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George C. Martin

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Kiyoshi Banno and George C. Martin

Olive fruit harvest by mechanical shaking continues to be limited by poor fruit removal - less than 60% removal for most varieties. Whereas foliar spray of ethylene releasing compounds such as ethephon increases fruit removal percent, excessive leaf loss following treatment precludes commercial acceptance of the treatment. A classic case of serendipity has led to the testing of phosphorus as an olive fruit loosening agent. Na2 HPO4 at 25 mM applied via the cut stem of explants leads to massive leaf and fruit abscission. When the P source is applied at 100 mM foliar spray, fruit removal is accomplished with minimal leaf loss. Results of this investigation will cover P source, concentration, genera with abscission response and some indication of mechanism of action.

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Hisashi Yamada and George C. Martin

The addition of Al2O3 to 8-hydroxyquinoline citrate (8-HQC) solution did not alter the sensitivity of the leaf abscission zone to external ethylene treatment. Exogenous ethylene at 791 nl·l-1 for 72 to 120h and at 193 nl·l-1 for 120h induced leaf abscission whereas at 47 nl·l-1 for 72 to 120h no leaf abscission occurred. Ethylene treatment at 791 nl·l-1 for 72 to 120h increased ethylene evolution, but the amount of ethylene evolved from the explants does not seem to be enough for leaf abscission induction. Three different ethylene inhibitors, aminooxyacetic acid (AOA), CoCl2 and am inoethoxyvinylglycine (AVG), were used to determine whether phosphorus-induced leaf abscission was mediated through elevated ethylene evolution. Although AOA and CoCl2 failed to inhibit ethylene evolution from the explants stem-fed with NaH2P O4, AVG inhibited ethylene evolution. Each of the inhibitors except for 5 mM CoCl2 promoted leaf abscission when administered alone or with phosphorus. Our results reveal that phosphorus induced olive leaf abscission occurs without elevated ethylene evolution, but that oxygen is required.

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Hisashi Yamada and George C. Martin

Adding Al2O3 to 8-hydroxyquinoline citrate (8-HQC) solution did not alter the sensitivity of the leaf abscission zone to external ethylene. Exogenous ethylene at 791 nl·liter-1 for 72 to 120 hours and at 193 nl·liter-1 for 120 hours induced leaf abscission, whereas no leaf abscission occurred at 47 nl·liter-1 for 72 to 120 hours. Ethylene at 791 nl·liter-1 for 72 to 120 hours increased ethylene evolution, but the amount of ethylene evolved from the explants does not seem to be enough to induce leaf abscission. Three different ethylene inhibitors—aminooxyacetic acid (AOA), CoCl2, and aminoethoxyvinylglycine (AVG)—were used to determine whether P-induced leaf abscission was mediated through elevated ethylene evolution. Although AOA and CoCl2 failed to inhibit ethylene evolution from the explants stem-fed with NaH2PO4, AVG inhibited ethylene evolution. Each inhibitor, except 5 mm CoCl2, promoted leaf abscission when administered alone or with P. Our results reveal that P-induced olive leaf abscission may occur without elevated ethylene evolution. At 40 or 75 mm NaH2PO4, abscission did not occur until explants were removed from N2 and placed in ambient air.

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Raphael Goren, Moshe Huberman and George C. Martin

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.

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James O. Denney and George C. Martin

Xenia and metaxenia are phenomena dealing with the effects that pollen from different sources have on certain characteristics exhibited by seeds and fruits in a variety of species. A review of dictionaries, textbooks, and the scientific literature reveals that there is widespread confusion with regard to the nature of these phenomena and how they are to be distinguished. This discussion will attempt to clarify the boundary between these related phenomena by examining both the origins of the terms and our present understanding of the metabolism and anatomy involved. From this perspective, we contend that xenia applies to pollen effects as exhibited in the syngamous parts of ovules, that is, the embryo and endosperm only. Metaxenia applies to such effects found in any structure beyond the embryo and endosperm, this is, in tissues which derive wholly from mother plant material. Metaxenia then encompasses effects found in seed parts such as the nucellus and testa as well as those found in carpels and accessory tissue.

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James O. Denney and George C. Martin

Xenia and metaxenia are phenomena dealing with the effects that pollen from different sources have on certain characteristics exhibited by seeds and fruits in a variety of species. A review of dictionaries, textbooks, and the scientific literature reveals that there is widespread confusion with regard to the nature of these phenomena and how they are to be distinguished. This discussion will attempt to clarify the boundary between these related phenomena by examining both the origins of the terms and our present understanding of the metabolism and anatomy involved. From this perspective, we contend that xenia applies to pollen effects as exhibited in the syngamous parts of ovules, that is, the embryo and endosperm only. Metaxenia applies to such effects found in any structure beyond the embryo and endosperm, this is, in tissues which derive wholly from mother plant material. Metaxenia then encompasses effects found in seed parts such as the nucellus and testa as well as those found in carpels and accessory tissue.

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Jack D. Early Jr. and George C. Martin

Photoperiod is an important environmental signal for regulating developmental patterns in many plant species. In several species, photoperiodic regulation of gibberellin A1 biosynthesis has been implicated as the mechanism by which photoperiod may alter development. To examine this phenomenon in strawberry, Fragaria virginiana plants grown under long day (LD) and short day (SD) conditions with equivalent total PAR were examined to determine changes in vegetative growth and GA1 biosynthesis.

LD conditions (16 hr) promoted vegetative growth. Runner production, total leaf area, area of individual leaves, and petiole lengths, all increased under LD conditions. No runner production occurred under SD conditions (8 hr); however, the number of branch crowns increased.

Gibberellins A44, A19, A20, and A1, all from the GA1 biosynthetic pathway, were identified in plants under both LD and SD conditions. However, SD conditions appeared to affect the 2β-hydroxylation of GA20 to GA1. Whereas levels of most GAs decreased under SD conditions, levels of GA20 increased, and only trace amounts of GA1 were found, indicating a possible blockage of the pathway at this point. As GA1 is considered the active component of the pathway, blockage of GA20 conversion under SD conditions may explain the concomitant reduction in vegetative growth.

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James O. Denney and George C. Martin

Record low temperatures were experienced in California during the last 10 days of December, 1990. Olive trees in both the Sacramento and San Joaquin Valleys suffered damage from the freeze. The lowest minimum recorded in these areas was -11.6C at Willows (Glenn Co.). Types of damage included death of succulent growing tips, defoliation, bark split, and bark and xylem discoloration. Tree death to the ground was uncommon. Defoliation continued throughout the growing season, and many leaves that persisted became chlorotic. Major outbreaks of olive knot disease caused by Pseudomonas savastanoi were seen in damaged trees, especially in `Manzanillo.' Anatomical studies showed evidence of ice nucleation events in the phloem, xylem, and leaves, but the cambium was usually left intact. Refoliation and healing of bark splits progressed rapidly once growth resumed in the spring, except in cases of olive knot infestation. Cultural practices that predisposed trees to freeze damage were those leading to late-season vegetative growth, namely fall pruning and late or excessive irrigation or fertilization. `Manzanillo' is the least cold-hardy of California cultivars and the most susceptible to olive knot. `Barouni' is the most hardy.

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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.