Polyploidy influences plant carbon/nitrogen balance and resistance to insect herbivory in Chamerion angustifolium
Anthropogenic inputs and increased soil microbial activity from rising temperatures are contributing to increases in plant available soil nitrogen (N). Enhanced soil N availability can increase plant N content and reduce carbon-based secondary defense compounds, making plants less resistant to some insect herbivores. Plant-herbivore interactions can also be influenced by genome duplication (polyploidy), which is common in plants and can alter gene expression including the production of secondary defense compounds. We cultivated diploid and autotetraploid Chamerion angustifolium (Fireweed) in a greenhouse under three soil N levels to examine whether (1) polyploidy influences the C/N balance of fireweed, (2) whether polyploidy influences fireweed resistance to insect herbivores, and (3) whether soil N and ploidy interact to influence herbivore resistance and plant fitness. We measured plant C/N content, above and belowground biomass, and proxies for male and female fitness (pollen and flower production). We estimated resistance as percent leaf area consumed by larval S. exigua, a generalist herbivore, in choice bioassay trials. We also used an ordinal scale of leaf damage to measure resistance to F. occidentalis (flower thrip), which invaded our greenhouse.
Both cytotypes responded to increasing soil N with increased tissue N concentrations, but tetraploids had significantly lower N concentrations and higher CN ratios. Overall tetraploids produced significantly more aboveground biomass and flowers, and we found no difference in pollen production. We did not find strong effects of soil N availability on cytotype biomass or fitness.
Tetraploids experienced significantly less damage by S. exigua, but significantly more damage by F. occidentalis. Interestingly, greater N availability was weakly correlated with less damage by S. exigua and significantly correlated with more damage by F. occidentalis. We found no interaction between N availability and ploidy on resistance to either insect species.
Polyploidy may alter fireweed resistance patterns by increasing the CN ratio; a trait associated with elevated C-based and reduced N-based secondary defense compounds. Observed differences in resistance patterns may be due to different nutrient requirements of insects, different deterrents (defense compounds), and/or different attractants. For example, F. occidentalis also feeds on flowers, and tetraploids produced more flowers which could attract these insects. We are currently conducting additional feeding bioassays and examining mechanisms of resistance by measuring secondary compounds in diploid and tetraploid fireweed grown under different soil N levels.