Aerial escape over aquatic acclimation: photosynthetic strategy and biomass allocation underlying the invasiveness of Alternanthera philoxeroides

The invasive alien species Alternanthera philoxeroides (Mart.) Griseb . exhibits extremely vigorous growth and regenerates clonally from stem fragments, forming dense mats in aquatic environments and causing serious impacts on agriculture and ecosystems. To understand the physiological mechanisms that drive its growth and invasiveness, we compared A. philoxeroides with two congeneric reference species across three complementary approaches: (1) growth analysis under emergent conditions, comparing leaf area and dry weight over 1–3 weeks; (2) gas exchange, chlorophyll fluorescence, and P700 absorbance (an indicator of photosystem I redox activity) measurements under emergent conditions; and (3) underwater photosynthesis measurements using leaves grown under emergent and submerged conditions. Alternanthera philoxeroides exhibited higher relative growth rate (RGR) than the control species, primarily attributable to a higher net assimilation rate (NAR) supported by greater CO 2 assimilation capacity in the leaves. The leaf weight ratio (LWR) was lower than that of the control species, indicating preferential biomass allocation to stems. Under submerged conditions, leaf function declined with no formation of new leaves, and A. philoxeroides did not actively utilize HCO₃⁻ for photosynthesis. These results suggest that A. philoxeroides prioritizes rapid stem elongation supported by high photosynthetic capacity. In aquatic environments, rather than relying on underwater photosynthesis, it adopts an escape strategy involving stem elongation to deploy leaves above the water surface. This coordinated physiological and morphological strategy likely underlies its successful invasion across aquatic and riparian habitats. This understanding of the invasiveness strategy A. philoxeroides provides a basis for the development of effective management and control measures.