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The prevalence of sour orange rootstock in the southern and eastern part of the Mediterranean Basin is presently threatened by the spread of Citrus Tristeza Virus (CTV) and its main vector Toxoptera citricida, combined with abiotic constraints such as drought, salinity and alkalinity. The search for alternative CTV-resistant rootstocks that also withstand the other constraints is now considered an urgent priority for a sustainable citrus industry in the area. Complementary progenitors can be found in citrus germplasm to combine the desired traits, particularly between Poncirus and Citrus genera. The production of somatic hybrids allows cumulating all dominant traits irrespective of their heterozygosity level, and would appear to be an effective way to solve the rootstock challenge facing the Mediterranean citrus industry. This paper presents the results obtained during a regional collaborative effort between five countries, to develop new rootstocks by somatic hybridization. New embryogenic callus lines to be used for somatic hybridization have been created. Protoplast fusions have been performed at CIRAD and IVIA laboratories, focusing on intergeneric combinations. Analysis of ploidy level by flow cytometry and molecular markers confirmed the acquisition of new interesting tetraploid somatic hybrids for six combinations. Diploid cybrids with intergeneric (Citrus × Poncirus) nucleus and C. reticulata or C. aurantifolia mitochondria were also identified for four combinations. The agronomical performance of a pre-existing somatic hybrid between Poncirus trifoliata and Citrus reticulata was validated in calcareous soils in Morocco. Somatic hybridization is now integrated into the breeding programs of the five Mediterranean countries.

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Grapefruit is the fourth economically most important citrus fruit in the world. In this research Inter-Simple Sequence Repeat (ISSR) markers were used to distinguish twenty-nine grapefruit (Citrus paradisi Macf.), five pummelo (Citrus maxima (Burm.) Merr.) and one Citrus hassaku Hort. Ex Tanaka accessions. Twelve ISSR primers produced a total of 100 fragments and 62 of them were polymorphic. The number of average polymorphic fragments per primer was 5.2. The mean polymorphism information content (PIC) was 0.37. The unweighted pair group method arithmetic average (UPGMA) analysis demonstrated that the accessions had a similarity range from 0.79 to 1.00. The accessions were separated into two main clusters; group A with five pummelos and group B with grapefruits. In the pummelo cluster, all pummelos were distinguished whereas in the grapefruit cluster some accessions were not clearly separated. There was a low level of variation in the grapefruits due to their mutation origin.

Genetic diversity was evaluated by sequencerelated amplified polymorphism (SRAP) and simple sequence repeat (SSR) markers among 45 lemons (Citrus limon (L.) Burm. f.), five citrons (Citrus medica L.), four rough lemons (Citrus jambhiri Lush), and two Citrus volkameriana accessions. Twenty-one SRAP primer combinations produced a total of 141 (77%) polymorphic fragments with an average of 6.7 fragments per primer combinations whereas 13 SSR primers produced a total of 26 (76%) polymorphic fragments with an average of 2.0 per primer. The unweighted pair-group method arithmetic average analysis as assessed with combined SRAP and SSR data demonstrated that the accessions had a similarity range from 0.65 to 1.00. Rough lemons and C. volkameriana accessions were relatively closely related. In lemon group, accessions from hybrid origin were distant from the others. We also applied principal components analysis (PCA) for a better presentation of relation among the accessions studies. Using PCA, 88.7% of the total variation in the original dimensions could be represented by just the two dimensions defined by the first two PCs. Although nearly all accessions could be distinguished, there was a low level of genetic diversity detected among lemon cultivars.

Citrus species are the most widely produced fruit crops in the world. Citrus fruits are mainly produced in coastal areas in several countries as well as Mediterranean region, and production in these regions is affected by both biotic and abiotic stresses, including drought, extreme temperature, salinity, citrus canker, citrus tristeza virus, citrus greening, and others. The use of rootstocks in fruit production includes not only stronger resistance against pathogens but also a higher tolerance to abiotic stress conditions such as salinity, heavy metals, nutrient stress, water stress, and alkalinity. There is extensive genetic diversity in citrus which provides several materials to be used as rootstocks against abiotic stress. In this work, we tried to provide an overview of the abiotic stresses in citrus by combining literature with our studies, role of citrus rootstocks commercially used against abiotic stresses and rootstock breeding in citrus.

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W. Murcott scion, budded onto two rootstocks, was evaluated under high pH conditions supplied with different Fe levels. Plant dry weight, leaf area, iron chlorosis symptom scale, leaf chlorophyll concentration, net photosynthetic rate, and ferric-chelate reductase (FCR) activity variables were investigated under high pH conditions. Control plants (T1) produced the most leaf area, whereas plants grown without Fe (T2) produced the least. Dry weight was highest in 'Volkameriana' T1 (control) plants and lowest in 'Swingle citrumelo' T3 (10 µM Fe and 7.8 pH) and T2 (0 µM Fe and 7.8 pH) treatments. Significant differences in SPAD and iron chlorosis scale reading were found between rootstocks and treatments. Treatments significantly affected the net photosynthetic rate of the W. Murcott mandarin. Moreover, it was found that tolerant rootstock had higher FCR activity in application T2 than in applications T3 and T4 (100 µM Fe and 7.8 pH). The data of the present study suggested that scion budded onto Volkameriana rootstock showed a higher tolerance to iron deficiency than those budded onto Swingle citrumelo under high pH conditions.