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.
Raphael Goren, Moshe Huberman and George C. Martin
Raphael Goren, Moshe Huberman and George C. Martin
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.
Yoel Bar, Akiva Apelbaum, Uzi Kafkafi and Raphael Goren
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.
Levava Roiz, Uzi Ozeri, Raphael Goren and Oded Shoseyov
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.
Ron Porat, Xuqiao Feng, Moshe Huberman, David Galili, Raphael Goren and Eliezer E. Goldschmidt
'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.
Jian Xin Shi, Joseph Riov, Raphael Goren, Eliezer E. Goldschmidt and Ron Porat
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.