Study On Physiological Response And Aquaporin Gene Expression Of Two Citrus Rootstocks Under Drought Stress

Drought seriously affects the yield and quality of citrus,restricts the development of citrus industry,and the use of drought resistant rootstock is an effective way to improve the drought tolerance of citrus.Sanhuhongju(Citrus tangerina Hort.ex Tanaka)and Sanhuhuahong(C.tangerina Hort.ex Tanaka×C.aurantium L.)differing in drought tolerance were used as materials in this study.The growth morphology,physiological and biochemical response,photosynthetic characteristic,and root growth of the two rootstocks under drought stress were compared.Then the aquaporin genes were studied,their sequence and expression response to drought were also analyzied.The main results are as follows:(1)The plant height,biomass and leaf area of Sanhuhongju and Sanhuhuahong decreased in different degrees by drought.But root shoot ratio of the two rootstocks increased under drought,while the Sanhuhongju increased higher.Proline contents in leaves and roots of these two cultivars were lower than that of control at 10 d ago of drought and conversely higher at 25 d of drought.Except to reduce the soluble sugar content in leaves of Sanhuhongju,both the contents of soluble sugar and soluble protein in roots of two rootstocks were increased by stress.The peroxydase(POD)activity in the leaves of Sanhuhongju increased first and then decreased under drought stress.The POD activity in root was significantly higher in treated Sanhuhongju but lower in Sanhuhuahong than that in control under drought stress.The activity of superoxide dismutase(SOD)decreased in leaves while increased in roots of both rootstocks.In addition,the catalase(CAT)activity decreased in the leaves of both rootstocks,but it increased in roots of Sanhuhuahong under drought stress.(2)Net photosynthetic rate,transpiration rate,stomatal conductance and intercellular CO2 concentration in leaves of two cultivars decreased under drought stress.After rewatering,Pn,Tr,Cs and Ci of Sanhuhongju were recovered but still were significantly lower than that of control group.The Sanhuhongju of Y(II),ETR,Fv/Fm had no significant difference in drought and rewatering.The Sanhuhuahong of photochemical quenching(qL)decreased faster in the later period of drought and there had no significant difference after rewatering.Its non-photochemical quenching(NPQ)was significant higher than that of control under drought and rewatering.But the Sanhuhongju of NPQ had no significant difference after rewatering.(3)The total root length and number of root tips of Sanhuhongju had no significant difference than that of control under drought and rewatering,but that of Sanhuhuahong were significant higher than control in later period of drought.The total root volume of Sanhuhongju reduced during the period of drought,but total root area decreased slightlyand had no big change after rewatering.The time of the one stage root length of Sanhuhongju began to increase at 10 d drought,but had no change of Sanhuhuahong at early stage of drought and decreased significantly at 20 d drought.In addition to the three level of lateral roots in Sanhuhongju,two rootstocks remaining stem lateral root growth was inhibited by drought.The root activities of two varieties in the later period of drought were significantly lower than the control.The root activity had no significantly difference in Sanhuhuahong,while that of Sanhuhongju were significant lower than control.(4)Four types of AQP gene family members and 35 members were identified,and different members have diversity in gene structure and chromosome location.Expression analysis showed that CiPIP1;3/2;2/2;3/2;5/2;7 ? CiNIP1;1/1;2/4;1/4;2/5;1/6;1 ?CiTIP2;2/3;1/3;2/4;1?CiSIP2;1 expression increased after drought in Sanhuhongju leaves,but CiPIP2;4/2;8?Ci NIP1;1?CiTIP1;2/3;1/5;1?CiSIP1;1 expression is lower than of the control in sanhuhuahong.Besides,CiPIP1;1/1;2/1;4/2;1/2;2/2;3/2;5/2;6/2;8/2;9/2;10 ?CiNIP1;1/1;2/4;2/5;1/6;1/7;1?CiTIP1;3/2;2/3;1/4;1/5;1?CiSIP1;1 expression increased after drought in Sanhuhongju roots,and CiPIP1;2/2;4/2;7/2;9/2;10 ?CiNIP1;2/2;1/5;1/6;1/7;1?CiTIP1;1/1;2/2;1/2;2/3;1/3;2/4;1/5;1?CiSIP1;1/2;1 expression descend after drought in Sanhuhuahong roots.Comparison,PIP1;2/2;1/2;2/2;3/2;9/2;10?NIP1;2/5;1/4;2?TIP2;2/3;2/5;1 in the opposite direction of expression after drought in the two Rootstocks.In summary,the drought resistance of Sanhuhongju stronger than Sanhuhuahong,which related to the change of morphological structure,such as decrease of leaf area and root architectures in arid condition.Osmotic substances especially the accumulation of proline and the increase of POD activity after drought stress contributed to the physiological adaptation to drought in Sanhuhongju.In addition,AQP gene such as PIP1;2/2;1/2;2/2;3/2;9/2;10?NIP1;2/5;1/4;2?TIP2;2/3;2/5;1 may also affect the drought resistance of two Rootstocks.