The chemical-mediated allelopathic interaction between rice and barnyard grass

Aims

The possible involvement of the chemical-mediated interaction in allelopathy between rice and barnyard grass was investigated.

Methods

Effcts of rice seedlings and rice root exudate on the alleloapthic activity of barnyard grass were determined and a key compound invovled in the allelopathic interaction between rice and barnyard grass was isolated.

Results

Allelopathic activity of barnyard grass was increased by the presence of rice seedlings. Rice root exudates also elevated the allelopahtic activity of barnyard grass. A key compound, which increased the allelopathic activity of barnyard grass, in the rice root exudates was isolated and determined as momilactone B. Momilactone B increased the allelopathic activity of barnyard grass at concentrations greater than 3 μM, and increasing the momilactone B concentration increased the activity.

Conclusions

Momilactone B is known to act as a potent rice allelochemical and to possess strong growth inhibitory activity against barnyard grass. The present research suggests that barnyard grass may response to the presence of neighboring rice by sensing momilactone B in rice root exudates and increase allelopathic activity. Thus, momilactone B may not only act as a rice allelochemical but also play an important role in rice-induced allelopathy of barnyard grass. The induced-allelopathy may provide a competitive advantage for barnyard grass through the growth inhibition of competing plant species including rice. Barnyard grass allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyard grass. Rice allelopathy was also reported to be increased by the presence of barnyard grass through increasing production and secretion of momilactone B into surrounding environments. During the evolutional process, rice and barnyard grass may have developed the chemical cross talk to activate the defense mechanisms against some biotic stress conditions by detection of certain key compounds.     

The relation between growth inhibition and secretion level of momilactone B from rice root

Abstract

The growth inhibitory activity of seven rice (Oryza sativa L.) cultivars and the secretion level of momilactone B from these rice cultivars were determined to understand chemical basis of the interaction of rice with other plant species. All rice cultivars inhibited the growth of hypocotyls and roots of lettuce (Lactuca sativa L.) seedlings when the lettuce was grown together with the rice, and showed different range of the inhibitory activity. These results suggest that all rice cultivars may possess allelopathic activity and the activity may be cultivar dependent. Momilactone B, which is a potent growth inhibitor, was found in root exudates of all rice cultivars, and the momilactone B concentration was also cultivar-dependent. The allelopathic activity of each rice cultivar was closely correlated with momilactone B concentration in the root exudates. The present results suggest that rice cultivars possess various allelopathic activities and their allelopathic activity may depend on the secretion level of momilactone B. Therefore, momilactone B may play an important role in rice allelopathy and in the chemical interactions of rice with other plant species.     

Stress-induced allelopathic activity and momilactone B in rice

Allelopathic activity of rice extracts and root exudates against Echinochloa crus-galli increased by heavy metal, cantharidin and jasmonic acid treatments. Since cantharidin (protein phosphatase inhibitor) acts as an elicitor and jasmonic acid is an important signaling molecule regulating inducible defense genes against the pathogen infection, heavy metal stress and pathogen infection may increase alleopathic activity of rice. These treatments also increased the concentrations of momilactone B in rice extracts and root exudates, suggesting that the production of momilactone B in rice and the secretion of momilactone B from rice into the rhizosphere may be enhanced by the treatments. As momilactone B possesses strong phytotoxic and allelopathic activities, the elevated production and secretion of momilactone B of rice by heavy metals, cantharidin and jasmonic acid may contribute to the increasing allelopathic activity of rice. Enhancement of the secretion of momilactone B into the rhizosphere may provide a competitive advantage for root establishment through local suppression of pathogen and inhibition of the growth of competing plant species. Therefore, allelopathy of rice may be one of the inducible defense mechanisms and may be regulated several environmental factors.     

UV-induced momilactone B accumulation in rice rhizosphere

UV-irradiation increased the concentration of momilactone B in shoots and roots of rice seedlings, and increasing the irradiation increased the concentration. The concentration in 90-min UV-irradiated shoots and roots, respectively, was 31.8- and 3.6-fold higher than that in non-irradiated shoots and roots. After UV-irradiation the concentration of momilactone B in rice shoots decreased. There was, however, an accumulation of momilactone B in the medium in which UV-irradiated seedlings were grown. Five days after UV-irradiation, momilactone B in the medium was at a level 2.5 times greater than on day 0, which was 47% of momilactone B in the seedlings, suggesting that rice may actively secrete momilactone B into medium. Therefore, UV-irradiation increased not only production of momilactone B in rice seedlings but also secretion of momilactone B into rice rhizosphere. As momilactone B acts as an antimicrobial and allelopathic agent, secretion of momilactone B into the rhizosphere may provide a competitive advantage for root establishment through local suppression of soil microorganism and inhibition of the growth of competing plant species.     

Rice seedlings release momilactone B into the environment

Since the growth inhibitor momilactone B was found recently in root exudates of rice (Oryza sativa L.), 3-day-old rice seedlings were transferred to hydroponic culture and the level of momilactone B released into the environment from the seedlings was measured. At day 15 after transfer, the level of momilactone B in the culture solution was 1.8 nmol per seedling compared with endogenous levels of 0.32 and 0.63 nmol per root and shoot, respectively, suggesting that rice seedlings actively releases momilactone B into the culture solution. This release must occur from the roots because only rice roots were immersed in the culture solution. Momilactone B inhibited the growth of ten cress (Lepidium sativum L.) seedlings at concentrations greater than 3 microM. Ten rice seedlings were incubated with ten cress seeds in a Petri dish containing 1 ml of medium, the medium contained 18 nmol of momilactone B, which came to 18 microM. This level of momilactone B was enough to reveal growth inhibition of the cress seedlings. Release level of momilactone B and its effectiveness as a growth inhibitor suggest that it may play an important role in rice allelopathy.     

Isolation and identification of a potent allelopathic substance in rice root exudates

A search for growth inhibitors in rice root exudates was undertaken in order to clarify the allelopathic system in rice ( Oryza sativa L.). Rice seedlings inhibited the growth of cress ( Lepidium sativum L.) and lettuce ( Lactuca sativa L.) seedlings when the cress and lettuce were grown with rice seedlings. The putative compound causing the inhibitory effect of rice seedlings was isolated from their culture solution, and the chemical structure of the inhibitor was determined by spectral data as momilactone B. Momilactone B inhibited the growth of cress and lettuce seedlings at concentrations greater than 3 and 30 µ M , respectively. The concentration of momilactone B was 3.4 and 1.1 nmol per seedling in the culture solutions of husked and non-husked rice seedlings, respectively. These results suggest that rice seedlings may release momilactone B into the environment and the stress caused by the husk-treatment may increase the amount of momilactone B released. Thus, momilactone B may play an important role in rice allelopathy.     

The chemical cross talk between rice and barnyardgrass

The chemical cross talk between rice and barnyardgrass which is one of the most noxious weeds in rice cultivation was investigated. Allelopathic activity of rice was increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. Rice allelochemical, momilactone B, concentration in rice seedlings and momilactone B secretion level from rice were also increased by the presence of barnyardgrass seedlings or barnyardgrass root exudates. As momilactone B possesses strong growth inhibitory activity and acts as an allelochemical, barnyardgrass-induced rice allelopathy may be due to the increased momilactone B secretion. These results suggest that rice may respond to the presence of neighboring barnyardgrass by sensing the chemical components in barnyardgrass root exudates and increase allelopathic activity by elevated production and secretion levels of momilactone B. Thus, rice allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyardgrass and the induced-allelopathy may provide a competitive advantage for rice through suppression of the growth of barnyardgrass.Key words: allelopathy, Echinochloa, chemical interaction, induced-allelopathy, momilactone, Oryza sativaThe chemical cross talk between host and symbiotic or parasitic plants is an essential process for the development of physical connections in symbiosis and parasitism.^1–3 Barnyardgrass is one of the most common and noxious weeds in rice paddy fields.⁴ Although barnyardgrass is adapted rice production system due to its similarity in growth habit, the reason why barnyardgrass so often invades into the rice paddy fields is unknown. There might be some special interactions between both plant species.Plants are able to accumulate phytoalexins around infection sites of pathogens soon after sensing elicitors of pathogen origin. This accumulation of phytoalexins can protect the plants from further pathogen infection.^5,6 Plants are also able to activate defense mechanisms against attacking herbivores by sensing volatile compounds, such as methacrolein and methyl jasmonate, released by herbivore-attacked plant cells. The volatile-sensed plants increase the production of phenolics, alkaloids, terpenes and defense proteins, which reduce herbivory attacks.^7,8 Therefore, plants are able to elevate the defense mechanisms against several biotic stress conditions by detection of various compounds.Allelopathy is the direct influence of organic chemicals released from plants on the growth and development of other plants.^9–11 Allelochemicals are such organic chemicals involved in the allelopathy.^12,13 Allelochemicals can provide a competitive advantage for host-plants through suppression of soil microorganism and inhibition of the growth of competing plant species because of their antibacterial, antifungal and growth inhibitory activities.^3,14,15Rice has been extensively studied with respect to its allelopathy as part of a strategy for sustainable weed management, such as breeding allelopathic rice strains. A large number of rice varieties were found to inhibit the growth of several plant species when these rice varieties were grown together with these plants under the field or/and laboratory conditions.^16–20 These findings suggest that rice may produce and release allelochemicals into the neighboring environments and may inhibit the growth of the neighboring plants by the allelochemicals.Potent allelochemical, momilactone B, was isolated from rice root exudates.²¹ Momilactone B inhibits the growth of typical rice weeds like barnyardgrass and Echinochloa colonum at concentrations greater than 1 µM and the toxicity of momilactone B to rice itself was very low.²² In addition, rice plants secrete momilactone B from the roots into the rhizosphere over their entire life cycle.²² The observations suggest rice allelopathy may be primarily dependant on the secretion levels of momilactone B from the rice seedlings.^22,23Allelopathic activity of rice exhibited 5.3- to 6.3-fold increases when rice and barnyardgrass seedlings were grown together. Root exudates of barnyardgrass seedlings also increased allelopathic activity and momilactone B concentration in rice seedlings. The increasing the exudate concentration increased the allelopathic activity and momilactone B concentration in rice.²⁴ Thus, the chemical components in barnyardgrass root exudates may affect gene expressions involved in momilactone B biosynthesis. However, effects of the barnyardgrass root exudates on the secretion level of mimilactone B from rice has not yet reported.Rice seedlings were incubated in the medium containing barnyardgrass root exudates for 10 d, and secretion level of momilactone B by rice was determined (Fig. 1). The root exudates increased the secretion level significantly at concentrations greater than 30 mg/L of barnyardgrass root exudates, and increasing the concentration increased the secretion level. At concentrations of 300 mg/L of the root exudates, the secretion level was 10-fold greater than that in control (0 mg of root exudate). There was no significant difference in the osmotic potential between the medium contained barnyardgrass root exudates and control medium (all about 10 mmol/kg), and pH value of the medium was maintained at 6.0 throughout the experiments.²⁵ These results suggest that unknown chemical components in the barnyardgrass root exudates may induce the secretion of momilactone B from rice. As momilactone B possesses strong phytotoxic and allelopathic activities,^21–23,25 the elevated production and secretion of momilactone B in rice may provide a competitive advantage for root establishment through local suppression of pathogens and inhibition of the growth of competing plant species including barnyardgrass. Thus, barnyardgrass-induced rice allelopathy may be caused by the chemical components in the barnyardgrass root exudates.Open in a separate windowFigure 1Effects of barnyardgrass root exudates on momilactone B secretion level in rice. Rice seedlings were incubated in the medium containing barnyardgrass root exudates for 10 d, and secretion level of momilactone B was determined as described by Kato-Noguchi.²⁴ The experiment was repeated six times with three assays for each determination. Different letters show significant difference (p < 0.01) according to Tukey''s HSD test.Although mechanisms of the exudation are not well understood, it is suggested that plants are able to secrete a wide variety of compounds from root cells by plasmalemma-derived exudation, endoplasmic-derived exudation and proton-pumping mechanisms.^3,15 Through the root exudation of compounds, plants are able to regulate the soil microbial community in their immediate vicinity, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species.^3,14,15 The present research suggests that rice may be aware of the presence of neighboring barnyardgrass by detection of certain key in barnyardgrass root exudates, and this sensorial function may trigger a signal cascade resulting in increasing rice allelopathy through increasing production of momilactone B and secretion of momilactone B into the rhizosphere. Therefore, rice allelopathy may potentially be an inducible defense mechanism by chemical-mediated plant interactions between rice and barnyardgrass.     

Diterpene Phytoalexins Are Biosynthesized in and Exuded from the Roots of Rice Seedlings

Rice (Oryza sativa L.) produces a variety of diterpene phytoalexins, such as momilactones, phytocassanes, and oryzalexins. Momilactone B was previously identified as an allelopathic substance exuded from the roots of rice. We identified in this present study momilactone A and phytocassanes A–E in extracts of, and exudates from, the roots of rice seedlings. The concentration of each compound was of the same order of magnitude as that of momilactone B. Expression analyses of the diterpene cyclase genes responsible for the biosynthesis of momilactones and phytocassanes suggest that these phytoalexins found in roots are primarily biosynthesized in those roots. None of phytocassanes B–E exhibited allelopathic activity against dicot seedling growth, whereas momilactone A showed much weaker allelopathic activity than momilactone B. The exudation of diterpene phytoalexins from the roots might be part of a system for defense against root-infecting pathogens.     

Diterpene phytoalexins are biosynthesized in and exuded from the roots of rice seedlings

Rice (Oryza sativa L.) produces a variety of diterpene phytoalexins, such as momilactones, phytocassanes, and oryzalexins. Momilactone B was previously identified as an allelopathic substance exuded from the roots of rice. We identified in this present study momilactone A and phytocassanes A-E in extracts of, and exudates from, the roots of rice seedlings. The concentration of each compound was of the same order of magnitude as that of momilactone B. Expression analyses of the diterpene cyclase genes responsible for the biosynthesis of momilactones and phytocassanes suggest that these phytoalexins found in roots are primarily biosynthesized in those roots. None of phytocassanes B-E exhibited allelopathic activity against dicot seedling growth, whereas momilactone A showed much weaker allelopathic activity than momilactone B. The exudation of diterpene phytoalexins from the roots might be part of a system for defense against root-infecting pathogens.     

Concentration and release level of momilactone B in the seedlings of eight rice cultivars

The release levels of a growth inhibitor, momilactone B, from rice (Oryza sativa L.) seedlings of eight cultivars were compared with the endogenous concentrations of momilactone B in their seedlings. All rice cultivars contained momilactone B in the seedlings, and their concentrations differed between the cultivars. Momilactone B was also found in all culture solutions in which these rice seedlings were grown, and the concentrations differed between the cultivars. The momilactone B concentrations in the culture solutions were reflected in the momilactone B concentrations in the seedlings. These results suggest that all rice cultivars may produce momilactone B and release momilactone B into the culture solutions. In addition, the release level of momilactone B may depend on the production level of momilactone B in the seedlings, which may affect allelopathic potential of these rice cultivars because as a growth inhibitor, momilactone B is able to act as an allelochemical.     

Allelopathic Substance Exuded from a Serious Weed, Germinating Barnyard Grass (Echinochloa crus-galli L.), Roots

The allelopathy of a serious weed, barnyard grass (Echinochloa crus-galli L.), was investigated. Root exudates of young barnyard grass showed allelopathic effects and plant-selective activity and inhibited root elongation of all plants tested. With respect to shoot growth, the exudates did not show inhibition of barnyard grass only. The allelopathic substance was isolated and identified as p-hydroxymandelic acid by NMR. p-Hydroxymandelic acid strongly inhibited shoot growth and root elongation of all plants tested. The effects of three congeners of p-hydroxymandelic acid were tested on rice shoot growth. In the biological activity exhibited in rice, shoot growth was related to the hydroxyl groups. Received October 7, 1998; accepted March 29, 1999     

Allelopathic substance in rice root exudates: rediscovery of momilactone B as an allelochemical

Much research on rice allelopathy has been directed toward the selection of allelopathic rice strains and the identification of allelochemicals in rice. This paper briefly summarizes recent progress in the rice allelopathy and focuses on rediscovery of momilactone B as an allelochemical. A large number of rice varieties were found to inhibit the growth of several plant species when grown together under field and/or laboratory conditions. These findings suggest that rice probably produces and releases allelochemical(s) into the environment. The putative compound causing the inhibitory effect of rice was recently isolated from rice root exudates, and the chemical structure of the inhibitor was determined by spectral data as momilactone B. In addition, it has been found that momilactone B is released from rice roots into the neighboring environment, and the release level of momilactone B from rice may be sufficient to cause growth inhibition of neighboring plants. These findings suggest that momilactone B may play an important role in rice allelopathy.     

Secretion of momilactone A from rice roots to the rhizosphere

The secretion levels of momilactone A from rice (Oryza sativa L.) seedlings of eight cultivars into the rhizosphere were compared with the endogenous momilactone A concentrations in their shoots and roots. All rice cultivars contained momilactone A in the shoots and roots, and concentrations differed among the cultivars. Momilactone A was also found in all culture solutions in which the rice seedlings were grown, and the concentrations differed among the cultivars. The momilactone A concentrations in the culture solutions were reflected in the momilactone A concentrations in the shoots. These results suggest that all rice cultivars may produce momilactome A and secrete momilactone A into the culture solutions. The secretion levels of momilactone A may be more dependent on their capacities for momilactone A production in the shoots than on their capacities for momilactone A transportation from the shoots into the environment via the roots. As momilactone A acts as an antimicrobial and allelopathic agent, the secretion of momilactone A into the rice rhizosphere may provide a competitive advantage for root establishment through local suppression of soil microorganisms and inhibition of the growth of competing plant species.     

Allelochemicals released by rice roots and residues in soil

A few rice (Oryza sativa L.) varieties or rice straw produce and release allelochemicals into soil in which interfere with the growth of neighboring or successive plants. Allelopathic rice PI312777 and Huagan-1 at their early growth stages released momilactone B, 3-isopropyl-5-acetoxycyclohexene-2-one-1, and 5,7,4′-trihydroxy-3′,5′-dimethoxyflavone into soil at phytotoxic levels, but non-allelopathic rice Huajingxian did not. Both allelopathic and non-allelopathic rice residues released momilactone B and lignin-related phenolic acids (p-hydroxybenzoic, p-coumaric, ferulic, syringic and vanillic acids) into the soil during residue decomposition to inhibit successive plants. The results indicated that allelochemicals involved in rice allelopathy from living and dead plants are substantially different. Interestingly, the concentrations of the allelochemicals released from the allelopathic rice seedlings in soil increased dramatically when they were surrounded with Echinochloa crus-galli. The concentrations of the allelochemicals were over 3-fold higher in the presence of E. crus-galli than in the absence of E. crus-galli. However, the same case did not occur in non-allelopathic Huajingxian seedlings surrounded with E. crus-galli. In addition to allelochemical exudation being promoted by the presence of E. crus-galli, allelopathic rice seedlings also increased allelochemical exudation in response to exudates of germinated E. crus-galli seeds or lepidimoide, an uronic acid derivative exuded from E. crus-galli seeds. These results imply that allelopathic rice seedlings can sense certain allelochemicals released by E. crus-galli into the soil, and respond by increased production of allelochemicals inhibitory to E. crus-galli. This study suggests that rice residues of both allelopathic and non-allelopathic varieties release similar concentrations and types of allelochemicals to inhibit successive plants. In contrast, living rice plants of certain allelopathic varieties appear to be able to detect the presence of interspecific neighbors and respond by increased allelochemicals.     

An allelopathic substance exuded from germinating watermelon seeds

When watermelon seeds were cultured in a Petri dish together with amaranth, barnyard grass, cockscomb, lettuce or tomato seeds, the shoot growth of amaranth and cockscomb was markedly promoted, whereas the shoot growth of lettuce and tomato was inhibited. The shoot growth of barnyard grass was not affected. These results suggest that plant-selective allelopathic substance(s) affecting the shoot growth of other plant seedlings were exuded from watermelon seeds. An allelopathic substance was isolated from the exudates of germinating watermelon seeds and identified as vanillic acid by its spectral analysis and Rf value on TLC. Vanillic acid promoted the shoot growth of cockscomb at the concentrations of 300 to 10 mg/l and that of amaranth at the concentrations of 30 to 3 mg/l, although the shoot growth of amaranth was inhibited by 300 mg/l of vanillic acid. The shoot growth of lettuce and tomato was inhibited at the concentrations higher than 30 mg/l by vanillic acid. However, the shoot growth of barnyard grass was not affected at the concentrations used. All these results suggest that vanillic acid may play as a major component of allelopathic substance(s), which shows plant-selective activity, in the exudates of germinating watermelon seeds.     

水稻和稗草共生土壤微生物生物量碳及酶活性的变化

以稻田稗草、化感水稻PI312777和普通水稻辽粳9为试材,研究了田间稗草和水稻1∶1共生条件下,土壤微生物生物量碳及脱氢酶、脲酶和转化酶活性的变化.结果表明：在稗草 的干扰下,化感水稻PI312777根区土壤微生物生物量碳含量比单作减少了 50.52％（P<0.01）,而行间土壤微生物生物量碳含量增加;普通水稻辽粳9根区土壤 微生物生物量碳含量比单作减少了38.99％（P<0.01）,但其行间土壤微生物生物量碳含量无明显变化.两个水稻品种根区土壤脱氢酶活性均被显著抑制（P<0.05）,下降率都在20％以上;PI312777根区土壤脲酶和转化酶活性均被显著促进（P<0.01）;而辽粳9根区土壤转化酶活性也被显著抑制（P<0.01）,但脲酶活性无明显变化.化感水稻根区土壤微生物生物量碳含量的显著减少及脲酶、转化酶活性的增加是其化感特性的表现,表明土壤微生物和酶均参与了水稻和稗草的种间作用,化感水稻具有抗稗草干扰的明显优势.     

The Allelopathic Effects of Sunflower and Wheat Root Exudates on <i>Sinapis arvensis and Sinapis alba</i>

In this study, we aimed to investigate the allelopathic effects of sunflower and wheat root exudates on the common weeds such as wild mustard and white mustard in our region. The root exudates which were obtained by soaking 8 weeks old sunflower and wheat seedlings (20 or 40 seedlings) in 100 mL of distilled water for 3 days were applied to the leaves of wild mustard and white mustard. In order to compare the allelopathic effect, the recommended dose (1 g.da^-1 ) and twice the recommended dose (2 g.da^-1 ) of Gromstor (Tribenuron-methyl), a herbicide preferred by farmers for the chemical control of these weeds was also applied. The allelopathy was performed for wild mustard and white mustard seedlings by the measurement of different physiological and biochemical parameters, such as chlorophyll a, chlorophyll b, total chlorophyll, carotenoid, proline, total protein amounts and superoxide dismutase enzyme activity. The amounts of total chl and carotenoid in wild mustard leaves decreased in all treatment groups compared to control. The highest decrease in total chl (50.93%) and carotenoid (46.69%) was oberved in the treatment of 40 wheat seedlings. 100 mL^-1 distilled water. In the white mustard leaves, the amount of total chl in all treatment groups except the treatment group of Gromstor 2 g.da^-1 and carotenoid in all treatment groups increased compared to the control. The highest increases again were observed in 40 wheat seedlings. 100 mL^-1 distilled water treatment. The proline amounts in wild mustard and white mustard increased in all treatment groups. The highest increase was observed for the treatment of 20 wheat seedlings. 100 mL^-1 distilled water in wild mustard (459.69%) and 40 sunflower seedlings. 100 mL^-1 distilled water in white mustard plant (104.70%). In superoxide dismutase enzyme activities, treatments decreased activity except treatment of 40 sunflower seedling root exudate in wild mustard, while increased activity outside commercial herbicide treatment in white mustard. The results showed that sunflower and wheat root exudates have allelopathic effects on wild mustard and white mustard weeds. It is thought that the study will be a reference for new studies that will enable the use of plant root exudates as bioherbicides or foliar fertilizers and will contribute to the fight against weeds in organic agriculture.     

Interference of allelopathic rice with paddy weeds at the root level

• Despite increasing knowledge of the involvement of allelopathy in negative interactions among plants, relatively little is known about its action at the root level. This study aims to enhance understanding of interactions of roots between a crop and associated weeds via allelopathy.
• Based on a series of experiments with window rhizoboxes and root segregation methods, we examined root placement patterns and root interactions between allelopathic rice and major paddy weeds Cyperus difformis, Echinochloa crus‐galli, Eclipta prostrata, Leptochloa chinensis and Oryza sativa (weedy rice).
• Allelopathic rice inhibited growth of paddy weed roots more than shoots regardless of species. Furthermore, allelopathic rice significantly reduced total root length, total root area, maximum root width and maximum root depth of paddy weeds, while the weeds adjusted horizontal and vertical placement of their roots in response to the presence of allelopathic rice. With the exception of O. sativa (weedy rice), root growth of weeds avoided expanding towards allelopathic rice. Compared with root contact, root segregation significantly increased inhibition of E. crus‐galli, E. prostrata and L. chinensis through an increase in rice allelochemicals. In particular, their root exudates induced production of rice allelochemicals. However, similar results were not observed in C. difformis and O. sativa (weedy rice) with either root segregation or root exudate application.
• The results demonstrate that allelopathic rice interferes with paddy weeds by altering root placement patterns and root interactions. This is the first case of a root behavioural strategy in crop–weed allelopathic interaction.