| With the vegetable industry developing, the area of protected cultivation of eggplant (Solanum melongena L.) is developing rapidly in China. As an important kind of fruit vegetables, protected eggplant cultivation has become an important way to increase farmers'income. However, excessive chemical fertilizer was applied usually, to achieve more economic benefit with high yield. Soil secondary salinization of the plastic film protected cultivation has become a serious problem, which were resulted both from a lack of leaching by rainfall and from strong evaporation of soil water due to long-term covering together with high fertilizer application rates, and severely limited the sustainable development of facility vegetable cultivation. In soil salinity in these systems, the most cation was Ca2+, and the most anion present was NO3- (67-76% of total anions). In recent years, a few researches on grafting techniques and salinity tolerance physiology of grafted eggplant have been carried out, which were mainly focused on NaCl stress, and few studies on physiological mechanisms of grafted eggplant in response to calcium nitrate stress have been reported. It has been well documented that grafting on salt tolerant rootstock can improve salinity tolerance of vegetable. In this study, we use Torvum Vigor' (Solanum torvum Swartz) a salt tolerant cultivar introduced from Japan as rootstock, and'Suqi Qie' a major cultivar in Nanjing area as scion, grafting was made to compare the differences in growth and physiological and biochemical mechanisms between hydroponically-grown grafted and non-grafted eggplant seedlings under 80 mmol·L-1 Ca(NO3)2 stress. The main results were as follows:1. The differences in photosynthetic pigments contents, photosynthetic characteristics and osmoregulation substances contents between grafted and non-grafted eggplant seedlings were observed under Ca(NO3)2 stress. The result showed that, the photosynthetic pigments contents in leaves of eggplant seedlings were significantly decreased, CO2 concentration (Ci) and stomatal conductance (Gs) were inhibited, net photosynthetic rate (Pn) was decreased under Ca(NO3)2 stress. The seedlings accumulated osmoregulation substances to reduce damage caused by Ca(NO3)2 stress. However, under Ca(NO3)2 stress, the plant height and biomass accumulation of grafted eggplant seedlings were significantly higher than those of non-grafted seedlings, photosynthetic pigments and osmoregulation substances contents in leaves of grafted seedlings were significantly higher than those of non-grafted seedlings, and grafted seedlings had significantly higher Pn, Tr, Ci and Gs than non-grafted seedlings, with significantly lower Ls in leaves of grafted seedlings. These indicated that, the adaptability of grafted seedlings to Ca(NO3)2 stress was more stronger than that of non-grafted plants.2. The differences of nitrogen metabolism and carbon metabolism between grafted and non-grafted eggplant seedlings were observed. The result showed that, under Ca(NO3)2 stress, the contents of NO3-N and NH4+-N in leaves of eggplant seedlings increased significantly at the early stage of treatment, then decreased significantly, however, the contents of NO3--N and NH4+-N in grafted seedlings were significantly higher than those of non-grafted seedlings in prolonged stress time; nitrate reductase (NR) activity in leaves of grafted seedlings was significantly higher than that of non-grafted seedlings since the 9th day of treatment; soluble protein content in leaves of grafted seedlings was significantly higher than that of non-grafted seedlings in the whole stress time; glutamine synthetase (GS) activity in leaves of eggplant seedlings increased significantly at the early stage of treatment, then decreased significantly, however, GS activity in grafted seedlings were significantly higher than those of non-grafted seedlings in prolonged stress time; soluble sugar content in leaves of grafted seedlings increased significantly at the early stage of treatment, then decreased significantly, but soluble sugar content in non-grafted seedlings decreased and was significantly lower than that of grafted seedlings in whole stress time; starch content in leaves of grafted seedlings was significantly higher than that of non-grafted seedlings, but activities of a-amylase and P-amylase were both significantly lower than those of non-grafted seedlings. The results indicated that nitrogen metabolism and carbon metabolism in grafted seedlings were more efficient than non-grafted plants, grafted seedlings were more tolerant to Ca(NO3)2 stress.3. In the aspect of characteristics of ion absorption and distribution, K+ contents decreased in all organs in eggplant seedlings, but grafted seedlings had significantly higher K+ content than that of non-grafted seedlings except in aged leaves and petiole, and K(G2/N2) ratios in root, young and functional leaves of grafted seedlings were higher; Ca2+ content increased in all organs in eggplant seedlings except in young leaves of non-grafted seedlings, but grafted seedlings had significantly higher Ca2+content than non-grafted seedlings in all organs, and Ca(G2/N2) ratio in young leaves was the highest; Mg2+content decreased in all organs in eggplant seedling, but Mg2+content in grafted seedlings was significantly higher than that of non-grafted seedlings except in aged leaves. These results indicated that, under Ca(NO3)2 stress, the ability of ionic selective absorption was stronger in root of grafted seedlings than that of non-grafted seedlings, and ions in grafted seedlings were mainly transported to vigorous meristem (young leaves) and efficient photosynthetic organ (functional leaves). Ionic selective absorption and distribution in grafted seedlings excelled than that of non-grafted seedlings under environmental stresses, and grafted seedlings showed stronger tolerance to Ca(NO3)2 stress.4. Under Ca(NO3)2 stress, in leaves of grafted eggplant seedlings, the antioxidant enzymes activities were significantly higher while the superoxide anion radicals (O2·) producing rate, hydrogen peroxide (H2O2) content, malondiadehyde (MDA) content and the electrolyte leakage rate were significantly lower than those of non-grafted seedlings. Ca(NO3)2 stress reduced the antioxidant enzymes activities, but increased O2·producing rate, H2O2 content, MDA content and the electrolyte leakage rate both in grafted and in non-grafted seedlings. However, grafted seedlings were less inhibited than non-grafted seedlings under Ca(NO3)2 stress, and the former had a stronger tolerance to Ca(NO3)2 stress owing to effectively scavenging of reactive oxygen species and preserving the integrity of membranes.5. The response of ascorbate-glutathione metabolism system of grafted and non-grafted eggplant seedlings under hydroponically-grown to Ca(NO3)2 stress was studied. The results showed that, under Ca(NO3)2 stress, activities of antioxidant enzymes [ascorbate peroxidase (APX), dehydro-ascorbate reductase (DHAR) and glutathione reductase (GR)], regenerating rates of ascorbate (AsA) and glutathione (GSH) and redox statuses (ratios of AsA/DHA and GSH/GSSG) were significantly higher in leaves of grafted seedlings than those of non-grafted seedlings. We concluded that grafted eggplant seedlings had efficient metabolism of ascorbate-glutathione cycle, which scavenged the H2O2 rapidly to alleviate the oxidative damage of Ca(NO3)2 stress; therefore, grafted seedlings had a stronger tolerance to salt stress.6. In polyamines metabolisms, under Ca(NO3)2 stress, free, soluble conjugated and insoluble bound polyamines (PAs) in leaves of grafted seedlings were significantly higher than those of non-grafted seedlings; total contents of diamine putrescine (Put), triamine spermidine (Spd) and tetraamine spermine (Spm) in leaves of grafted seedlings were significantly higher than those of non-grafted seedlings; grafted seedlings had significantly higher ratio of free Put/(Spd+Spm) and significantly lower ratio of soluble conjugated Put/(Spd+Spm), with similar level of insoluble bound Put/(Spd+Spm); activities of diamine oxidase (DAO) and polyamine oxidase (PAO) in leaves of grafted seedlings were significantly lower than those of non-grafted seedlings. We concluded that accumulations of three kinds of PAs in grafted seedlings were closely related to salinity tolerance under Ca(NO3)2 stress; however, accumulation of free Put might be important to salinity tolerance, moreover, the conversions from soluble conjugated Spd and Spm might play fundamental roles to improve the tolerance of grafted seedlings under Ca(NO3)2 stress.
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