Do winter and summer cohorts of the invasive weed Parthenium hysterophorus differ in seed germination and seedling growth?

Abstract

The highly invasive weed Parthenium hysterophorus flowers during both hot, humid summers and cold, dry winters in Nepal, yet it remains unknown if there are differences in seed germination and seedling growth during these contrasting seasons. We analysed seed germination and seedling growth of summer and winter cohorts of P. hysterophorus in Kathmandu Valley. Seeds were germinated at low (25/15°C) and high (30/20°C) temperatures, 12/12 h photoperiod and complete dark, and at different levels of water stress. Seedlings were grown in a greenhouse to determine biomass allocation and relative growth rate (RGR). Winter seeds had higher seed biomass than summer seeds, and both seed types germinated under complete dark and 12/12 h photoperiod. Summer seeds germinated at significantly lower rates and percentage than winter seeds at the low temperature, but this difference in germination was not present when summer and winter seeds were incubated at the high temperature. Winter seeds had higher germination (percentage and rate) and took a longer time to reach maximum germination than summer seeds at different levels of water stress. Seedling emergence of both seed types declined with increasing soil depth. Seedling RGRs were nearly equal (summer: 73 mg g−1 day−1, winter: 77 mg g−1 day−1). Biomass allocation to stem and leaves were similar in both cohorts but root biomass allocation was higher in the winter than in the summer cohort. Overall, the maternal environmental effect was moderate on seed germination, and weak on seedling growth. Lack of a pronounced difference in the germination and seedling growth of summer and winter cohorts of P. hysterophorus suggests a wide environmental tolerance of the species, and this could be one of the reasons for the species' ability to invade diverse climatic and geographic regions globally.