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Abstract
The science and practice of growing plants and the study of their usable parts and products are traditionally two separate fields of horticulture. The former is the domain of production physiologists, the latter of postharvest physiologists. Nevertheless, it is common knowledge now that preharvest conditions are of the outmost consequence to the quality of postharvest products. Indeed, in the past 25 years, reports on studies linking preharvest factors and postharvest behavior have become more frequent (3, 5, 20, 28), but we know of a few attempts to survey their interaction in a general way.
Abstract
Flowering, peel growth and abscission provide examples of processes regulated by endogenous hormonal balance and susceptible of reacting to applied growth substances. Knowledge, accumulated during recent years, of internal balances and of reactions elicited by exogenous factors, has been integrated. Aspects of mode of action, dynamics of processes, the time component, non-hormonal factors and their interaction with internal hormonal balance and applied regulators are discussed in relation to application.
Previous studies, in which the role of phosphorus in abscission of olive leaves was examined in the presence of ethylene biosynthesis inhibitors, have suggested that phosphorus induces abscission directly, without involvement of ethylene. In the present study, this possibility was further explored by comparing the effects of an ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), and an ethylene action inhibitor, 2,5-norbornadiene (NBD), in olive [Olea europaea (L) cv. Manzanillo] and citrus [Citrus sinensis (L.) Osbeck cv. Shamouti]. In olive, leaf abscission was always induced in the presence of KH2PO4, with or without AVG and NBD (alone or in combination), but was more pronounced when KH2PO4 was applied alone. In citrus, the effect of KH2PO4 alone on the induction of leaf abscission and ethylene production was much stronger than that observed in olive. However, in the presence of NBD, KH2PO4 did not induce leaf abscission in citrus during the first 60 hr. Similar results were obtained when NBD was replaced by AVG, but, in this case, abscission was inhibited for only 48 hr. In both cases, ethylene was detected after the inhibitory period had ended. The results obtained with citrus indicate that the observed effect of KH2PO4 on the ethylene-independent induction of leaf abscission in olive is not a general phenomenon and may differ in different species.
Abstract
Olive (Olea europaea L.) leaves are characterized by their ability to respond to exogenous ethylene by a 100- to 400-fold enhanced ethylene production irrespective of leaf age or time of year when sampled. The autoenhancement of ethylene production from intact or detached leaves is positively correlated with the concentration of external ethylene. A lag time of 72 to 120 hr occurred before the autoenhancement of ethylene production could be observed. An autoinhibition of ethylene production was usually observed during the first 24 to 48 hr. The effect was, however, much less pronounced. This autoinhibition of ethylene production apparently does not involve wound ethylene. Olive fruit normally produce only negligible amounts of ethylene, and the enhanced ethylene evolution, which was observed after the fruits were exposed to exogenous ethylene, was found to be exogenous ethylene that was trapped by the fruit tissue during its exposure to ethylene. In leaves, however, autoenhancement of ethylene production evidently is a physiological response that may induce a senescing process in the leaves rather than abscission.
A study was conducted to elucidate the effects of chloride in the irrigation water on growth and development of two citrus rootstocks. `Cleopatra' mandarin (Citrus reshni Hort. ex Tan) is salt tolerant and `Troyer' citrange (Poncirus Citrus sinensis) is salt sensitive. Increasing chloride from 2 to 48 mm in the irrigation water resulted in increased leaf chloride levels, more severe damage of the leaves, and reduced branch growth. High chloride in the irrigation water also caused increased putrescine (PUT) and decreased spermine (SPM) contents of the leaves. These effects were slight in `Cleopatra' but highly apparent in `Troyer'. The symptoms caused by high chloride were associated with high PUT and low SPM levels in the leaves. PUT may be involved in the development of chloride toxic symptoms, and SPM may protect or have no effect on chloride plant injury. The leaf polyamine profiles of `Troyer' and `Cleopatra' under nonstress chloride conditions were different. In `Troyer' leaves, PUT level was 9-fold higher than in `Cleopatra'; in `Cleopatra' leaves, SPM level was 25-fold higher than in `Troyer'. Nitrate supplement to saline water reduced chloride accumulation in the leaves and reduced the increase in PUT. The possible connection between ethylene production and PUT and SPM levels in the leaves of stressed plants is discussed.
Aspergillus niger B-1 (CMI CC 324626) extracellular RNase (RNase B1) was purified to homogeneity. It was found to contain two isoforms of 32- and 40-kDa glycoproteins, sharing a 29-kDa protein moiety. Optimal RNase activity was observed at 60 °C and pH 3.5. In `Almog' peach [Prunus persica (L.) Batsch (Peach Group) `Almog'] and `Murcott' tangerine (Citrus reticulata Blanco `Murcott') the enzyme inhibited pollen germination and pollen tube growth in vitro as well as in vivo. In field experiments, spray application of the RNase caused a reduction in `Fantasia' nectarine [Prunus persica (L.) Batsch (Nectarine Group) `Fantasia'] fruit set and interfered with embryo development. The biological effect of the RNase may be of horticultural value, due to its potential to control fertilization.
Previous studies have demonstrated that phosphorus, which stimulates ethylene biosynthesis, induces abscission of olive leaves directly without the involvement of ethylene. In the present study this possibility was further explored by comparing the effects of an ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), and an ethylene action inhibitor, 2,5-norbornadiene (NBD), in olive [Olea europaea (L.) `Manzanillo'] and citrus [Citrus sinensis (L.) Osbeck `Shamouti']. In olive, leaf abscission was always induced in the presence of KH2PO4 with or without AVG and NBD (alone or in combination), but it was much more pronounced when KH2PO4 was applied alone. In citrus, KH2PO4 did not induce leaf abscission in the presence of NBD during the first 48 (detached shoots) or 60 hours (leaf explants) despite the high levels of ethylene production by the tissues. Our results demonstrate that phosphorus can, at least partly, act independently of ethylene action in inducing leaf abscission in olive but not in citrus.
Abstract
A comparative study of fruit set of an alternate bearing cultivar of Citrus (‘Murcott’, a C. reticulata hybrid of unknown origin) and a nonalternating cultivar [C. sinensis (L.) Osbeck ‘Shamouti’] was conducted. ‘Murcott’ set a higher percentage of flowers (13.6%) than ‘Shamouti’ (3.8%), because of a higher abscission rate in ‘Shamouti’ throughout the fruit set period. Prebloom girdling increased fruit set significantly in ‘Shamouti’ but had a smaller effect in ‘Murcott’. The nonstructural carbohydrate balance of ‘Murcott’ and ‘Shamouti’ fruiting branches during fruit set as affected by girdling also were investigated. ‘Murcott’ generally had a higher starch content in the fruiting twigs and a higher starch and soluble sugars content in the mature, source leaves. Girdling increased the starch content in mature leaves and twigs of both ‘Murcott’ and ‘Shamouti’. This increase was observed in ‘Murcott’ 2 weeks after girdling, but only 4 and 8 weeks after girdling in ‘Shamouti’ leaves and twigs, respectively. ‘Shamouti’ mature leaves and twigs also had increased soluble sugars content because of girdling. In expanding new leaves, ‘Shamouti’ had similar or higher amounts of nonstructural carbohydrates than did ‘Murcott’, but during maturation, ‘Murcott’ young leaves developed a higher sugar starch content than ‘Shamouti’ young leaves, similar to the situation in mature one-year-old leaves. These results are discussed in terms of source-sink relationships and fruit set.
'Oroblanco' is an early-maturing pummelo-grapefruit hybrid (Citrus grandis Osbeck × C. paradisi Macf.). The fruit are usually picked and marketed while the peel color is still green; however, in some cases they can lose this green color during postharvest shipping and storage, which diminishes their commercial value. The effects of storage temperatures, gibberellic acid (GA), ethylene, and 1-methylcyclopropene (1-MCP) on the degreening of 'Oroblanco' fruit were examined. Storage temperature was critical for retaining fruit color: at 2 °C the fruit remained green for a period up to 5 weeks, whereas at storage temperatures of 6, 12, and 20 °C there was a progressive increase in the rate of degreening. Applications of GA, either as preharvest sprays or as postharvest dip treatments, effectively retained the green fruit color. Ethylene exposures up to 100 μL·L-1 for 3 days had only a slight effect on fruit degreening, and 1-MCP treatments up to 200 nL·L-1 for 16 hours had no effect at all. The slight influence of ethylene and the ineffectiveness of 1-MCP on fruit color change can not be attributed to difficulties in their application, since in the same experiments ethylene markedly induced peduncle abscission, and 1-MCP effectively inhibited this ethylene effect. Accordingly, ethylene had only a relatively small effect on the induction of chlorophyllase enzyme activity in green 'Oroblanco' peel tissue.
Molecular aspects of ethanol fermentation in citrus fruit were investigated in immature and mature ‘Star Ruby’ grapefruit (Citrus paradisi Macf.) and ‘Murcott’ mandarin (Citrus reticulata Blanco). Transcript levels of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH), which play a central role in ethanol fermentation, were detectable in all stages of fruit development, but accumulation of acetaldehyde (AA) and ethanol was evident only as fruit approached maturation or after several weeks of storage. Treatment of mature fruit with ethylene enhanced ethanol fermentation in grapefruit but not in mandarin. Immature fruit of both cultivars, on the other hand, responded to ethylene by prominent, although transient, enhancement of ethanol fermentation. Exposure of mature or immature fruit to anaerobic conditions (N2 atmosphere) upregulated the expression of PDC and ADH, and increased the levels of AA and ethanol. Exposure of mature fruit to anaerobic conditions also increased the enzymatic activities of PDC and ADH. The data indicate that the potential for ethanol fermentation exists in citrus fruit throughout development, even under aerobic conditions, but AA and ethanol are detected mainly toward maturation or under prolonged storage. However, prominent, long-term molecular induction of ethanol fermentation occurs only under anaerobic conditions imposed by N2 atmosphere.