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Frank G. Dennis Jr.

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Frank G. Dennis Jr.

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Frank G. Dennis Jr.

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Frank G. Dennis Jr.

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Ahmed Mahhou and Frank G. Dennis Jr.

Morocco ranks fifth among the nations of the world in almond (Prunus dulcis L.) production, and contains many zones where climatic conditions are ideal for this species. Seedling trees are responsible for more than half the total production, although grafted trees are usually much more productive per hectare. A large seedling population represents an important gene pool, both for a breeding program and for selection of superior genotypes adapted to Moroccan conditions.

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John C. Neilsen and Frank G. Dennis Jr.

Chan and Cain (Proc. ASHS 91:63-68, 1967) demonstrated that seeded apple fruits inhibited flowering, whereas seedless ones did not. `Spencer Seedless' spurs bearing seeded or seedless fruits were defruited at various times after anthesis in 1989-1991 and fruit weight, seed number and bourse shoot length recorded, as well as repeat bloom. Similar defruiting treatments were also applied to entire `Paulared (all seeded fruits) trees in 1991 and 1992. In all years spurs bearing seedless fruits flowered the following year, regardless of defruiting time, shoot length or fruit weight per spur. Flowering of spurs bearing seeded fruits decreased as defruiting was delayed. In all years over 90% of spurs bearing fruits containing a total of 5 seeds or less flowered, whereas 90% of those with more than 5 seeds remained vegetative. Flowering was not correlated with shoot length or fruit weight. Bearing spurs of `Paulared' flowered when whole trees were defruited within 60 days after anthesis, but flowering was greatly inhibited when fruits were left on for 97 and 74 days in 1991 and 1992, respectively.

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Jun Ban and Frank G. Dennis Jr.

Some apple cultivars are biennial. Experiments with the facultatively parthenocarpic apple cultivar `Spencer Seedless' indicated that seeds are responsible for inhibiting flowering (Chan and Cain, Proc. ASHS 91:63-68, 1967). Because gibberellin inhibits flowering in apple, seed GAs may be responsible for biennial bearing, or seeds may affect metabolism of GAs in bourse shoots. To obtain additional information, 14C - G A12 was injected into seeds or bourse shoots of `Spencer Seedless' and the metabolites were extracted after 24 or 48 hr. Metabolites moved from seeds to bourse shoots and vice versa in 1992, only the most polar compound(s) being transported. However, transport was not observed in 1993. Five metabolites of 14C - G A12 were found in apices following injection, 5 to 7 in seeds. Two metabolites occurred in fruit flesh following seed treatment. The presence of seeds appeared to have no qualitative effect on metabolism in the bourse shoot, although the metabolites have not yet been identified.

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Ahmed Mahhou and Frank G. Dennis Jr.

Siberian C peach (Prunus persica L.) seeds were stratified at 5 and 20C. DWs and soluble protein content remained constant regardless of stratification temperature and duration. Seed extracts subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a decrease in the intensity of nine polypeptides in the cotyledons of seeds held at 5C during weeks 5 through 8, coinciding with an increase in germination capacity. These changes were confined to cotyledons held at 5C, and were observed only when the seeds were able to germinate. The effects of stratification and the imbibition degree on changes in the protein content of seeds of two additional peach biotypes (`Farouki' and `Maloussi') were also evaluated. Germination of fully imbibed seeds at 20C increased steadily as stratification time at 5C increased. Partially imbibed seeds (25 % or 50% of full imbibition) did not germinate regardless of stratification time. However, when these seeds were soaked in water after stratification, their germination paralleled that of fully imbibed seeds. Thus, dormancy was broken, even though the seeds could not germinate. Changes in protein profiles in fully imbibed seeds confirmed those previously reported for Siberian C seeds. Similar changes occurred in cotyledons of partially imbibed seeds during stratification at 5C, but at a slower rate. Those changes were, however, delayed by partial imbibition, whereas germination capacity (ability to germinate when fully imbibed) was not. Changes in cotyledon protein profiles were not affected by removing the embryonic axis before stratification, a result indicating that such changes are not controlled by the axis. Gibberellic acid (GA3 induced 35 % to 40% germination of nonchilled seeds. It hastened the loss of protein band intensity in `Farouki' but not in `Maloussi'. However, GA3-treated seeds germinated before any visible changes occurred in protein profiles. We conclude that the effects of chilling on breaking dormancy are independent of its effects on the protein changes observed in this study.

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Frank G. Dennis Jr. and John C. Neilsen

The evidence for several hypotheses regarding the mechanism(s) controlling biennial bearing in apple (Malus×domestica Borkh.) are reviewed, citing relevant evidence from work with citrus (Citrus sp.) species and pear (Pyrus communis L.). The view that flowering is inhibited by withdrawal of nutrients, primarily carbohydrates, by apple fruit is questionable, given the effects of seed development in inhibiting flowering in facultatively parthenocarpic (normally seedless) apple cultivars. The hypothesis that seeds inhibit flowering by exporting hormones, chiefly gibberellins (GAs), is an attractive one, given a) the effects of application of GAs in inhibiting flowering and b) the high concentrations of GAs in seeds. However, an alternative hypothesis, namely that seeds compete with apices for hormones that are required for flowering, is equally tenable.