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The mechanism of growth amelioration of NaCl-stressed rice (Oryza sativa L. cv. Nipponbare) by d-aminolevulinic acid (ALA) was investigated. Rice growth recovered when plants were pretreated with 0.1 or 1 mM ALA before treatment with 50 mM NaCl. There was no increase in chlorophyll content by ALA treatment, indicating that the growth recovery was not due to increased chlorophyll content although ALA is a precursor of chlorophyll biosynthesis. The activities of antioxidative enzymes, including catalase (CAT), glutathione reductase (GR), ascorbate peroxidase (APx), and superoxide dismutase (SOD), increased with ALA treatment. In particular, stimulation of CAT, GR, and APx activities by ALA was significant. The hydrogen peroxide (H 2 O 2 ) content decreased after ALA treatment under NaCl-stress conditions. Peroxidation of membrane lipids, as measured by ethane evolution, also decreased after ALA treatment. These results suggest that ALA induces growth amelioration in NaCl-stressed rice by stimulating antioxidative enzyme activity, which resulted in decreased reactive oxygen generation and lipid peroxidation.

The extracts of wood, bark, and leaves of Haldina cordifolia were evaluated for their phytotoxicity on seed germination, seedling growth, and root cell viability in two weeds (Mimosa pigra and Cenchrus echinatus) and two crop plants (Vigna radiata and Oryza sativa cv. Khao Dawk Mali 105). Seeds were grown in petri dishes and treated with 5 ml of extracts at various concentrations: 0.5, 1.0, 5.0, and 10.0 mg/ml. The inhibitory effect on seed germination increased with increasing concentration of the extract treatment. Bark extract was the most toxic at the highest concentration, causing total inhibition of germination in all tested seeds except in V. radiata. Low concentrations (0.5 and 1.0 mg/ml) of wood extract inhibited shoot and root growth in C. echinatus by 31.0%-56.0% and 67.0%-71.0%, respectively. Interestingly, it promoted root growth in M. pigra by 106.9%-108.8% (at low concentrations) and in V. radiata (at all concentrations) by 108.1%-108.9% (shoot) and 108.8%-120.1% (root). Bark extract inhibited seedling growth in all tested plants at different levels. Strong inhibition was found in roots of O. sativa (3.0%-4.0%). The result from Evans blue uptake study suggested that the H. cordifolia extract did not directly affect the root cell viability. Surprisingly, we found that M. pigra and V. radiata treated with the extracts at low concentrations had increasing number of lateral roots, suggesting that H. cordifolia extract could act as a plant growth regulator (PGR) and an herbicide at the same time, depending on concentration and target plant.

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