Differential allelopathic expression of bark and seed of Tamarindus indica L.

Allelopathic performance of the bark and seed of Tamarindus indica L. tree was evaluated through bioassay-guided studies using seven common agronomic crops ( asparagus, cucumber, lettuce, radish, sesame, tomato and welsh onion) and seven weed species ( barnyard grass, Chinese milk vetch, perennial ryegrass, phacelia, timothy grass, white clover and wild ginger) under laboratory conditions. As demonstrated by a sandwich method, the bark of the tamarind tree caused strong growth inhibition ( compared to the corresponding controls) in both radicles and hypocotyls of the species tested, and the inhibitory effect was highest in barnyard grass (52 - 65%) and lowest in welsh onion (19 - 13%). The crude-water soluble extracts of bark at different concentrations (1, 5 and 10%) (w/v) exhibited a strong growth inhibition in all the plant species tested, and a proportional increase in the percentage of growth inhibition was observed with an increase in the concentrations of the extracts. The magnitude of inhibition in weed species was higher (5 - 60%) than those of agronomic crop species (3 - 40%). The growth of all the weed species tested was strongly inhibited (17 - 56%), while the agronomic crop species showed both inhibited (5 - 21%) and stimulated (5 - 27%) growth due to the effect of crude-water soluble exudates of tamarind seed. Among the agronomic crop species tested, lettuce (22 - 27%) followed by radish (20 - 25%) and sesame (5 - 8%) showed stimulatory growth with the crude-water soluble exudates of seed. In the pot culture experiments using four agronomic crops ( lettuce, radish, tomato and cucumber) and two weed species ( barnyard grass and white clover), spraying of crude-water soluble extracts of tamarind seed-coat at three different concentrations (1, 5 and 10%) (w/v) showed that the growth of lettuce ( 35 - 62%) and radish ( 32 - 56%) was stimulated, while all other species tested showed growth inhibition (29 - 61%). When the spraying of crude extracts of seed-coat was turned off, the growth of both lettuce and radish continued to be stimulated (4 - 7%) and all other previously inhibited species recovered well, the recovery percentage ranging between 78 and 82%. However, when spraying of crude extracts of seed-coat was continued, growth increased (10 - 14%) in lettuce and radish, and reduced (37 - 76%) in four other species tested. The inhibitory or stimulatory effects of the crude extracts on agronomic crop and weed species were higher in the radicle than the hypocotyl and reached a peak with 10% ( w/v) concentrations. These results clearly demonstrated the differential allelopathic effects ( inhibitory and excitatory) of bark and seed of tamarind tree in the species tested. Thus, it is evident that these two organs contain certain biologically active true growth regulator(s) and are either additively or synergistically involved in the plant-specific expression, particularly by the seed-coat.