Gabriella Kazinczi

The role of allelopathy in host-virus relations

Introduction The term allelopathy was introduced by Molish (1937) at the first time. Earlier it was considered as a type of interference among higher plants, where products of secondary metabolism inhibit (less promote) the development and physiological processes of neighbourhood plants (Rice 1984). The term allelopathy has been extended recently, including not only plant-plant, but also plant microorganism interactions. It is considered as a new alternative way for biological weed control (Duke et al. 2002, Dikic et al. 2003), in order to reduce environmental pollution (Nemeth Konda 2003). Plant viruses make up about 15-30% out of the whole plant diseases. Virus particles create extremely close biological units with the host cell. The biosynthesis of viruses is done by the organelles of the host cell. Therefore chemical protection against viruses is unsuccesful in vivo and causes the death of the host plant cell at the same time. In spite of this, some natural substances are known to inhibit the replication and cellto cell movement of viruses and to reduce virus concentration (Moraes et al. 1974, Baranwal and Verma 1997, Manickam and Rajappan 1998, Vivanco et al. 1999). The mode of action of natural substances is not yet known exactly, but it can be presumed, that these substances may modify special receptor places on the plant cell surface, therefore adhesion of virus particles can not be happened (Gaborjanyi and Tobias 1986). The aim of our investigations was to study the effect of allelopathic weed extracts on host-vims relations.

Herbicide-Affected Plant Metabolism Reduces Virus Propagation

Abstract It has been previously shown that certain herbicides or plant extracts inhibited the viral infection. The goal of this study was to investigate the effect of Obuda pepper virus (ObPV) infection and herbicide or plant extract treatments on the photosynthetic processes of the host plants to get informations about the interactions of these factors. In Capsicum annuum- ObPV host-virus relations the virus infection slightly increased the activity of photosystem II (PSII), as it was supposed from fluorescence induction parameters. Chlorophyll content of leaves was also elevated probably due to virus-induced growth inhibition. The herbicide Stomp (active ingredient: pendimethalin) incorporated into the soil one week before planting (preplant treatment) together with virus infection even strengthened these effects in agreement with previous observations that this herbicide always did not prevent virus infection or reduce virus concentration in hosts. In ObPV-infected Nicotiana tabacum the structural changes showed similar tendency like in ObPV-infected C. annuum, but PSII efficiency did not significantly differ from that of the control. However, non-photochemical quenching (NPQ) increased because of the strongly decreasing CO2 fixation activity. Though simultaneous application of a water extract of Cirsium arvense shoot caused a little stronger inhibition of CO2 fixation, little loss in production was obtained due to significant reduction in virus concentration. In Solanum nigrum-ObPV relation the slightly increasing tendency of the values of actual PSII quantum efficiency could be related to the probably elevated ratio of reaction centre components (increased chlorophyll a/b ratio) in the thylakoids. Application of the herbicide Fusilade S (active ingredient: fluazifop-P-butyl) at 4-6 leaf stage as a postemergence treatment practically prevented systemic virus infection and the virus-induced changes of photosynthesis are probably due to inhibiting the virus infection/replication process.