Responses of citrandarins (Poncirus trifoliata x tangerine Sunki) to infection by ‘Candidatus Liberibacter asiaticus’

Currently, Huanglongbing has been considered one of the most important diseases that affects citrus plants worldwide, including Brazil. The causal agent is the Gram-negative bacterium 'Candidatus Liberibacter asiaticus', which occurs restricted to the phloem vessels of the host plant. Diaphorina citri is the insect vector associated with HLB disease. Despite the wide susceptibility, CLas multiplication is a contrasting characteristic among Citrus species. For example, Poncirus trifoliata and some of its hybrids have been shown to be a possible source of tolerance to the bacteria. In addition, these genotypes have presented a great importance when used as citrus rootstocks, due to the qualities they confer to the fruits. In this context, the present work aimed to challenge, under continuous exposure to CLas, 14 citrandarins genotypes (P. trifoliata × Citrus sunki) that showed negative results for natural CLas infection. Therefore, the study also investigated the effect of CLas on one of these genotype when used as a rootstock and intergraft for the CLas-infected ‘Valência’ orange tree. For that, two experiments were carried out under greenhouse conditions. In experiment I, the buds of the hybrids were overgrafted on the canopy of ‘Valência’ orange plants with high CLas titer. CLas detection and quantification were performed using total DNA samples extracted from the leaf petioles, followed by quantitative PCR (qPCR). The quantification of starch and the presence of callose was also evaluated. The results indicated, at 360 days of infection, there was a decrease in the number of CLas infected plants, since the complete absence of infection for the H106 genotype in all replicates analyzed was observed. Furthermore, low starch concentrations (17 mg/g tissue) and lower callose deposition (8 fluorescence units/mm2) were observed in H106 hybrids. In experiment II, the citrandarin H222, which was pre-selected in previous studies, was used as rootstock and intergraft for ‘Valência’ orange, CLas inoculations were performed via grafting of infected tissue. Detection and quantification were performed using samples of total DNA extracted from the petioles of leaves and roots. The volume, diameter, dry and wet weight of the root system of infected plants and control treatments were quantified. The results showed it was not possible to detect CLas in the roots of the plants graft onto H222, this fact occurred in within three to four biological repetitions. The multivariate hierarchical analysis (Ward statistic) considering the mean variables of above-ground stem diameter, root volume and weight, and CLas infection rate, analyzed jointly, grouped H222 in the same group of the uninoculated plants. Plants inoculated, which had Swingle citrumelo as rootstock, and those with 5 and 30 cm H222 intergrafts clustered in another cluster. Therefore, based on the set of variables analyzed, it is concluded that the H222 rootstock, even with the scion infected with CLas, was not significantly affected, on this basis considered similar with the non-inoculated plants. So, we concluded that H222 rootstock presented superior performance when compared to the citrumelo Swingle regarding the development of the root system. However, intergrafts with this genotype did not bring gains to the plants, being even deleterious. (AU)