth substances can be regarded from the horticultural point of view as complementary to those based upon exogenously supplied growth regulators, leading to better understanding of the developmental processes involved and, eventually, facilitating the practical control of horticultural production.
The share of brand-name fruit and vegetables is much lower than that for general foods or processed industrial foods. The paper analyzes consumers' choice between generic and brand-name products and shows that consumers' preferences for horticultural brands increase with their appreciation of quality and with a low quality of the generic products, and that brand preferences are affected by socio-economic variables. The theoretical findings are supported by an orange (Citrus sinensis) consumer survey held in the UK and Israel. We found that in both countries consumers assign a relatively low value to orange brands. Consumers who are willing to pay for better quality are those who perceive brands as more important.
Two alternate-bearing ‘Wilking’ mandarin (Citrus reticulata Blanco) trees, an “on” and an “off” tree, were uprooted and dissected into 11 organ types during the time of fruit maturation (February). Total or water-soluble N, P, K, Ca, and Mg (nitrate for soluble N) content were determined for each organ. Calcium was separated further into water-soluble, exchangeable, insoluble, and oxalate fractions. Leaves and twigs of on trees had reduced N, P, and particularly K levels. Organs of on trees, including minor roots and fruit, contained increased nitrate levels. Leaves, twigs, and minor roots of on trees accumulated Ca, mainly as Ca oxalate. The amounts of N, P, and K present in the fruit represented 32.2%, 43.7%, and 57.8%, respectively, of the total for an on tree. Fruit were very low in Ca, containing only 6% of the tree total. A 15-month follow-up of seasonal fluctuations in minerals of on and off ‘Wilking’ trees showed that the mineral composition of leaves and twigs was rapidly changed by the presence or absence of fruit. Defruiting in July prevented mineral depletion and permitted reasonable flower-bud differentiation on subsequent year. Leaf disks of on trees had low initial nitrate–reductase activity but responded weakly to nitrate induction. Leaves of overloaded on trees appear to suffer from a metabolic disorder involving relatively high nitrate levels, Ca accumulation, and loss of the nitrate–reductase induction response.
Mature green ‘Valencia’ orange (Citrus sinensis (L.) Osbeck) fruit were painted with solutions of 25 ppm 14C-gibberellic acid and harvested from the tree 1, 3, 10, 30, and 100 days after application. Radioactivity recovered from the flavedo after 1 day amounted to 2% of that applied and more than 1% could still be recovered after 100 days. About 50% of the radioactivity partitioned at pH 3.0 into ethylacetate in all flavedo samples (day 1 to 100). Migration of the ethylacetate radioactivity in thin layer chromatography (TLC) indicated that it may still be in the form of gibberellic acid.
The chlorotic appearance of mineral-deficient citrus leaves presumably reflects degradative changes in chloroplast components, most of which have nitrogen as a principal constituent. To examine this assumption the size of some major nitrogen pools, the SDS-PAGE pattern of soluble and chloroplast membranal proteins, and the activities of nitrate reductase and ribulose bisphosphate carboxylase (RuBPcase) were determined in leaves of rough-lemon (Citrus volkameriana Ten. & Pasq) plants grown hydroponically for 3 to 10 months under K, Mg, and Ca deficiencies. Plants grown under minerally deficient conditions produced less biomass. Leaves developing under K, Mg, and Ca-deficient conditions had significantly reduced concentrations of the respective elements. Chlorophyll levels of the chlorotic Mg and Ca-deficient leaves were lower than those of control leaves but chlorophyll a/b ratios were not markedly different. Calcium deficiency caused significant decreases in total nitrogen, nitrates, and the free amino acid pool. Proline which is the major component of the free amino acid pool decreased by 82.5%. Calcium-deficient leaves had significantly lower nitrate reductase and RuBPcase activities. The level of RuBPcase holoenzyme and its subunits were also reduced. Protein levels of K, Mg, and Ca-deficient leaves were not significantly altered. The SDS-PAGE patterns of soluble and chloroplast membranal proteins did not reveal major qualitative changes. In conclusion, the data do not demonstrate a general close link between chlorosis of minerally deficient citrus leaves and nitrogen metabolism. Calcium deficiency seems to specifically interfere with early stages of nitrogen assimilation and free amino acid accumulation but the metabolic integrity of the leaf is apparently maintained even under severe nutritional stress conditions.
Two alternate bearing ‘Wilking’ mandarin (Citrus reticulata Blanco) trees, an “on” and an “off” tree were uprooted and dissected into 11 organ types. Starch and soluble sugar concentrations were determined for each organ. Starch concentrations were 3.6 (leaves) to 17.4 (medium roots) times higher in the “off” tree. Soluble sugar concentrations were 1.5 (leaves) to 1.9 (medium roots) times higher in the “off” tree. A total dry matter, starch and soluble sugar balance was compiled for each tree. The “off” tree contained 13.26 kg starch and 10.66 kg soluble sugars; as against 2.95 kg starch and 6.75 kg soluble sugars in the “on” tree (excluding the fruit). The majority of this reserve pool would presumably be recycled and used for next year’s crop. Removal of fruit by mid-summer permitted reasonable flower bud differentiation the following year, connected with a build up of the starch reserve levels.