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Since red pine (Pinus densiflora Sieb. et Zucc.) often forms sparse forest floors where herbaceous plants do not grow well, allelopathy of red pine was investigated. A growth inhibitory substance was isolated from an aqueous methanol extract of red pine needles and determined by spectral data as abscisic acid-β-d-glucopyranosyl ester (ABA-GE). This substance inhibited root and shoot growth of cress and E. crus-galli seedlings at concentrations greater than 0.1 μM. The concentrations required for 50% growth inhibition on roots and shoots of cress were 0.23 and 0.61 μM, respectively, and those of E. crus-galli were 1.1 and 2.8 μM, respectively. The activity of ABA-β-d-glucosidase, which liberates free ABA from ABA-GE, in cress and E. crus-galli seedlings was 13–29 nmol mg−1 protein min−1. Endogenous concentration of ABA-GE in the pine needles was 4.1–21.5 μmol kg−1 and the concentration in soil water of the pine forest was 2.5 μM. The effectiveness of ABA-GE on growth inhibition and the occurrence of ABA-GE in pine needles and soil water suggest ABA-GE may play an important role in the allelopathy of red pine resulting in the formation of sparse forest floors.

Aqueousmethanol extracts of Cit rus junos, C. unshiu and C. sudachi fruit peel in hib ited the growth of the roots and hypocotyls of alfalfa (Medicago sativa L.), cress (Lepidiumsativum L.) and lettuce (Lactuca sativa L.) seedlings. Signifi cantreductions in the root and hypocotyl growth were observed as the extract concentration in creased in all bioassays. The in hibitory activity of C. junos ex tract on the growth of test plants was 3.3-to 17.9-fold and 3.6- to 20.6-fold greater than that of C. unshiu and C. sudachi extracts, respectively. Theconcent rationin C. junos was 3.5- and 4.9-fold greater than that in C. unshiu and C. sudachi, re spectively. Thus, there was a good correlation-between abscisic acid-b-D-glucopyranosylester (ABA-GE) concentrationsin C. junos, C. unshiu and C. sudachi fruit peel and theinhibito ryactivities of the irextracts. The concentration of ABA-GE in C. junos fruit peel was in creased with fruit maturityasgrowthinhibit oryactivity of C. junos fruit peel was reported to be in creased with fruit maturity, in dicating that the concentrations of ABA-GEin C. junos fruit peel was correlated withgrowthinhibit oryactivity of C. junos fruit peel in time course of fruit maturation. These findings suggest that ABA-GE may be involved in the growth inhibitory effect of C. junos, C. unshiu and C. sudachi fruit peel.

The allelopathic potential of an aqueous methanol extract of Citrus junos leaves was investigated under laboratory conditions. The extract inhibited the growth of roots and shoots of cress (Lepidium sativum L.), lettuce (Lactuca sativa L.) and alfalfa (Medicago sativa L.) seedlings. Significant reductions in the root and shoot growth were observed as the extract concentration increased. Since abscisic acid -β-D-glucopyranosyl ester (ABA-GE) was found to be a major inhibitor in fruit peel of C. junos, ABA-GE in C. junos leaves was determined and the concentration was 540 μg kg-1 fresh weight in the leaves harvested on September 26, 2002. These results suggest that C. junos leaves possess allelopathic activity and could work as a weed inhibiting agent, which should be investigated further in the field for practical application.

The allelopathic potential of Citrus junos fruit waste after juice extraction was investigated. Aqueous methanol extracts of peel, inside and seeds separated from the fruit waste inhibited the growth of the roots and shoots of alfalfa (Medicago sativa L.), cress (Lepidium sativum L.), crabgrass (Digitaria sanguinalis L.), lettuce (Lactuca sativa L.), timothy (Pheleum pratense L.), and ryegrass (Lolium multiflorum Lam.). The inhibitory activity of the peel extract was greatest and followed by that of the inside and seed extracts in all bioassays. Significant reductions in the root and shoot growth were observed as the extract concentration was increased. The concentrations of abscisic acid-β- d -glucopyranosyl ester (ABA-GE) in peel, inside and seeds separated from the C. junos fruit waste were determined, since ABA-GE was found to be one of the main growth inhibitors in C. junos fruit. The concentration was greatest in the peel, followed by the inside and seeds; there was a good correspondence between these concentrations and the inhibitory activities of the extracts. This suggests that ABA-GE may also be involved in the growth inhibitory effect of C. junos waste. These results suggested that C. junos waste may possess allelopathic potential, and the waste may be potentially useful for weed management.

Exogenously applied ABA-β-d-glucopyranosyl ester (ABA-GE) inhibited shoot growth of alfalfa (Medicago sativa L.), cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), Digitaria sanguinalis L., timothy (Pheleum pratense L.) and ryegrass (Lolium multiflorum Lam.) seedlings at concentrations greater than 0.1 μM. The growth inhibitory activity of ABA-GE on these shoots was 26–40% of that of (+)-ABA. ABA-β-d-glucosidase activities in these seedlings were 11–31 nmol mg−1 protein min−1. These results suggests that exogenously applied ABA-GE may be absorbed by plant roots and hydrolyzed by ABA-β-d-glucosidase, and liberated free ABA may induce the growth inhibition in these plants. Thus, although ABA-GE had been thought to be physiologically inactive ABA conjugate, ABA-GE may have important physiological functions rather than an inactive conjugated ABA form.

The inhibitory effect of Citrus junos peel on plant growth using lettuce (Lactuca sativa L.) as a bioassay material was investigated, since the powder of the peel had been found to inhibit growth of weeds. Basic, neutral and acidic fractions were separated from the aqueous fraction obtained from the methanol extract of C. junos peel. All fractions inhibited the growth of lettuce seedlings, but by far the greatest inhibition was observed with the neutral fraction. Thus, the latter was further purified and an allelopathically active substance was isolated. The structure of the substance was determined from high-resolution MS and 1H and 13C NMR spectral data as abscisic acid-beta-D-glucopyranosyl ester (ABA-GE). ABA-GE inhibited hypocotyl and root growth of lettuce seedlings at concentrations greater than 0.3 microM, and the concentrations for 50% inhibition of hypocotyl and root growth were 2.3 and 1.4 microM, respectively. The effectiveness of ABA-GE on inhibition of growth and the occurrence of ABA-GE in the peel itself suggested that ABA-GE may play an important role in the allelopathic potential of C. junos peel. The peel may be potentially useful for weed management in a field setting.