GENOMIC TOOLS TO IMPROVE PEACH (P. PERSICA L. BATSCH) RESILIENCE TO CLIMATE CHALLENGES

As one of the earliest flowering fruit trees, peach is constitutively subjected to yield loss caused by freezing temperature occurrence at blooming time. However, higher winter average temperatures recorded in recent years may lead to anticipated budbreak increasing the magnitude of this phenomenon. The counterpart of winter warming is that, especially in tropical and subtropical growing areas, peach and other Prunus species are experiencing erratically or abnormal flowering mainly referrable to inadequate chill satisfaction and incomplete dormancy release. Dormancy is a finely regulated mechanism developed by temperate fruit trees to synchronize their vegetative and reproductive phenology with the cyclic availability of favourable temperature conditions in the growing environment. The interest of understanding the mechanisms underlying dormancy set and release have been driven, since a long time, by the necessity to manipulate the dormant period in order to avoid spring frost and to select the appropriate cultivar to be grown in a certain environment. Nowadays, renewed interest is rising in this topic in light of projected changes in thermal availability and distribution during winter season that may affect fruit tree phenological behaviour, thus threatening the sustainability of fruit production in several growing areas. Timing of bud burst and other spring phenological events are crucial in the climatic adaptation of fruit production. In the near future, agriculturists may encounter the need of changing the cultivars historically grown to maintain satisfactory production and new cultivars, more adapted to the mutated temperature asset, should be developed soon. The first part of the thesis is mainly dedicated to the phenotyping of thermal requirements in peach, aimed to inform breeding of new cultivars more adapted to the current and future climatic conditions across the leading fruit growing areas. In Chapter 1 the thermal requirements of a large number of accessions belonging to the peach germplasm reference collection the ‘PeachRefPop’, representing the major possible peach diversity and breeding history, have been studied by chilling-forcing experiments. Temperature-based models have been compared for their suitability to the phenotyping in a representative peach growing area located into the continental climate zone of Europe, for their sensitivity in describing the effect of temperature variations on tree phenology and for their predictive performance. In Chapter 2 the same phenotyping approach was adopted to study the segregation of reproductive phenology traits in three highly related F2 populations, derived from the cross between ‘Contender’ and ‘Elegant lady’ characterized by contrasting chill requirements. Thermal requirements are finely genetically regulated and are inherited as quantitative traits. Peach has been widely used as model species to study the genetic determinants of these traits and several segregating populations have been developed for that purpose. However, genetic analysis needs accurate, robust and replicable phenotypic data. We have been therefore comparing the performance of different temperature-based models at the QTL analysis in order to clarify which model should be used for phenotyping in our field study area. Together with winter warming, another harmful issue that is nowadays threatening peach fruit production, especially through the Mediterranean areas, is the enhanced recurrency of late frost events that is causing severe injuries to buds, flowers and developing fruitlets, extremely resulting the entire crop loss. Even if many freeze protection techniques are nowadays available for the management of late frosts and for preventing them from causing irreversible damages to the fruiting structures, they’re still too expensive and often technically non feasible in an economically sustainable tree cropping system. As far as alternative and result-oriented solutions urge, researchers are now focusing on the one hand, on improving forecasting and modelling tools for more accurate prediction of frost events, on the other hand, they’re pushing the selection of late frost tolerant cultivars as a general priority of breeding programmes, setting the ambitious challenge of making frost tolerance a preferred asset for the next future orchards. Frost damage is mainly caused by ice formation rather than by low temperatures per se. During spring freezing events, the cellular damage is usually produced by extra-cellular freezing and results in a series of anatomical effects, that can be reflected in diverse morphological manifestation, depending on the phenological stage reached by the tree at that time. Ice crystals formation throughout the extra-cellular regions, on the one hand causes direct damage to the cellular membrane but on the other hand causes a rapid rise in the solute concentration of the extracellular solution and establishes a water potential gradient through the plasmalemma increasing the osmotic concentration inside and leading to cell dehydration resulting in the coagulation of the protoplasm. Flower buds are hardiest during the winter, when they are fully dormant. During dormancy the lack of active xylem connection may protect flower organs thus ice formation is only possible within the bud scales. Upon dormancy breaking xylem continuity is reestablished and ice can propagate into flower structures. As the pistil is the most frost-tender organ in the flower before fertilization and ovarian locule is particularly sensitive to freezing temperatures, frost occurrence at blooming time may determine either stylar necrosis and abscission or ovule abortion. Very late frost events occurring after fruit set in a phenological phase of intense cell division, when the developing endocarp is particularly sensitive, may result in apparently no external manifestation but can kill the embryo affecting the normal development of the fruit or even cause abscission. On field characterization of late frost tolerance have been facing many troubles as it is very difficult to discriminate the genotype and the environmental contribution to freeze injury susceptibility. Laboratory based freeze assays have been, therefore, recently adopted for the determination of critical lethal temperatures for different Prunus species. In the second part of the thesis, we developed and validated a methodology for the characterization of late frost tolerance under controlled conditions. We used a climatic chamber able to simulate the humidity and temperature profile of a real late frost that allowed us to compare plenty of different genotypes at the same most critical phenological stages, something not possible in open field conditions. We rated the degree of frost tolerance of the genotypes under study by calculating simple indexes of injury. In Chapter 3 we screened accessions belonging to the PeachRefPop (used in Chapter 1) for their frost tolerance in order to explore if the trait is available into the current peach germplasm. We studied the relationships between reproductive phenology traits and frost tolerance of the reproductive organs both from the phenotypic and from the genetic perspective. In the last Chapter 4, we consider the possibility to introduce frost tolerance trait into the peach background by the hybridization with other Prunus species such as almond, Prunus davidiana and other wild relatives of the cultivated stone fruits.