Fruit Self-Thinning: A Trait to Consider for Genetic Improvement of Apple Tree

Abstract

In apple (Malus×domestica Borkh), as in many fruiting crops, fruit maintenance vs abscission is a major criteria for production profitability. Growers routinely make use of chemical thinning agents to control total fruit load. However, serious threats for the environment lead to the demand for new apple cultivars with self-thinning properties. In this project, we studied the genetic determinism of this trait using a F1 progeny derived from the cross between the hybrid INRA X3263, assumed to possess the self-thinning trait, and the cultivar ‘Belrène’. Both counting and percentage variables were considered to capture the fruiting behaviour on different shoot types and over three consecutive years. Besides low to moderate but significant genetic effects, mixed models showed considerable effects of the year and the shoot type, as well as an interaction effect. Year effect resulted mainly from biennial fruiting. Eight Quantitative Trait Locus (QTL) were detected on several linkage groups (LG), either independent or specific of the year of observation or the shoot type. The QTL with highest LOD value was located on the top third of LG10. The screening of three QTL zones for candidate genes revealed a list of transcription factors and genes involved in fruit nutrition, xylem differentiation, plant responses to starvation and organ abscission that open new avenues for further molecular investigations. The detailed phenotyping performed revealed the dependency between the self-thinning trait and the fruiting status of the trees. Despite a moderate genetic control of the self-thinning trait, QTL and candidate genes were identified which will need further analyses involving other progenies and molecular investigations.

Precocity and a long-term cropping in apple progenies grown on M 9 rootstock

“…Our results concerning the effect of high and extremely high fruit set on biennial bearing occurrence are also in agreement with data in existing literature (Guitton et al 2012;Durand et al 2013;Celton et al 2014).…”

Optimizing Crop Load for New Apple Cultivar: “WA38”

“…It is well known that management practices in the orchard influence tree canopy balance and the production of high apple fruit quality. For this reason, defining the optimal crop load of a new cultivar characterized by the trait of setting one or two fruits per cluster requires a fine-tuning of all the management techniques [7]. In fact, pruning trees helps maintain a sufficient and well-illuminated canopy to act as a source for photosynthate production [8], whereas apple crop load management adjusts the number of sinks to reduce resource competition and have enough carbon for developing fruits [9].…”

Additional Amphivasal Bundles in Pedicel Pith Exacerbate Central Fruit Dominance and Induce Self-Thinning of Lateral Fruitlets in Apple

“…Despite the characterization of the genetic determinism of fruitlet self-thinning (Celton et al, 2014b), the selection of elite cultivars exhibiting this characteristic remains largely empirical. The quasi-systematic acquisition of dominance of central fruitlets over the lateral ones allows the setting up of experimental designs aimed at deciphering the processes involved in the natural acquisition of this dominance and the activation of fruitlet AZ.…”

“…If all pedicels of the corymb have amphivasal bundles, the central fruit dominance may not be exacerbated, so what is the tree bearing behavior then? Further investigation should focus on the particular role of these additional bundles and determine how the expression of this characteristic is controlled in a dedicated segregating population, such as the one used by Celton et al (2014b). Such study may confirm whether the selfthinning trait is associated with the presence of amphivasal bundles in the central pedicel only.…”

“…Recently, a study aimed at characterizing the genetic determinism of apple fruitlet self-thinning in a segregating population was undertaken, and several quantitative trait loci controlling this trait were identified (Celton et al, 2014b). In this study, the self-thinning capacity of the apple tree hybrid X3177 (obtained from the Institut National de la Recherche Agronomique) was characterized from the tree scale to the molecular level.…”