Evaluation of Three New Citrus Rootstocks under Boron Toxicity Conditions

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.

Water-Stress Influences on Three New Promising HLB-Tolerant Citrus Rootstocks

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.

Feeding and Oviposition Behaviour of Trioza erytreae (Hemiptera: Triozidae) on Different Citrus Rootstock Material Available in Europe

Trioza erytreae (Del Guercio, 1918) (Hemiptera: Triozidae) is a vector of Candidatus Liberibacter spp., the causal agent of Huanglongbing disease (HLB). This study evaluates the preference of T. erytreae in different citrus seedlings. Thus, six different non-grafted citrus rootstocks were used for these experiments: (a) Carrizo citrange; (b) Citrus macrophylla; (c) ‘Cleopatra’ mandarin; (d) Forner-Alcaide No. 5; (e) Forner-Alcaide No. 517, and (f) Poncirus trifoliata (‘Flying Dragon’). The behaviour and survival of this psyllid was evaluated through the feeding preference of T. erytreae adults for different rootstocks (in a choice trial under greenhouse conditions) and oviposition and survival of T. erytreae adults on the different citrus material (in a no-choice trial under laboratory conditions). Trioza erytreae showed a clear preference for hosting and feeding on C. macrophylla, and Carrizo citrange was the most suitable rootstock for insect reproduction and survival followed by C. macrophylla. Conversely, Poncirus trifoliata was the least attractive rootstock to T. erytreae adults in the greenhouse trial and led to significantly lower T. erytreae survival. Our results suggest that conventional citrus rootstocks, such as Carrizo citrange and C. macrophylla, could increase T. erytreae populations.

Citrus Varieties with Different Tolerance Grades to Tristeza Virus Show Dissimilar Volatile Terpene Profiles

Plants produce considerable amounts of volatile organic compounds (VOCs) with several biological functions, including protection against biotic agents such as viruses and their vectors. In citrus species, these metabolites can be related with their different susceptibility/tolerance toward the Tristeza virus (CTV), one of the main biotic constraints for the citrus industry. The objective of this study was to compare the VOCs pattern from the leaves of a CTV-susceptible citrus variety such as Citrus aurantium and from three CTV-tolerant varieties: Citrus volkameriana, Carrizo citrange, and Forner-Alcaide no. 5. The VOCs emitted were analyzed via the headspace SPME method, while plant metabolites sequestered in the leaves were analyzed by heptane extraction followed by GC-MS. The results indicated that the majority of the VOCs emitted and sequestered in the leaves of the varieties tolerant and susceptible to CTV are constituted mainly by volatile terpenes (VTs) that exhibit strong qualitative/quantitative differences among the profiles of the four citrus species. In detail, the VOC emission indicated different patterns between C. aurantium and C. volkameriana and from both of them in comparison with Forner-Alcaide no. 5 and Carrizo citrange that exhibited more similarities, with the last two characterized by a higher presence of sesquiterpenes. The data obtained from the analysis of the VOCs sequestered in leaf tissues of the CTV-tolerant varieties indicated a higher presence of monoterpenes such as limonene, α-pinene, and p-cymene, known to be the main components of several plant extracts showing deterrent properties toward viruses and insect vectors. As VOC evaluation is a fast and noninvasive measure of phenotypic dynamics, allowing the association of plant phenotypes in accordance to plant disease resistance and/or stress tolerance, the possible implications of such differences in terms of tolerance grade to CTV and/or its related vectors are discussed.

Tetraploid Citrumelo 4475 rootstocks improve diploid common clementine tolerance to long-term nutrient deficiency

Nutrient deficiency alters growth and the production of high-quality nutritious food. In Citrus crops, rootstock technologies have become a key tool for enhancing tolerance to abiotic stress. The use of doubled diploid rootstocks can improve adaptation to lower nutrient inputs. This study investigated leaf structure and ultrastructure and physiological and biochemical parameters of diploid common clementine scions (C) grafted on diploid (2x) and doubled diploid (4x) Carrizo citrange (C/CC2x and C/CC4x) and Citrumelo 4475 (C/CM2x and C/CM4x) rootstocks under optimal fertigation and after 7 months of nutrient deficiency. Rootstock ploidy level had no impact on structure but induced changes in the number and/or size of cells and some cell components of 2x common clementine leaves under optimal nutrition. Rootstock ploidy level did not modify gas exchanges in Carrizo citrange but induced a reduction in the leaf net photosynthetic rate in Citrumelo 4475. By assessing foliar damage, changes in photosynthetic processes and malondialdehyde accumulation, we found that C/CM4x were less affected by nutrient deficiency than the other scion/rootstock combinations. Their greater tolerance to nutrient deficiency was probably due to the better performance of the enzyme-based antioxidant system. Nutrient deficiency had similar impacts on C/CC2x and C/CC4x. Tolerance to nutrient deficiency can therefore be improved by rootstock polyploidy but remains dependent on the rootstock genotype.

Salinity Effect on Plant Physiological and Nutritional Parameters of New Huanglongbing Disease-Tolerant Citrus Rootstocks

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.

In Planta Localization of Endophytic Cordyceps fumosorosea in Carrizo Citrus

Entomopathogenic fungi can be a useful resource for controlling insect vectors of citrus plant pathogens, such as the Asian citrus psyllid (Diaphorina citri) associated with huanglongbing or the citrus root weevil (Diaprepes abbreviatus) associated with the spread of Phytophtora spp. In this study, Cordyceps fumosorosea (Cfr) was investigated in planta as a potential endophytic entomopathogenic fungus and various inoculation techniques were used to determine if it would colonize the Carrizo citrange (Citrus × insitorum) seeds and plants. The four inoculation methodologies evaluated were seed soaking, stem injection, foliar spray, and soil drench. Seed immersion trials demonstrated that the roots of the Carrizo citrange plant can be inoculated successfully with Cfr. Stem injection, foliar spray, and soil drench also provided successful inoculation of Cfr. However, this fungus was only endophytic in the plant stem. Sand cores indicated that Cfr moved down through the sand column and was able to inoculate the roots. Given the prevalence of Cfr in the soil during the drench experiment, and that the fungus was able to colonize Carrizo citrange roots through seed immersion, this finding provides evidence of the potential endophytism of this fungus when applied to citrus plant species.