Preliminary Studies On Drought Resistance Of Different Walnut Cultivats

In order to study the response of different walnuts to drought stress and provide basis fordrought resistance mechanism, screen drought-resistant walnut cultivars, make contribution tothe production of “Xiangling” under drought stress and provide theoretical basis for watermanagement of “Xiangling” walnut, this paper addresses the method of drought simulationexperiment and comprehensive evaluation on drought resistance of different walnut cultivarsand on the study of “Xiangling” seedlings physiological and biochemical response to differentwater stress. Main conclusions are as follows:(1) The response of6walnut cultivars “(Xiangling”,“Xilin2”,“Liaohe4”,“Zhonglin1”,“Qingxiang” and “Luguang”) under drought stress was studied by the method ofdifferent polyethylene glycol6000(PEG-6000) levels (0.00,-0.50,-0.75,-1.00,-1.50, and-2.00MPa), the results indicated that, compared with the control (0.00MPa), the average seedgermination rate of all walnut cultivars under drought stress reduced in different degrees, andthe stronger the stress level, the lower the average germination rate. Under the conditions ofall the stress levels, the average root length of “Luguang” and “Liaohe4” offspringseedling was longer than other cultivars, which indicated that the effect of drought stress onthe two cultivars rooting ability was relatively weaker, the drought resistance ability of thetwo cultivars were stronger. At the same time, the relative water content (RWC) in theoffspring seedling roots of walnuts was greatly reduced with the increase of stress degree. Byanalysis of the results, the offspring seedlings of “Xiangling” and “Qingxiang” weregreatly influenced by the drought stress, and the offspring seedlings of “Luguang” walnutwere less affected.(2) Under osmotic stress conditions, the roots and shoots free proline content of allwalnut offspring seedlings showed growth trend, and the soluble sugar content increased withthe stress levels, while the soluble protein content increased first and then decreased. In theprocess of the stress treatment, the free proline and soluble protein contents in “Luguang”seedlings were always the highest.(3) With stress levels increasing, roots and shoots superoxide dismutase (SOD) activityof6walnut cultivars decreased after an initial increase, and presented a peak under the-1.00MPa. Peroxidase (POD) activity showed a similar trend with SOD activity, while the catalase(CAT) activity showed a gradual decreasing trend. The activities of the three kinds of protective enzyme in “Luguang” walnut offspring seedling were the highest, the ones in“Xiangling” walnut seedlings was the lowest, indicating the drought resistance of “Luguang”walnut was stronger, and “Xiangling” was weaker.(4) With the increase of the stress degrees, the chlorophyll a, b, and total contents of allwalnut cultivars showed a decreasing trend, while the magnitude reduced different. Underdrought stress conditions,“Qingxiang” and “Xiangling” chlorophyll content were greatlyinfluenced by stress, and the impact on “Xilin2” and “Luguang” was smaller, indicating that“Xilin2” and “Luguang” showed strong resistance to drought. The malondialdehyde (MDA)contents in all the cultivars offspring seedlings roots and shoots under stress conditions weresignificantly increased. Shoots relative conductivity increased with increasing of the stresslevels. The relative conductivity in “Luguang” offspring seedling shoots was always thelowest, while “Xiangling” walnut was the highest. The results indicated that the effect ofdrought stress on “Xiangling” walnut was the greatest, that is, its performance was moresensitive to drought, and the drought resistance was weak;“Luguang” walnut was lessaffected by drought stress, showing strong drought resistance.(5) Moreover, by subordinate function value method and clustering analysis, the rank ofdrought-resistant ability of the6walnut cultivars was:“Luguang”>“Liaohe4”>“Zhonglin1”>“Xilin2”>“Qingxiang”>“Xiangling”. Analysis by cluster analysis indicated that:“Luguang” was drought-resistant cultivar,“Liaohe4” and “Zhonglin1” were moderatedrought-resistant;“Xilin2” was medium not drought-resistant,“Qingxiang” and “Xiangling”were not drought-resistant. Both the methods have better consistency in drought resistanceevaluation of6walnut cultivars, the results showed that drought tolerance of “Luguang”walnut was the strongest, and “Xiangling” walnut was the weakest.(6) Grafted seedlings of “Xiangling” walnut cultivar main cultivated in Shaanxi Provincewere used as perimental materials, by the research of the dynamic changes in physiologicaland biochemical indicators of them under continuous stress for40d and rehydration at4different water treatments, the results indicated that: compared with the control (soil relativewater content80%), with the time going on, the leaf relative water content decreased undermild (60%), moderate (40%) and severe (20%) stress, and the water saturation deficit, relativeelectrical conductivity and MDA content showed an growth trend; protective enzymesactivities, osmotic adjustment substances, chlorophyll a, b and total chlorophyll contentsincreased at the first then reduced or directly reduced. Rehydration after40d under mildstress, the growth of “Xiangling” seedlings recovered back to the level of control; while itcould not be restored under moderate and severe stress for24d. This determination provides atheoretical basis for water management of the cultivation in “Xiangling” walnut. As a consequence, CAT activity, free proline, soluble sugar and MDA content could beused as effective indicators for walnuts drought resistance.