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Abstract

Translocation of ^l4C-sucrose from leaf to fruit was measured in ‘Golden Delicious’ and ‘Staymared’ apple (Malus domestica Borkh.) and ‘Redhaven’ peach (Prunus persica (L.) Batsch) following the application of compounds reported to influence fruitlet abscission. Succinic acid-2,2-dimethylhydrazide (daminozide) reduced ¹⁴C translocation in both apple cultivars but reduced fruit set only in ‘Golden Delicious’. Ethephon reduced ¹⁴C translocation and fruit set of peach. Abscisic acid (ABA) and 2,4-dinitrophenol (DNP) reduced ¹⁴C-sucrose translocation in apple. Enclosing apple limbs in black cloth bags reduced fruit set and naphthaleneacetic acid (NAA) increased leaf water potential. One method of preliminary screening of compounds for apple fruit thinning may be based on their effect on ¹⁴C-sucrose translocation from foliage to fruit.

Abstract

This study was initiated to determine if differences exist in the rates at which reducing sugars accumulate in the flavedo tissue of interior and exterior canopy ‘Marsh’ grapefruit (Citrus paradisi Macf.) during low-temperature storage and if intermittent warming treatments alter the conversion of sucrose to reducing sugars. Sucrose concentrations decreased and reducing sugar concentrations increased in the flavedo tissue of grapefruit stored at 5C. Relative changes in sucrose and reducing sugar concentrations were similar in fruit harvested from interior and exterior canopy positions. Thus, differences in chilling sensitivity of interior and exterior canopy fruit were not related to the concentrations of sugars in the peel tissue. Reducing sugars accumulated and sucrose decreased more rapidly in the flavedo tissue of intermittently warmed grapefruit than in similar tissue of grapefruit held constantly at 5C. Exogenous glucose did not stimulate O₂ uptake by grapefruit flavedo tissue in air (21% O₂) or in 100% O₂ at any time during the harvest season. Dinitrophenol stimulated O₂ uptake by tissue in 100% O₂, but not in air. Thus, sugar substrate appeared to be adequate to support the respiratory rates of grapefruit flavedo tissue and O₂ availability limited respiration. Chilling sensitivity of grapefruit is not directly related to the concentration of reducing sugars per se.

The mechanism by which diphenylamine (DPA) controls superficial scald in apples and reduces chilling injury in green bell peppers [Capsicum annuum L. var. annuum (Grossum Group)] has been assumed to be related to its antioxidant activity. In the present study, DPA inhibited the respiratory activity of green bell pepper fruit as well as oxygen uptake by the mitochondria isolated from them. When the alternative oxidase was inhibited with n-propyl gallate or disulfiram during state 4 respiration, DPA did not further inhibit O₂ uptake. Treating green bell peppers with DPA before storage did not alter the induction and abundance of the alternative oxidase protein in mitochondria which was maximally induced in peppers stored at 4 °C. Whether added before or after the uncoupler, 2,4-dinitrophenol, DPA negated the enhanced O₂ uptake associated with uncoupling of electron transfer in isolated mitochondria. These observations indicate that DPA inhibits the flow of electrons through the cytochrome path, probably somewhere in the cytochrome bc ₁ complex. Although the secondary amine function of DPA makes it a powerful antioxidant, the effectiveness of DPA in reducing chilling injury in green bell peppers and superficial scald in apples [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] also may be due, in part, to its inhibition of respiration.

Abstract

Chilling of 8-day-old cucumber (Cucumis sativus L.) seedlings for 48 hours at 2°C caused reversible damage; chilled seedlings collapsed but regained turgor within 6 hours at 25°. Hypocotyl segments from chilled seedlings exhibited increased solute leakage, ethylene production, and O₂ uptake after transfer to 25°. Pretreatment of seedlings with 10⁻⁴ m abscisic acid generally reduced symptoms of damage but did not prevent chilling injury. Pretreatment with 10⁻³ m CaCl₂ was generally ineffective, and pretreatment with 10⁻³ m ascorbate generally intensified symptoms of injury. Oxygen uptake increased during the first 6 hours at 25° but returned to the level of unchilled seedlings within 24 hours. Chilled tissue was sensitive to 2,4-dinitrophenol during this period, although sensitivity was less than in controls during the first 12 hours after chilling. Unchilled tissue was insensitive to SHAM, but O₂ uptake was inhibited by SHAM during the respiratory burst following chilling. Tissue was no longer SHAM-sensitive after 24 hours at 25°, by which time the respiratory burst had subsided. Thus, the alternative respiratory pathway was apparently operating only during the time of the respiratory burst, when the seedlings were recovering from chilling injury.