Crop Load Overwhelms Effects of Gibberellic Acid and Ethephon on Floral Initiation in Apple

Abstract

<jats:p>Potential strategies against biennial bearing in apple [<jats:italic>Malus</jats:italic> × <jats:italic>sylvestris</jats:italic> (L.) Mill. var. <jats:italic>domestica</jats:italic> (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’; GA<jats:sub>4 + 7</jats:sub> 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. GA<jats:sub>4 + 7</jats:sub> 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 GA<jats:sub>4 + 7</jats:sub> 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 GA<jats:sub>4 + 7</jats:sub> applications generally promoted shoot extension, whereas heavy crops and ethephon had the opposite effect.</jats:p>

Manipulating Regular Bearing in ‘Golden Delicious’/M9 Apple Trees using GA4+7and Ethephon

“…Standard practices, such as thinning, pruning, nutrition, and irrigation, contribute to the establishment of equilibrium between vegetative and generative growth but generally fall short of modifying the alternate bearing (Schmidt et al, 2009). The irregularities in a flowering cycle can be theoretically reduced since potential effects of plant growth regulators (PGRs) on promotion or inhibition of flowering were considered (Bukovac et al, 2006;Duyvelshoff, 2011;Schmidt et al, 2009).…”

“…The irregularities in a flowering cycle can be theoretically reduced since potential effects of plant growth regulators (PGRs) on promotion or inhibition of flowering were considered (Bukovac et al, 2006;Duyvelshoff, 2011;Schmidt et al, 2009).…”

“…In fact, the reduction of flower bud formation in apple by GAs provides a different crop load management, which is considered as an indirect thinning method (Garcia-Pallas et al, 2001). External GAs applied especially throughout the flower bud formation significantly reduced flowering in apples (Schmidt et al, 2009). Davis (2002) listed the inhibitory effect of GAs in apples in the following order: GA 7 > GA 3 > GA 4 .…”

“…GA 4+7 can be used ranging between 100 and 400 ppm (McArtney and Hoover, 2005). The most appropriate application time for gibberellins has been determined as fruits 10 mm in diameter for apples (Schmidt et al, 2009).…”

“…Ethylene regulates flowering and many other physiological processes in almost all plants (Bukovac et al, 2006;Cline, 2006). However, since ethephon is very sensitive to environmental factors, particularly to the temperature after application, there are some contrary findings regarding its flowering promoting potential (McArtney et al, 2007;Schmidt et al, 2009). Therefore, it is recommended to experiment ethephon in different cultivars and diverse environments to better understand its flowering promoting potential in apples (Schmidt et al, 2009).…”