The methanol extracts of 168 plant species from 68 families were evaluated for their inhibitory activity against lettuce seedling elongation. Among the plant species tested, 12 species had EC50 values for radicle growth inhibition ranging from 0.01 to 5.00 mg fresh weight equivalent mL-1. Enterolobium contortisiliquum, a traditionally used herbal medicine, exhibited the strongest inhibitory activity (estimated EC50: 0.28 fresh weight equivalent mL-1). Among the 12 species, Pachysandra terminalis, Tamarindus indica, and Albizia guachapele required investigation, because only little has been reported about their chemical constituents to date. The data in the present study would be useful in finding new lead compounds for natural herbicides.
Weed control is a major problem in agriculture. Uncontrolled weeds cause enormous reductions in crop yields [1,2]. Synthetic herbicides play an important role in weed suppression [
In the present study, we evaluated the plant growth-inhibitory activity of the methanol extracts of 168 plant species to estimate their potential as sources of bioactive compounds. While allelopathy was the main purpose of the screening studies reported by our laboratory [11-13], we intended to identify potential species for the isolation and characterization of plant growth-inhibitory compounds in the present work; therefore, plant extracts were used for bioassays instead of living plants or dried leaves [14, 15]. To investigate the active compounds in the potential species, the availability of plant material was considered the top priority when the species were selected. Therefore, we collected the leaves of weeds and ornamental plants that are common in the Kanto region of Japan together with those of herbaceous plants and trees grown in a botanical garden. The results allowed us to estimate the inhibitory potential of various species, leading to the application of plant growth-inhibitory compounds in weed management for sustainable agriculture.
The aerial parts of common weeds and ornamental plants were collected in Matsudo and Tsukuba, Japan. Those of tropical and subtropical plants were sampled in greenhouses of the National Institute for Agro-Environmental Sciences, Tsukuba, Japan and Tsukuba Botanical Garden, National Museum of Nature and Science, Tsukuba, Japan. The plant materials were immediately soaked in methanol and left at room temperature for 2 weeks. The methanol extracts were used for bioassays after filtration.
BioassayFilter paper (27 mm ø, Toyo Roshi Kaisha, Ltd, Tokyo) was placed in a glass Petri dish (27 mm ø). Test solution was added to the filter paper in the Petri dish and dried completely in vacuo. After adding distilled water (0.7 ml), five pregerminated (20 h at 20˚C in the dark) seedlings of lettuce (Lactuca sativa cv. Great Lakes 366) were placed on the filter paper and incubated for 52 h at 20˚C in the dark. The final concentration ranged from 0.43 to 1429 mg fresh weight equivalent mL−1 (F.W. eq. mL−1). Inhibition of radicle elongation was determined by comparing the radicle lengths of treated plants with that of control plants. The effective concentrations required to induce half-maximal inhibition of growth (EC50) and 95% confidence intervals were calculated by the probit method using SPSS for Windows statistical software (ver. 11.0.1J, IBM, Armonk, NY, USA).
The estimated EC50 values of 168 plant species against elongation of the radicles of lettuce seedlings are shown in
In this section, we describe the physiological and chemical information of the 12 species with EC50 values of elongation inhibition against lettuce radicle of 0.01 - 5.00 mg F.W. eq. mL−1. Enterolobium contortisiliquum (Mimosaceae) displayed the strongest inhibitory activity (estimated EC50: 0.28 F.W. eq. mL−1) among the 168 species tested in the present study. It is a herbal medicine traditionally used to treat parasitism and gonorrhea in Brazil [
Spiraea thunbergii (Rosaceae) displayed the second strongest inhibitory activity (estimated EC50: 0.32 F.W. eq. mL−1). We selected this species as a positive control because it is already established that S. thunbergii exhibits strong plant growth-inhibitory activity [
Convallaria keiskei (Liliaceae) had an estimated EC50 value of 0.87 F.W. eq. mL−1. Convallaria species produce white flowers with a mild fragrance and contain cardiac glycosides; therefore, many studies have been performed to evaluate the horticultural, medical, chemical, and physiological properties of this genus [20-22]. However, the plant growth-inhibitory activity of this species has not been reported thus far.
Gliricidia sepium (Fabaceae; estimated EC50: 1.78 F.W. eq. mL−1) is distributed throughout seasonal dry forests in Mexico and other Central American countries [
Pachysandra terminalis (Buxaceae; estimated EC50: 1.92 F.W. eq. mL−1) is a small shrub (approximately 30 cm in height) distributed in Japan. It is used as a decorative plant, and the dried plant has been used as folk medicine for stomachache in the northern part of Japan [
Samanea saman (Mimosaceae; estimated EC50: 2.20 F.W. eq. mL−1) is native to tropical America. It has now become widespread throughout the humid and sub-humid tropics. Commonly referred to as “monkey pod” or “cow tamarind,” this plant is traditionally used to treat diarrhea, intestinal diseases, stomach aches, colds, and headaches [
Tamarindus indica (Caesalpiniaceae) exhibited strong
inhibitory activity against lettuce radicle elongation (estimated EC50: 2.51 F.W. eq. mL−1). A detailed study was performed on the allelopathic potential of this species; the leaves of T. indica reportedly suppressed the growth of several weed and edible crop species [
Albizia guachapele (Fabaceae; estimated EC50: 3.08 F.W. eq. mL−1) is one of the species selected as potential trees for reforestation in the study of Wishnie et al. [
Indigofera kirilowii (Fabaceae; estimated EC50: 3.08 F.W. eq. mL−1) contains toxic glucose conjugates of 3-nitropropanoic acid in its roots [
The estimated EC50 value of Caesalpinia pluviosa var. peltophoroides (Caesalpiniaceae) was 3.76 F.W. eq. mL−1. According to the literature, several Caesalpinia species have been studied phytochemically and pharmacologically. Cassane-type diterpenes [37-39], flavonoids [40, 41], biflavonoids [
Acacia podalyriifolia (Fabaceae; estimated EC50: 4.75 F.W. eq. mL−1) is native to Australia and is cultivated in southern Brazil as an exotic ornamental tree. Several species in the genus Acacia have important medicinal applications [44,45]. They possess phenolic compounds, for which countless biological activities are described [45,46].
Kalmia latifolia (estimated EC50: 4.79 F.W. eq. mL−1), a species of the Ericaceae family, is an evergreen understory shrub found in abundance on xeric southwest slopes in the southern Appalachian Mountains. It typically forms a dense understory in mixed pine/hardwood stands and occasionally in association with Rhododendron maximum [47,48]. Another species in this genus, K. angustifolia, contains toxic phenolic compounds [
The results presented in this study provide essential information for further research on potential natural herbicides. For example, a series of studies have been conducted on cis-cinnamoyl derivatives isolated from S. thunbergii and S. prunifolia [50,51]. The chemical structure that is essential for the inhibitory activity is the cis-cinnamoyl moiety [
This research was supported by the Programme for Promotion of Basic and Applied Researches for Innovations in Bio-oriented Industry (BRAIN).