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Cycle 8 (2019 Deadline)

Evaluation of new citrus rootstocks for their adaptation in different growing environments in Tunisia

PI: Hajer Snoussi Ep. Trifa (, National Institute of Agronomy Research of Tunisia
U.S. Partner: Violeta Tsolova, Florida A&M University (original partner: Anthony Ananga)
Project dates:  March 2020 - December 2023

Project Overview:

8-94 Field Data Collection
The team collects samples in the field (photo courtesy of Dr. Snoussi).
The change in the global climate causes a big threat to the production of agricultural crops, and as a result, crops have to cope up with a combination of novel and complex environmental conditions1. Therefore, there is a need to develop strategies that can mitigate climate change, and improve the yield, resilience and productivity of crops under both abiotic and biotic stress conditions. In Tunisia, citrus production is facing a complex combination of abiotic constraints like drought and salinity, as well as biotic threats such as Citrus Tristeza Virus (CTV). The most sustainable and environmental-friendly strategy is to promote combinations of suitable gathering of a given commercial variety with a specific rootstock, conferring adaptation to the abiotic stresses and tolerance to CTV. This project team will use morphological, physiological, and molecular analysis on various rootstock/cultivar combinations in multi-location field trials to gain a better understanding of the impact of the rootstock/cultivar relationship on physiological processes such as toxic ion uptake, ion translocation, ion homeostasis, water uptake, osmotic regulation, and photosynthesis resilience. Commercial orchards usually contain more than one variety and/or citrus specie. This is done because farmers often have to fulfill contracts to provide products over an extended period. In addition, this allows better management of the human resources at the farm, particularly for harvesting. Nurserymen are therefore called on to provide several location-specific rootstock/cultivar combinations in order to answer farmers’ demands. The goal of the project is to provide to farmers the best behaving citrus combinations (rootstock/variety) that could replace the current ones while ensuring the adaptation of these new combinations to local conditions to secure a good profit.

In Tunisia, the citrus sector is a strategic component of the overall national economic development program. Citrus production covers 27,000 hectares and is the main source of income for more than 12,000 farmers. Citrus fruits are among the most consumed fruits in Tunisia and constitute the most common and most affordable fruit in the Tunisian diet during the 9 months from September through May. However, citrus growing farmers would be poorly resilient in the case of a breakthrough of CTV, and potential risks are increasing given the current climate change trajectory. If this problem is not tackled, it would impact the entire citrus value chain, including producers, retailers, exporters, and consumers and would be a matter of food security as well. The Tunisian National Strategy for Citrus Development (2020-2025) is built on several pillars, among which are the replacement of sour orange to cope with the CTV threat, the development of new areas of citrus production, and the increase of the sector’s resilience to climate change. This new PEER project project fits within these orientations and proposes to work with both the public and the private sectors to analyze the behavior of a selection of CTV-resistant rootstocks/varieties in a multiple location trials in the traditional and new cultivation areas. Through seminars and field days, Dr. Snoussi and her colleagues will share the project’s results with government stakeholders at the Ministry of Agriculture, as well as citrus farmers’ cooperatives and farmers’ unions, with which the research team has longstanding previous cooperation.

Summary of Recent Activities

During the fourth quarter of 2022, Dr. Snoussi and her team worked on Work Package 1, which involves characterizing the irrigation water, soil, and nutritional status of citrus combinations (scion-graft) in four experimental sites (PROMAG Mabrouka, SEDAN Bousalem, CRDA Kairouan, and OTD Intileka Beni Khalled), which vary in their pedoclimatic conditions and their cultivation techniques. Symptomatic observations and physiological and biochemical measurements supplemented these parameters in order to identify the morpho-physiological response of new combinations under abiotic stress, particularly salt stress. The activities planned for February through April 2023 will concern laboratory analyses on contrasting pairs (tolerant/susceptible) of combinations taken from trial sites different in salinity: (1) OTD Intileka Beni Khalled (high salinity), (2) SMVDA Bousalem (low salinity) and (3) PROMAG Mabrouka (intermediate salinity). The analyzes will concern mineral (Na+, Cl-, K+) and biochemical (chlorophyll content, sugar content, MDA, starch and proline) analysis. They will be carried out at the laboratory of valorization of non-conventional waters at the National Institute of Research in Rural Engineering, Water and Forests (INRGREF) in Tunisia. Assays of antioxidant enzymes, antioxidant molecules (ascorbic acid and glutathione), and hydrogen peroxide will be done in Florida at the FAMU Viticulture Center when Dr. Snoussi and her colleague Dr. Hend Aski visit from late February through late March.

Work Package 2 on this project looks at comparative RNA-seq analysis and physiological studies of citrus rootstock combinations under different soil and water irrigation salinity. This part of the study looks at field citrus rootstock/variety combinations in three different pedoclimatic sites with varying irrigation and soil salinity conditions. This study aims to enhance understanding of the adaptation mechanisms and salt resistance regulation through which citrus tristeza virus (CTV)-tolerant rootstock/variety combinations respond to different salinity levels in different growing areas. While focusing on the transcriptome, associated physiological traits of salinity response will also be achieved by different biochemical, ion content analyses and water relation measurements to decipher the mechanisms involved in salt response in field conditions. For that, they team will have to take into account three changing factors: the site (slightly and moderately saline irrigation water), rootstock salinity tolerance (contrasting tolerant and sensitive pairs), and the variety (mandarin and orange). The workplan of this activity includes two parts: the comparative transcriptome analysis (RNAseq), and (2) the biochemical analysis complementary and essential for the interpretation of the molecular data and the rootstock adaptative behavior/environmental constraint analysis overall. This study will be achieved in cooperation with U.S. partner Prof. Violeta M. Tsolova and Dr. Islam El Sharkawy at in the Center for Viticulture and Small Fruit Research, Florida A& M University.

On Work Package 4, “Farmers survey and economic studies,” during this quarter the team continued formatting the database collected from surveys carried out during May 2022 with a set of 25 adopters of new rootstocks. The researchers have identified interesting data on characteristic of the surveyed farms, management variables, and farmers’ perceptions of CTV risks and experience in adopting tolerant rootstocks. The teams have also collected economic data that must be verified, as the farmers do not always have consistent accounting records or methods. Therefore, the team will continue to seek this data from other farmers and/or adoptive companies with solid experience to assess the economic impact. In conjunction with the visit of Dr. Snoussi and Dr. Askri, two other team members, Dr. Fraj Chemak and Dr. Asma Ben Abdrabou Ep Najar will also spend a week in Florida visiting citrus growers and processors to learn more about industry operations in the United States.

To help disseminate their interim findings, Dr. Snoussi and her colleague organized a two-day workshop entitled “Adoption of alternative rootstocks to Sour orange in Tunisia: A sustainable solution at the heart of national virus control strategy tristeza on citrus.” The first day, held at the National Institute of Agronomic Research on January 25, 2023, featured presentations from the researchers on various aspects of their work. On January 27, interested farmers and other stakeholders were hosted at the Mabrouka Nursery for demonstrations of the various alternative rootstocks and cultivation techniques.

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