The Role of Spur Leaves, Bourse Leaves, and Fruit on Local Flower Formation in Apple: An Approach to Understanding Biennial Bearing

Abstract

<jats:p>Lack of consistent flower formation is the underlying cause of biennial bearing. Flower formation in apple (<jats:italic>Malus</jats:italic> ×<jats:italic>domestica</jats:italic> Borkh.) has been associated with different factors, such as leaf area, shoot growth, bourse length, crop load, and seed number. However, it is unclear how these different factors interact to promote or inhibit flower formation. The effect of spur defoliation, fruit removal, and their interaction were evaluated on spur flower formation and bourse length in annual-bearing ‘Gala’ and the biennial-bearing ‘Honeycrisp’. Eight different combinations of spur defoliation and fruiting treatments were applied in three consecutive springs, 2013–15. Bourse shoot defoliation and fruiting treatments inhibited spur flower formation in both cultivars, but in different patterns from year to year. In addition, spur leaf defoliation did not affect flower formation in either cultivar. Furthermore, local defoliation and fruiting treatments did not affect bourse length. We propose that bourse leaves play a major role in both producing and transporting flower formation signals, but the effect depends on cultivar.</jats:p>

Molecular Basis of Flower Formation in Apple Caused by Defoliation and Gibberellins

“…This highlights the very biennial nature of 'Honeycrisp' and may have been due to the time of thinning (petal fall), as earlier blossom thinning has been shown to be more effective at promoting return bloom than later thinning as practiced here (Tromp, 2000). Two major flower formation inhibitors have been previously described in apple: GAs (Marino and Greene 1981;McArtney and Li, 1998;McLaughlin and Greene, 1984) and defoliation (Elsysy and Hirst 2017;Fulford, 1960;Harley et al, 1942). In most of these previous studies, treatments were applied either on whole trees or to major branches.…”

“…In addition to leaf area, the types of leaves are also important for flower formation. Our recent work showed that bourse leaves, but not spur leaves, were important in the regulation of flower formation in apple (Elsysy and Hirst, 2017). Moreover, the effect of local inhibitions such as fruiting and defoliation could be affected by the availability of flowering promotion signals in the whole tree (Elsysy and Hirst, 2017).…”

“…Flower induction genes have been shown to be active in the leaves (Hanke et al, 2007). Flower formation is affected by leaf area (Sahulka, 1967), and defoliation decreased flower formation (Elsysy and Hirst, 2017;Fulford, 1960;Harley et al, 1942). Flower formation was inhibited with partial and whole tree defoliation in 'Braeburn', 'Golden Delicious', 'Ramey York', and 'Fuji' (Davis, 2002).…”

Effects of Different Crop Loads on Physiological, Yield and Fruit Quality of ‘JoyaTM’ Apple Trees: High Crop Load Decreases Maximum Daily Trunk Diameter and Does Not Affect Stem Water Potential

“…It influences many physiological processes in fruit orchards such as source-sink carbon relationships, water status and water consumption (Bustan et al, 2016). The fruit are strong carbohydrate sink, and high crop load levels can stunt the vegetative growth (Yuri et al, 2011) and return bloom of trees (Elsysy and Hirst, 2017).…”

Effect of Fruiting and Biennial Bearing Potential on Spur Quality and Leaf Gas Exchange in Apple

“…Biennial bearing is ''the fluctuation in cropping from year-to-year caused by irregular flowering'' (Hirst, 2017). Considerable research has been conducted to investigate biennial bearing; however, it remains a complicated problem (Bangerth, 2009;Elsysy and Hirst, 2017;Monselise and Goldschmidt, 1982). A number of highvalue apple cultivars exhibit biennial bearing, including Cameo, Fuji, and Honeycrisp.…”