Some very large anatomical and morphological differences (cell and organ size) exist between diploid and tetraploid Citrus. They are reflected in a very different physiology, notably lower growth of tetraploid rootstocks. However, these phenotypic differences are not due to strong differences in the expression of genomes at leaf level (Allario et al., 2011). In the field, tetraploid rootstock/clementine combinations display less growth and lower yield, without any incidence for fruit quality. Lastly, tetraploid rootstocks would appear to afford a greater ability to adapt to environmental conditions (Hussain et al., 2012). In situations of saline stress or water stress, tetraploids are clearly more tolerant than the respective diploids (Mouhaya et al., 2010; Podda et al. 2013; Oliveira 2017). This greater tolerance is also seen when rootstocks are grafted (Allario et al., 2013; Dutra de Souza et al., 2017). At root level, tetraploid rootstocks constitutively over-express many genes involved in stress tolerance (genes involved in detoxification, osmoregulation and ABA synthesis). The greater tolerance of tetraploid rootstocks would seem to be linked to more efficient regulation of transpiration flows, notably due to increased synthesis of root ABA. Better tolerance of cold has been found for clementine trees grafted onto Carrizo citrange (Oustric et al., 2017). Likewise, the allotetraploid somatic hybrid FLHORAG1 displays increased tolerance of cold and light stress (Oustric et al., 2018). Work is under way to assess to what extent polyploid material might afford better adaptation to a disease that decimates citrus fruit orchards: Citrus greening.

References cited

• Allario T., Brumos J., Colmenero-Flores JM, Tadeo F, Froelicher Y., Talon M., Navarro L., Ollitrault P., Morillon R. (2011). Large changes in anatomy and physiology between diploid Rangpur lime (Citrus limonia) and its autotetraploid are not associated with large changes in leaf gene expression. J. Exp. Bot. 62: 2507-2519.
• Allario T, Brumos J, Colmenero-Flores JM, Iglesias DJ, Pina JA, Navarro L, Talon M, Ollitrault P, Morillon R. (2013) Tetraploid Rangpur lime rootstock increases drought tolerance via enhanced constitutive root abscisic acid production. Plant Cell Environ. 36: 856-68.
• Dutra de Souza J, de Andrade Silva EM, Coelho Filho MA, Morillon R, Bonatto D,  Micheli F, da Silva Gesteira A. (2017) Different adaptation strategies of two citrus scion/rootstock combinations in response to drought stress Plos one. https://doi.org/10.1371/journal.pone.0177993.
• Hussain S, Curk F, Dhuique-Mayer C, Urban L, Ollitrault P, Luro F, Morillon R. (2012) Autotetraploid trifoliate orange (Poncirus trifoliata) rootstocks do not impact clementine quality but reduce fruit yields and highly modify rootstock/scion physiology. Scientia Horticulturae 134:100–107
• Mouhaya W, Allario T, Brumos J, Andrés F, Froelicher Y, Luro F, Talon M, Ollitrault P, Morillon R (2010) Sensitivity to high salinity in tetraploid citrus seedlings increases with water availability and correlates with expression of candidate genes. Functional Plant Biology, 37: 674-685
• Oliveira TM, Ben Yahmed J, Dutra J, Maserti BE, Talon M, Navarro L, Ollitraut P,  Gesteira A, Morillon R.  (2017) Better tolerance to water deficit in doubled diploid ‘Carrizo citrange’ compared to diploid seedlings is associated with more limited water consumption and better H2O2 scavenging.  Acta Physiologia Plantarum 39 :204.
• Oustric J, Morillon R, Luro F, Herbette S, Lourkistia R, Giannettinia J, Berti L, Santini J. (2017) Tetraploid Carrizo citrange rootstock (Citrus sinensis Osb. × Poncirus trifoliata L. Raf.) enhances natural chilling stress tolerance of common clementine (Citrus clementina Hort. ex Tan). Journal of Plant Physiology 214: 108–115.
• Oustric J, Morillon R, Ollitrault P.,, Herbette S, Luro, F., Froelicher Y., Tur I., Dambier D., Giannettini J, Berti L, Santini J. (2018) Somatic hybridization between diploid Poncirus and Citrus improves natural chilling and light stress tolerances compared with equivalent doubled-diploid genotypes. Trees, /doi.org/10.1007/s00468-018-1682-3.
• Podda A, Checcucci G, Mouhaya W, Centeno D, Rofidal V, Del Carratore R, Luro F,  Morillon R, Ollitrault P, Maserti B E. (2013) Salt-stress induced changes in the leaf proteome of diploid and tetraploid mandarins with contrasting Na(+) and Cl(-) accumulation behaviour. J Plant Physiol. 170: 1101-1112