Drought and flooding conditions are increasingly common abiotic factors that affect citrus crops in both the Mediterranean Basin and Florida. Furthermore, emerging diseases, such as Huanglongbing (HLB), are a potential risk for these crops in those producing areas. This study aimed to evaluate the behavior under water-stress treatments of three new citrus rootstocks (UFR-6, B11R5T60, and 2247 x 6070-02-2) with reported tolerance of HLB, comparing them with a common commercial citrus rootstock (Carrizo citrange). Four water conditions were established: Control, Medium Water Stress (MWS), Drought, and Flooding. Chlorophyll index (SPAD), growth in height, relative growth rate, biomass (fresh and dry weight) and plant water status were evaluated. Citru rootstock response were different for each genotype; Carrizo citrange was negatively affected by all water treatments in the chlorophyll index (SPAD) and biomass production. By contrast, UFR-6 showed a positive response in SPAD and growth under MWS and Drought, B11R5T60 displayed similar behavior to Control under all water stresses, and the response of 2247 x 6070-02-2 under MWS treatment was adequate but was not under Drought or Flooding conditions. Our study describes the behavior of these promising new citrus rootstocks against water stress; B11R5T60 exhibiting the best performance. These results can be useful for the citrus industry to address water-stress problems in these crops.

Salinity is a major agriculture problem for citrus in the Mediterranean basin, which is a major global producer region. Citrus crops are also threatened by emerging diseases such as Huanglongbing (HLB). The use of different rootstocks increases the variability of citrus plant material in orchards, thus preventing extensive damage caused by abiotic and/or biotic diseases. In this work, we have evaluated the salinity response of five citrus rootstocks (US942, US897, X639, Forner Alcaide No. 5 and Carrizo citrange) some of which have known tolerance to HLB, under Mediterranean conditions. Four treatments with different salt concentrations (0, 25, 50 and 75 mM of NaCl) were applied by watering the plants three times per week for eleven weeks. Chlorophyll index (SPAD), growth and plant symptom parameters were recorded on a biweekly basis. At the end of the trial, roots, stem and leaves biomass and plant mineral content were obtained. The increasing concentration of NaCl resulted in visible leave damage symptoms for all citrus rootstocks assayed, hindering plant growth in all citrus rootstocks assayed, except for X639. The highest concentration of toxic ions in leaves was detected in Carrizo citrange and US897 for Cl⁻, while the lowest concentration of Na⁺ was obtained in X639. These results provide growers with information about the sensitivity to salinity of different citrus rootstocks.

Boron (B) toxicity is a common limiting factor both in arid and semiarid regions, such as the Mediterranean Basin. Citrus trees are sensitive to B-toxicity, which generates a negative impact in citrus orchards. In this work, two promising citrus rootstocks (UFR-6 and 2247 x 6070-02-2) have been assessed against B-toxicity and compared with Carrizo citrange, a common commercial citrus rootstock in Mediterranean Basin. Three B concentration treatments (Control, 1 and 2.5 mM H₃BO₃) were established, irrigating the plants three times per week for 21 days under greenhouse conditions. During the assay, above-ground symptoms, and chlorophyll index (SPAD) were recorded. At the end of the experiment, stomatal conductance, relative water content, and B concentration in leaves and roots were determined. The increasing B concentration in plants generates visual damage in leaves for all citrus rootstocks assayed. Carrizo citrange displayed the greatest visual symptoms, decreased its chlorophyll index (SPAD), and stomatal conductance throughout the B-treatment. However, UFR-6 and 2247 x 6070-02-2 displayed less symptoms than Carrizo citrange and only reduced its parameters under the 2.5 mM H₃BO₃ treatment. These results can aid citrus grower rootstock planting decisions with under B-toxicity conditions.

Phytophthora spp. are one the most common soil-borne pathogens in citrus crops, in which Phytophthoranicotianae and P. citrophthora are the most relevant species, causing disease problems worldwide, such as foot rot and gummosis of the trunk, branch canker, brown rot of fruit, feeder root rot in orchards, and seedling damping-off in nurseries. Phytophthora-tolerant citrus rootstocks are essential for its control and for the success of the citrus industry. The aim of this study was to determine the susceptibility of new citrus rootstocks with low HLB incidence to Phytophthora diseases. Thus, plants of several commercial and new citrus rootstocks originating in different breeding programs were inoculated with an isolate of P. nicotianae. Thirty days post inoculation (DPI), the damage of lesion length in stem was measured for each plant. These results displayed a different susceptibility response to the damage caused by P. nicotianae among the citrus rootstocks tested. Thus, eleven new citrus rootstocks (B11R3T25, B11R5T25, B11R5T49, B11R5T60, B11R5T64, N40R1T18, N40R1T19, N40R3T25, WGFT + 50-7, UFR-6, and CL-5146), which have not been previously studied against Phytophthora diseases, improved the tolerance effect of Carrizo citrange. Our findings provide useful information for citrus growers on rootstock selection to address incidence problems caused by Phytophthora spp.

Citrus are one of the most relevant fruit crops in the Mediterranean basin, which is the second-highest citrus-growing region in the world. However, these crops are mainly grown under Mediterranean semi-arid conditions, characterized by long periods of drought and torrential rain. In this work, we have assessed the response of three promising HLB-tolerant citrus rootstocks (Orange-14, UFR-1, and B11R3T27) to the application of four water stress conditions (Control, Mild water stress, Drought, and Flooding), comparing them with Carrizo citrange. Aerial plant symptoms were recorded during the experimental period, whereas plant water parameters, including stomatal conductance, leaf water potential, and relative water content, were obtained at the end of the assay. For all assessed rootstocks and variables, drought treatment was the most limiting factor, with Carrizo citrange being the most suitable rootstock under this condition. Flooding was the second restrictive treatment, in which UFR-1 was the least affected rootstock. Mid-water stress with 50% water requirements did not differ from the Control treatment, which can help save water resources in semi-arid regions. This information can be helpful for the citrus industry to increase the efficiency of citrus crops subject to water stress in semi-arid regions.

Citrus orchards are highly affected by iron chlorosis in Mediterranean Basin, which is the second‐highest producing region worldwide. In addition, citrus orchards from the Mediterranean Basin are exposed to emerging diseases, such as Huanglongbing, the most devastating citrus disease worldwide. Consequently, citrus rootstock can serve as an interesting tool to reduce crop losses caused by these abiotic and biotic stresses. In our study, we evaluated the effect of four Fe‐chlorosis stress treatments on six new citrus rootstocks under greenhouse conditions and compared with Carrizo citrange and Forner‐Alcaide 5. Above‐ground symptoms and chlorophyll index (SPAD) were recorded on a regular basis during the experiment. At the end of the assay, root, stem, and leaves biomass and Fe concentration in leaves were recorded. Plant symptoms were detected under Fe‐chlorosis stress treatments, in which 2247 × 6070‐02‐2 and UFR‐6 displayed lower incidence than other citrus rootstocks. These plant symptoms were in keeping with the SPAD response per citrus rootstock. Almost all citrus rootstock reported the highest biomass reduction under the treatment of 1 g CaCO ₃ /l pH 9. Only two citrus rootstocks (Carrizo citrange and B11R5T49) achieved the highest Fe concentration on leaves under control conditions compared with the other treatments per citrus rootstock.