Allelopathic potential of Wedelia trilobata L.: effects on germination, growth and physiological parameters of rice
1 Institute of Crop Genetics and Breeding, Foshan University, Foshan, 528231 China; www.fosu.edu.cn Email niecr@scau.edu.cn
2 Institute of Tropical and Subtropical Ecology, South China Agricultural University, Guangzhou, 510642, China; www.scau.edu.cn Email smluo@scau.edu.cn
3 Food Science College of South China Agricultural University, Guangzhou, 510642, China; www.scau.edu.cn Email mindymo@scau.edu.cn
The allelopathic potential of aqueous extracts of Wedelia trilobata L. on rice (Oryza sativa L.) were studied. The aqueous extracts of root, stem, leaf and of the whole plant of W. trilobata reduced the activities of some hydrolytic enzymes and some protective enzymes in geminating rice seeds. The geminating seeds treated with W. trilobata had higher membrane permeability, lower respiratory rate and vitality. Aqueous extracts of W. trilobata reduced rice plant height, tillers, root activity, fresh or dry root weight, leaf chlorophyll content, net photosynthetic rate and fresh or dry weight per plant. Extracts also significantly reduced the activities of nitrate reductase and glutamine synthetase, and the total nitrogen content in the leaves. As a comprehensive effect, watering rice seedlings with aqueous extracts of W. trilobata significantly reduced yield and yield components of rice.
An important weed in South China, Wedelia trilobata L.was found to have a negative allelopathic effect on rice.
Key words
Rice, Allelopathy, Wedelia trilobata L.
Introduction
Wedelia trilobata L. is perennial herbage with crawal plant. It distributes in the tropical regions, including China, Vietnam, Kampuchea, Burma and Philippine. It often grows luxuriantly in valleys, raceways, field edge, roadway and humid lands. It is an important weed and garden herb in South China (Li 1998). It is treaded into paddy field as an organic fertilizer or taken as green manure of dry land crops by local peasants in Guangdong, Fujian, Taiwan and Liaonin province (Nie et al. 2002). We were astonished to find that the crops would have severe lack of seedlings, poor growth and low yield after the using of W.trilobata as green manure (Nie et al. 2002;2004; Luo et al. 1994; Zeng et al. 1994). In order to learn the influence of W.trilobata on rice(Oryza sativa L.), the effects of W.trilobata aqueous extracts on germination, growth, and physiological parameters of rice were studied.
Methods
W. trilobata was collected from the field at Foshan University. When fresh plants of W. trilobata were cut down, the yellow leaves and dead sticks were discarded, and the plant washed and cut it into pieces of approximately 2 cm. Water at a volume 2 times that of the material was poured over. The material was dipped in water for 36 hours, then the solution filtered, and poured into some water so that the aqueous extracts were prepared in the concentration of 0.4 g FW/mL. Separated the root, stem and leaf of W. trilobata and then extracted them with water as the method described above.
The variety of rice used was Qixianzhan, provided from the Rice Institute, Guangdong Agricultural Academy, which was planted with large area in Guangdong province.
Effect on germinating seeds of rice
Put a filter paper in each culture dish whose diameter was 15 cm as the culture bed, add 200 rice seeds of healthy and satiety, and pour into 50 ml aqueous extractions of the root, stem, leaf or the whole plant of W.trilobata L. respectively. Seed were germinated under the room temperature (25~30 C) and change the aqueous every day. Each treatment had three replications. After germination, seed vitality, α-Amylase activity, membrane permeability, respiratory rate and peroxidase activity were all detected according to the methods described in reference (Zhu et al, 1900; Zhou, 2000)).
Effect on seedlings growth and yield components of rice
Five rice seedlings with 3 leaves for each seedling were planted in each pot. Every treatment had 9 pots and all pots were arranged completely random. After planting, aqueous extract of different organs and that of the whole plant of W.trilobata were added. Three pots were used to investigate the dynamic of plant height and tillers, and 50 days after planting measure the dry weight of the plant for each treatment. Another 3 pots were used to detect root activity and some physiological characters in the 3rd upper leaf 50 days after planting for each treatment. The rest 3 pots were left mature and analysis of the yield component. Root activity was detected by the method of TTC (Zhou, 2000), chlorophyll content by the method of Arnon, net photosynthesis rate by the modified half leaf method, and total nitrogen content by Kjeldahl method (Zhou 2000). Activities of nitrate reductase and glutamine synthetase were also detected (Zhou 2000)
Statistics
Analysis of variance (ANOVA) was carried out using SAS8.1, and significant treatment differences were tested at 0.05 level using Duncan’s multiple range test.
Results
Effect on physiological parameters of germinating rice seeds
Aqueous extracts of W. trilobata reduced α-Amylase activities of germinating rice seeds significantly (Table 1). They may inhibit the germination of rice seed by inhibiting the activities of hydrolytic enzymes. W.trilobata extracts significantly reduced the peroxidase activity and significantly increased the membrane permeability of germinating rice seeds. The metabolism intensions of germinating rice seeds depend on the integrality of the membrane. Peroxidase can protect the membrane from peroxidation. Because the aqueous of W.trilobata reduced the peroxidase activity, the germinating seeds cannot get rid of the active oxygen in time so that the membrane was damaged and its permeability increased. Aqueous extracts of W.trilobata also significantly reduced respiratory rate and vitality of the germinating rice seeds.
Table 1 Effect of W. trilobata aqueous extract on physiological parameters of germinating rice seeds
Extracted organs |
α-Amylase activity |
Peroxidase activity |
Membrane permeability |
Respiratory rate |
Seed vitality |
Control |
26.9±2.13a |
3.98±0.53a |
100±6.32a |
7.25±1.68a |
103.0±11.6a |
Root |
17.6±1.56b |
2.15±0.32b |
116.4±12.1b |
3.52±1.46b |
72.7±8.6b |
Plant |
15.3±1.29b |
1.36±0.23c |
125.6±13.4bc |
2.21±0.61b |
68.5±5.7b |
Stem |
13.6±0.97b |
0.95±0.11c |
134.8±11.37c |
1.76±0.85b |
59.7±8.3b |
Leaf |
14.8±1.35b |
1.21±0.26c |
128.3±12.35bc |
2.05±0.93b |
63.1±9.2b |
Notes: Different letters in the same column indicate significant difference at 0.05 level, and so do the following tables.
Effect on growth of rice seedlings
Aqueous extract of W.trilobata reduced the tillers of rice significantly (Figure 1). The plants of the control tillered rapidly, while the plants treated had no tillers for about 20 days after planting. At harvest, plants treated with aqueous extracts of root, stem, leaf and of the whole plant of W.trilobata had 45.45%, 19.09%, 25.45% and 34.55% tillers of the control, respectively.
Figure 2. Dynamic of tillers of rice affected by W. trilobata
W. trilobata extracts also inhibited the growth of the plant. After return green, the plants of the control grew rapidly, while the plants treated with aqueous of W. trilobata grew slowly. During the whole observation, height of the plants treated were significantly lower than that of the control (Figure 2). The plants treated were always dwarf in appearance.
Figure 2. Dynamic of height of rice affected by W. trilobata
Aqueous extracts of different organs and of the whole plant of W. trilobata reduced root activity and dry root weight significantly. Aqueous extract of leaf had the largest effect (Table 2). Results indicated that W. trilobata inhibited the growth of roots and the plant cannot build a strong root system or grow healthy. W. trilobata extracts significantly reduced leaf chlorophyll content, net photosynthesis rate and dry weight per plant. Leaf extract also had the largest effect.They significantly inhibited the activity of nitrate reductase and glutamine synthetase. These results indicated that aqueous extracts of W. trilobata reduced the efficiency of plant using nitrate nitrogen. As a comprehensive indicator of nitrogen metabolism, the total nitrogen contents in the plant treated with W. trilobata was significantly lower than that in control plant.
Table 2 Effects of aqueous extracts of W. trilobata on the growth and physiological activities of rice seedlings
Extracted organs |
Dry root weight(g) |
Root activity |
Dry weight per plant (g) |
Chlorophyll content |
Net photosynthesis rate |
Nitrate reductase |
Glutamine synthetase |
Total nitrogen % |
Control |
15.76±1.31a |
65.2±8.54a |
45.36±8.63a |
2.86±0.32a |
21.15±3.22a |
12.38±1.12a |
0.968±0.09a |
2.56±0.23a |
Root |
5.85±0.83b |
45.3±5.67b |
20.36±6.42b |
2.03±0.25b |
15.32±1.86b |
9.89±1.10b |
0.732±0.08b |
1.96±0.28b |
Plant |
4.36±0.62b |
34.9±3.73c |
13.27±2.92b |
1.86±0.32b |
13.56±1.63b |
9.53±0.89b |
0.675±0.08b |
1.75±0.21b |
Stem |
3.22±0.76c |
29.8±1.55d |
12.86±2.37b |
1.85±0.24b |
13.25±1.53b |
8.32±0.62c |
0.636±0.06b |
1.78±0.23b |
Leaf |
4.18±0.64b |
34.2±2.63c |
8.36±1.13c |
1.72±0.31b |
10.35±0.82c |
7.98±0.91c |
0.615±0.07b |
1.35±0.12c |
Notes: Chlorophyll content: mg/gFW; Net photosynthesis rate: mg/dm2/h; Nitrate reductase: µmolNO2/g/min; Glutamine synthetase: µmolNADH/mg protein/min
Effect on yield components of rice
Aqueous extracts of W. trilobata had no significant effect on the grain weight of rice (Table 3). Result indicated that grain weight of rice was a steady character mainly dominated by genetic factors. But W.trilobata extracts reduced the numbers of effective panicles, spikelet per panicle, filled grains per panicle and panicle weight. Except for the root aqueous of W. trilobata, other extracts had significant negative effect on the seed setting rate.
Table 3 Effects of aqueous extracts of W. trilobata on the yield components of rice
Extracted organs |
No. of effective panicles |
Spikelets per panicle |
Filled grains per panicle |
Seed setting rate (%) |
1000 Grain weight (g) |
Panicle weight (g) |
Control |
11±1.3a |
163.5±20.3a |
148.6±16.4a |
90.9±3.3a |
18.6±0.3a |
2.75±0.23a |
Root |
5±0.8b |
121.8±13.6b |
106.3±8.1b |
87.3±3.4a |
18.6±0.2a |
2.02±0.16b |
Plant |
2.1±0.3c |
113.2±7.2b |
91.8±6.2c |
80.6±2.6b |
18.3±0.3a |
1.68±0.13c |
Stem |
2.8±0.6c |
95.1±6.8c |
76.5±8.6d |
80.4±3.1b |
18.5±0.3a |
1.43±0.14c |
Leaf |
3.8±0.6bc |
106.3±8.9bc |
86.7±8.6cd |
81.6±2.5b |
18.8±0.3a |
1.68±0.14c |
Discussion and Conclusion
Results showed that the allelopathic effects of W. trilobata on rice included several aspects on the physiological level. Reduction in protective enzyme activities made the plants treated cannot effectively protect the membrane from the active oxygen and led to the damage of the membrane, so that the seed vitality decreased or even the seed decayed. The decrease of hydrolytic enzyme activities made the seeds not able to provide enough energy and materials for germination so that the germination rate decreased and strong seedlings cannot be built.
The aqueous extracts of W. trilobata inhibited the growth of the root and its activity. This not only restricted the absorption of water and nutrition by root system, but also had negative effect on the healthy growth of the aerial part. The inhibition of W.trilobata on the photosynthesis causes the lack of material and energy for the normal growth of plant. The plant treated with W. trilobata always had etiolated leaves and nanism plants (Nie et al. 2002).
As to the nitrogen metabolism, W. trilobata reduced rice’s activities of nitrate reductase and glutamine synthetase, the two key enzymes in nitrogen metabolism of plant. These inhibitions may affect the nitrogen metabolism and some carbon metabolism deeply, because the nitrate nitrogen can be reduced into ammonia nitrogen only by the catalysis of nitrate reductase, which has inducibility. Glutamine synthetase is an enzyme with multi-functions in the center of nitrogen metabolism, which participates in the regulations of many sorts of nitrogen metabolism. The assimilation of ammonia in the cell was mainly through the approach of GS—GOGAT (Lea et al. 1974).
Irrigating rice with the aqueous extracts of W. trilobata had significant negative effect on the biomass and the yield components. The reduction of crop yield was the comprehensive effect of the allelopathy of W. trilobata. So it is unsuitable for peasants to tread the W. trilobata directly into paddy field as organic fertilizer when they weed out W. trilobata in the ridge of field.
Nowadays, the research of allelopathy is focusing on the separation, purification and identification of allelochemicals (Zeng et al. 1996; Kong 1998). Although this work is necessary, more attention should be paid to their allelopathic mechanism to the acceptor plants, especially on the level of physiological level and molecular level. Another field should strengthen is the effect of allelopathy on the crop yield and quality.
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