The chemical constituents identified in the essential oils hydrodistilled from the leaves of Amomum gagnepainii T.L.Wu, K.Larsen and Turland and Amomum repoense Pierre ex Gagnep (Zingiberaceae) of Vietnam origin are reported. The chemical analyses were performed by means of gas chromatography-flame ionisation detector (GC-FID) and gas chromatography coupled with mass spectrometry (GC-MS). The main compounds of A. gagnepainii were farnesyl acetate (18.5%), zerumbone (16.4%) and β-caryophyllene (10.5%). On the other hand, Amomum repoense comprised of monoterpenes dominated by β-pinene (33.5%), (E)-β-ocimene (9.6%), γ-terpinene (9.1%) and α-pinene (8.4%). This is the first report on the essential oils of A. gagnepainii and A. repoense grown in Vietnam or elsewhere.

Infestation of cowpea aphid (Aphis craccivora Koch) induced oxidative stress in leaves of soybean (Glycine max (L.) Merr. cv. "Nam Dan") with a burst in generation of reactive oxygen species (ROS) products such as superoxide anion radical (O 2 .-) and hydrogen peroxide (H 2 O 2 ) recorded around 24 hours after aphid feeding. A continuous increase in content of thiobarbituric acid reactive substances (TBARS) in lipid peroxidation and a defined percentage of injury in aphid-infested leaves (2.20 -8.79 %) were resulted from the cellular oxidative damage. An enhanced activity of the antioxidant enzymes such as superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6) was recorded in leaves of soybean "Nam Dan" from 24 to 48 hours post-infestation (hpi). The enhancement of these enzymes function as the antioxidative response that controlled both ROS-accumulation playing as defensive element and ROSdetoxifying, therefore, reduce the oxidative damage within 48-96 hpi. Activity of SOD and CAT also can improve the tolerance of soybean "Nam Dan" to infestation from A. craccivora.

The differential accumulation of endogenous flavonoids was recorded in soybean (G. max (L.) Merr. cv. "Nam Dan") under infestation of cowpea aphid (Aphis craccivora Koch) and/or treatment of the crude extract of cyanobacterium strain Nostoc calcicola HN9-1a. During A. craccivora infestation, flavonoids in the aphid-infested leaves were strongly induced to high contents, which were 1.84-2.21 -fold higher than that observed in control. Those bioactive substances were suppressed by the single treatment of N. calcicola crude extract, however, were enhanced by the cross-talk interactions of N. calcicola HN9-1a and A. craccivora. Flavonoids in the double factors-effected soybean leaves were remarkably increased to high level since 24 hours after cowpea aphid attack, having by 2.99 -3.06-fold higher than that in control. It was the important evidences to suggest that flavonoids may function in the defense mechanism of soybean "Nam Dan" against A. craccivora; and N. cacicola HN9-1a crude extract improved the accumulation of flavonoids in soybean response to cowpea aphid infestation.

Strain Nostoc calcicola HN9 of Cyanobacteria, which was in static culture, was collected biomass in log phase and death phase to exam its effect on the antioxidant system of soybean [Glycine max (L.) Merr. “Nam Dan”] in the vegetative stages. Suspension of HN9 in death phase plays as an unfavourable factor that caused to oxidative stress in soybean leaves with a burst of superoxide anion radical (O2.-) in stage V1 (each seedling has one unrolled trifoliolate leaf) and hydrogen peroxide (H2O2) in stage V3 (seedlings have two unrolled trifoliolates enclosing auxillary buds). In HN9-treated plants, total phenolics, a non-enzymatic antioxidant, was early induced to high content, whereas generation of ascorbic acid was inhibited; parallel, activity of enzymatic antioxidants such as superoxide dismutases (SOD), catalase (CAT), ascorbate peroxidases (APX) and polyphenol oxidase (PPO) were induced during the vegetative stages. An enhancement of SOD, CAT, and APX firmly regulated ROS products to avoid oxidative damage, reduced cellular damage, and allowed them to play crucial functions in signal transduction to increase soybean tolerance. The defensive role of PPO should be more clarified in the prospective studies. Different from above effects of cyanobacteria in death phase, suspension of HN9 in log phase only raised content of ascorbic acid, and induced activity of SOD and APX, however, its stimulated level was lower than that in death phase. Lack of convinced evidences discribed how strain HN9 in log phase affected (stimulated or inhibited) to biosynthesis of total phenolics and activity of CAT and PPO.

Changes in contents of glycosylated flavonoids in leaves of soybean (Glycine max (L.) Merr. cv. “Nam Dan”) in the flowering stage under infestation of cowpea aphid (Aphis craccivora Koch) was investigated. A strong decrease in the level of glycosylated flavonoids, e.g., genistein 7-O-glucoside, 2’OH genistein 7-O-glucoside, 2’OH genistein malonyl glucoside, after 48-96 hours post-infestation (hpi) of aphid. Levels of these compounds in aphid-infested leaves were remarkably lower than those in controls. Whereas, genistein and total flavonoids in the aphid-infested leaves were induced to higher contents than that observed in control. Activity of important enzymes regarding biosynthesis of flavonoids such as phenylalanine ammonia-lyase (PAL, EC. 4.3.1.5), chalcone synthase (CHS, EC 2.3.1.74) and chalcone isomerase (CHI, EC 5.5.1.6) in the aphid-infested leaves strongly increased during experiemntal time. An increase in acitivity of those enzymes was proportional with the induced decrease of the glycosylated flavonoids and an accumulated increase of genistein and total flavonoids. These results indicate that, changes in the glycosylated flavonoids occured in soybean “Nam Dan” leaves infested by cowpea aphid may connect to a protective mechanism in this soybean cultivar in the flowering stage.