| In recent years,protected cultivation of vegetables has expanded rapidly in China. However,due to the protected cultivation is a closed system,the temperature inside the greenhouse is higher than outside,in addition,because there is less rain rinsing,the upward movement of soil water inside the greenhouse is more greatly than outside.Protected cultivation usually bring excessive nitrogen fertilization into soil and the mineralization of soil inside the greenhouse is more greatly than outside,all these will make the salt concentration of soil surface increasing easily,which inhibits vegetable growth and development,causes decline of yield and quality,brings a negative effect on further development of protected cultivation of vegetables.It has been well documented that grafting can improve vegetable tolerance to environmental stresses,become an effective approach to overcome salt stress.Tomato(Lycopersicon esculentum Mill.) is one of the most important vegetables in protected cultivation,in recent years,researches on grafting techniques and grafting improving tomato salt tolerance have been carried out all over the world,but all these researches focused on NaCl stress.In greenhouse salt soil,the most cation is Ca2+,taking up about 60%of all cations,the most anion is NO3-,taking up 67%～76%of all anions,but few reports on physiological and biochemical changes in grafted tomatoes during Ca(NO3)2 have been published.In this study,we use hydroponically-grown grafted and own-root tomato(cvs.'Kagemusya' and 'Baoda 903' as rootstock and scion respectively) seedlings as experimental materials,comparisons of physiological and biochemical changes were made between grafted and own-root tomato seedlings under 80 mM Ca(NO3)2 stress.The results showed that under Ca(NO3)2 stress,superoxide dismutase(SOD) activities of both grafted and own-root tomato seedlings increased significantly,peroxidase(POD) activities decreased during the first 2 d,then increased significantly,catalase(CAT) activities decreased significantly,but every index of grafted seedlings was significantly higher than that of own-root seedlings.Under Ca(NO3)2 stress,O2- producing rate, contents of hydrogen peroxide(H2O2) and malondialdehyde(MDA) of both grafted and own-root tomato seedlings increased significantly,but every index of grafted seedlings was significantly lower than those of own-root seedlings.The proline and soluble protein contents of grafted tomato seedlings increased significantly,proline contents of own-root tomato seedlings and soluble protein content in own-root tomato roots increased significantly,however,no significant difference was observed in soluble protein content in own-root tomato leaves,but each index of grafted seedlings was significantly higher than that of own-root seedlings,exhibited that grafted seedlings have a stronger osmotic adjustment ability.Under Ca(NO3)2 stress,the activities of antioxidant enzymes(APX,DHAR and GR) were significantly higher,the regenerating rates of AsA and GSH were significantly higher, and redox statuses(ratios of AsA/DHA and GSH/GSSG) were also significantly higher in leaves and roots of grafted seedlings than those of own-root seedlings,and H2O2 content was significantly lower in leaves and roots of grafted seedlings than that of own-root seedlings.These results indicated that under Ca(NO3)2 stress grafted tomato seedlings had an efficient metabolism of ascorbate-glutathione cycle,which scavenged the H2O2 rapidly to alleviate the oxidative damage,thus grafted seedlings had a stronger tolerance to Ca(NO3)2 stress.Polyamines are low-molecular-weight biologically active compounds,play important roles in seedlings growth and development,configuration formation and response to environmental stresses.Under Ca(NO3)2 stress,contents of three forms of putrescine(Put), spermidine(Spd) and spermine(Spm) in leaves of grafted seedlings increased significantly in the whole process of stress,the contents of three forms of Put in leaves of own-root seedlings decreased significantly,the contents of free and conjugated Spd and Spm increased significantly,but the contents of bound Spd decreased significantly,however,no significant difference was observed in bound Spm after treatment.Under Ca(NO3)2 stress, contents of three forms of Put,Spd and Spm in roots of grafted seedlings increased significantly in the whole process of stress,the contents of free Put,Spd and Spm in roots of own-root seedlings increased significantly,the contents of conjugated Put,Spd and Spm in roots of own-root seedlings decreased significantly,no significant difference was observed in contents of bound Put at the early stage of stress,then decreased significantly, however,no significant difference was observed in bound Spd and Spm after treatment. During the whole course of stress,the contents of whole polyamines in grafted seedlings are significantly higher than those of own-roots seedlings,arginine decarboxylase(ADC) activities in grafted seedlings are significantly higher than those of own-roots seedlings,but polyamine oxidase(PAO) activities in grafted seedlings are significantly lower than those of own-roots seedlings.These results indicated that under Ca(NO3)2 stress,the synthesis of polyamines in grafted tomato seedlings is higher than that of own-root seedlings,but the degradation of polyamines in grafted tomato seedlings is lower than that of own-root seedlings,which showed the characteristics of stronger Ca(NO3)2 tolerance of grafted seedlings.In the aspect of characteristics of ion absorption and distribution,under Ca(NO3)2 stress,the contents of K+ in grafted seedlings increased significantly and the increase in roots was most obvious,however,the contents of K+ in own-root seedlings decreased significantly and the decrease in roots was least.The contents of Ca2+ in both grafted and own-root seedlings increased significantly and the increase in roots was most obvious, however,the increase in grafted seedlings was more significant than that in own-root seedlings.The contents of Mg2+ in both grafted and own-root seedlings decreased significantly and the decrease in functional leaves was most obvious,however,the decrease in own-root seedlings was more significant than that in grafted seedlings.The contents of Fe2+ in functional leaves,old leaves,petiole and stem of grafted seedlings decreased significantly and the decreased in stem was most obvious,however,no significant difference was observed in the Fe2+ contents in young leaves and roots,the contents of Fe2+ in all parts of own-root seedlings decreased significantly and the decreased in stem was most obvious.The contents of Mn2+ in grafted seedlings increased significantly and the increase in stem was most obvious,contents of Mn2+ in functional leaves,old leaves and petiole of own-root seedlings decreased significantly,but Mn2+ contents in young leaves, stem and root of own-root seedlings increased significantly and the increase in stem was most obvious.All these results indicated that the selective absorption,transportation of ion in grafted seedlings were stronger and the compartmentalization of ion was better than those in own-root seedlings,thus had a stronger tolerance to Ca(NO3)2 stress.Nitrogen metabolism and carbon metabolism are the most important two metabolic processes in crops,during the development of crops they must keep homeostasis in order to achieve maximal yield.Under Ca(NO3)2 stress,the contents of NO3- and NH4+ increased significantly at the early stage of stress,then decreased significantly,each index of grafted seedlings was significantly higher than that of own-root seedlings.Nitrate reductase(NR) activities of both grafted and own-root seedlings increased significantly at the early stage of stress,then decreased and the activity of NR in grafted seedlings was significantly higher than that of own-root seedlings.Glutamine synthetase(GS) activities in leaves of grafted seedlings increased significantly during the whole stress,GS activities in own-root seedlings and roots of grafted seedlings increased significantly at the early stage of stress, then decreased significantly.The contents of starch in own-root seedlings and leaves of grafted seedlings decreased significantly,however,the starch content in roots of grafted seedlings increased slowly.The contents of soluble sugar in grafted seedlings and leaves of own-root seedlings increased significantly,the contents of soluble sugar in roots of own-root seedlings increased significantly at the early stage of stress,then decreased. Amylase activities in grafted seedlings and roots of own-root seedlings increased significantly in the whole process of stress,however,no significant difference was observed in amylase activities in roots of grafted seedlings.All these results indicated that under Ca(NO3)2 stress,nitrogen metabolism in grafted seedlings were stronger than that of own-root seedlings and the inhibition of nitrogen metabolism to carbon metabolism was less than that in own-root seedlings,thus grafted seedlings were more tolerant to Ca(NO3)2 stress.
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