No abstract

Parthenium weed (Parthenium hysterophorus L.) is a globally invasive plant with significant negative impacts on the environment, agriculture, economy, and health. This invasion problem is likely to be exacerbated by climate change. Elevated atmospheric carbon dioxide concentration ([CO 2 ]), high ambient temperatures, heatwaves, and droughts will facilitate the establishment, range expansion, and interference potential of this invasive species while making its management more difficult. Here, for the first time, we review the potential impact of climate change elements on parthenium weed biology and management at regional and global levels. We establish that (1) parthenium weed plants morphologically and physiologically adapt to sustain or promote their growth, fecundity, competitive ability, and allelopathic effect under most climate change scenarios; (2) climate change elements can reduce the relative vigor of neighboring plants, especially C4 grasses, in competition with parthenium weed, providing this invasive species a competitive edge; (3) elevated [CO 2 ] has the most pronounced positive effect on parthenium weed performance compared to other climate change elements; (4) the efficacy of a widely used herbicide, glyphosate, may be reduced on parthenium weed grown under elevated [CO 2 ]; (5) mixed effects have been reported for biological control agents, with some losing ground, while others are maintaining or improving their efficacy; and (6) a greater risk of spread and range expansion under sustained climate change will pose greater challenges for management in natural and managed ecosystems. There has been a disturbing transition of this weed from noncropped landscapes to agroecosystems in response to recent climatic and land use changes. All evidence presented in this review suggests that the situation will be greatly aggravated if drastic management interventions are not implemented. Therefore, the research, policy, and outreach focus must be revisited and revamped considering the impacts of climate change to develop pragmatic solutions to combat this "superweed".

Parthenium hysterophorus L. (Asteraceae) utilises multiple mechanisms to facilitate its dispersal. It has been speculated that the cypsela, the propagule of this species, can be dispersed by water under varying environmental conditions. Four experiments were conducted to test this hypothesis, using simulated shaking and immersion to test floating ability and viability of the propagule in water. The influence of the acidity of the immersion medium on cypsela viability was also examined. Our results revealed that the freshly harvested cypselae could float on river water for at least 20 days, although around 80% sank within a week if moderate or severe turbulence was applied. Sinkage was observed to be more rapid in naked seeds (within a day) than in cypsela (within a week). On still water surfaces, germination occurred within a week but extended to 1.5 weeks under turbulent conditions due to sinkage. In river water, initial germination of floating cypselae was greater (70%) under illuminated conditions as compared to dark conditions (20%). The viability of immersed cypselae was found to remain high in distilled water for 45 days, when immersion was in cool conditions (10 or 15°C). However, in moderate (20 and 24°C) or warm (25 and 30°C) conditions, the rate of viability loss increased, and at 34°C, around 50% of the cypselae died after 20 days of immersion. Similar trends for cypselae longevity were observed in studies using river and pond water; viability loss was faster, especially in pond water. In summary, a proportion of cypselae will float in turbulent water and could be carried significant distances in river systems. Immersed cypselae can remain viable for weeks and can germinate on contact with soil. Water bodies or floods are therefore considered as important pathways in parthenium weed dispersal; hence, post-flood monitoring is strongly recommended to minimise its spread.