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Tory Schmidt, Don Elfving and Jim McFerson

In three trials, 2004 crop loads were adjusted at the balloon stage of blossom development on heavily cropped whole apple trees by clipping all flower pedicels within a cluster while leaving the spur intact. Trees were adjusted to 0% crop (all flowers removed), 50% crop (half of all clusters removed), or left as 100% crop (no flowers removed). On `Cameo'/Bud.9, 400 ppm GA4+7 were applied to trees of each crop level at petal fall, 10 mm, 20 mm, or left unsprayed. At each crop load, GA4+7 marginally diminished the 2005 return bloom regardless of application timing, but the 2004 crop level was far more influential in 2005 flowering. In a second `Cameo'/Bud.9 trial, 0, 300, 600, or 900 ppm ethephon were applied to whole trees of each crop level at 45 DAFB. Ethephon generally demonstrated a rate response in improving the 2005 return bloom, but the 2005 flowering was more dramatically influenced by 2004 crop levels. On `Honeycrisp'/M.9, 300 ppm GA4+7 were applied to whole trees of each crop level at 10 mm. GA4+7 diminished 2005 return bloom at the 50% crop load, but spray effects were not as clear at the extreme 2004 crop levels. These results suggest that commercial floral inhibitors and promoters have difficulty overcoming endogenous effects of heavy or light bloom and crop in severely alternating apple trees. In a fourth trial, lightly cropped organic `Fuji'/MM.106 trees were sprayed with 0, 150, 300, or 450 ppm GA4 at petal fall, 10 mm, or 20 mm timings in 2004. The 2005 return bloom was inversely correlated with spray rates, with 10 mm showing more floral inhibition than other timings. Overall, `Cameo' was less sensitive to GA and ethephon than `Honeycrisp' or `Fuji'.

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Steven McArtney, Duane Greene, Tory Schmidt and Rongcai Yuan

‘Golden Delicious’ and ‘York Imperial’ are apple cultivars that are prone to develop a biennial bearing habit. A successful chemical thinning program with carbaryl plus 6-benzyladenine applied at the 10-mm fruit diameter stage reduced cropload and increased return bloom of ‘York Imperial’, although the improvement in return bloom resulting from chemical thinning was insufficient to ensure a commercial cropload in the year after treatment (fewer than 10% of spurs developing flowers). A chemical thinning program with multiple applications of a naphthaleneacetic acid (NAA) and ethephon mixture during the period from 36 to 73 days after bloom increased return bloom of ‘York Imperial’ trees to commercially acceptable levels (25% or greater of spurs flowering). NAA applied during the period from 50 to 100 days after bloom (summer NAA program) or from 110 to 140 days after bloom (preharvest NAA program) increased return bloom of ‘Golden Delicious’. When aminoethoxyvinylglycine (AVG) was included with the first NAA spray in a summer program, the efficacy was reduced, indicating that ethylene may be partly involved in the florigenic activity of NAA. Dissection of ‘Golden Delicious’ buds sampled from three locations (Asheville, NC; Amherst, MA; Wenatchee, WA) at ≈14-day intervals beginning 50 days after bloom indicated that the time of floral transition (doming of the meristem apex) occurred during the period from 65 to 105 days after bloom at each location. Thus, NAA applications in a summer program for return bloom coincided with the period when floral determination normally occurred. Preharvest NAA programs effectively promoted return bloom in the experiments where a summer NAA program was also effective. These responses indicate that NAA can trigger floral development within vegetative buds relatively late in the summer and outside of the time period when it is generally believed possible to influence flower bud formation.

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Giverson Mupambi, Stefano Musacchi, Sara Serra, Lee A. Kalcsits, Desmond R. Layne and Tory Schmidt

Globally, apple production often occurs in semiarid climates characterized by high summer temperatures and solar radiation. Heat stress events occur regularly during the growing season in these regions. For example, in the semiarid eastern half of Washington State, historic weather data show that, on average, 33% of the days during the growing season exceed 30 °C. To mediate some of the effects of heat stress, protective netting (PN) can be used to reduce the occurrence of fruit sunburn. However, the impacts of reduced solar radiation in a high light environment on light-use efficiency and photosynthesis are poorly understood. We sought to understand the ecophysiological response of apple (Malus domestica Borkh. cv. Honeycrisp) under blue photoselective PN during days with low (26.6 °C), moderate (33.7 °C), or high (38.1 °C) ambient temperatures. Two treatments were evaluated; an uncovered control and blue photoselective PN. Maximum photochemical efficiency of PSII, or photosystem II (Fv/Fm) was significantly greater at all measurement times under blue photoselective PN compared with the control on days with high ambient temperatures. Fv/Fm dropped below 0.79, which is considered the threshold for stress, at 1000 hr in the control and at 1200 hr under blue photoselective PN on a day with high ambient temperature. On days with low or moderate ambient temperatures, Fv/Fm was significantly greater under blue photoselective PN at 1400 hr, which coincided with the peak in solar radiation. ‘Honeycrisp’ apple exhibited dynamic photoinhibition as shown by the diurnal decline in Fv/Fm. Quantum photosynthetic yield of PSII (ΦPSII) was also generally greater under blue photoselective PN compared with the control for days with moderate or high ambient temperatures. Photochemical reflectance index (ΔPRI), the difference in reflectance between a stress-responsive and nonstress-responsive wavelength, was greater under PN compared with the control on the day with high ambient temperatures, with no differences observed under low or moderate ambient temperatures. Leaf gas exchange did not show noticeable improvement under blue photoselective netting when compared with the control despite the improvement in leaf-level photosynthetic light use efficiency. In conclusion, PN reduced incoming solar radiation, improved leaf-level photosynthetic light use efficiency, and reduced the symptoms of photoinhibition in a high-light, arid environment.

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Keith Yoder, Rongcai Yuan, Leon Combs, Ross Byers, Jim McFerson and Tory Schmidt

Effects of temperature and the combination of liquid lime sulfur (LLS) and fish oil (FO) applied during bloom on pollen germination and pollen tube growth in flowers and fruit set were examined in apples (Malus ×domestica Borkh.). Percent germination of pollen of ‘Manchurian’ crabapples and ‘Golden Delicious’ apple flowers on the stigmatic surface of ‘Golden Delicious’ pistils increased with increasing temperature from 13 to 29 °C in the first 24 and 48 h after pollination, respectively, but not thereafter. Pollen tube growth rate in the style increased quadratically with increasing temperature from 13 to 29 °C. ‘Manchurian’ was a more effective pollenizer of ‘Golden Delicious’ than was ‘Golden Delicious’ pollen. For example, at 24 or 29 °C, some ‘Manchurian’ pollen tubes grew to the base of ‘Golden Delicious’ styles by 24 h after pollination. On the other hand, no ‘Golden Delicious’ pollen tube grew to the base of a ‘Golden Delicious’ style regardless of temperature and time. Pollen tube growth rate in the style increased with increasing day/night temperature from 7/0 to 24/7 °C. The time required for pollen tubes to grow to the base of styles decreased with increasing day/night temperature from 13/2 to 24/7 °C. Only ≈36 h was required for pollen tubes to grow to the base of style at 24/7 °C, whereas pollen tubes grew very slowly and no pollen tubes grew to the base of style at 7/0 °C regardless of pollen source. LLS + FO, applied 4 or 24 h after pollination, inhibited pollen germination, pollen tube growth in the style, fertilization, and fruit set, but it had no effect when applied 48 h after pollination. These results suggest that LLS + FO applied at this bloom stage causes flower or fruit abscission most likely by inhibiting pollen germination, pollen tube growth in the style, and fertilization.

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Tory Schmidt, Don C. Elfving, James R. McFerson and Matthew D. Whiting

Potential strategies against biennial bearing in apple [Malus × sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] include promotion of return bloom with an “on”-year application of ethephon or inhibition of return bloom with an “off”-year application of gibberellic acid (GA), but the influence of initial crop load on the efficacy of these bioregulators is poorly understood. In 2004 and 2005, six total trials were initiated in which whole trees were manually adjusted shortly before anthesis to one of three levels of crop load (100%, 50%, 0%) in ‘Cameo’, ‘Honeycrisp’, and ‘Fuji’; GA4 + 7 was overlaid on trees of each crop level in four trials and ethephon in two. In all trials, initial crop load was the primary determinant of return bloom; proportional influence on flower density, fruit density, and yield was generally most pronounced at the 50% crop level. GA4 + 7 consistently reduced floral initiation, whereas ethephon promoted it. Flowering responses from a historically alternating ‘Cameo’ trial site showed greater sensitivity to ethephon and less sensitivity to GA4 + 7 than did responses from parallel trials established in an annually bearing ‘Cameo’ block, suggesting a predilection of nascent buds to a specific fate before the influence of exogenous bioregulators or gibberellins from seeds produced in developing fruit. Light crop loads and GA4 + 7 applications generally promoted shoot extension, whereas heavy crops and ethephon had the opposite effect.