[Influence of exogenous gamma-aminobutyric acid (GABA) on GABA metabolism and amino acid contents in roots of melon seedling under hypoxia stress].

This paper investigated the influence of gamma-aminobutyric acid (GABA) on GABA metabolism and amino acid content under hypoxia stress by accurately controlling the level of dissolved oxygen in hydroponics, using the roots of melon 'Xiyu 1' seedlings as the test material. The results showed that compared with the control, the growth of roots was inhibited seriously under hypoxia stress. Meanwhile, the hypoxia-treated roots had significantly higher activities of glutamate decarboxylase (GAD), glutamate dehydrogenase (GDH), glutamate synthase (GOGAT), glutamine synthetase (GS), alanine aminotransferase (ALT), aspartate aminotransferase (AST) as well as the contents of GABA, pyruvic acid, alanine (Ala) and aspartic acid (Asp). But the contents of glutamic acid (Glu) and alpha-keto glutaric acid in roots under hypoxia stress was obviously lower than those of the control. Exogenous treatment with GABA alleviated the inhibition effect of hypoxia stress on root growth, which was accompanied by an increase in the contents of endogenous GABA, Glu, alpha-keto glutaric acid and Asp. Furthermore, under hypoxia stress, the activities of GAD, GDH, GOGAT, GS, ALT, AST as well as the contents of pyruvic acid and Ala significantly decreased in roots treated with GABA. However, adding GABA and viny-gamma-aminobutyric acid (VGB) reduced the alleviation effect of GABA on melon seedlings under hypoxia stress. The results suggested that absorption of GABA by roots could alleviate the injury of hypoxia stress to melon seedlings. This meant that GABA treatment allows the normal physiological metabolism under hypoxia by inhibiting the GAD activity through feedback and maintaining higher Glu content as well as the bal- ance of carbon and nitrogen.

[Effects of exogenous gamma-aminobutyric acid on polyamine metabolism of melon seedlings under hypoxia stress].

Taking melon cultivar 'Xiyu No. 1 ' as test material, a hydroponic experiment was conducted to investigate the effects of exogenous gamma-aminobutyric acid (GABA) on the seedlings polyamine metabolism under hypoxia stress. Compared with the control in normoxic treatment, the seedlings under hypoxia stress had significantly higher glutamic acid decarboxylase (GAD) activity and GABA content, and their polyamine synthesis enzymes activities all enhanced significantly, which led to a marked increase of polyamines contents. Meanwhile, the seedlings leaf- and root diamine oxidase (DAO) and polyamine oxidase (PAO) activities also had a significant increase. The increment of root arginine decarboxylase (ADC) activity was higher, which induced a higher content of free putrescine (Put) in roots, while the increment of leaf ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC) activities were higher, inducing a higher content of free spermidine (Spd) in leaves. The PBs-extractable DAO and PAO activies in roots were significantly lower than those in leaves, but the cell wall-bound PAO activity was in adverse. Under hypoxia stress, the addition of exogenous GABA increased the leaf- and root GABA and glutamic acid contents and decreased the GAD activity significantly. The increase of arginine, ornithine, and methionine contents promoted the activities of polyamines synthesis enzymes, which led to the significant increase of polyamines contents and the significant decrease of DAO and PAO activities.

[Effects of gamma-aminobutyric acid on the photosynthesis and chlorophyll fluorescence parameters of muskmelon seedlings under hypoxia stress].

By the method of hydroponic culture, this paper studied the effects of exogenous gamma-aminobutyric acid (GABA) on the photosynthetic pigment contents, photosynthesis, and chlorophyll fluorescence parameters of muskmelon seedlings under hypoxia stress. Hypoxia stress induced a significant decrease of photosynthetic pigment contents, resulting in the decrease of photosynthesis. Applying GABA could significantly increase the photosynthetic pigment contents, net photosynthetic rate (P(n)), stomatal conductance (G(s)), intercellular CO2 concentration (C(i)), carboxylation efficiency (CE), maximal photochemical efficiency of PS II (F(v)/F(m)), photochemical quenching (q(P)), apparent photosynthetic electron transfer rate (ETR), and quantum yield of PS II electron transport (phi(PS II)), and decrease the stomatal limitation value (L(s)), minimal fluorescence (F(o)), and non-photochemical quenching (NPQ) under both hypoxic and normal conditions. The alleviation effect of GABA on photosynthetic characteristics was more obvious under hypoxia stress. However, simultaneously applying GABA and VGB could significantly decrease the alleviation effect of GABA under hypoxia stress.