Knowing how plants forage for food could help farmers place fertilizer more efficiently and allow researchers to breed more successful crops
Biologists have long known about fairly complex animal behaviour, like risk assessment. But complex plant behaviour — like foraging for food or recognizing friends and foes — seems more like science fiction than science. However, plants may behave with more purpose than we have been giving them credit for.
Dr. James (JC) Cahill is an experimental plant ecologist at the University of Alberta. Cahill says plant biologists have focused on short-term responses to stimuli, but animal biologists have studied both short-term responses and the deeper questions around why animals behave the way they do.
Understanding plant behaviour may help the agriculture industry harness a plant’s natural potential to feed and protect itself, increasing yields, says Cahill.
“Nobody’s tried to do this. There’s great genetic variation sitting out there for breeders. There are great potential direct changes in agronomic practices to enhance some of these behaviours.”
Plants can defend themselves against insects by releasing chemicals. For example, wheat breeders have developed midge-tolerant wheat varieties that produce phenolic acid when attacked. The wheat midges basically starve to death after biting the kernel. Other plants issue a chemical SOS to attract insects that will prey on the insects eating the plant.
But fluctuating insect populations complicate a plant’s defence strategy. Insect defences require resources, so “many species will mount a defence only when there’s a proven threat of being consumed,” says Cahill.
Cahill says some plants will also work with neighbours to trigger a collapse in the insect population. Researchers defoliated an alder tree and studied leaf damage in surrounding alders. Cahill says there was no pattern in leaf damage before the alder was defoliated.
“But once that first tree was damaged, all of a sudden, other trees near the damaged tree didn’t get damaged. The trees far away did,” Cahill says. A beetle, which feeds and lays its eggs on the trees, was unable to lay eggs on trees near the defoliation.
“This is communication. The plants were somehow telling their neighbours that there’s a threat and those neighbours (were) now inducing their defences even though they hadn’t yet been attacked.”
Knowing your neighbours
As any farmer knows, bugs aren’t the only pests plants have to fend off. Weeds compete with plants for sunlight, water and nutrients.
Some plants growing in shade respond by growing tall and thin, while plants with abundant light grow short and fat. But growing taller sacrifices resources in other areas, such as seed production, so it makes sense for plants to only grow as tall as needed.
Cahill says whether or not a plant can detect its neighbours has agricultural implications.
“That’s going to influence how a crop interacts with weed species. And whether it lets itself… get overtopped.”
Plants can win the weed battle through suppression, avoidance, or tolerance. Herbicides and planting systems use suppression to fight weeds. “But we often forget there are two other ways to deal with these pests. You can win through avoidance and tolerance, not just suppression.”
Cahill says in theory weeds should only be a problem if they are suppressing yield or affecting the crop’s purity. Crops may be able to avoid competition with weeds by sending their roots to weed-free parts of the soil.
Foraging affects nutrient uptake
Cahill’s main expertise is foraging in plants. He says plant biologists have focused on what plants do once they have food, but they haven’t studied how plants actually find food.
“And this is important, because when we talk about different fertilization placement plans — banding, near the seed, far from the seed — those are foraging issues.”
Understanding how roots find nutrients would lead to better fertilizer placement, and less fertilizer wasted or used by weeds, Cahill says.
Grasses don’t tend to concentrate their roots in nutrient patches, says Cahill. Canola roots, on the other hand, zoom in on nutrient-rich patches. Roots hosting micorrhizae grow much more slowly towards nutrient patches than micorrhizae-free roots.
Cahill’s research with yarrow shows that some roots stick to nutrient-rich patches.
“What this means is that if your nutrients are right near the plant, that root system is not going to expand. Your roots are going to be right near the base of the plant. That’s efficient, but it may also make it susceptible to lodging.”
Cahill and his colleagues published a paper in the journal Science showing velvetleaf, a weed affecting soybeans and corn, integrates information about nutrient placement and competitors. When grown alone, the weed sent out many lateral roots, whether nutrients were spread evenly or concentrated in patches.
If velvetleaf had neighbours, it would compete by sending roots to nutrient-rich patches. But when nutrients were spread evenly and the plant had neighbours, lateral root growth stopped.
“Does that have agricultural implications? Yeah, because that means if you’re putting any nutrients where those roots aren’t, they’re wasted. The only things that will get them are weeds. So not understanding how a plant sees the soil environment likely leads to lost money.”
Some plants can even identify their kin, and will treat their relatives differently than neighbouring strangers, Cahill says. Ontario researchers studied searockets, an annual found in dunes, to see whether it modified its root growth based on neighbours. Searockets planted near strangers grew many more roots than searockets growing next to kin.
“These plants were enhancing competition with strangers and minimizing it with kin,” says Cahill.
Researchers don’t yet know which species acknowledge kin and which don’t. Cahill’s research has shown that while some plants won’t compete with kin when nutrients are plentiful, if nutrients are scarce, family ties don’t tone down competition.
But improving kin recognition in crops might bump yields, Cahill says.
Crop research planned
So far Cahill’s research hasn’t included crops. He says it made more sense to start in a simple lab system before researching complex agricultural systems.
“It doesn’t really matter what plant species we start with. We just need to understand what plants are capable of. And once we understand the basic functions of plants, we can then begin to move into a more applied context.”
Cahill hopes to expand his research into crops in the next five years or so. He’s now talking with colleagues in the University of Alberta’s agricultural school about collaborating.
Cahill is flexible in terms of which crops he’ll study. But canola is an obvious choice because it’s widely grown and because it doesn’t host micorrhizae. No micorrhizae means the plant relies entirely on its roots to secure nutrients, making root activity particularly important.
Though the idea of plants recognizing and reacting to kin, competitors, and pests may seem weird to us, Cahill says these processes exist. “And if we ignore it because it’s an idea we’re uncomfortable with, we’ve probably lost some yield potential.”
For more information about Cahill’s research, visit hocking.biology.ualberta.ca/bioscholar/cahill_lab. †