Studies On Physiological Mechanisms Of Salt Tolerance Improved By Grafting With Salt-Tolernat Rootstock In Eggplant (Solanum Melongena L.) Seedlings

Eggplant (Solarium melongena L.) is considered to be moderate sensitive to salinity, soil salinity deteriorates severely the growth and yield of eggplant. Grafting plays an important role in eggplant cultivation and can effectively enhance the ability of eggplant to resist adversity and has been applied widely in practice. However, studies were mainly focused on increasing disease and cold resistance by grafting, there are few studies on improving salt tolerance by grafting. Furthermore, soil secondary salinization has been becoming the main factor which severely limited the sustainable development of facility vegetable cultivation. Therefor, it is very important to investigate the mechanism of salt tolerance by grafting. In this study, we use 'Torvum Vigor' (Solarium torvum Swartz)which is a special cultivar introduced from Japan as rootstock and eggplant cultivar 'Suqiqie' as scion, grafting was made to compare the differences in growth and metabolism of physiology and biochemical between grafted and own-rooted eggplant seedlings under salt stress, in order to better elucidate the physiological and biochemical mechanisms of improving salt tolerance by grafting. The results of this study were as follows:1.Biological characteristics of 'Torvum Vigor' were observed and salt tolerance was evaluated. The results showed that 'Torvum Vigor' displayed stronger growth vigor and flowered at the end of August. Flower color is white and inflore scene is in clusters. Fruit number per plant is great and fruit is small. These are similar to wild eggplant cultivar in morphological character. It is difficult to obtain seeds in Nanjing region under open cultivation since flower period is very later. Under 80 mmol·L^-1 sodium chloride stress, 'Torvum Vigor' seedlings growth was less affected than that of 'Suqiqie' ones, root activity of 'Torvum Vigor' seedlings was stronger than that of 'Suqiqie' ones. Contents of Na⁺ and Cl^- in roots of 'Torvum Vigor' were higher than those of 'Suqiqie', while content of Na⁺ and Cl^- in leaves of 'Torvum Vigor' was lower than those of 'Suqiqie' seedlings. The above results suggested salt tolerance of 'Torvum Vigor' is super to 'Suqiqie'.2.Growth and photosynthetic characteristics of grafted and own-root eggplant seedlings were studied under 80 mmol·L^-1 NaCl stress. The results showed that plant growth, contents of chlorophyll a and chlorophyll b, net photosynthetic rate (Pn), transpiration rate (Tr),stomatal conductance (Gs) were significantly decreased and stomata limited value (Ls) was significantly increased by NaCl stress, intercellular CO₂ concentration (Ci) first were decreased and then were increased. NaCl stress had little effect on content of carotenoid in grafted and own-root eggplant seedlings. Under the same stress conditions, Pn, Tr, Gs and contents of photosynthetic pigment in grafted seedlings were significantly higher than those of own-root ones. The growth of grafted seedlings was less affected by NaCl stress than that of own-root ones, which was related to keep higher Pn in grafted seedlings.3.The following results were obtained. Contents of Na⁺ and Cl^- of grafted seedlings were significantly lower than those of own-root seedlings in shoot except in petiole, and they were accumulated in roots of grafted seedlings. Under NaCl stress, grafted seedlings kept high values of K⁺/Na⁺ and Ca²⁺/Na⁺ in young leaves and roots; selective absorption values of S_K,Na and S_Ca,Na in roots and selective transport values of S_K,Na and S_Ca,Na in young leaves of grafted seedlings were significantly higher than those of own-root seedlings. Based on these results, it could be concluded that, mechanisms of salt tolerance of grafted eggplant seedlings were attributed both to accumulation of Na⁺ and Cl^- in roots to alleviate the injury effect of salt ions on shoot growth, and to increasing the values of K⁺/Na⁺ and Ca²⁺/Na⁺ in young leaves and roots by accumulating K⁺ and Ca²⁺.4.The activities of antioxidant enzymes, contents of osmotic adjustment substances and metabolism of reactive oxygen in grafted and own-root eggplant seedlings were investigated. The results showed that NaCl stress significantly reduced superoxide dismutase (SOD) activity and increased (?) producing rate, contents of free proline,H₂O₂ and MDA in leaves of both grafted and own-root eggplant seedlings, while peroxidase (POD) and catalase(CAT) activity increased firstly and then decreased. Compared with own-root eggplant seedlings, the grafted seedlings had a higher antioxidant enzymes activities and contents of osmotic adjustment substances and a lower (?) producing rate, contents of H₂O₂ and MDA under NaCl stress. NaCl stress induced lipid peroxidation in leaves of both grafted and own-root eggplant seedlings. However, the grafted seedlings were less affected by NaCl stress than that of own-root seedlings. It was concluded that higher activities of antioxidant enzymes and less oxidative damage might be involved in stronger salt tolerance of grafted eggplant seedlings.5.The response of the enzymes and metabolites of the ascorbate-glutathione pathway to oxidative stress caused by salt stress was studied under matrix cultivation. The results showed that NaCl stress induced generation of H₂O-2 and accumulation of malondiadehyde(MDA) both in grafted and own-root seedlings. However, contents of MDA and H₂O₂ in leaves of grafted seedlings were significantly less than those that in own-seedlings. Activities of glutathione reductase (GR) and ascorbate-specific peroxidase (APX) in leaves of grafted seedlings were significantly increased whereas own-seedlings decreased under NaCl stress. NaCl stress promoted reduced ascorbic acid (AsA) and reduced glutathione (GSH) synthesis in grafted eggplant seedlings. Contents of AsA and GSH in leaves of grafted seedlings were significantly higher than those in own-seedlings. It was concluded that a more active AsA-GSH cycle may be involved in stronger salt tolerance of grafted eggplant seedlings, which ensures regeneration of AsA and GSH to reduce oxidative damage.6.Polyamines metabolism and abscisic acid (ABA) content in leaves of grafted and own-root eggplant seedlings were studied. The results showed that plant growth, contents of three forms of polyamines (free, conjugated and bound), and ABA content in grafted seedlings were significantly higher than those in own-root seedlings under NaCl stress. NaCl stress significantly increased contents of spermine (Spm) and ABA in leaves, putrescine (Put) and spermidine (Spd) contents increased then decreased in both grafted and own-root seedlings. However, the reduction of Put and Spd contents in leaves of grafted seedlings was lower than that of own-root seedlings, while the increase of Spm and ABA contents of grafted seedlings was higher than that of own-root seedlings. The polyamine metabolism of grafted seedlings was less affected by NaCl stress than those of own-root seedlings. It was concluded that both the rapid accumulation of ABA and higher polyamine contents might be involved in stronger salt tolerance of grafted eggplant seedlings.7.Using high-pressure liquid chronatography (HPLC) method, differences in root polyamines metabolism between hydroponically-grown grafted and own-root eggplant seedlings were studied under 80 mmol·L^-1 NaCl stress. The results showed that contents of free spermidine (Spd), conjugated polyamines in roots of grafted eggplant seedlings were significantly higher than those of own-root seedlings, while contents of free putrescine (Put) and bound polyamines of grafted seedlings were significantly lower than those of own-root seedlings and there was no difference in free spermine (Spm) between grafted and own-root seedlings at 2 d after NaCl treatment. NaCl stress reduced the contents of three forms of polyamines in roots of own-root seedlings at 10 d after NaCl treatment. Whereas, the contents of free Spm, conjugated Put and bound polyamines in roots of grafted seedlings were increased. Polyamine oxidase (PAO) activities were significantly increased under NaCl stress. However, the elevation of PAO in own-root seedlings was higher than that of grafted seedlings. The above results indicated that both accumulation of conjugated polyamine at early stage of stress and higher Spm content at later stage were important to the improvement of salt tolerance in grafted seedlings.