Specific Physiological and Anatomical Traits Associated With Polyploidy and Better Detoxification Processes Contribute to Improved Huanglongbing Tolerance of the Persian Lime Compared With the Mexican Lime

Huanglongbing (HLB) is presently a major threat to the citrus industry. Because of this disease, millions of trees are currently dying worldwide. The putative causal agent is a motile bacteria belonging to Candidatus Liberibacter spp., which is transmitted by psyllids. The bacteria is responsible for the synthesis of callose at the phloem sieve plate, leading to the obstruction of the pores that provide connections between adjacent sieve elements, thus limiting the symplastic transport of the sugars and starches synthesized in leaves to the other plant organs. The Persian triploid lime (Citrus latifolia) is one of the most HLB-tolerant citrus varieties, but the determinants associated with the tolerance are still unknown. HLB-infected diploid Mexican lime (Citrus aurantiifolia) and Persian lime were investigated. The leaf petiole was analyzed using scanning electron microscopy (SEM) to observe callose deposition at the phloem sieve plate. Leaf starch contents and detoxification enzyme activities were investigated. In the field, Persian lime leaves present more limited symptoms due to HLB than the Mexican lime leaves do. Photosynthesis, stomatal conductance, and transpiration decreased compared with control plants, but values remained greater in the Persian than in the Mexican lime. Analysis of the petiole sieve plate in control petiole samples showed that pores were approximately 1.8-fold larger in the Persian than in the Mexican lime. SEM analyses of petiole samples of symptomatic leaves showed the important deposition of callose into pores of Mexican and Persian limes, whereas biochemical analyses revealed better detoxification in Persian limes than in Mexican limes. Moreover, SEM analyses of infected petiole samples of asymptomatic leaves showed much larger callose depositions into the Mexican lime pores than in the Persian lime pores, whereas biochemical traits revealed much better behavior in Persian limes than in Mexican limes. Our results reveal that polyploids present specific behaviors associated with important physiological and biochemical determinants that may explain the better tolerance of the Persian lime against HLB compared with the Mexican lime.

Molecular, phenotypical, and host-range characterization of Robbsia andropogonis strains isolated from Bougainvillea spp. in Mexico.

Characteristic leaf spot and blight symptoms caused by Robbsia andropogonis on bougainvillea plants were found in three locations in different provinces of Mexico from 2019 to 2020. Eleven bacterial isolates with morphology similar to R. andropogonis were obtained from the diseased bougainvillea leaves. The isolates were confirmed as R. andropogonis by phenotypic tests and 16S rRNA, rpoD and gyrB gene sequencing. In addition to bougainvillea, the strains were pathogenic to multiple agriculturally significant crops, including maize (Zea mays), sorghum (Sorghum bicolor), barley (Hordeum vulgare), coffee (Coffea arabiga), carnation (Dianthus caryophilus), Mexican lime (Citrus x aurantifolia), common bean (Phaseolus vulgaris), broadbeans (Vicia faba) and pea (Pisum sativum), but not runner bean (Phaseolus coccineus). The presence of this bacterium represents a challenge for plant protection strategies in Mexico.

Effect of harvest time and fruit size on seed germination and seedlings growth of sour orange and Mexican lime under in vitro conditions

The aim of this research was to determine the best time to harvest the fruits for seed production which would ultimately lead to the production of citrus rootstocks of optimum quality. The sour orange and Mexican lime fruits were harvested on 7 and 5 occasions, respectively. The very first fruits were harvested 80 days after flowering and subsequent harvests were gathered every 30 days. An in vitro experiment was carried out in a completely randomized design, with four replications and 20 seeds in each replication. Based on fruit growth curve the time of fruit harvest affected seed germination (percentage and rate) and seedling growth (stem and root length, fresh and dry weight of stems, roots and leaves). The results showed that the best time to harvest the fruits of sour orange and Mexican lime was 230 and 170 days after flowering, respectively, which led to maximum seed germination (Mexican lime 100% and sour orange 85%) and seedling growth. The highest root, stem and leaf fresh and dry weight was also obtained at 230 and 170 days after flowering in sour orange and Mexican lime respectively.

Effects of flowering and production inducers in the Mexican lime (Citrus aurantifolia Swingle)

Objective: To evaluate new hormonal and biostimulant products to induce flowering of Mexican lime (Citrus aurantifolia Swingle) for winter production at the state of Guerrero, Mexico. Design/methodology/approach: Prohexadione calcium (PHD-Ca), cytokinins, paclobutrazol, biuretic urea, granulated urea (46%) and a control were evaluated. Results: There are differences between treatments for the number of flowers and number of fruits (Pr>0.0411 and 0.048); regard the “number of flowers” variable, paclobutrazol, prohexadione calcium and granulated urea (46%) stood out; while, prohexadione calcium, paclobutrazol and cytokinins favor the “number of fruits” variable. The yields show statistical differences (Pr>0.0332) and the cytokinins and paclobutrazol stood out. Findings/conclusions: The usage of cytokinins for induction and winter production of Mexican lime is suggested.