Unravelling the Physiological and Genetic Adaptation of Grafted Pepper under Saline and Hydric Stresses

Pepper culture is economically very important worldwide, although it is very sensitive to suboptimal conditions of water and high salinity. However, the tolerance to these stresses can be improved by the grafting technique. Previous studies of the Valencian Institute for Agricultural Research and the Polytechnic University of Valencia have been conducted to select pepper accessions that showed tolerance to both stresses, after which a further selection of them was used as rootstocks to find physiological mechanisms of tolerance and to increase its agronomic profit. However, after all these studies, the available information in this regard is still scarce. Therefore, the objectives of this thesis were to: I) screen new tolerant pepper accessions under high salt concentrations and suboptimal water conditions, to increase the availability of tolerant genotypes to be used in future breeding programmes, with the final aim of obtaining new and improved tolerant rootstocks; II) identify the short-term physiological mechanisms of water stress tolerance of a tolerant accession (A25) used as a rootstock; III) identify the physiological mechanisms of short-term tolerance to salinity of a new tolerant hybrid rootstock (NIBER®); and IV) find the main molecular pathways of salinity tolerance of a tolerant accession (A25) compared to a sensitive one (A6) by a transcriptomic approach. After conducting these studies, we firstly found a positive relationship between photosynthetic capacity and growth maintenance in plants that were tolerant to water or salt stress, both grafted or ungrafted; indeed, based mainly on this relationship, we selected accessions A34 and A31 as tolerant to salt and water stress, respectively. In addition, we were able to demonstrate that the main role of proline under salinity and water scarcity is not linked herein to the drop in osmotic potential; on the contrary, we identified different protective roles that, together with other antioxidant protective molecules such as phenols, contribute to the tolerance of pepper plants to these environmental stresses. Moreover, hydrogen peroxide, a reactive oxygen species, was found to play important roles in the antioxidant capacity of pepper, working as a signalling molecule under salinity stress. Furthermore, the drop in abscisic acid concentration and its signalling deregulation were also shown to maintain stomatal aperture and thus the growth of the scion when grafted onto tolerant rootstocks and ungrafted accessions under high salt concentration conditions. It was also demonstrated that a limitation of Na+ transport to leaves, as well as a more efficient transport and accumulation of K+ in roots and leaves, are essential to reach ion homeostasis and, thus, tolerance in pepper plants grafted onto tolerant rootstocks. Finally, the study of the molecular pathways of tolerance was a useful tool to confirm the physiological and agronomical behaviour of a pepper accession previously classified as tolerant, although new mechanisms were also found. The differentially expressed genes found were linked to hormonal signalling, plant growth and development, photoprotection, regulation of ion transporters and ROS detoxification