The Mechanism And Measures Of Iron Deficiency Chlorosis In Huangguan/Quince A Combinations Grown In Calcareous Soils

Pear tree is the third main fruit tree species in China.Pear industry plays an important role in the national economy.Dwarfing cultivation represents the trend of pear cultivation.Quince(Cydoniao blonga Mill.)are commonly used as dwarfing rootstocks for dwarfing cultivation in the world,among which quince A is widely used.However,the promotion of dwarfing rootstocks in pear production in China is seriously hampered due to iron(Fe)deficiency in pear trees especially Pyrus bretschneideri Rehd.grafted onto quince rootstocks grown in calcareous soils in Northern China.In order to analyse the mechanism of Fe deficiency in Pyrus bretschneideri Rehd.grafted onto quince A in calcareous soil,and to promote the development of pear dwarfing cultivation,field experiments and pot experiments were carried out.Dwarfing rootstock quince A,vigorous rootstock Pyrus betulaefolia and their combination with Huangguan pear(Hardy as interstock).The processes of Fe uptake,transport,and unloading were focused as well as some effective measures to alleviate Fe deficiency.Key results of this study were as follows:(1)Since the root adaptation strategies of quince A and Pyrus betulaefolia under bicarbonate stress were similar,the difference in Fe efficiency between Huangguan grafted onto quince A and Huangguan grafted onto Pyrus betulaefolia was not due to the difference in typical Fe deficiency adaptation strategies of Strategy I plants,but rather due to the greater ability of Huangguan grafted onto Pyrus betulaefolia in activating Fe in leaves under bicarbonate stress compared with Huangguan grafted onto quince A.(2)The main limiting process of Fe deficiency chlorosis in Huangguan grafted onto quince A grown in calcareous soils was the unloading or distribution process in leaves,which can be explained as follows: the increased apoplastic p H and inhibited ferric chelate reductase activity in leaves due to high bicarbonate concentration in calcareous soil rather than high p H,resulting in the deposition of a large amount of Fe in the apoplastic region near the midvein and greatly reduced Fe mobility in leaves.(3)Ammonium application alleviated bicarbonate-induced Fe deficiency chlorosis in Huangguan grafted onto quince A thanks to the improved transport efficiency of Fe from roots to shoots,and the reduced leaf apoplastic p H and increased leaf ferric chelate reductase activity which enhanced the mobility of Fe in leaves.However,the effect of ammonium fertilizer application on Fe deficiency chlorosis was not obvious in calcareous soil.(4)The treatment of flattening without landscape fabric mulching which can improve soil aeration increased the Fe concentration in leaves of Huangguan grafted onto quince A by increasing root ferric chelate reductase activity.Manure application had positive effects on both soil Fe increase and leaf Fe utilization even though the leaf Fe concentration was not improved.Therefore,it was suggested that the orchards with dwarfing rootstocks or with vigorous rootstocks that were prone to Fe deficiency could try to adopt the cultivation mode of flattening without mulching,and the appropriate amount of manure could also be used as a combined strategy.In summary,the main factor that induced Fe deficiency chlorosis in Huangguan grafted onto quince A grown in calcareous soils was the high concentration of bicarbonate.Bicarbonate caused Fe inactivation in the leaves of Huangguan grafted onto quince A,which resulted in Fe deposition around the leaf veins.Rootstock-scion combinations differ in their ability to adapt to bicarbonate,in which rootstock plays a crucial role.The poor adaptation of quince A to bicarbonate compared with Pyrus betulaefolia resulted in higher leaf apoplastic p H and lower leaf ferric chelate reductase activity which was the underlying cause of the Fe inefficiency of Huangguan grafted onto quince A in calcareous soils.In pear production,in addition to improving Fe uptake,activating the accumulated Fe in leaves and promoting Fe distribution in leaves are the keys to solve Fe deficiency chlorosis in Huangguan grafted onto quince A grown in calcareous soils.