The low ferric chelate reductase activity and high apoplastic pH in leaves cause iron deficiency chlorosis in 'Huangguan' pears grafted onto quince A grown in calcareous soil

Quince (Cydonia oblonga Mill.) is a dwarfing rootstock often used in pear cultivation. However, the pear cultivar 'Huangguan' (Pyrus bretschneideri Rehd. cv) grafted onto quince A (HG-QA) suffered iron (Fe) deficiency chlorosis in the calcareous soils of North China, while 'Huangguan' grafted onto Pyrus betulifolia (HG-PB) did not. We hypothesized that the higher leaf apoplastic pH of HG-QA than HG-PB causes Fe deficiency chlorosis by restricting Fe transportation within leaves. Three-year-old rootstocks quince A and Pyrus betulifolia and the combinations HG-QA and HG-PB were grown in the same orchard in a calcareous soil. Newly expanded leaves were analysed for Fe related properties. Micro X-ray fluorescence analysis was used for Fe mapping within the individual leaves. The extractable Fe(II) concentrations in quince A and HG-QA leaves were much lower than those in Pyrus betulifolia and HG-PB, even though the total leaf Fe concentrations were similar. More Fe was located near the leaf midrib in quince A and HG-QA than in Pyrus betulifolia and HG-PB. These were correlated with the lower ferric chelate reductase (FCR) activity in the chlorotic leaves of the quince A rootstock than Pyrus betulifolia and the lower FCR activity and higher apoplastic pH in the chlorotic leaves of the HG-QA rootstock -scion combination than HG-PB. In conclusion, the reduced Fe availability within the leaves caused by the lower FCR activity and higher apoplastic pH compared with those of HG-PB led to HG-QA leaf chlorosis in calcareous soils. This result implies that rootstocks are involved in the regulation of leaf Fe utilization by scions.