The phytotoxic potential of plants and their constituents against other plants is being increasingly investigated as a possible alternative to synthetic herbicides to control weeds in crop fields. In this study, we explored the phytotoxicity and phytotoxic substances of Schumannianthus dichotomus, a perennial wetland shrub native to Bangladesh, India, and Myanmar. Leaf extracts of S. dichotomus exerted strong phytotoxicity against two dicot species, alfalfa and cress, and two monocot species, barnyard grass and Italian ryegrass. A bioassay-driven purification process yielded two phenolic derivatives, syringic acid and methyl syringate. Both constituents significantly inhibited the growth of cress and Italian ryegrass in a concentration-dependent manner. The concentrations required for 50% growth inhibition (I50 value) of the shoot and root growth of cress were 75.8 and 61.3 μM, respectively, for syringic acid, compared with 43.2 and 31.5 μM, respectively, for methyl syringate. Similarly, to suppress the shoot and root growth of Italian rye grass, a greater amount of syringic acid (I50 = 213.7 and 175.9 μM) was needed than methyl syringate (I50 = 140.4 to 130.8 μM). Methyl syringate showed higher phytotoxic potential than syringic acid, and cress showed higher sensitivity to both substances. This study is the first to report on the phytotoxic potential of S. dichotomus and to identify phytotoxic substances from this plant material.

Wedelia chinensis (Asteraceae) is a wetland herb native to India, China, and Japan. It is a valuable medicinal plant recorded to have pharmaceutical properties. However, the phytotoxic potential of Wedelia chinensis has not yet been examined. Thus, we carried out this study to establish the allelopathic effects of Wedelia chinensis and to identify its phytotoxic substances. Extracts of Wedelia chinensis exhibited high inhibitory activity against the root and shoot growth of cress, alfalfa, rapeseed, lettuce, foxtail fescue, Italian ryegrass, timothy, and barnyard grass. The inhibition was varied with species and was dependent on concentrations. The extracts were separated through several purification steps, and the two effective substances were isolated and characterized as vanillic acid and gallic acid using spectral analysis. Vanillic acid and gallic acid significantly arrested the growth of cress and Italian ryegrass seedlings. The concentrations of vanillic acid and gallic acid needed for 50% inhibition (I50 values) of the seedling growth of the cress and Italian ryegrass were 0.04–15.4 and 0.45–6.6 mM, respectively. The findings suggest that vanillic acid and gallic acid may be required for the growth inhibitory activities of Wedelia chinensis.

The research work was conducted at the Agronomy Field Laboratory, Bangladesh Agricultural University, Mymensingh during aman season from July to December, 2015 to study the effect of nitrogen fertilizer and weed management on the growth and yield of transplant aman rice cv. BRRI dhan46. The experiment consisted of four fertilizer treatments viz. 0 kg N ha −1 (N 0 ), 40 kg N ha −1 (N 1 ), 80 kg N ha −1 (N 2 ) and 120 kg N ha −1 (N 3 ) and four weeding treatments viz. one hand weeding at 20 DAT (W 1 ), two hand weedings at 20 and 35 DAT (W 2 ), three hand weedings at 20, 35 and 50 DAT (W 3 ), weeding by Japanese rice weeder twice at 20 and 35 DAT (W 4 ) and unweeded control (W 5 ). The experiment was laid out in randomized complete block design with three replications. The highest plant height (113.00 cm), number of total tillers hill −1 (8.74), number of effective tillers hill −1 (6.18), panicle length (21.98 cm), number of grains panicle −1 (114.20), grain yield (4.00 t ha −1 ), straw yield (5.25 t ha −1 ) and biological yield (9.25 t ha −1 ) were recorded in N 2 (80 kg N ha −1 ) treatment. The lowest plant height (106.00 cm), number of total tillers hill -1 (7.20), number of effective tillers hill −1 (5.00), panicle length (20.70 cm), number of grains panicle -1 (97.60), grain yield (3.52 t ha −1 ), straw yield (4.46 t ha −1 ) and biological yield (7.97 t ha −1 ) were recorded from N 0 (No nitrogen fertilizer control) treatment. On the other hand, the highest grain yield 4.23 t ha −1 was observed in three weedings condition because of the highest number of effective tillers hill −1 (6.81), number of grains panicle −1 (111.10). The highest straw yield (5.51 t ha −1 ) was also found in three weeding condition. The lowest grain yield (3.40 t ha −1 ) was recorded in W 5 (unweeded control) treatment. The lowest straw yield (4.31 t ha −1 ) was also observed in W 5 (unweeded control) treatment because of the smallest plant (106.97 cm) and lower number of total tillers hill −1 (7.20). Therefore, 80 kg N ha −1 along with three hand weedings at 20, 35 and 50 DAT may be used for obtaining the highest grain and straw yields of BRRI dhan46.