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  • Author or Editor: Martin J. Bukovac x
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The effect of temperature on uptake of C-labeled NAA was determined using detached apple leaves. Uptake by both adaxial and abaxial surfaces was measured at 15 and 35C over a 24-hotm period. Foliar absorption of NAA by the abaxial surface was greater than that by the adaxial surface. Absorption by the abaxial surface increased linearly (P < 0.001) with temperature over the range of 15 to 35C. These results are discussed in relation to fruit thinning. Chemical name used: 2-(1-naphthyl)acetic acid (NAA).

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NAA is commonly used for post-bloom apple thinning. However, occasionally with `Delicious', excessive small fruit (50 - 67 mm) may be produced, regardless of crop load. This effect has been associated with late and/or low volume applications, however the cause is not known. A preliminary study (1991; fruit diam., 11 mm; 15 mg·L-1 equivalent at 234 - 2104 L·ha-1) resulted in a high incidence (19 - 40 %) of small fruit. A later application (fruit diam., 20 mm, 701 L·ha-1) had a lower incidence and fruit size approached that of hand thinned controls. In 1992, NAA (15 mg·L-1,equiv.) was applied (fruit diam., 8 mm) at 250 - 2000 L·ha-1. Time of application was assessed by applying NAA (15 mg·L-1, high vol.) at 5 - 21 mm fruit diameter. At harvest, there was no significant amount of small fruit in any treatment. Fruit from NAA treatments were smaller than hand-thinned, but larger than non-thinned controls. Fruit size distribution showed no significant effect of spray volume or time of application. In a related study, a higher concentration (17 mg·L-1) of NAA induced small fruit. The possible involvement of seasonal/environmental factors will be discussed.

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Parthenocarpic fruit development was induced in ‘Montmorency’ sour cherry (Prunus cerasus L.) with AC 94377. The biological activity of AC 94377 was increased significantly by NAA. The development (Stages I, II, III, “June drop”, ripening) of parthenocarpic fruit was similar to that of open pollinated controls except for smaller fruit size. Ovules enlarged during Stage I in AC 94377-induced parthenocarpic fruit, but lacked embryos; all ovules aborted during Stage II or III of fruit development. The primary action of NAA appeared to reduce abscission of AC 94377-treated ovaries with no significant effect on fruit size. Chemical names used: 1-naphthaleneacetic acid (NAA); N-(phenylmethyl)-1H-purin-6-amine (6-BA); 1-(3-chlorophthalimido)-cy-clohexanecarboximide (AC 94377); polyoxyethylene sorbitan monooleate (Atlox BI).

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14C-urea penetration of isolated tomato (Lycopersicon esculentum Mill. cv. `Pik Red') fruit cuticular membranes (CM) was studied as a function of concentration and temperature. There was no significant effect of cuticular wax on urea penetration at 25C, permeances for the CM being 8.4 × 10-10 and dewaxed CM (DCM) 11.1 × 10-10·m·s-1. Time lags were near zero for both CM and DCM. Steady-state diffusion analysis suggests that the relatively low cuticular permeance of urea is due to low partitioning that offsets high diffusivity. Urea flux through the CM and DCM showed ≈1.5- and 1.9-fold increases, respectively, for each 10C increase between 5 and 45C. Urea flux across CM and DCM increased linearly with concentration (10 μm to 1 m) and, thus, was a first-order process.

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The cuticle is the primary barrier to the penetration of foliar applied chemicals. Recent studies indicate that octylphenoxy polyethoxylated surfactants (Triton X, TX) directly enhance cuticular permeability. An infinite dose diffusion system, which consisted of donor and receiver cells interfaced by a cuticular membrane (CM) enzymatically isolated from mature tomato fruit, was used to determine surfactant effect on growth regulator penetration. Steady state penetration of CM and dewaxed CM by the nondissociated forms of radiolabelled NAA and BA was followed successively after. (1) NAA or BA addition to donor cell, then (2) surfactant (0.1% w/v) addition to donor or receiver cell, and finally (3) surfactant addition to the other cell. The ratio of growth regulator permeances after to before surfactant addition (Pa/b)was used to quantify the surfactant effect In general, surfactants increased growth regulator penetration regardless of the location of surfactant addition, presence of waxes, or cuticle orientation to the donor solution (average Pa/b: NAA + TX-45, 6.1; BA + TX-100, 2.1). Surfactantenhanced penetration did not depend on growth regulator/surfactant copenetration since the enhancement was observed when the two components were in opposite cells. This suggested that the surfactant acted directly on cuticle. Transient state analysis of growth regulator sorption/desorption did not indicate a clear surfactant effect on either diffusivity or partitioning. However, latent effects of TX-45 on NAA penetration were seen also for sorption and desorption.

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