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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: Anthony Ananga, Florida A&M University
Project dates:  March 2020 - November 2022

Project Overview:

8-94 Field Data Collection
The team collects data samples in the field (photo courtesy of Dr. Trifa).
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

After several months of delays due to administrative processes for setting up the grant account within her institution, Dr. Snoussi and her team finally had the chance to launch their PEER project as of November 3, 2020. The COVID pandemic also presented many challenges due to restrictions on travel and in-person work in the labs, and some technicians on the project unfortunately had to quarantine due to illness or potential exposure. However, Dr. Snoussi notes that the delays have actually had some positive effect. Rootstock experiments can last up to 10 years, and some of the trees involved in the PEER study are only 3 years old and just now beginning to bear fruit, so extending the project will mean additional time for the trees to grow and produce more fruit for analysis. The PEER team is working on 6 rootstock trials based at both state- and privately-owned orchards in various soil and climate zones. They are making regular measurements of tree growth parameters, elemental content of leaves, soil and water properties, and physiological response of rootstock/scion combinations to saline and drought stress. The researchers are also gathering data on weather conditions and agricultural practices at the orchard sites. The project also involves a farmer survey and economic study to collect and analyze data on citrus orchard area, cultivated varieties, production evolution, and the market structure and the added value of the citrus sector. Due to COVID restrictions, however, this last component has been postponed until the second or third quarter of 2021.

During the first half of this year, Dr. Snoussi hopes that conditions will allow her team to make more frequent visits to their study sites to continue the measurements described above. They will also add on spring and summer leaf sampling to perform mineral and biochemical analyses (sugars, prolines, chlorophylls, nitrogen) and will test juice from the citrus crop for phenolic compounds and biological and microbial activity. The portion of the project involving molecular sequencing and analysis will be carried out during a planned visit by Dr. Snoussi to the lab of U.S. partner Dr. Anthony Ananga at Florida A&M University. Unfortunately, that activity must be postponed until international travel restrictions are lifted. A no-cost extension has been issued through November 2022 to allow more time for the visit to be carried out.

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