Plant communication southwest trees and turf june 2015

Plant Health: “Just Do It” Bob Morris Horticulture Consultant for Viragrow, Inc. Plants are competitors in the most extreme sense of the word. They don’t think about competition. They don’t have brains. Using the Nike catchphrase, they “Just Do It.”. If they are going to “Just Do It”, plants must be in good health.

This means a “healthy” soil.

Most of our ideas of what competition is comes from the animal world. Most of that relates to our sense of vision. The opponents size, color how it poses itself toward others helps determine if competition might lead to success or not. To a lesser degree, perhaps, our sense of hearing contributes as well but certainly vision dominates our Figure 1. Nike's logo is a popular phrase for getting things done. interpretation of competition.

Do plants compete? If they do, inherent in the word “competition” means they must “sense” their opponents. They can’t “see” or “hear” their competition like animals. They must use other “senses”. To decide if plants do compete or not, it is important to look at what senses they might use to “see” and “hear” their competition.

Unlike animals, plants live in two totally different “worlds” or environments; above-ground and below-ground. It is almost as if plants are two different creatures, above and below ground creatures, linked together physically and genetically.

The above-ground environment is much different from the below-ground environment. The above ground plant creature is easier for us to understand Figure 2. Tree root spread is related to the genetics of the tree, type because that’s where we live; air, light, of soil, the size of the tree and distribution of rainfall. movement of air, hot and cold temperatures, moisture present in the air as well as rain stimulate our senses to name a few. Air and light are primary “rivers” that plants and animals use for “communicating”.

The below ground environment is more stark and more difficult for us to understand. First and foremost, there is no light and so communication using light is much more limited. Likewise, variations in temperature are much less dramatic below ground.

But a significant difference between these two environments lies in chemistry and the presence or absence of air and water which impacts soil chemistry. The movement of water, and to a lesser degree air, are primary “rivers” of communication used by this underground plant creature.

The easiest of these plant “creatures” to study, and the one we relate to best, lives above ground. Less is known and understood about the below ground plant creature, the root environment.

Scientists call this underground plant environment the “rhizosphere” (coining it from the word “atmosphere”). When we think about it, the rhizosphere is a very busy place. It can be densely populated with microorganisms like bacteria and fungi, macro organisms like worms and insects as well as roots from 3. What told this purple leaf plum to send up these neighboring plants. Some of these organisms Figure green suckers from its rootstock? Communication? Maybe not are “friends” and others might be considered how we define communication but communication was involved through edaphic factors. “foes”.

Organisms and plant roots living in this rhizosphere are in competition for space, water and nutrients. Each of them wants to reproduce and grow to their fullest potential; their measurement of “success”. If resources like space, water and nutrients are limited, there is going to be competition for them. If there is competition, there must be communication. If there is communication, plant roots and soil organisms must be able to send and receive “signals” from each other.

There is a growing body of scientific evidence over the last 30 years that this method of communication is chemical and that plants can actively change the chemistry surrounding their roots in a number of different ways. In the case of plant communication with organisms and other plants, scientists believe plant roots release chemical signals broadly called exudates. Root exudates are not simple and few. Collectively, root exudates may number 100,000 different kinds or more and range in complexity from single protons to large molecules.

Production of root exudates, and the kind produced, is determined by what plants sense is surrounding them. Exudate production is greatly affected by environmental factors surrounding the root, termed by scientists as edaphic factors; nutrients in short supply, drought, quality of soil

water, soil air, opportunities to “partner” with beneficial organisms and possible invasion by disease organisms. These edaphic factors communicate to the plant if and how the plant needs to respond chemically and physically.

Scientists believe that plants can take an active role in “signaling traffic” between roots of competing plants and soil microorganisms. Neighboring roots and soil microorganisms send one way signals to roots that communicate the nature of the soil chemistry and whether something needs to be done or not.

Figure 4. The significance of root exudates as belowground defense substances has long been underestimated, presumably due to being buried out of sight. http://www.sciencedirect.com/science/article/pii/S1360138513002598

Scientists are suggesting that roots may have a very significant role in regulating the populations of microorganisms. Root exudates play an important role in “encouraging” beneficial microorganisms such as mycorrhiza to colonize them and may stimulate soil microorganisms that are antagonistic to pathogens if those pathogens are nearby. It has been known for a number of years that plant roots can inhibit the growth of other plant roots through a process called allelopathy.

Plant health and development, both above ground and below ground, depends on the ability of roots to communicate with microbes. But the opposite is also true. Many soil bacteria and fungi

depend on their associations with plants and these associations are impacted by root exudates. In order for beneficial soil microorganisms to invade plant roots and become colonized, the plant must signal and allow for this invasion to occur. This is done by roots secreting organic and amino acids, basically a “welcome sign” for these organisms.

Scientists have shown that chemicals released by plant roots may not be the only factor involved. Electrical charges on the surface of roots may also play a significant role. By changing the electrical signals on root surfaces, research scientists have been able arbitrarily “turn off” or “turn on” this welcome sign to invading organisms.

Scientists believe these electrical charges may signal to microorganisms whether a root is living and “ripe” for invasion or not.

Root exudates also act as “repellents” or antimicrobials that can ward off unwanted microorganisms, insects and even other plants giving the plant defensive advantages. Root exudates have both anti-bacterial and antifungal properties and, regulated by the plant, may work Figure 5. together to give a desired result. http://rydberg.biology.colostate.edu/Phytoremediation/20 Different plants release different types of 08%20websites/Alford%20Phytostimulation%20Webpage root exudates but frequently these exudates are /rhizosphere.htm similar among plants in the same family. Successful invasion by a micoorganism depends on its ability to “detoxify” the root exudates produced by that plant, circumvent its defenses and gain entry.

Plant roots are under constant harassment by soil microorganisms and macro organisms.

Plants must be able to determine whether the attack is “friendly” or “unfriendly” and are constantly secreting exudates in response to the presence of specific soil organisms. Scientists have shown that roots can selectively sense a pathogen in the vicinity, resulting in the production of exudates that ward off the pathogen.

Another mechanism to cope with these stresses is the production of plant cells in front of very susceptible portions of the plant roots. These specialized cells, border cells, are not attached to the root. Border cells’ primary responsibility is to provide “perimeter security” or a “front defensive line” that protects plant roots. Border cells continuously produce and secrete exudates into the rhizosphere. Scientists have demonstrated that these chemicals do in fact influence the behavior of soil microorganisms.

Obviously, plants must be in good health for these things to occur. Total plant health is reliant on “soil health”.

Bob Morris is a Horticulture Consultant for Viragrow, Inc. in Las Vegas, Nevada.