N-P-K ratio affects exudation of germination stimulants and resistance of tobacco seedlings to broomrapes

Nutrient deficiency is known to aggravate the infestation of broomrape in a number of plant species. This effect is hypothesised to result from the promotion of the release of germination stimulants from the roots of the host plant. In the present study, three experiments (hydroponic culture, Petri dish culture and soil culture) were conducted to investigate the effects of macro-nutrient ratios on the production and release of germination stimulants and the resistance of host tobacco cultivars to broomrapes. The hydroponic culture study showed that nitrogen/phosphate deficiency and potassium excess increased the germination-inducing activity of root exudates on Phelipanche aegyptiaca and Orobanche cumana seeds, with two tobacco cultivars showing different responses to nutrient ratios. The production of germination stimulants was significantly increased under nitrogen- and phosphate-deficient soil conditions. In the Petri dish study, increased vulnerability to P. aegyptiaca infection was observed in RG tobacco seedlings under nitrogen- and potassium-deficient conditions and in WF tobacco seedlings under nitrogen- and phosphate-deficient conditions. The results showed that macro-nutrient ratios can affect both the exudation of germination stimulants and the resistance of tobacco seedlings. Finally, we suggest that the classification of the resistance or tolerance of host species should fully consider the nutritional conditions of cultivation and the strategy of fertilisation in terms of the potential control of parasitic weeds prior to the identification of the key mechanism of parasite resistance.