A Novel Citrus Rootstock Tolerant to Iron Deficiency in Calcareous Soil

Iron (Fe) deficiency caused by calcareous soil is a serious problem in the cultivation of citrus (Citrus L.) trees. In this study, we report that 'Zhique' (Citrus wilsonii Tanaka) citrus rootstock from Chenggu county of Shaanxi province, China, shows tolerance to Fe deficiency under calcareous soil conditions. In the same calcareous field conditions, 'Miyagawa Wase' Satsuma mandarin (Citrus unshiu Marc.) grafted on trifoliate orange [Poncirus trifoliate (L.) Raf.] rootstock, the most commonly used rootstock, showed obvious interveinal chlorosis in young leaves, though some leaves or branches are asymptomatic, whereas no symptoms were found on those grafted on 'Zhique' rootstock. This was further evidenced by the fact that the chlorophyll concentration in chlorotic leaves of 'Miyagawa Wase' grafted on trifoliate orange was significantly lower than in those grafted on 'Zhique'. In addition, transmission electron microscopy (TEM) analysis revealed a significant reduction of grana and stroma thylakoid of chloroplasts in chlorotic leaves. Measurement of Fe concentrations revealed that the total Fe and cell wall Fe showed no difference between 'Zhique' and trifoliate orange roots, whereas the ferrous Fe was significantly higher in 'Zhique' than trifoliate orange roots. Interestingly, both total Fe and ferrous Fe concentrations in chlorotic leaves were significantly lower than in green leaves of 'Miyagawa Wase' grafted on either 'Zhique' or trifoliate orange, whereas the cell wall Fe concentration of 'Miyagawa Wase' leaves only showed significant difference between the 'Zhique' and trifoliate orange samples. Further transcript assessment found that the Fe acquisition-related genes FIT, HA, FRO, and NRAMP were upregulated in roots of 'Zhique' compared with trifoliate orange, thus suggesting 'Zhique' might be more capable of Fe uptake under calcareous soil conditions. The novel citrus rootstock reported here could be used as an ideal material for Fe-uptake research, and as a Fe-deficiency-tolerant rootstock for citrus cultivation in calcareous soils.