Momilactones A and B, the major phytotoxins and phytoalexins in rice plants, were quantified by a HPLC-APCI-MS-MS (APCI-MS-MS) system under multiple reaction monitoring conditions. Since MA and MB were found to be easily extracted with water, these phytotoxic compounds may affect germination and growth of other plant species when the rice straws were left in the fields.

Oryzalexine A, a potent growth inhibitor against several weeds such as Digitaria sanguinalis (L) Scop., and Amaranthus lividus L. was purified by conventional solvent partitioning and column chromatographies. This substance showed strong inhibitory activity on several weeds: Germination of seeds of Poa annua L. was inhibited by 36.5 % at 1.0 mM and Amaranthus lividus L. by 56.1% at 2.5mM. Growth of root and shoot of Digitaria sanguinalis (L.) Scop. by 10.2% and 22.4% at 2.5mM, respectively. These suggest that Oryzalexine A in rice straw might affect the germination and growth of susceptible weeds and other plants.