缺锌胁迫对‘天红2号/冀砧2号’苹果幼树生长、光合特性和内源激素含量的影响

Effects of zinc deficiency on growth,photosynthetic characteristics,and endogenous hormone content of apple sapling‘Tianhong 2/Jizhen 2’

【目的】研究‘天红2号/冀砧2号’幼树生长、光合特性及内源激素变化对缺锌胁迫的响应。【方法】以1年生‘天红2号/冀砧2号’砧穗组合苹果幼苗为试材,采用营养液浇灌方式进行盆栽试验。营养液设置供锌浓度分别为0μmol/L(Zn0)、2μmol/L(Zn2)、4μmol/L(Zn4,对照),共3个处理。处理后40天,测量幼树株高、叶面积,取样测定叶片锌含量、光合特性、叶绿素含量、叶绿素荧光参数、内源激素含量等指标,并分析叶绿素合成关键酶基因MdHEMA1、叶绿素降解关键酶基因M d P A O、生长素合成基因MdYUCCA4a和MdYUCCA6a相对表达量。【结果】‘天红2号/冀砧2号’幼树的株高、叶面积及叶片锌含量随供锌浓度的下降而显著降低;供锌浓度越低,生长素合成基因MdYUCCA4a和MdYUCCA6a相对表达量越低,植株体内IAA和GA3水平越低;与对照(Zn4)相比,净光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)在缺锌条件下显著降低,胞间CO_(2)浓度(Ci)升高;缺锌胁迫下,叶绿素a、叶绿素b、总叶绿素含量下降,叶绿素a/b值上升,叶绿素合成关键酶基因MdHEMA1和叶绿素降解关键酶基因MdPAO均显著升高;‘天红2号/冀砧2号’幼树叶片叶绿素荧光参数Fv/Fm、ΦPSⅡ、qP及ETR值在缺锌胁迫下显著下降,Fo和NPQ显著上升。【结论】缺锌胁迫使生长素合成基因MdYUCCA4a和MdYUCCA6a表达量显著降低,IAA合成受阻,导致IAA和GA3水平降低,抑制幼树生长。PSⅡ反应中心和PSⅡ氧化传递链在缺锌胁迫下受到一定损伤,造成PSⅡ活性下降,并加剧叶绿素降解,导致光合作用降低;同时,叶绿素降解基因MdPAO的高表达引起树体产生反馈调节机制来进行自我保护,诱导叶绿素合成代谢途径中的关键酶基因MdHEMA1表达量升高,但所合成的叶绿素依然不能补偿由缺锌所造成的叶绿素降解损失。

【Objectives】The study investigates the growth response,photosynthetic characteristics,and changes in endogenous hormones in young‘Tianhong 2/Jizhen 2’trees due to zinc deficiency.【Methods】One-year-old‘Tianhong 2/Jizhen 2’rootstock-spike combined apple saplings were used as experimental materials.Three zinc concentrations were set up in nutrient solution:0μmol/L(Zn0),2μmol/L(Zn2),and 4μmol/L(Zn4,control).We measured the plant height and leaf area of young trees and determined zinc content,photosynthetic characteristics,chlorophyll content,chlorophyll fluorescence parameters and endogenous hormone in leaves at 40 days after treatment.We studied the relative expression levels of the enzyme encoding gene MdHEMA1,chlorophyll degradation key enzyme encoding gene MdPAO,and auxin synthesis genes MdYUCCA4a and MdYUCCA6a.【Results】The plant height,leaf area,and leaf zinc content of‘Tianhong 2/Jizhen 2’saplings(P<0.05)decreased with decreasing zinc concentration.The lower the zinc concentration,the lower the relative expression of auxin synthesis genes MdYUCCA4a and MdYUCCA6a and IAA and GA3 levels in plants.Compared with the control,net photosynthetic rate(Pn),transpiration rate(Tr),and stomatal conductance(Gs)(P<0.05)decreased,while intercellular CO2 concentration(Ci)increased under zinc deficiency.Under zinc stress,chlorophyll a,b,and total chlorophyll content decreased,and the chlorophyll a/b ratio increased.Chlorophyll synthesis enzyme encoding gene MdHEMA1 and chlorophyll degradation enzyme encoding gene MdPAO(P<0.05)increased.The chlorophyll fluorescence parameters Fv/Fm,ΦPSⅡ,qP and ETR of‘Tianhong 2/Jizhen 2’seedlings(P<0.05)decreased under zinc deficiency,while Fo and NPQ increased.【Conclusions】The expression of auxin synthesis genes MdYUCCA4a and MdYUCCA6a decreased under zinc deficiency,and IAA synthesis was blocked.This decreased IAA and GA3 levels,inhibiting the young tree growth.The PSⅡreaction center and PSⅡoxidation transfer chain were damaged to some extent under zinc deficiency,resulting in decreased PSⅡactivity,chlorophyll degradation,and decreased photosynthesis.Simultaneously,the high expression of the chlorophyll degradation gene MdPAO caused the tree to produce a feedback regulation mechanism to protect itself.It induced the expression of the key enzyme gene MdHEMA1 in the chlorophyll synthesis metabolism pathway.However,the synthesized chlorophyll did not compensate for the loss of chlorophyll degradation caused by zinc deficiency.