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Key messageThe activation of the antioxidant system under stress combination is a transmissible trait from the rootstock to the scion. Therefore, rootstock selection is key to improve crop performance and a sustainable production under changing climate conditions.Climate change is altering weather conditions such as mean temperatures and precipitation patterns. Rising temperatures, especially in certain regions, accelerates soil water depletion and increases drought risk, which affects agriculture yield. Previously, our research demonstrated that the citrus rootstock Carrizo citrange (Citrus sinensis × Poncirus trifoliata) is more tolerant than Cleopatra mandarin (C. reshni) to drought and heat stress combination, in part, due to a higher activation of the antioxidant system that alleviated damage produced by oxidative stress. Here, by using reciprocal grafts of both genotypes, we studied the importance of the rootstock on scion performance and antioxidant response under this stress combination. Carrizo rootstock, under stress combination, positively influenced Cleopatra scion by reducing H2O2 accumulation, increasing superoxide dismutase (SOD) and ascorbate peroxidase (APX) enzymatic activities and inducing SOD1, APX2 and catalase (CAT) protein accumulations. On the contrary, Cleopatra rootstock induced decreases in APX2 expression, CAT activity and SOD1, APX2 and CAT contents on Carrizo scion. Taken together, our findings indicate that the activation of the antioxidant system under stress combination is a transmissible trait from the rootstock to the scion and highlight the importance of the rootstock selection to improve crop performance and maintain citrus yield under the current scenario of climate change.

This work was supported by Ministerio de Economía y Competitividad (MINECO) and Universitat Jaume I through grants No. AGL2013-42038-R and P1IB2013-23, respectively. SIZ was supported by a predoctoral grant from Universitat Jaume I.

Under field conditions high temperatures are usually associated to high light intensity for periods of time that are getting longer because of global warming caused by climate change. These adverse conditions lead to significant reductions in yield and fruit quality in crops of great economic relevance such as citrus. In this work, the effect of high temperatures and high light intensity occurring alone or simultaneously has been studied in Carrizo citrange plants, a citrus genotype widely used as a rootstock, and the mitigating effect of kaolin (KL) evaluated. The combined stress conditions affected the plants in a unique manner at both, physiological and hormonal regulation levels, different to the effects of each individual stress. KL alleviated the deleterious effect of combined stress in different ways: (i) reducing leaf damage and abscission, (ii) improving physiological and gas exchange parameters, (iii) decreasing foliar proline content, (iv) increasing chlorophyll content, (v) preventing carotenoid degradation, and (vi) modulating levels of hormones and their precursors involved in plant responses to stress [abscisic acid (ABA), salicylic acid (SA), cinnamic acid (CA), indole-3-acetic acid (IAA), jasmonic acid (JA) and 12-oxophytodienoic acid (OPDA)].

The selection of a suitable rootstock is critical for sustainable citrus production, particularly in regions facing abiotic constraints like calcareous soils and cold damage. This study aimed to identify viable rootstock alternatives to the standard sour orange (C. aurantium L.) for ‘W. Murcott’ mandarin cultivation in the calcareous soils of Adana, Turkey. The performance of four rootstocks “Carrizo citrange (C. sinensis × Poncirus trifoliata), Volkameriana (C. volkameriana), sour orange, and Flying Dragon (P. trifoliata var. monstruosa)” was evaluated based on fruit yield, quality parameters, leaf chlorophyll content (SPAD), chlorophyll fluorescence ( Fv’/Fm’ ), and gas exchange measurements. The results demonstrated significant rootstock-induced variation across all measured parameters. Volkameriana produced the highest fruit yield (58.33 kg/tree) and exhibited superior leaf chlorophyll content (73.63 SPAD) and photosynthetic rates. However, it yielded fruits with lower total soluble solids (TSS) and higher titratable acidity (TA) compared to other rootstocks. Carrizo citrange provided an optimal balance, ranking second in yield (48.33 kg/tree) while producing the largest fruits (151.13 g) and the highest TSS (13.63%). It also supported high photosynthetic activity and strong juice color intensity (indicated by a positive a* value). Flying Dragon consistently demonstrated the poorest performance across all metrics, including yield, fruit size, and physiological activity, attributed to its sensitivity to calcareous conditions. Sour orange performed reliably in fruit quality. Correlation analysis revealed strong positive relationships between chlorophyll content, photosynthetic rate, and yield. In conclusion, while Volkameriana maximizes vegetative growth and yield, Carrizo citrange is recommended as the most suitable rootstock for ‘W. Murcott’ in this region due to its synergistic combination of high yield, excellent fruit quality, and physiological efficiency.

Daisy mandarin is a commercially important cultivar of citrus, well known for its superior fruit quality and adaptability to subtropical climates. However, the fruit splitting and imbalances in leaf nutrient content often limits its productivity. To address this issue, a field experiment was carried out during 2021 and 2022 at the Fruit Research Station, Jallowal-Lesriwal, Jalandhar, to evaluate the influence of different growth regulators on the incidence of fruit splitting and leaf nutrient composition of Daisy mandarin budded on two rootstocks viz., Rough lemon and Carrizo Citrange. Maintaining the optimal foliar nutrient status is essential, as it plays a vital role in sustaining tree vigor, development of fruits and overall yield. This study involved the foliar application of gibberellic acid (GA 3 ) at 10, 20 and 30 ppm, naphthalene acetic acid (NAA) at 10, 20 and 30 ppm; ethephon at 10, 20 and 30 ppm; and salicylic acid (SA) at 100 and 200 ppm concentration, applied at different phenological stages. The trial was conducted on Daisy mandarin trees budded on two different rootstocks i.e. Rough lemon and Carrizo citrange. Fruit splitting was observed to be more pronounced in Carrizo citrange rootstock than in Rough lemon, which may be attributed to higher soluble solids accumulation, weaker anatomical structure, and greater stress susceptibility. The pooled analysis over two consecutive years revealed that foliar application of SA at 100 ppm significantly reduced the fruit splitting incidence (3.14% and 3.60%) and enhanced the concentrations of nitrogen (2.65% and 2.66%), phosphorous (0.15% and 0.16%), potassium (1.65%), calcium (3.58% and 3.52%), magnesium (0.35%), and other micro nutrient content of Daisy mandarin in the leaves followed by other growth regulators as compared to other untreated (control) plants. These results suggest that SA at 100 ppm is a promising foliar treatment for improving the nutritional status of Daisy mandarin in the subtropical conditions of Punjab.

Shortening the juvenile stage in citrus and inducing early flowering has been the focus of several citrus genetic improvement programs. FLOWERING LOCUS T (FT) is a small phloem-translocated protein that regulates precocious flowering. In this study, two populations of transgenic Carrizo citrange rootstocks expressing either Citrus clementina FT1 or FT3 genes under the control of the Arabidopsis thaliana phloem specific SUCROSE SYNTHASE 2 ( AtSUC2 ) promoter were developed. The transgenic plants were morphologically similar to the non-transgenic controls (non-transgenic Carrizo citrange), however, only AtSUC2-CcFT3 was capable of inducing precocious flowers. The transgenic lines produced flowers 16 months after transformation and flower buds appeared 30–40 days on juvenile immature scions grafted onto transgenic rootstock. Gene expression analysis revealed that the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 ( SOC1 ) and APETALA1 ( AP1 ) were enhanced in the transgenics. Transcriptome profiling of a selected transgenic line showed the induction of genes in different groups including: genes from the flowering induction pathway, APETALA2/ETHYLENE RESPONSE FACTOR ( AP2 / ERF ) family genes, and jasmonic acid (JA) pathway genes. Altogether, our results suggested that ectopic expression of CcFT3 in phloem tissues of Carrizo citrange triggered the expression of several genes to mediate early flowering.

Background The citrus genus comprises a number of sensitive tropical and subtropical species to cold stress, which limits global citrus distribution to certain latitudes and causes major economic loss. We used RNA-Seq technology to analyze changes in the transcriptome of Valencia delta seedless orange in response to long-term cold stress grafted on two frequently used citrus rootstocks: Carrizo citrange (CAR), considered one of the most cold-tolerant accessions; C. macrophylla (MAC), a very sensitive one. Our objectives were to identify the genetic mechanism that produce the tolerant or sensitive phenotypes in citrus, as well as to gain insights of the rootstock-scion interactions that induce the cold tolerance or sensitivity in the scion. Results Plants were kept at 1 ºC for 30 days. Samples were taken at 0, 15 and 30 days. The metabolomic analysis showed a significant increase in the concentration of free sugars and proline, which was higher for the CAR plants. Hormone quantification in roots showed a substantially increased ABA concentration during cold exposure in the CAR roots, which was not observed in MAC. Different approaches were followed to analyze gene expression. During the stress treatment, the 0-15-day comparison yielded the most DEGs. The functional characterization of DEGs showed enrichment in GO terms and KEGG pathways related to abiotic stress responses previously described in plant cold adaption. The DEGs analysis revealed that several key genes promoting cold adaption were up-regulated in the CAR plants, and those repressing it had higher expression levels in the MAC samples. Conclusions The metabolomic and transcriptomic study herein performed indicates that the mechanisms activated in plants shortly after cold exposure remain active in the long term. Both the hormone quantification and differential expression analysis suggest that ABA signaling might play a relevant role in promoting the cold hardiness or sensitiveness of Valencia sweet orange grafted onto Carrizo citrange or Macrophylla rootstocks, respectively. Our work provides new insights into the mechanisms by which rootstocks modulate resistance to abiotic stress in the production variety grafted onto them.

Herbivore-induced plant volatiles (HIPVs) are known to activate immune signaling in plants; however, their effectiveness can vary depending on the genotype and the signaling pathway involved. In this study, we evaluated the transcriptional response of four citrus rootstocks (Carrizo citrange, Forner-Alcaide 5 (FA5), Forner-Alcaide 74 (FA74), and Microcitrus australasica ) to six synthetic HIPVs [(Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate, (Z)-3-hexenyl butyrate, (Z)-3-hexenyl propanoate, methyl jasmonate, and methyl salicylate]. We focused on genes associated with the salicylic acid (SA) and jasmonic acid (JA) pathways, as well as the susceptibility gene CsPUB21. Overall, the SA pathway was more consistently activated than the JA pathway, with upstream and intermediate genes induced across most genotypes and treatments. In contrast, downstream markers showed more variable expression, suggesting that synthetic HIPVs may induce a primed rather than fully activated defense state. Among the volatiles tested, (Z)-3-hexenyl propanoate and (Z)-3-hexen-1-ol were the most effective, activating genes in both pathways. Importantly, these two compounds also consistently repressed CsPUB21 expression, a gene recently associated with huanglongbing (HLB) susceptibility, through coordinated transcriptional and post-translational regulation. Carrizo citrange showed the strongest transcriptional response, while FA74 exhibited more moderate activation, emphasizing the influence of genetic background on HIPV perception and signaling. These findings highlight the potential of selected synthetic HIPVs as sustainable defense priming agents capable of enhancing citrus immunity by simultaneously activating immune pathways and repressing susceptibility genes such as CsPUB21 . This dual mode of action offers promising tools for the integrated management of HLB and other citrus diseases.

Citrus holds the key position in horticulture sector of Pakistan in terms of area and production. Kinnow is considered as the trademark of Pakistan’s citriculture industry. Cultivation experiment was conducted to evaluate the endogenous plant hormones, leaf gas exchange, photosynthetic pigments, vegetative performance, and yield of Kinnow mandarin grafted on nine rootstocks (Rough lemon, cox mandarin, Fraser hybrid, Troyer citrange, Cleopatra mandarin, Poncirus trifoliata, Benton, C-35, and Carrizo citrange). Maximum indole acetic acid (IAA), gibberellic acid (GA3), and zeatin (ZT) levels were recorded in scion and rootstock of Poncirus trifoliata followed by Fraser hybrid. Troyer citrange rootstock depicted maximum amount of abscisic acid in scion and rootstock. Maximum leaf gas exchange in Kinnow mandarin was recorded on Fraser hybrid rootstock. Kinnow grafted on Troyer citrange rootstock had minimum stomatal conductance and photosynthetic activity because of high ABA level. Photosynthetic pigments of Kinnow mandarin were maximum on Poncirus trifoliata and minimum on Troyer citrange. A positive correlation was observed among the levels of IAA, GA, ZT, and vegetative growth of Kinnow mandarin. It is concluded that endogenous hormones modulated the growth of Kinnow mandarin. Poncirus trifoliata and Fraser hybrid rootstocks could be an alternate rootstocks of traditional Rough lemon for Kinnow mandarin.

Bacterial diseases like huanglongbing (HLB) and citrus canker severely impact citrus production. HLB, caused by “Candidatus Liberibacter asiaticus” (CLas), leads to tree decline, while citrus canker, caused by Xanthomonas citri pv. citri (Xcc) causes necrotic lesions on leaves and fruit. Many bacterial pathogens secrete effector proteins that suppress host plant immunity and promote pathogenesis through the upregulation of host‐encoded susceptibility genes. Xcc uses the type III secretion system to introduce effector proteins such as the transcription factor‐like (TAL) effector PthA4 that can directly activate host susceptibility gene expression. In contrast, CLas lacks most bacterial secretion systems and relies predominantly on the Sec secretion system for pathogenesis. While some Sec‐secreted proteins have been identified in CLas, their direct role in causing HLB symptoms remains unproven. Several Sugars Will Eventually be Exported Transporter (SWEET) genes, encoding sucrose transporters, are candidate susceptibility genes. Here we investigate the roles of the citrus SWEET10, SWEET12 and SWEET15 genes and show that mutations of SWEET15 resulted in reduced susceptibility to citrus canker in three different citrus cultivars: Carrizo citrange (Citrus sinensis ‘Washington’ sweet orange × Poncirus trifoliata), ‘Limoneria 8A’ Lisbon lemon (Citrus limon) and ‘Pineapple’ sweet orange (C. sinensis). Furthermore, Lisbon lemon plants mutated for SWEET15 also showed reduced CLas titre in infected plants. These results suggest that SWEET15 may act as a broad‐spectrum susceptibility gene, and disruption of SWEET15 gene activity could be a viable approach to mitigating bacterial diseases such as citrus canker and HLB in a variety of citrus cultivars. Assays of knockouts in several SWEET genes show that SWEET15 mutations reduce levels of Xanthomonas citri pv. citri and “Candidatus Liberibacter asiaticus” in infected citrus plants, suggesting that SWEET15 is a susceptibility gene.