2. Allelopathy
Allelopathy is chemical interaction among plants and caused by allelochemicals
[27], which are produced in plants and released into the vicinity of the plants including rhizosphere soil either by root exudation, decomposition of plant litter and residues, and rainfall leachates and volatilization from the plant parts
[28][29][30].
2.1. Plant Extract
Allelopathic activity of the extracts of leaves, seeds, bark, and aerial parts of
L. leucocephala on crops and weeds were determined since allelochemicals are synthesized and accumulated in certain plant parts
[27][28][29][30]. Aqueous extracts of the leaves of
L. leucocephala suppressed the radicle growth of
Lactuca sativa L. and
Oryza sativa L. seedlings
[31], and the seedling growth of
Ischaemum rugosum Saisb and
Vigna radiata (L.) R. Wilczek
[32]. Its aqueous leaf and seed extracts showed the inhibitory activity on the germination and seedling growth of three weed species,
Ageratum conyzoides L.
Tridax procumbens L., and
Emilia sonchifolia (L.) DC. Ex Wight
[33]. Aqueous leaf, seed, and bark extracts of
L. leucocephala inhibited the germination, growth, and crop yield of
Zea mays L. under pot culture conditions
[34].
2.2. Leachate
For the simulation of rainfall conditions, plant tissues were soaked in water, and its supernatant was used as leaches from the tissues by rainfall
[35][36][37][38][39]. The senescent leaves of
L. leucocephala was soaked in water for 48 h, and its supernatant showed inhibitory activity on the germination and growth of
Raphanus sativus L.
[37]. The soaking water of
L. leucocephala leaves also enhanced electrolyte leakage from the leaf cells of
Eichhornia crassipes (Martius) Solms. and increased the activities of catalase and ascorbate peroxidase in the leaves
[38].
2.3. Plant Litter and Residue
Leaf litter of
L. leucocephala was mixed with soil, and the seeds of a woody plant,
Albiza procera (Roxb.) Benth., and crop plants,
Vigna unguiculata (L.) Walp.,
Cicer arietinum L. and
Cajanus cajan (L.) Millsp. were sown into the mixture. The treatments resulted in the suppression of the germination and growth of these test plant species
[39]. Soil mixture with decomposing leaves of
L. leucocephala increased the mortality of five tree species,
Alnus formosana (Burkill) Makino,
Acacia confusa Marr.,
Liquidambar formosana Hance,
Casuarina glauca Sieber, and
Mimosa pudica L.
[31].
Aqueous extracts of
L. leucocephala litter, which accumulated on the forest floors, showed the suppression of the germination and radicle growth of
Lolium multiflorum Lam.
[31], and
Ageratum conyzoides L.
Tridax procumbens L., and
Emilia sonchifolia (L.) DC. ex Wight
[40]. Leaf mulch of
L. leucocephala covered on soil surface or mixed with soil inhibited the germination and growth of
Vigna unguiculata (L.) Walp., and the root nodulation of
V. unguiculate [41]. Those observations indicate that leaf litter and residues of
L. leucocephala may contain certain allelochemicals, and some of them may be liberated into the soil during their decomposition processes.
2.4. Rhizosphere Soil and Root Exudate
The seeds of
Ageratum conyzoides L.,
Tridax procumbens L., and
Emilia sonchifolia (L.) DC. ex Wight were sown into the soil collected from
L. leucocephala infested areas. The treatments resulted in the suppression of the germination and growth of those plant species
[40]. Rhizosphere soil of
L. leucocephala also inhibited the germination and growth of
Vigna radiata (L.) R.Wilczek and
Glycine max L.
[42]. Aqueous extracts of the soil of the forest floors under
L. leucocephala trees showed the inhibition of the radicle growth of
Lactuca sativa L.
[31]. In addition, root exudates from
L. leucocephala showed the suppression of the germination and growth of
Ageratum conyzoides L.,
Tridax procumbens L., and
Emilia sonchifolia (L.) DC. ex Wight
[40]. Those observations suggest that rhizosphere soil of
L. leucocephala may contain certain allelochemicals, which may be supplied through root exudation, decomposition of plant litter and residues, and rainfall leachates.
3. Allelochemical
Phenolic acids, flavonoids, and mimosine were isolated and identified from
L. leucocephala as its allelopathic agents (
Figure 2).
Figure 23. Allelochemicals identified in L. leucocephala. (1) p-hydroxybenzoic acid, (2) protocatechuic acid, (3) vanillic acid, (4) gallic acid, (5) p-hydroxyphenylacetic acid, (6) p-hydroxycinnamic acid, (7) caffeic acid, (8) ferulic acid, (9) epicatechin, (10) epigallocatechin, (11) gallocatechin, (12) quercetin, (13) mimosine.
3.1. Phenolic Acid
Phenolic acids such as p-hydroxybenzoic acid (1), protocatechuic acid (2), vanillic acid (3), gallic acid (4), p-hydroxyphenylacetic acid (5), p-hydroxycinnamic acid (6), caffeic acid (7), and ferulic acid (8) were identified in the leaves of L. leucocephala. Total concentrations of those phenolic acids in young leaves were 2-fold greater than those in mature leaves [31]. The concentration of total phenolic compounds in L. leucocephala plants was estimated to be 1.3 to 2.8 mg g−1 of dry weight of the plants [43]. Phenolic acids have been found in a wide range of plants, plant residues, and soils, and their involvement has been often mentioned in the allelopathy of those plant species [44][45].
3.2. Flavonoid
Flavonoids such as epicatechin (
9), epigallocatechin (
10), and gallocatechin (
11) were identified in
L. leucocephala roots. Those compounds inhibited the nitrification process, which is an important step in the nitrogen cycle in soil
[46]. Quercetin (
12) and other 16 flavonoids were identified in
L. leucocephala leaves, and some of them showed antioxidant activity
[47].
L. leucocephala was reported to contain condensed tannins, which may contribute towards the plant resistance to pathogens and insects.
[48][49][50].
3.3. Mimosine
Mimosine (
13); L-mimosine, synonym; leucenol) is a non-protein amino acid. It was first isolated from
Mimosa pudica L
[51], and found in some other species of the genus,
Mimosa and
Leucaena, including
L. leucocephala [52][53][54]. Mimosine possesses a wide range of pharmacological and biological properties, such as anti-tumor, apoptotic, anti-inflammation, anti-viral, and cell cycle blocking effects
[55]. It also possesses the inhibitory activity on the germination and growth of several plant species
[31][56][57]. Therefore, mimosine is possibly involved in the allelopathy of
L. leucocephala.
4. Conclusions
Although the economic value of
L. leucocephala is widely recognized, the species is listed in the world’s 100 worst invasive alien species. It is an aggressive colonizer and forms dense monospecific stands. It interferes the regeneration and replacement of native plant species in its dominant forests. The species richness in
L. leucocephala invaded forests was lower than that in its uninvaded forests, and seedling establishment of native plant species under
L. leucocephala invaded areas was also low. Sunlight intensity and other conditions of the forest floors between
L. leucocephala forests and native forests were not apparent. In addition, plant extracts, leachates, root exudates, plant litter and residues, and rhizosphere soil of
L. leucocephala showed the enhancement of the mortality and suppression of the germination and growth of several plant species including weeds and woody plants. Those observations suggest that
L. leucocephala is allelopathic and contains allelochemicals which affect the plant mortality, germination, and growth, and some of the allelochemicals may be released into the vicinity of
L. leucocephala, including its rhizosphere soil.
L. leucocephala produces a large amount of mimosine and accumulates it in almost all parts of the plants. Mimosine showed growth inhibitory activity against several plant species including some shrubs and another invasive plant species. Mimosine blocked cell division of protoplasts from
Petunia hybrida between G
1 and S phases, and disturbed some enzyme activities such as peroxidase, catalase, and IAA oxidase. In addition, several phenolic acids and flavonoids were identified in
L. leucocephala. However, the concentrations of mimosine, phenolic acids, and flavonoids in the rhizosphere soil and vicinity of
L. leucocephala have not yet been reported. The information is essential to evaluate the contribution of mimosine, phenolic acids, and flavonoids to the allelopathy of
L. leucocephala.