Cobo-Medina, Magdalena
(2024)
Combining root architecture, root function and soil management to improve production efficiency and quality of apples.
PhD thesis, University of Nottingham.
Abstract
Rootstock-induced dwarfing is a complex mechanism that causes a reduction in the size of the grafted scion by altering the floral and vegetative balance. Dwarfing rootstocks are essential to intensive production methods since they crop more and earlier. The impact of rootstock-induced dwarfing has previously been widely studied in scions but little is known about the role of dwarfing on root architecture. With the increase in food demand and climate change, it is essential to understand the impact of dwarfing on root architecture and how rootstocks can be optimised for both productivity and resilience to better adapt to future climate conditions.
Several QTL mapping studies have been conducted in apples to identify QTL linked to rootstock-induced dwarfing. However, the genetic basis of this complex trait remains unknown. A previous study which performed QTL mapping for root bark percentage, a trait associated with rootstock-induced dwarfing, identified three QTL named Rb1, Rb2 and Rb3 in Chromosomes 5, 11 and 13, respectively. In this thesis, fourteen SSR markers spanning Rb1 and Rb2 QTL were developed to fine-map these large QTL areas. The Rb1 QTL region has been successfully reduced from 4.4 Mb to 2.2 Mb. Regarding Rb2, the analysis interestingly suggested that there were actually two QTL in that region, located between 6.9 Mb and 7.5 Mb, and between 10.9 Mb and 12.7 Mb. In addition, this thesis has generated useful markers linked to dwarfing that are currently used by breeders to accelerate the breeding process.
Another aspect of this thesis identified QTL for rooting ability using stoolbeds that colocalized with the dwarfing QTL. However, when winter hardwood cuttings were utilised no QTL were identified for rooting ability. This revealed that rooting ability in apples is impacted by dwarfing and/or the physiologically active processes associated with dwarfing. Furthermore, seedling root architecture was studied to determine if early selection of deep-rooted (and therefore more climate resilient) varieties is feasible. In this study, three QTL linked to primary root length in rootstock seedlings were identified. However, no correlation was found between the seedling root architecture and the grafted rootstocks propagated using stoolbeds. This indicates that for apples, seedlings are not a good tool for predicting future root architecture.
Lastly, a selection of these rootstocks with different levels of dwarfing were collected from stoolbeds, grafted with Gala and planted in rhizotrons to analyse root system architecture changes over a season. It was found that dwarfing rootstocks exhibited a reduced maximum root system depth and convex hull area compared to vigorous rootstocks at the end of the first growing season. The great variability of data, especially in the dwarfing group, suggested that either dwarfing genotypes are more susceptible to environmental factors or that there are other genes influencing root architecture, opening the possibility of decoupling dwarfing and root system architecture.
Root bark QTL fine mapping, together with the identification of rooting ability QTL colocalizing with the dwarfing QTL, and the root architectural responses to dwarfing, have advanced our understanding of the genetics and physiological processes occurring in dwarfing rootstocks and their root systems. In future investigations, more genetic markers spanning the refined QTL could be tested in the key genotypes generated in this study to further reduce the size of these areas. Moreover, the advances in the physiological understanding of dwarfing root systems will help to better design future experiments for QTL mapping of relevant root traits that will improve rootstock resilience in a changing climate.
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