ScienceWise - Sep/Oct 2009

Plastic Plants

Assessing the Threat of Weeds with Phenotypic Plasticity

Amy Davidson is into plastic weeds. Well, not so much weeds made of plastic as invading plants that possess phenotypic plasticity. And she believes a better understanding of phenotypic plasticity in plants will help us manage weeds in a world undergoing climate change.

“Phenotypic plasticity is when the genotype of an individual plant is able to respond differently to varying environmental conditions,” explains Ms Davidson, a PhD student in the Research School of Biology. “In other words, the plant can respond with greater flexibility as conditions change to maximise its chances.”

A plant’s genotype is the information it carries in its genes. Its phenotype is how this information is expressed as the plant grows. Measuring a plant’s plasticity, however, is far more challenging than simply measuring its leaves or the size of its seeds. It requires the measurement of a range of physical and physiological attributes of one plant and then comparing it with these same attributes with another plant species with similar genes to see how they respond to environmental conditions as they change.

“It’s long been believed that weeds might be more plastic than native species,” says Ms Davidson. “Indeed, it’s this property that enables them to do so well, to be an effective invader.

“When a weed comes into a new area, a place where it hasn’t evolved, it often arrives with only a small number of individuals and not a lot of genetic diversity. So, it’s been postulated that one of the reasons weeds do so well, especially in novel or changing environments, is that they are more plastic than the natives already in that area.”

As part of her research, Ms Davidson has trawled through the literature looking at plasticity between native plants and closely related invasive plants. To her surprise, she found that for many plant species, and regardless of what treatment the native and the invasive were put through, the invasive species were always significantly more plastic than the native species.

Plasticity looks like being an important characteristic of invasive plants, but to explore how it works, in what circumstances it comes into play, Ms Davidson has set up her own growth experiments using two species of a small herb from the geranium family.

“I’m looking at an Australian native called Erodium crinitum and comparing it with the exotic weed Erodium cicutarium, which is native to the Mediterranean. Both species co-occur around Canberra and can be found growing west of here out to Mildura. That’s great for me because it means they are found along a gradient of decreasing rainfall.
“So, I’m growing several hundred plants in a glasshouse under five different water treatments that represent some of the rainfall regimes they experience in the wild or may experience as the climate changes. These range from frequent water, more than they would experience in the wild, down to only watering when the plants gets to wilting point (with a range of treatment in between these extremes).”

To examine and compare plasticity, Amy will measure her test plants for a range of characteristics including phenology (timing and duration of flowering and seeding), reproductive effort (how many seeds they produce and how viable those seeds are), leaf shape and area, physiological measures like stomatal conductance and photosynthetic rates, and basic anatomical measures such as rooting depth, root biomass, shoot biomass and leaf and seed biomass. She’ll see how the native compares with the invasive under different water availability treatments and, from this, assess their plasticity and fitness.

“There is some evidence to suggest that plants with phenotypic plasticity will cope better with novel conditions that might be increasingly experienced with climate change,” Amy says. “In south east Australia one of the biggest changes we’ll be dealing with is changing rainfall, and especially lower rainfall and more variable rainfall.

“So, my starting hypothesis is that the native species are used to slightly more predictable rainfall so it would have specialised, it would be locally adapted along that natural rainfall gradient. The invasive plant, on the other hand, hasn’t had as much time to evolve so it might be dealing with that natural rainfall gradient by being phenotypically plastic, and that’s what I’m testing in the glass houses right now.”

“This knowledge might provide us with an idea of when being plastic is particularly beneficial for an invasive species, and that’s something we need to take into consideration. Often when we’re mapping out where we think invasive species will be able to move to, we don’t take into account phenotypic plasticity. Also if we understand under which circumstances phenotypic plasticity is likely to be an important factor in the invasion process, then it’s something we can start to look for.”

Improving our capacity to understand and manage weeds could be vital in a climate change world in which novel environments will be increasingly common. Weeds already cost Australian farmers over $4 billion every year (in terms of the cost of control and reduced crop yield), as well having enormous impacts on many native plant and animal species. Under climate change, many scientists and managers believe that the impact of weeds will dramatically rise because of their ability to adapt to novel environments; something many crop and native species don’t possess.

So plastic weeds may be on the rise, and plasticity is one aspect of plant biology that we need to pay more attention to. Amy Davidson’s work on a little geranium weed should give us a much better handle on what it means to be plastic.

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