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Piperine is the bioactive constituent of black pepper (Piper nigrum). For quality control of piperine content, expensive instruments are usually used. A simple and inexpensive TLC image-analysis method for quantification of the piperine content of the traditional medicinal preparations of Bhutan has been established in this study. An image of the TLC chromatogram, under UV light at 254 nm, was taken by use of a digital camera and was further transformed to a density profile plot, along the direction of development, by use of Scion Image software. The concentration of piperine was calculated by comparing its peak area with a calibration plot established by chromatography of piperine standards on the same TLC plate. Linear regression analysis of the calibration data revealed a good linear relationship (R 2 = 0.9962) between response and amount of piperine in the range 0.34-1.03 μg per spot. The specificity, precision, accuracy, and recovery of the method are satisfactory. The analytical results obtained by use of the method were not significantly different from those obtained by use of densitometric TLC. A disadvantage of the method is its low sensitivity -the limits of detection (LOD) and quantification (LOQ) were 15.36 and 46.54 ng per spot, respectively.

The study was designed to evaluate the inhibitory effects of selected herbs on plant growth. Three insecticides containing plants (leaf of Azadirachta indica A. Juss var indica (AI), Nicotiana tabacum L. (NT), and root of Derris elliptica (Roxb.) (DE) were selected for various solvent extractions. Five extraction solvents: methanol, hexane, dichloromethane, butanol and aqueous were used in this study. The test method was lemna phytotoxicity assay which measured the inhibitory effect on duckweed, Lemna minor growth. This toxicity test proved to be a practical bioassay method because this method is simple, sensitive and cost effective. Butanol extracts of AI were shown to have more inhibition activity on duckweed growth than NT (% inhibition growth rate is 66.08, 27.08 respectively). Butanol extracts of AI had inhibitory effect (EC10) of 40.87 μg/ ml while dichloromethane extracts had EC10 of 131.72 μg/ ml. It could therefore be concluded that butanol extracts of AI showed the greatest inhibitory effects. The results from this study suggest that butanol extracts of AI were the most promising candidates for biological weed control and might be used as potential natural herbicides or as alternatives for the reduction of chemical herbicides. The study is worthy of further investigation since this could provide potential bioherbicide and may lead to the discovery of new effective and applicable bioherbicide.

The phytotoxic effects of leaves and seeds from Annona squamosa extracts on lettuce (Lactuca sativa L.) seeds and Mimosa pigra were evaluated. A. squamosa leaf dichloromethane extracts reduced germination and seedling growth of lettuce seeds. In field study, A. squamosa leaf dichloromethane extracts was applied on M. pigra after growing. Fresh weight of M. pigra was decreased with increasing concentrations of leaf dichloromethane extracts. These results suggest that dichloromethane extracts from leaves of A. squamosa may have growth inhibitory substances. Thus, our study indicated that leaf dichloromethane extracts have phytotoxic activity which could be utilized as bioherbicide for future weed control.