Parthenium avoids drought: understanding the morphological and physiological responses of the invasive herb Parthenium hysterophorus to progressive water stress

Parthenium hysterophorus L. is a noxious annual invasive herb which threatens biodiversity, food security and human and animal health in various tropical and sub-tropical regions globally. Given that these regions, including South Africa, experience frequent drought events, it is important that the invasive potential of this weed be better understood in the context of water stress. This study aimed to assess the physiological and morphological responses of P. hysterophorus to water stress. To elicit these responses, juvenile and adult plants were progressively water stressed over a six-week period. Despite significant reductions in leaf water potential, relative leaf water content and stomatal conductance from severe soil water stress, plants maintained positive net photosynthesis, albeit at a severely reduced rate. Concomitantly, water-use efficiency of P. hysterophorus increased by more than 300%. Morphologically, water-stressed plants exhibited reductions in leaf area and leaf number, with adults accelerating flower production when compared to well-watered plants. Linked to this, were alterations in biomass, with juveniles and adults reducing above-ground biomass by 75% and 35%, respectively. Overall, many of the physiological and morphological responses are indicative of a largely drought avoidance strategy, allowing P. hysterophorus to persist during periods of water-stress as leafy basal rosettes (juveniles) or as reproductive herbs (adults). This research yielded valuable insights into the underlying water relations of P. hysterophorus, and how these may relate to the potential spread, detriment and management of the weed in various sub-tropical environments