Nitrogen form affects physiological responses and root expansigenous honeycomb aerenchyma in the emergent macrophyte Acorus americanus

High nitrogen (N) concentrations and high NH4+:NO3− ratios that are characteristic of heavily eutrophic and constructed wetlands may be detrimental to the growth and establishment of macrophytes in wetlands formed in the oil sands reclamation areas. This study investigates the effects of N form on the physiology, growth, and root expansigenous honeycomb aerenchyma structure of Acorus americanus (Raf.) Raf., an important macrophyte targeted for wetland reclamation in Canada. Three populations of A. americanus were grown in aerated solution culture and provided for up to 3 months with two different concentrations of N (2 mmol·L−1, 8 mmol·L−1) as NH4+, NO3−, or NH4+ + NO3−. Fresh mass and physiological parameters including gas exchange, chlorophyll fluorescence, stomatal traits, nitrogen and chlorophyll concentration, root morphology, and aerenchyma structure were examined. The effects of N form were concentration-dependent. At 2 mmol·L−1 N, NH4+ + NO3− plants had higher fresh mass, photosynthetic rates, number of first-order roots, root diameter, and maximum root length than those provided solely with NH4+ or NO3−. At 8 mmol·L−1 N, both NH4+ and NH4+ + NO3− treatments had a negative impact on growth, net photosynthesis, and chlorophyll concentrations, and they also led to thinner and shorter roots with necrotic tips, a significant reduction in fractional root porosity, and a denser aerenchyma with smaller lacunae. The results indicate that high levels of NH4+-N may negatively affect the establishment of A. americanus plants in constructed wetlands through its impact on growth, net photosynthesis, and root morpho-anatomy.

Comparative genomic analyses in Asparagus

Garden asparagus (Asparagus officinalis L.) belongs to the monocot family Asparagaceae in the order Asparagales. Onion (Allium cepa L.) and Asparagus officinalis are 2 of the most economically important plants of the core Asparagales, a well supported monophyletic group within the Asparagales. Coding regions in onion have lower GC contents than the grasses. We compared the GC content of 3374 unique expressed sequence tags (ESTs) from A. officinalis with Lycoris longituba and onion (both members of the core Asparagales), Acorus americanus (sister to all other monocots), the grasses, and Arabidopsis. Although ESTs in A. officinalis and Acorus had a higher average GC content than Arabidopsis, Lycoris, and onion, all were clearly lower than the grasses. The Asparagaceae have the smallest nuclear genomes among all plants in the core Asparagales, which typically have huge genomes. Within the Asparagaceae, European Asparagus species have approximately twice the nuclear DNA of that of southern African Asparagus species. We cloned and sequenced 20 genomic amplicons from European A. officinalis and the southern African species Asparagus plumosus and observed no clear evidence for a recent genome doubling in A. officinalis relative to A. plumosus. These results indicate that members of the genus Asparagus with smaller genomes may be useful genomic models for plants in the core Asparagales.Key words: asparagus, GC content, duplication.