Physiological And Related Gene Expression Responses Of Pingyitiancha (Malus Hupehensis) Differ During Heterogeneous Irrigation Placement

PRD is a partial root-zone irrigation that can improve the efficiency of the special water-saving irrigation.Understanding the water-stress related genes expression response to differed partial rootzone drying is important in making sustainable irrigation scheduling.Physiological and gene expression responses of apple rootstock（Malus hupehensis）to partial rootzone drying were investigated using a split root system across two compartments.Four treatments were implemented:（1）control irrigation treatment（CK,irrigation volume of 100%evaporation）;（2）alternating partial rootzone drying（APRD,irrigation volume of 50%of the evapotranspiration）;（3）deficit irrigation（DI,irrigation volume of 50%of the evapotranspiration）fixed partial rootzone drying（FPRD,irrigation volume of 50%of the evapotranspiration）.The main results are as follows:（1）Compared with CK,the net photosynthesis（Pn）,stomatal conductance（Gs）,transpiration rate（Tr）,leaf water potential(Ψ_leaf)and evaporation of three drought stress treatments were significantly decreased.There was no significant difference in the soil water content（V_θ）between the three drought treatments.There was a significant positive correlation between Gs andΨ_leaf.And there was also a positive correlation between Gs and V_θ,indicating that the size of Gs was affected by degree of drought.Pn,Gs,Tr andΨ_leaf of APRD plants were higher than those of DI and FPRD,which indicated that the APRD showed favorable physiological water status under the same irrigation amount.（2）Compared with CK,the ABA produced by three drought stress plants was significantly increased.The order of ABA content of different treated plants was in the leaves:CK<APRD<DI<FPRD,in the roots:CK<APRD<FPRDw<DI<FPDRd.The trend of ABA content in leaves and roots was consistent.The order of IAA content of different treated plants was,in the leaves:DI<FPRD<APRD<CK,in the roots:DI<FPRDd<FPRDw=CK<APRD.Compared with DI and FPRD,APRD maintains a relatively higher IAA content and a lower ABA content,indicating that APRD plants have a relatively better physiological growth.There was no significant difference in the GA content(GA_root)between CK and FPRDw,but were significantly higher than that of APRD and DI.The content of Zr(Zr_root)in roots was significantly higher than that in FPRDd,DI and APRD,and there was no significant difference between Zr_root of FPRDd,DI and APRD plants.The correlation analysis showed that there was a negative correlation between Gs and ABA_leaf concentration,and V_θwas negatively correlated with ABA_leaf.ABA_leaf/Zr_leaf has a significant negative correlation with Gs andΨ_leaf.（3）The expression of all genes studied was significantly higher in trees roots than in leaves.The expression level of MpNCED1 and MpNCED2 in the roots was thousands times than that in the leaves.But ABA_leaf was 3-7 times than ABA_root,which indicated that ABA was transported from roots to the leaves after synthesis in roots.The ABA synthesis genes and metabolic genes（MdCYP707A1和MdCYP707A2）expression of DI and FPRD plants were significantly higher than that of APRD and CK,which indicated that ABA had a faster metabolic rate in DI and FPRD plants.The expression of MpNCED2 and MdCYP707A2 in the root positively correlated with ABA_root/Zr_root.The expression of dehydrins（MdDHN2 and MdDHN5）were up-regulated in the root system after three drought stress treatments,and gene expression in the root of DI and FPRDd were significantly higher than that of CK and APRD.The expression of the dehydrin genes were the highest in the leaves of DI treated plants.The expression of aquaporin gene（MpPIP2;1）in leaves and roots of APRD,FPRD and DI plants was significantly different,in which the lowest expression was in the CK plants.However,the expression of MpTIP2;2 in the leaves and roots of CK was significantly higher than that of the other three groups,suggesting that MpPIP2;1 by up-regulated expression and MpTIP2;2 by down-regulated expression could reduce the water loss.These results suggest that gene expression induction by heterogeneous irrigation placement depended on the location and intensity of the water deficit in the entire rootzone.This study presents that APRD induced a weaker genetic response and could be potentially effective water-saving irrigation strategy.