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The effect of four applications of gibberellin A4+7 [GA4+7 (10 mg·L−1 at 10-day intervals beginning with petal fall)] on water-induced russeting, formation of microcracks. and on fruit growth and deposition of the cuticular membrane (CM) was studied in developing ‘Golden Delicious’ fruit (Malus ×domestica Borkh.). Submerging developing apple fruit in deionized water for 48 h induced russeting in untreated control but not in GA4+7-treated fruit. Immersing in water during early fruit development, 19 days after full bloom (19 DAFB), resulted in more russeting than immersions occurring later (139 DAFB). Water on the outer surface of epidermal segments increased the frequency of microscopic cracks in untreated controls but to a lesser degree in GA4+7-treated fruit. The effect of GA4+7 on water-induced russeting and formation of microcracks was larger during early as compared with later stages of fruit development. Fruit treated with GA4+7 consistently had fewer microcracks as compared with non-treated control fruit. GA4+7 had no effect on amounts or rates of cutin or wax deposition, strain, or mechanical properties of the CM as compared with the non-treated control. Thus, the decrease in russeting and formation of microcracks in the cuticle of GA4+7-treated fruit must be accounted for effects on underlying epi- and hypodermal tissues.
The effects of NAA, BA, or Accel on CO2 assimilation of shoot leaves of mature bearing Redchief `Delicious' and `Empire' apple (Malus ×domestica Borkh.) trees were evaluated over two seasons. BA at 50 mg·L-1 did not significantly affect any of the gas-exchange parameters measured. NAA (15 mg·L-1) consistently suppressed CO2 assimilation rate (from ≈10% to 24% below that of the control). This suppression was NAA-concentration dependent, continued for >15 days after treatment, and was completely overcome in `Empire', but only partially or not at all in `Delicious' when BA was combined with NAA. These results are discussed in relation to fruit thinning and NAA-induced inhibition of fruit growth in spur-type `Delicious'. Chemical names used: 2-(1-napthyl) acetic acid (NAA); N-(phenyl)-1H-purine-6-amine (BA); BA + gibberellin A (GA)4+7 (Accel).
Benzyladenine (BA), reported to increase fruit growth in apples, was evaluated with NAA to overcome NAA-induced inhibition of fruit growth. High volume sprays of NAA (15 mg·liter-1), BA (25 to 100 mg·liter-1) and combinations were applied to Redchief `Delicious' (king fruit = 10 mm). Yield was not significantly reduced. The combinations (NAA + BA 25, 50 or 100 mg·liter-1) resulted in the highest percentage of small fruit (39% < 70 mm) and the lowest percentage of large fruit (35% > 77 mm) compared to NAA, BA and hand thinned control. There was no significant effect of NAA or BA on size of king fruit in absence of lateral fruit competition on a given spur, while the combinations decreased (P = 0.01) king fruit size. NAA, but not BA, reduced growth of lateral fruit, with or without competition. However, the combinations caused marked suppression of lateral fruit growth and reduced seed content. With `Empire', both NAA (10 mg·liter-1) and BA (25 to 150 mg·liter-1) effectively thinned. Fruit size was greater with BA than NAA. The combinations (NAA, 10 mg·liter-1 + BA, 25 or 50 mg·liter-1) over-thinned and did not increase the amount of small fruit as in `Delicious'.
Abscission of apple (Malus ×domestica Borkh.) fruitlets is associated with increased ethylene evolution, although the role of ethylene is not clear. Fruitlet ethylene evolution and expression of ethylene-related MdACO1, MdACS5A, and MdACS5B genes were followed in the abscission zone (AZ) and fruit cortex (FC) of king and lateral fruitlets of ‘Golden Delicious’/M.9 after spraying with 15 mg·L−1 naphthaleneacetic acid (NAA), 150 mg·L−1 6-benzyladenine (BA), 500 mg·L−1 ethephon, and after 3 days 96% shading, all applied at a mean 10 mm fruitlet diameter. Lateral fruitlets (LF) were more susceptible to abscission after treatment with chemical thinners or shading compared with king fruitlets (KF) in which only ethephon-treated KF abscised later. On KF, only ethephon increased fruitlet ethylene evolution, MdACO1 expression in FC and AZ 8 days after treatment (DAT), and abscission of KF 10 DAT, but the other treatments did not. Although ethephon increased MdACO1 expression in FC of KF 8 DAT, the expression of MdACS5A and MdACS5B remained unchanged. On LF, all treatments at 8 DAT increased ethylene evolution and MdACO1 expression in FC and enhanced abscission 10 to 22 DAT. Expression of MdACS5A and MdACS5B in FC of LF 8 DAT was expressed at a lower level compared with MdACO1. In the AZ of both KF and LF, only ethephon increased expression of MdACO1, MdACS5A, and MdACS5B.