Physiological regulation of γ-aminobutyric acid on the salt tolerance of grafted tomato seedlings

Objectives  γ-Aminobutyric acid (GABA) plays an important role in enhancing crop quality and stress resistance. We studied the effects of exogenous GABA on salt tolerance of plants through the growth and the physiological responses under salt stress, to provide theoretical base for the application of small molecular weight amino acid against salt stress in vegetable production.

Methods  Soilless culture method was used for this research. The salt-tolerant tomato seedlings were grafted the scions of salt-sensitive cultivar ‘Zhongza 9’ onto the rootstocks of salt-tolerant cultivar ‘OZ-006’ (RS), the less tolerant seedlings were self-grafted of ‘Zhongza 9’ (SS). The salt stress condition was made by adding 175 mmol/L of NaCl in the basic Hoagland nutrient solution, and 5 mmol/L of GABA was added as treatment. Since the tomato seedlings were treated three days, the growth, Na⁺ accumulation, amino acid content and active oxygen metabolism indices of the seedlings were measured.

Results  Both SS and RS seedlings were significantly injured by salt stress with the extension of NaCl stress time, however, the RS seedlings had significantly lower salt damage index and Na⁺ content, significantly higher seedling growth rate, chlorophyll content and total amino acid content, and significantly lower \rmO_\small 2^\overline \,\cdot\, production rate and MDA content, compared with those of SS seedlings. The addition of GABA significantly increased the fresh weights, seedling growth rates, chlorophyll contents and the amino acid contents (especially GABA, glutamic acid and proline), antioxidant enzyme (SOD, POD and CAT) activities, while significantly reduced Na⁺ content, \rmO_\small 2^\overline \,\cdot\, production rate and MDA contents in roots and leaves of both SS and RS seedlings, compared with respective controls. Furthermore, the effect of exogenous GABA on improving the salt tolerance of SS was greater than that of RS.

Conclusions  Salt stress significantly affects the growth of tomato seedlings, especially the salt intolerant varieties. GABA could effectively improve the salt tolerance of both salt tolerant and sensitive tomato seedlings. The reason for this effect may be that, GABA provides nitrogen nutrition for seedlings and promotes plant growth and chlorophyll synthesis under salt stress. The addition of GABA induces the increase of multiple amino acid contents in cells and the enhancement of osmotic adjustment ability of leaves, thus inhibits the accumulation of Na⁺ and alleviates the damage caused by the accumulation of intracellular reactive oxygen species. In addition, the mitigation effect of the salt damage is more obvious in salt sensitive seedlings. Therefore, under salt stress conditions, the application of exogenous small molecular weight amino acids, such as GABA, may be an effective measure to improve the salt tolerance of crops.