Analysis of phenolic acids and terpenoids in rhizosphere soils of different allelopathic rice varieties under dry field conditions

Although phenolic acids and secondary terpenoid metabolites respectively from shikimate pathway and isoprene metabolic pathway are the main plant allelochemicals, it is far from conclusion in academic circles on what allelochemicals cause rice allelopathy. Thus far, most studies on rice root exudates have been conducted in laboratory conditions. Furthermore, few reports have been made on the changes in allelopathic substances in the rhizoshpere soil of different allelopathic potential rice cultivars especially under stressful field conditions. This paper studied the differences in allelopathic compounds including phenolic acids and terpenoids extracted from the rhizosphere soils of strong allelopathic rice cultivar 'PI312777' and its counterpart 'Lemont' and those from the control soil without any plants under dry and wet soil conditions in the seedling nursery. Putative allelochemicals of rhizosphere soil extracts were then identified via GC-MS (Gas Chromatograph-Mass Spectrometer-computer) and HPLC (High-performance liquid chromatography). The results showed that the compositions of phenolic acids and terpenoid compounds were similar but the contents of them were different in different treated rhizosphere soils and controls. Under moderate drought stress, the total content of five phenolic acids such as caffeic acid, 4-hydroxybenzoic acid, vanillic acid, ferulic acid and cinnamic acid, increased in all treated rhizosphere soils, and allelopathic rice 'PI312777' showed the highest increases in the total content of the five phenolic acids, which was 2.84 times higher than that of control soil under wet treatment. In addition, among 27 detected and identified terpenoids, 17 were oxygenic monoterpenoid compounds in the extracts of rhizosphere soil samples. Under drought stress, the contents of monoterpenes, oxygenated monoterpenes, oxygenated sesquiterpenes and total terpene showed the changing patterns with different extents and different trends in different treated rhizosphere soils. The relative content of monoterpene was significantly increased in rhizosphere soils under rice 'PI312777', but decreased in rhizosphere soils under the counterpart 'Lemont'. Based on the results, the cause and its mechanism of the weed-suppression in the two rice accessions under the field condition were further discussed.