Several mechanisms are involved in resistance of Helianthus to Orobanche cumana Wallr.

Orobanche cumana (broomrape) poses a serious threat to sunflower cultivation in many countries since there is no effective method of controlling the pathogen. It is known that wild species of the genus Helianthus show resistance to different pathogens. Our goal was to identify resistant wild species and derived interspecific hybrids (wild Helianthus x sunflower) and to characterize the resistance mechanisms involved. After identification of key stages (germination, fixation, tubercule formation and development) in the resistance of Hetianthus to O. cumana, several germplasms were tested. Development of a hydroponic co-culture system allowed rapid screening of new Hetianthus germplasms and a more accurate study of the resistance phenomenon. Several resistance mechanisms, such as cell wall deposition, vessel occlusion and broomrape cellular disorganization were demonstrated in the resistant sunflower genotype LR1 by histological observation of the host-parasite interface. In contrast, no reaction was observed in the susceptible cultivated genotype 2603. H. debilis debitis (the wild parent of LR1) exhibited an encapsulation layer which prevented intrusion of the parasite into the cortex. Severe reduction in the root and apex biomass of sunflower was observed in the susceptible line (2603) indicating strong competition between broomrape and sunflower sinks. This large loss of sunflower biomass was compensated by broomrape biomass. In resistant genotypes LR1 and 92B6, the effect of broomrape infestation on sunflower biomass was much more limited. 14CO2 radiotabelling showed differences between resistant and susceptible sunflower genotypes in nutrient flux from the host to broomrape. © 2001 Annals of Botany Company.