Plant growth and physiological responses of the invasive plant Acmella radicans to contrasting light and soil water conditions

Acmella radicans (Jacquin) R.K. Jansen is a new invasive species record for Yunnan Province, China, as of 2017 and little is known about its invasion mechanisms. To better understand its invasive strategies, we investigated the growth, physiological and soil nutrient use parameters of the invader under combined conditions of light (25%, 50%, 75%, and 100% of light availability) and soil water content (full, high, medium, and low soil water content) in the glasshouse. The results showed that light level, soil water content and their interaction had a significant effect on all plant morphological, physiological and soil nutrient parameters for A. radicans (P < 0.05). For the most part, plant height, total branch length, leafstalk length, leaf area, inflorescence number, seed number, leaf biomass, stem biomass, aboveground biomass, and total biomass of A. radicans were significantly increased with increased shading rate and soil water content and were generally greater at intermediate light levels and intermediate to high soil water levels. Under high-irradiance conditions, the chlorophyll content was greatly reduced compared to other treatments. We also observed that leafstalk length, leaf area, Pn, chlorophyll a, and chlorophyll b of A. radicans were markedly increased with increased shading rate and soil water content, and were generally highest under intermediate irradiance and soil water conditions, but many plant physiological parameters also exhibited relatively high values under waterlogging conditions. The concentrations of organic matter, available N and available K of A. radicans soils at high-irradiance and full-high soil water content treatments and medium-irradiance and high-medium soil water content treatments were often less than those of other treatments indicated that hypothetically soil absorption was increased by moderate shading and high soil water. This was the first study demonstrating that A. radicans thrives best under moderate light conditions in combination with high soil water. Furthermore, we surmised that its higher phenotypic and physiological plasticity contributes to its invasion success.