Corn plants growing among weeds
THERE GOES THE NEIGHBORHOOD! These corn plants are reacting to an abundance of weeds in the neighborhood. Corn plants sense weeds in the same way they sense other corn plants.

How do plants know they have neighbors?

Corn Illustrated: Plants sense reflected light and know whether or not they are alone.

It’s well-known that plants compete with their neighbors for light. They try to grow taller until they reach the top of the canopy to gain a competitive advantage. But how do they know there are other plants in their neighborhood with which to compete?

Let’s first look at the origin of plant life. It’s generally accepted that after the Big Bang about 13.5 billion years ago, the Earth formed about 4.6 billion years ago. Through the study of rocks and fossils, scientists claim that the first photosynthetic bacteria appeared on Earth about 3.4 billion years ago.

Studies of fossils indicate that fungi first appeared on land about 1,300 million years ago, and the first plants or photosynthetic organisms evolved about 700 million years ago. However, the first land plants with vascular systems that could transport water to different parts of their bodies appeared about 0.475 billion years ago. There were plants like ferns, grasses and trees that could grow tall canopies to capture more sunlight. In competing for light, plants try to grow taller than their neighbors until they reach the top of the canopy.  

Neither plant leaves nor roots touch each other, so how do they know early on that they have competition and start reacting to it? Many plant physiologists in the U.S., Canada and Europe have tried to understand how plants learn about their neighbors. Research conducted by CL Ballare and co-workers published in 1990 indicated that the far-red radiation reflected from adjacent plants is an early signal of competition in plant canopies.

Light reflectance
Light is made up of three major colors: green, blue and red. Red light has two major components: red and far-red. The ratio of red to far-red components lets plants know about neighboring plants.

Shade created by the vegetation canopy is characterized by a reduced ratio of red light to far-red light. Plants detect the competition in part by measuring the relative proportion of red to far-red light in their microenvironment.

Under full sunlight, plants are exposed to relatively equal fluxes of red light and far-red light. However, most red light is absorbed by vegetation, while far-red is reflected. Under persistent far-red rich light conditions, plants initiate a shade avoidance response that includes reduced branching or tillering, enhanced apical dominance, increased plant height, decreased leaf size and early flowering.

Weed factor
This is a brief explanation of how plants learn about their competition and how they respond to beat the competition. Ultimately, the purpose of all plants and other organisms is to produce as many progeny as possible, depending on their environment.

It is obvious how and why early weeds can hurt yield. Crop plants can’t distinguish between other crop plants or weeds. They respond to the light reflected from the chlorophyll of green weeds, too.

It’s so important to control early weeds in order to maximize yields. It’s also important that most seeds are equally spaced within rows and germinate at about the same time. Otherwise, latecomers won’t have much chance to produce progeny!

Nanda is president of Agronomic Crops Consultants LLC. Email him at [email protected] or call 317-910-9876.

TAGS: Weeds
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