Mineral nutrient concentration influences sunflower infection by broomrape (Orobanche cumana).

Orobanche cumana Wallr., a root parasitic angiosperm, causes severe yield losses in Helianthus annuus L. (sunflower) in Europe. Until now, the only effective method of controlling this parasite has been the use of resistant sunflower genotypes. Broomrape resistance is, however, poorly understood even though previous studies have revealed several defence mechanisms. The study of a susceptible (2603) and a resistant (LR1) sunflower genotype in hydroponic co-culture showed that the degree of infection by broomrape is influenced by the concentration of nutrients in the growth medium. For the susceptible genotype, an increase in broomrape necrosis was observed when the nutrient concentration was increased. In the resistant genotype LR1, the rate of infection was reduced by increasing the concentration of mineral nutrients, measured as a decrease in broomrape attachments and a lack of underground stem development. When sunflowers were cultivated in full-strength medium, these findings correlated with a lower 14C incorporation in broomrape and a change in carbon allocation to host plant organs with a reinforced "shoot apex sink strength". Results demonstrated that in controlled conditions, the nutrient concentration directly affects sunflower resistance potential towards broomrape. [ABSTRACT FROM AUTHOR]