Alkaloid production changes due to nutrient deficiencies in Senecio grisebachii inflorescences.
Senecio grisebachii Baker is a weed that invades natural pastures and crops in southern Brazil, Uruguay, the mesopotamic provinces and Buenos Aires in Argentina, and is considered to be toxic because of the presence of pyrrolizidine alkaloids (PAs) in its tissues. The effects of nitrogen and phosphorus deficiency was evaluated on the APs patterns of the weed inflorescences. A completely randomized design with 10 repetitions was used in an experiment where plants growing in hydroponics from May to October were irrigated with a nutrient solution containing 50% P or N concentrations with respect to the control treatment. PAs quantification on S. grisebachii inflorescences by GC and GCMS showed a significant increase in total PAs content in treatments with N or P deficiencies (1,33 and 1,34 mg gr-1 dry weight, respectively, compared to the control with 0,35 mg g-1). Seven PAs were identified with relative concentrations that varied according to treatments. Senecionine was the mayor alkaloid in treatments with N deficiency, while senecifiline was the most abundant in the control and in P- deficient treatments, followed in all cases by integerrimine and minor amounts of spartiodine, jacobine, jacozine and retrorsine. Agricultural intensification during the last decades, in addition to the introduction of soybean cropping in Argentina, resulted in soil degradation of the agroecosystems. This is evidenced by a decline in soil organic matter and nutrient (nitrogen, phosphorous) contents, among other factors. Moreover, new weed communities appeared, Senecio grisebachii Baker among them. Our results demonstrate that this invasive weed increases its toxicity when grown in continuously cropped lands, and its presence in crops and forage lands should be considered as an threat to human and cattle health.