Leaf nitrogen partition between photosynthesis and structural defense in invasive and native tall form Spartina alterniflora populations: effects of nitrogen treatments

Among the mechanisms underlying biological invasions, the evolution of increased competitive ability (EICA) hypothesis, proposing that invasive plants may become superior competitors in their introduced ranges because they are able to reduce resource allocation to defenses while increasing allocation to growth in response to low herbivory, is one of the most common explanations for the success of invasive plants because EICA is a mechanism added to other mechanisms currently well known in invasion biology such as enemy release hypothesis (ERH), genetic admixture and propagule pressure and so on. Nitrogen is a primary resource limiting the growth of plants. In this study, we investigated nitrogen allocation to the photosynthetic protein Rubisco and to cell walls in invasive and native tall form Spartina alterniflora to explore the mechanisms behind its successful invasion. We conducted an experiment with seeds from three populations collected respectively from its introduced range in Jiangsu Province, China, and from its native range in Georgia, USA. Seeds were germinated, and plants from all populations were grown in a greenhouse at low and high levels of nitrogen for several months. We found an increased allocation of leaf nitrogen to Rubisco of invasive tall form S. alterniflora relative to its native conspecifics in the limited nitrogen condition. Nitrogen addition had significant effect on leaf nitrogen allocation in Rubisco and cell walls of native S. alterniflora, but did not influence those of invasive populations. Invasive S. alterniflora had higher Rubisco content and photosynthetic rate than native S. alterniflora in both nitrogen treatments, but no significant differences were found in cell wall content, fraction of leaf nitrogen in cell walls and photosynthetic nitrogen use efficiency. The increased photosynthetic capacity of invaders relative to native plants resulted from the significantly higher fraction of nitrogen allocated to Rubisco and higher nitrogen content in the low nitrogen treatment, but only higher nitrogen content in the high nitrogen treatment. Our results suggest that shifts in these leaf traits following its introduction to a new range may play an important role in invasion success of tall form S. alterniflora.