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- Author or Editor: F. G. Dennis Jr. x
Went and Thimann (50) defined a hormone as ‘a substance which, being produced in one part of an organism, is transferred to another part and there influences a specific physiological process.’ In the strict sense of the word, growth substances extracted from plant tissues cannot be called hormones. In this paper, however, the term will be used in a general sense to refer to auxins, gibberellins (GAs), cytokinins, inhibitors, and ethylene occurring in plant extracts, diffusates, exuded sap, or intercellular spaces.
Growth inhibitors have been implicated in the control of many physiological processes, including dormancy of seeds and buds (35, 36, 45), apical dominance (43), root initiation (12), flowering (13, 16, 44), abscission (5, 8, 25), fruit development (11), dwarfism (29, 40, 50) and senescence (26). Despite the abundance of correlative evidence obtained, no cases are known in which irrefutable proof exists for such control. The purpose of this paper is to discuss how correlations may be extended to proof of a causal relationship. For convenience, most of the illustrations will be drawn from studies of seed and bud dormancy.
Fall applications of several chemicals were evaluated for their effects in delaying bloom of tree fruits. Sprays of gibberellic acid (KGA3) had no effect on time of bloom of sour cherry (Prunus cerasus L.) or sweet cherry (Prunus avium L.) but resulted in moderate to severe winter injury to the cambium and flower buds of the latter. Neither succinic acid-2,2-dimethylhydrazide (SADH) nor α-cyclopropyl-α-(4-methoxyphenyl)-5-pyrimidinemethanol (ancymidol) had observable effects on sour cherry, apple (Malus domestica Bork.), or apricot (Prunus armeniaca L.). (2-Chloroethyl)phosphonic acid (ethephon) delayed bloom in several stone fruits, including sweet cherry and plum (Prunus domestica L.), and thereby reduced spring freeze injury. Beneficial effects were offset, however, by deleterious side effects such as gummosis, bud abscission or failure to open, and reduced fruit set.
Gibberellin (GA)-like substances in extracts of immature apple (Malus domestica Borkh.) seeds and fruits were partially purified by paper and column chromatography. Following silicic acid column chromatography, two zones of activity (S-I and S-II) were observed in seed extracts, occurring at the approximate elution volumes of GA4/7 and GA5/6 respectively. On thin layer chromatograms, most of the biological activity of S-I occurred at the Rf of GA4/7, although an additional less polar GA-like component appeared to be present; that of S-II occurred at the Rf of GA3. Both S-I and S-II were active in all 5 bioassays used. Levels of both components were at a maximum in early July, shortly after June drop. Bioassay of extracts prepared at this time indicated that over 95% of the activity occurred in the endosperm. Extracts of fruit flesh contained two GA-like components, both more polar than GA4/7. Activity per unit fresh weight was approximately 3000 fold higher in seeds than in fruit tissue. The possible roles of these compounds in fruit development and flowering are discussed.
Gibberellic acid (GA3) has little effect on fruit set in seeded commercial varieties of apple unless pollination is prevented. To determine whether seedless apples might respond to GA, blossoms of several apetalous clones were treated with gibberellins. In addition, seeded and seedless fruits were sprayed several weeks after bloom with naphthaleneacetic acid (NAA), a fruit-thinning agent, to ascertain the role of seeds in auxin-induced fruit abscission.
GA3 was effective in increasing fruit set in 2 of the 6 clones tested, but had no effect on the remaining 4, only one of which crops heavily under orchard conditions. GA7 and GA4+7 were more active than GA3 in inducing fruit set in the 2 responsive clones. NAA thinned both seeded and seedless fruits of one clone. Fruit growth was inhibited in seedless fruits of 2 other clones, but abscission was not promoted. These results support the view that NAA-induced fruit abscission is not a result of seed abortion.
In 1994, benzyladenine (BA, formulated as Accel, containing 1.8% BA and 0.18% GA4+7) was evaluated as an apple fruit-thinning agent. Naphthaleneacetic acid (NAA, 10 ppm) and carbaryl (60 g·liter–1) were also used, as well as combinations of these chemicals with BA. Whole trees were treated with either an airblast sprayer or a hand gun, BA being used at 15–20 g/acre. Good responses to BA were obtained in one of two trials, with both `Empire' and `Gala', but `Jonagold' and `Jonathan' were not responsive (one trial each). In general, response to NAA and carbaryl was more consistent. In only one orchard (`Gala') did BA appear to increase fruit size without reducing crop load. Combinations of BA with NAA or carbaryl were generally no more effective than one chemical alone, but such combinations overthinned in one experiment with `Empire'.
Production of temperate-zone fruits in the tropics and subtropics may seem fanciful to the uninitiated, but it is a practice that has existed in localized regions for generations. Seedling peaches are grown in Venezuela (7), northern Thailand (11), and southern Mexico (5). No one knows precisely when these species were introduced, but local selection has produced cultivars adapted to areas with little or no chilling temperatures. Rest either does not occur (5) or is sufficiently shallow to be broken by stress induced by defoliation or drought.
Exposure of stratified apple (Malus domestics Borkh. cv. Golden Delicious) seeds to 30C induces secondary dormancy. To determine if an increase in abscisic acid (ABA) content was associated with the loss in germination capacity, stratified seeds (3,- 6, or 9 weeks at 5C) were held at 30C for 0, 3, or 6 days. Stratification at 5C either had no effect or increased ABA content in embryonic axes, cotyledons, and seed coats. Exposure to 30C after stratification either did not affect or decreased ABA content of embryonic axes and seed coats; in contrast, cotyledonary ABA was increased. Seed coats, cotyledons, and embryonic axes stratified for 3, 6, or 9 weeks at 20C contained the same or higher levels of ABA in comparison with nonstratified seeds or seeds stratified at SC. Changes in ABA levels were not consistently correlated with changes in germination capacity during stratification or after exposure to 30C. These data suggest that changes in ABA are not related to changes in dormancy. Chemical names used: abscisic acid (ABA); butylated hydroxy-toluene (BHT); n-(trichloromethyl) thio-4-cyclohexene-1,2-dicarboximide(Captan).