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Plants face a wide range of biotic and abiotic stress conditions, which are further intensified by climate change. Among these stressors, increased irradiation in terms of intensity and wavelength range can lead to detrimental effects, such as chlorophyll degradation, destruction of the PSII reaction center, generation of ROS, alterations to plant metabolism, and even plant death.Here, we investigated the responses of two citrus genotypes, Citrus macrophylla (CM), and Troyer citrange (TC) to UV‐B light‐induced stress, by growing plants of both genotypes under control and UV‐B stress conditions for 5 days to evaluate their tolerance mechanisms.TC seedlings had higher sensitivity to UV‐B light than CM seedlings, as they showed more damage and increased levels of oxidative harm (indicated by the accumulation of MDA). In contrast, CM seedlings exhibited specific adaptive mechanisms, including accumulation of higher levels of proline under stressful conditions, and enhanced antioxidant capacity, as evidenced by increased ascorbate peroxidase activity and upregulation of the CsAPX2 gene. Phytohormone accumulation patterns were similar in both genotypes, with a decrease in ABA content in response to UV‐B light. Furthermore, expression of genes involved in light perception and response was specifically affected in the tolerant CM seedlings, which exhibited higher expression of CsHYH/CsHY5 and CsRUP1‐2 genes.These findings underscore the importance of the antioxidant system in citrus plants subjected to UV‐B light‐induced stress and suggest that CsHYH/CsHY5 and CsRUP1‐2 could be considered genes associated with tolerance to such challenging conditions. [ABSTRACT FROM AUTHOR]

The effect of Citrus tristeza virus ( CTV) infection on photosynthetic activity and antioxidant metabolism was analysed in plants of the highly susceptible citrus genotype Mexican lime ( Citrus aurantifolia). Two virus isolates differing in their virulence (the severe T318 and the mild T385) were used in the experiments. CTV infection caused a reduction in photosynthetic capacity in infected plants. This limitation was mainly due to a reduction in the carboxylative efficiency whereas the limitation of CO2 diffusion through the stoma had lower impact. The virus did not damage the antennae and did not reduce the efficiency of light harvesting complexes. Oxidative damage occurred in infected plants, as evidenced by the increase in malondialdehyde levels. Indeed, CTV infection caused an increase in ascorbate peroxidase activity in new shoots developed in infected plants during the 2 years of the experiment. Data suggest that the H2O2 removal machinery was not damaged as a result of stress but the defence mechanism was overwhelmed with time due to the continuing pressure of biotic stress. [ABSTRACT FROM AUTHOR]

Industry residues, phosphate fertilisers and wastewater as a source of irrigation have considerably increased levels of heavy metals in the soil, mainly cadmium (Cd2+). To test the effects of a calcium (Ca2+) treatment on Cd2+ accumulation and plant tolerance to this heavy metal, plants of two citrus genotypes, Cleopatra mandarin ( CM) and Carrizo citrange ( CC), were watered with increasing concentrations of Cd2+, and phytochelatin ( PC) and glutathione ( GSH) content were measured. Both genotypes were able to synthesise PCs in response to heavy metal intoxication, although CM seems to be a better Cd2+ excluder than CC. However, data indicate that CC plants had a higher capacity for regenerating GSH than CM plants. In this context, the effects of Ca2+ treatment on Cd2+ accumulation, plant survival and PC, GSH and oxidised glutathione ( GSSG) content were assessed. Data indicate that treatment with Ca2+ had two positive effects on citrus physiology: it reduced Cd+2 uptake into roots and also increased GSH content (even in the absence of Cd2+). Overall, the data indicate that although Cd2+ exclusion is a powerful mechanism to avoid heavy metal build-up into photosynthetic organs, the capacity to maintain optimum GSH levels to feed PC biosynthesis could also be an important factor in stress tolerance. [ABSTRACT FROM AUTHOR]

The effect of increased Cd concentrations in the watering solution on citrus physiology was studied by using two citrus genotypes, Cleopatra mandarin and Carrizo citrange. Cadmium content in roots and leaves was tested together with measurements of leaf damage, gas exchange parameters, and hormonal contents. Citrus roots efficiently retained Cd avoiding its translocation to the shoots and Cleopatra mandarin translocated less Cd than Carrizo. With increasing Cd concentration all gas exchange parameters were decreased more in Carrizo than in Cleopatra mandarin. Cd-induced increases in abscisic acid and salicylic acid contents were observed in leaves but not in roots of both genotypes. [ABSTRACT FROM AUTHOR]

WRKY transcription factors (TF) family is involved in a huge variety of plant processes, including seed germination, plant development, phytohormone signalling, and defence against both biotic or abiotic stresses. In this work, WRKY TF family has been characterized in citrus. In a first experiment, the relative expression of CsWRKYs was analyzed in shoots and roots of plants treated with abscisic acid (ABA), salicylic acid (SA), and methyl jasmonate (MeJA) under in vitro conditions. Expression of CsWRKYs was also determined in roots of commercial citrus rootstocks subjected to osmotic and salt stresses. A total amount of 50 CsWRKYs has been found and devided into different groups of WRKY family according to the WRKY domain sequences. In response to the applications of phytohormones, the highest differences were observed in roots, and it was found that ABA and SA generally repressed CsWRKYs expressions, but MeJA induced their overexpression. Osmotic stress repressed the expression of most of the CsWRKYs, whereas salt stress induced their expression. Moreover, salt stress induced higher increase in CsWRKY expressions in the tolerant rootstock Citrus macrophylla than in the sensitive rootstock Carrizo citrange, suggesting that these TFs may play an important role in response to this stress. [ABSTRACT FROM AUTHOR]