Several Mechanisms are Involved in Resistance of Helianthus to Orobanche cumana Wallr.

Orobanche cumana(broomrape) poses a serious threat to sunflowercultivation in many countries since there is no effective methodof controlling the pathogen. It is known that wild species ofthe genusHelianthus show resistance to different pathogens.Our goal was to identify resistant wild species and derivedinterspecific hybrids (wild Helianthus x sunflower) and to characterizethe resistance mechanisms involved. After identification ofkey stages (germination, fixation, tubercule formation and development)in the resistance of Helianthus to O. cumana, several germplasmswere tested. Development of a hydroponic co-culture system allowedrapid screening of new Helianthus germplasms and a more accuratestudy of the resistance phenomenon. Several resistance mechanisms,such as cell wall deposition, vessel occlusion and broomrapecellular disorganization were demonstrated in the resistantsunflower genotype LR1 by histological observation of the host-parasiteinterface. In contrast, no reaction was observed in the susceptiblecultivated genotype 2603. H. debilis debilis(the wild parentof LR1) exhibited an encapsulation layer which prevented intrusionof the parasite into the cortex. Severe reduction in the rootand apex biomass of sunflower was observed in the susceptibleline (2603) indicating strong competition between broomrapeand sunflower sinks. This large loss of sunflower biomass wascompensated by broomrape biomass. In resistant genotypes LR1and 92B6, the effect of broomrape infestation on sunflower biomasswas much more limited.¹⁴CO₂radiolabelling showed differencesbetween resistant and susceptible sunflower genotypes in nutrientflux from the host to broomrape. Copyright 2001 Annals of BotanyCompany Carbon transfer, Helianthus, Orobanche cumana, resistance, sunflower