Response Characteristics Of Three Apple Rootstocks To Long-term Water Stress

Apple(Malus pumila)is a pillar industry in the northwest Loess Plateau that is mostly rain-fed agriculture area.Also,drought is the main factor of restricting the production of local crops and fruit trees.The drought resistance of apple tree depends on the rootstock.It is important to understand the response characteristics of main rootstocks to water stress,which is the theoretical basis of the drought resistance mechanism,the exploitation and application of resistance genes.In this experiment,the native apple rootstocks,such as Malus halliana(CS),Malus sieversii(Ldb.)Roem.(XJ),Malus baccata(SDZ),were used as experimental materials.The pots experiments were carried out to make clear the anatomical characteristics of drought resistance of different rootstocks by observing root tissues,stem segments,microstructure and ultrastructure of leaves,and to clarify the effects of photosynthetic characteristics of different rootstocks under water stress by determining photosynthetic pigment content,photosynthetic rate,chlorophyll fluorescence parameters.In addition,q RT-PCR technique was used to analyze the expression of resistance genes in different tissues and different stress conditions.The main results were as follows:1.Under water stress,The damage of chloroplast ultrastructure in mesophyll cells of three rootstocks was different,and the damage degree of CS mesophyll cells was significantly lower than that of XJ and SDZ;The palisade tissue cells of three kinds of apple rootstocks became shortening and the leaves became thinner,also the leaf microstructure changes of CS was significantly less than that of others.The xylem vessel density of the three rootstocks showed a decreasing trend,and the xylem vascular density of CS was remarkably less affected by drought than the other two rootstocks.The microstructure of the root system of the three rootstocks also changed,and the volume of XJ' and SDZ' endodermis cells greatly decreased,but the shape of CS' endodemis cells was not obvious and the degree of lignification was lower.2.The net photosynthetic rate(Pn)of leaves decreased with the increase of water stress time,and the decrease of Pn in CS leaves under severe stress was lower than that of other treatments.The changes of stomatal conductance(Gs)and transpiration rate(Tr)of the three rootstocks under water stress were similar to that of Pn,and the intercellular CO2 concentration(Ci)showed an increasing trend.After a long period of water stress(28d),the photosynthetic parameters of CS affecting degree were significantly lower than those of XJ and SDZ.3.Under the condition of short term and mild stress,the primary light energy conversion efficiency of PSII(Fv/Fm)and the potential activity of PSII(Fv/Fo)were picked up,while they were decreased under the severe water stress.Under the condition of long term mild and severe stress,the fluorescence parameters all caused a sharp decrease except the NPQ,and the NPQ deserved a significant increase.Also,the decrease of CS was significantly lower than those of SDZ and XJ.4.The activities of SOD and CAT in the leaves of the three rootstocks showed the trend of increasing first and then decreasing under water stress.And under the heavy water stress,the activity of SOD and POD in CS was always higher than those of XJ and SDZ.The decrease of LRWC in SDZ leaves was significantly higher than that of the other two rootstocks when treatment time lengthened.The WUEL of the three kinds of rootstocks were significantly lower than those of normal water supply,among which CS had the smallest variation,followed by XJ,and the difference of SDZ was the highest.5.Taking CS and BL as experimental materials,the content of different hormones in leaves after the long-term water stress was analysed.The results showed the ABA in the leaves showed an increasing trend,and the CTK and IAA showed a decreasing trend.After 28 d treatment,the ABA content in the leaves of CS was significantly higher than BL;the ZT,IAA of CS leaves which are two growthpromoting hormone levels were significantly lower than BL.ZT / ABA,ZT / IAA and(ZT + IAA)/ ABA showed a decreasing trend,and the decreasing rate of CS was faster than that of BL.6.Under long-term water stress,the expression of drought resistant genes,such as Md RBP7 and Md RBP3,were up-regulated,and Md RBP1 expression was down regulated.The expression of three genes of CS was significantly different before and after water stress.