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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 - June 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

As 2021 drew to a close, Dr. Snoussi reported her recent progress on the four work packages (WP) that comprise her PEER project:
  • WP1: Growth and physiology studies on rootstocks/varieties combinations in various environments: The PI and her team monitored and evaluated agronomic and morphological parameters of citrus tristeza virus (CTV)-tolerant rootstocks achieved in the different ecological sites (BouSalem, Cap Bon, Kairouan, Khelidia, and Dégache). The work is aimed at assessing how different varieties behave on the various rootstocks used and in the different regions, taking into account the variability in different environments and pedoclimatic conditions. Another goal is to characterize the physiological behavior of the different combinations of citrus fruits in the five cultivation sites. The researchers are pursuing this objective by testing irrigation water and soil at the five experimental sites. Mineral, biochemical analysis and stomata morphology of the sampled leaves are also in progress.
  • WP2: Transcriptome differences in the leaves of orange grafted onto different rootstocks. This work has not yet started, as it is planned to be carried out in collaboration with the U.S. partner Dr. Ananga. The necessary exchange visits in both directions have been postponed until the COVID situation improves and international travel restrictions are eased.
  • WP3: Production parameters and fruit quality assessment: Dr. Snoussi and her colleagues are working to estimate yield in different growing regions, taking into account the management of the trial and the impact of factors like salinity, irrigation, fertilization, and soil type. They are also performing organoleptic quality assessments (juice yield, pH, content of soluble solids, titratable acidity, vitamin C). They are also testing the biological and antioxidant properties of phenolic compounds in the citrus juice.
  • WP4: Farmer survey and economic studies: The team gathered updated statistical data on citrus orchard area under cultivation, cultivated varieties, production evolution, market structure, and added value of the entire sector. Their economic assessment of the adoption of CTV-resistant rootstocks is in progress.

In addition to causing the postponement of exchange visits on the project, the pandemic continues to create other problems on the project, including with the purchase of equipment and supplies and the organization of in-person seminars. The researchers have also encountered an obstacle in carrying out their survey under WP4 due to the reluctance of some farmers to provide data. The team is trying to find a solution to ease the farmers’ concerns. In 2022, the PI and her team will continue their field and lab work on all work packages. They also plan to disseminate their findings and recommendations to various stakeholders, including the Tunisian Ministry of Agriculture and its dependent agencies. In the national strategy for the development of the citrus production (2019- 2026), CTV has been declared a major threat if it were to arrive in Tunisia. Science-based recommendations on the exclusion of sour orange-grafted citrus and the promotion of identified CTV-tolerant combinations could have great impact. A no-cost extension has been issued on this project through June 2023 to allow additional time to complete the planned activities, including the exchange visits, data analysis, and dissemination.

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