Screening of ‘King’ mandarin (Citrus nobilis Lour)×Poncirus trifoliata ((L.) Raf.) hybrids as citrus rootstocks tolerants to iron chlorosis

Iron (Fe) chlorosis is one of the commonest problems in calcareous soils and plant tolerance is determined mainly by rootstocks. This work evaluates new citrus hybrids’ tolerance to Fe chlorosis compared with the most widely used rootstocks in Spain: Citrus macrophylla (CM, Fe chlorosis-tolerant) and Carrizo citrange (CC, Fe chlorosis-sensitive). Growth parameters, Fe concentration, photosynthetic parameters, ferric chelate reductase (FC-R) activity and proton (H+) extrusion capacity were assessed in plants irrigated with 20 (control, Ct) or 0 (Fe-deficient, -Fe) μM FeEDDHA. Some—Fe hybrids presented marked Fe chlorosis symptomatology reflected by the root:shoot ratio and Chl a and b concentrations sharply dropping, and an increase in the Chl a/b ratio. These effects were very strong in the 050119, 050124-B and 050110—Fe species. The net CO2 assimilation rate significantly lowered in the 05019, 050131, 050125, 050112 and 05013—Fe species. The 050120, 050125, 050112 and 050124-B—Fe plants presented lower stomatal conductance than the Ct ones. Conversely, the internal CO2 concentration (Ci) tended to increase in Fe-deprived plants. Fe-deficiency increased FC-R activity and H+ extrusion in some 0501 species. Both responses were significantly induced in the CM, 050114, 05019, 050131 and 050125—Fe plants. Fe2+ accumulation in—Fe plants related inversely with FC-R activity. Most—Fe species with the greatest FC-R activity also accumulated the most Fe3+ ions and, therefore, the Fe pool was much larger in the root apoplast. Collectively, the main trait that determined Fe-chlorosis tolerance among these genotypes was the ability to: (1) boost Fe3+ reduction in response to Fe-deficiency; (2) acidify root media; (3) benefit the Fe amounts stored in the root apoplast.