Leaf phenology and plant invasions: a role for genome size?

Background/Question/Methods

Invasive plants are often thought to have relatively small nuclear genomes due to the association of fast growth rates with rapid cell division.  However, cool season growth in temperate environments is often accomplished via cell expansion rather than cell division, and invaders have also been associated with early or late season growth phenology (e.g., early budbreak in woody invaders).  Furthermore, herbaceous species of early spring growth phenology have been shown to have large genomes, potentially because the larger cell sizes associated with higher nuclear DNA content allow greater capacity for growth via cell expansion, but such research in woody species is lacking.  I measured the nuclear DNA content of 56 species of native and invasive shrubs and lianas of Eastern U.S. deciduous forests, and explored connections between genome size, cell size, spring growth phenology, and summer growth rate.  Spring budbreak phenology, summer stem extension rate, and photosynthetic capacity were monitored over three years in a common garden, and nuclear DNA content and cell size were measured via flow cytometry and image analysis of stomatal guard cells.

Results/Conclusions

Nuclear DNA content was a strong predictor of spring budbreak phenology and guard cell size in 18 species of the honeysuckle genus Lonicera, where early leafing species, mostly natives, had larger cells of higher DNA content.  Across all 56 species from several genera, high DNA content was associated with early leafing behavior and was greater in native species, but with a weaker association to cell size.  For Lonicera, all seven native species had genome sizes greater than 2 pg, and all 11 invasive species had genome sizes less than 2 pg, a correlation that was phylogenetically independent within the honeysuckle clade.  Genome size, in turn, was a strong negative predictor of summer growth rate and photosynthetic capacity for this genus.  Although genome size was not correlated with summer growth rate across all 56 species, invaders exhibited significantly higher summer growth rates and significantly lower genome size than natives.  Results point toward a potential tradeoff in temperate woody plants between early season growth, accomplished via cell expansion in plants with large genomes, and mid-season growth rate, maximized in plants with small genomes capable of rapid cell division.  Invaders show a tendency of maximizing summer growth rather than earlier spring leaf emergence.