Due to climate change, we are forced to face new abiotic stress challenges like cold and heat waves that currently result from global warming. Losses due to frost and low temperatures force us to better understand the physiological, hormonal, and molecular mechanisms of response to such stress to face losses, especially in tropical and subtropical crops like citrus fruit, which are well adapted to certain weather conditions. Many of the responses to cold stress that are found are also conserved in citrus. Hence, this review also intends to show the latest work on citrus. In addition to basic research, there is a great need to employ and cultivate new citrus rootstocks to better adapt to environmental conditions.

The Spanish Mediterranean region concentrates the largest producers of lemons (Citrus limon Burm. f.) at the national level where the 98.4% of the cultivated area of lemons corresponds to the varieties “Verna” and “Fino”. In this study, the morphological and chemical variations of the fruits obtained in five variety/rootstock combinations were investigated in order to determine the influence and impact of the rootstock on the physicochemical properties of the fruits. The assay was carried out using three lemon varieties (“Fino 95”, “Fino 49” and “Verna”) grafted onto two different rootstocks (Citrus macrophylla and Citrus aurantium). The varieties were selected due to be consolidated commercial varieties, while the rootstocks are the most commonly used in the world. Both the morphological characteristics of the fruits (colour, weight, size) as well as their physicochemical characteristics (total soluble solids, titratable acidity, maturity index, antioxidant activity, sugars, and organic acids) were evaluated. Based on the results, the lemons with the best physicochemical and the best compositional characteristics were obtained in the “Fino 95” and “Fino 49” lemons grafted onto C. aurantium rootstock presented the highest quality fruits.

Background and aims: This work evaluates the regulation of iron uptake responses in Citrus leaves and their involvement in the Fe paradox effect.Methods: Experiments were performed in field-grown ‘Navelina’ trees grafted onto two Cleopatra mandarin ×Poncirus trifoliata(L.) Raf. hybrids with different Fe-chlorosis symptoms: 030146 (non-chlorotic) and 030122 (chlorotic).Results: Chlorotic leaves were smaller than non-chlorotic ones for both dry weight (DW) and area basis, and exhibited marked photosynthetic state affection, but reduced catalase and peroxidase enzymatic activities. Although both samples had a similar total Fe concentration on DW, it was lower in chlorotic leaves when expressed on an area basis. A similar pattern was observed for the total Fe concentration in the apoplast and cell sap and in active Fe (Fe²⁺) concentration.FRO2gene expression and ferric chelate reductase (FC-R) activity were also lower in chlorotic samples, whileHA1andIRT1were more induced. Despite similar apoplasmic pH, K⁺/Ca²⁺was higher in chlorotic leaves, and both citrate and malate concentrations in total tissue and apoplast fluid were lower.Conclusion: (1) The rootstock influences Fe acquisition system in the leaf; (2) the increased sensitivity to Fe-deficiency as revealed by chlorosis and decreased biomass, was correlated with lower FC-R activity and lower organic acid level in leaf cells, which could cause a decreased Fe mobility and trigger other Fe-stress responses in this organ to enhance acidification and Fe uptake inside cells; and (3) the chlorosis paradox phenomenon in citrus likely occurs as a combination of a marked FC-R activity impairment in the leaf and the strong growth inhibition in this organ.