Alterations in phytotoxicity and allelochemistry in response to intraspecific variation in Parthenium hysterophorus

Allelopathy plays a crucial role in providing competitive advantage to several alien invasive species, and assists in their establishment beyond native boundaries. Role of allelopathy in the invasion success of the alien weed, Parthenium hysterophorus is well established; however, the ecological and evolutionary factors that could affect its allelopathic interactions are relatively unexplored. In our earlier findings, we suggested the presence of two morphotypes (P_A and P_B) in the population of P. hysterophorus, with variable morphology, physiology, and level of ecological impacts. Here, we hypothesize that phenotypically distinct morphotypes of P. hysterophorus may vary in their phytotoxicity and allelochemistry, thereby producing differential ecological impact. To test this hypothesis, effects of rhizospheric soil (RS) and plant amended soils (PAS) of the two morphotypes of P. hysterophorus (P_A and P_B) were studied on selected dicot (Bidens pilosa and Senna occidentalis) and monocot species (Phalaris minor and Avena fatua). Also, the composition of allelochemicals in P_A and P_B was assessed using Liquid Chromatography and Mass Spectrometry (LC-MS). The study revealed that the phytotoxic effect of P_B was greater than P_A in all the parameters measured for dicot species and in most of the parameters studied in monocot species. A generalized dose-dependent response was observed in the test species (PAS0 < PAS10 < PAS20 < PAS40) and the effect of RS was comparable to PAS20 and PAS40. A greater number of allelochemicals were reported from P_B, which is in accordance with the growth studies. The study concludes that intraspecific variations account for differential phytotoxicity and allelochemistry in P. hysterophorus.