The Roles And Key Components Of Root Exudates In Interspecific Root-root Interactions Of Intercropping Systems

Increasing evidence suggests that plant roots can directly respond to the identity of neighbor plant species with potential consequences for changes in root growth and morphology.The previous field experiments showed that wheat roots spread under maize plants,whereas the lateral spread of maize roots under wheat plants was significantly limited during co-growing period.However,faba bean and maize roots were intermingled in soils during co-growing period.To elucidate the mechanisms underlying the difference in the two systems,wheat/maize,barley/maize and faba bean/maize intercropping systems were studied with helps of various devices in greenhouse,and focused on root distribution and root-root recognition between different crop species,effects of root exudates on root growth and root morphology of neighbored crops,and the key compounds in root exudates.The main results and conclusions were as follows:(1)Plants are able to response to identities of neighbors,alter their root distribution.The roots of wheat,barley,faba bean or maize plant distributed equally in self growing areas and common growing areas when it was neighbored with plant of the same species.When wheat was neighbored with maize plant,wheat roots tended to grow towards maize roots,while maize roots tend to avoid wheat roots.When barley was neighbored with maize plant,barley roots tended to avoid maize roots,whereas maize roots distributed equally in self growing areas and common growing areas.When faba bean plant was neighbored with maize plant,the root distribution of faba bean and maize was similar to that of faba bean and maize plant grown with the same species plant.(2)The roles of root exudates in behaviors driven by interspecific root interactions differed among different intercropping systems.Root exudates of wheat/maize intercropping inhibited the root growth of maize,but promoted root growth of wheat.Adding activated carbon to the compartments where wheat plants grown in significantly increased maize root length by 40%when wheat and maize grown without root separation.Adding activated carbon to the compartments where wheat and maize plants grown in significantly decreased wheat root length by 27%and 15%when wheat and maize grown without root separation.Root exudates of faba bean/maize intercropping promoted root growth of faba bean and maize plants.Adding activated carbon to both compartments,where barley and maize grown in,significantly decreased faba bean root length by 46%under plastic mesh treatments.Adding activated carbon to the compartments where barley plants grown in significantly increased maize root length by 45%when barley and maize grown without root separation.(3)The effects of root exudates on growth and root morphology of crops are species-specific.The effects of root exudates collected from different cereal crops on biomass and root morphological parameters differed.Root exudates of wheat significantly reduced maize shoot dry weight by 59%,root dry weight by 55%,and root length by 64%,while neithor maize nor and barley root exudates had similar effects on neighboring different plant speices.The root length of faba bean was increased by maize root exudates by 142%,but not by wheat and barley root exudates.(4)6-Methoxy-benzoxazolin-2-one(MBOA)may be a key substance in wheat root exudates that affects the interspecific competition between wheat and maize plants.The concentration of MBOA in the mixture(maize + wheat)was significantly 477%higher than that of wheat grown alone,224%higher thanthat of maize grown alone,and 315%higher than that of the weighted means of wheat and maize grown alone.The root length of intercropped wheat plants treated with 1,5,10 ?g/L MBOA was 55%,58%and 52%higher than that of control.The root length of intercropped maize treated with 1?g/L MBOA was 24%less than that of control.(5)The root exudates of maize significantly increased the nodulation of faba bean.Flavonoid such as genistein may be the key substances that affect the interspecific facilitation on nudulation of faba bean and maize plants.Adding activated carbon to self(faba bean)compartments,neighbor(maize)compartments or both compartments reduced the number of nodules in faba bean by 42%,24%and 64%,respectively.In addition,the number of nodules in faba bean was increased by 137%by adding maize root exudates to the rhizosphere of faba bean,compared to control.Genistein,quercetin and Iuteolin were detected in both faba bean root exudates and maize root exudates.Within this PhD thesis,we found that key compounds,for example MBOA,in wheat root exudates significantly inhibit the root growth and root length of maize in competitive intercropping systems(i.e.wheat/maize intercropping).In addition,maize root exudates significantly promoted the growth and nodulation of faba bean plants.Flavonoids(i.e.genistein,luteolin and quercetin)were detected in maize root exudates,and play an important role in promoting root growth and nodulation of faba bean plants.These findings explained the phenomenon observed under field conditions and have importance in understanding interspecific root interactions below-ground.