Ensuring good growth

THE ROLE OF SEED TREATMENTS IN PLANT COMPETITION

recent research shows that seed treatments can play an important role in plant competition, says Clarence Swanton, a Professor of Crop Science in the Department of Plant Agriculture at the University of Guelph.

Traditionally, we think about competition in terms of competing for resources — light, water and nutrients – but other non-direct sources of competition affect plants as well. We know, for instance, that weeds that emerge with a crop are the most competitive and have an effect on   overall yields.

“If you control the weeds really early in the cycle of a plant, the yield potential of that crop is changed,” says Swanton. “It’s on a totally different trajectory. Even though you’ve done it very early and you still have 90% of the growth ahead of the plant, you have within a few days – or a few leaf stages – affected the actual growth and yield potential of that plant.”

That yield loss is permanent, he says. But what happens if those same weeds emerge three leaf stages later? There is, in fact, no yield loss – even at the same densities.

“What could have possibly changed in a seedling at such speed, in such early timing, to have changed the plant’s entire response to stress?” asks Swanton. If you’ve done everything right then the plants aren’t competing for light, there are ample nutrients available, and you’ve planted to moisture depths. So what else can it be? Can the plant detect its environment? Can it adjust quickly? And is it possible that there’s actually early communication going on?

Biological and physiological plant changes
Shade avoidance is an evolutionary trait in plants, which is the ability to detect changes in light quality. Swanton and his team looked at how shade avoidance affects plants and whether or not it affected yields down the line. To do this, they ran a series of experiments using perennial rye grass. “The far-red reflecting off of the weed surface is picked up by phytochrome in the actual corn plant, and that triggers a whole cascade of biological and physiological changes that occur in the plant,” says Swanton.

If the corn plant detects a weed, two things happen: the plant’s production of anthocyanin decreases, and its production of lignin increases. Anthocyanin acts as a sunscreen for the plant and is important in protecting it against sun damage. It also acts as a powerful antioxidant, which can aid in disease resistance.

Shade avoidance also causes the plant to increase the lignin content in its stem tissue in preparation for the competition or stress it senses. It also increases ethylene content in the presence of weeds, particularly in the crown roots. The crown roots will determine the plant’s yield, and ethylene will cause aging and a change in the physiology of the plant.

“Getting this explosion of ethylene into the root system does create a problem in terms of recovery for the plant,” says Swanton.

Finally, shade avoidance also up-regulates the auxin transporter gene in the stem, which changes the entire translocation of hormones in the plant. Basically, it alters hormonal distribution in the plant.

“You can see that the plant is changing dramatically and then you start to think, well, maybe all of these start to play a role in the loss of yield in a plant,” says Swanton.

responding to stress
One of the ubiquitous responses to stress in humans, animals and plants is the explosion of free radicals. There are different types of free radicals; Swanton looked specifically at hydrogen peroxides.

“These free radicals are found normally in plants. Under stress, though, there’s a threshold, and suddenly rather than being a facilitator of physiology, it turns into a damaging component,” says Swanton.

Although they don’t know where that threshold is yet, they do know the kinds of damage it causes. “It damages DNA, proteins, and lipids, inactivates enzymes, alters plant metabolism, and you actually close the stomatal, and then as a result you get photosynthesis down – you get a whole series of things happening all because of that explosion of free radicals.”

All of these things together have to be repaired by the plant. You can take away the weed, but there’s still a physiological price to pay.

“I believe that those plants are now on a totally different growth trajectory,” says Swanton. “In fact, I think it’s like a stairway. I think a plant begins to grow in terms of biomass, it hits a stress and that stress causes a loss of energy that has to repair it. There’s a step there, then it starts on a new trajectory and continues to grow. We always do it with a nice smooth curve, showing you how the plant grows, and I don’t think that’s correct. I think it’s really like a stairway to its final yield potential, and each time it’s on a step, there’s a loss of yield potential because of energy loss.”

seed treatments play a new role
Swanton has discovered, though, that seed treatments can do more than protect crops against disease and insects. Using Syngenta’s Cruiser®, or a basic component of it (thiamethoxam), Swanton designed an experiment, which was replicated five times, where he took treated and untreated seeds and grew them under weed-free and weedy conditions. When exposed to aboveground neighbouring weeds, corn seedlings that originated from seeds treated with thiamethoxam did not express typical morphological shade avoidance responses, says Swanton. In fact, seeds treated with thiamethoxam increased the percent of germination and the length of the root radicle.

“When we treated the seed with thiamethoxam, we negated this shade avoidance effect completely, which has never been recorded in the literature at all before.”

On top of that, where seeds that were treated with thiamethoxam were used, crown root growth was unaffected in weedy conditions. In fact, everything that we know about how plants react physiologically under stress and pressure is negated when seeds treated with thiamethoxam are used.

Under non-limiting resources, this insecticide enhanced seed germination and negated the entire far-red effect. It maintained anthocyanin content, and it activated the antioxidant’s systems so that you have reduced accumulation of hydrogen peroxides. Seeds treated with thiamethoxam not only prevented the loss of anthocyanins caused by neighbouring weeds, but increased the amount of anthocyanins in the plant. Treated seeds accumulated less hydrogen peroxide when exposed to aboveground weed pressure, which means the presence of free radicals is minimized overall.

“Suddenly, you start to think about the role of seed treatments in an integrated weed management system,” says Swanton. “Now you start to think about seed treatments as a way to trigger genes. I think that’s an exciting possibility for the future of seed treatments.” •