- Plants are not alone. A terrestrial plant is surrounded by other plants, countless soil microbes, and indeed pests, pathogens, and humans.
- Just like humans, plants can be choosy about their neighbours and have a love-hate relationship with plants next to them.
- This occurs via allelopathy, a mechanism by which plants produce chemicals that influence the growth of other plants nearby.
Plants are deceptive in their sessile life; it is not obvious that they are busy chemical factories, producing an arsenal of compounds for their survival. While many of these compounds function to ward off pests and pathogens, some are produced by plants to deter neighbouring plant growth. This phenomenon, termed allelopathy, can be exploited in agriculture towards weed suppression.
Location, location, location
A plant knows where it can thrive. Field plants may thrive and yield well in one place, but not in an adjacent position in the same location. Soil quality and neighbours determine a plant’s fate.
Plants are busy chemical factories, producing an arsenal of compounds for their survival.
The black walnut tree earmarks its space by producing chemicals that inhibit growth of surrounding plants. Plants can also thrive in some soils where a different species could have been growing in a previous season. Legume plants (peas and beans), for example, use specialised soil bacteria to increase available nitrogen, which enhances the subsequent growth of a non-legume (maize) in the same soil. The success of a plant therefore depends on chemical interactions.
Choosy chemistry
Plants release chemicals into their surroundings, which then influence the growth of another plant, or other organisms. These chemicals are secondary metabolites, which are unnecessary for primary metabolic processes involved in plant development, but make plants competitive in their environment.
Allelochemicals are those secondary metabolites produced by allelopathic plants that have both positive and negative effects on the receiving plant. Allelochemicals inhibit seed germination, reduce growth, and lower seed production of the receiving plant. In addition, soil microbes influence the secretion and effects of allelochemicals on recipient plants, thereby enhancing or suppressing plant growth. Advances in analysis techniques enable the detection of minute quantities of various types of allelochemicals, providing a good resource to discover sustainable chemicals to use on weeds.
Worrisome weeds
A weed is an undesirable (for humans) plant that competes with a cultivated plant. Compared with pests and pathogens, weeds cause maximum yield losses in agriculture. Historically, agriculture has depended on the use of pesticides, which are categorised as plant protection products. Herbicides fall under this remit, defined as chemicals used to kill undesirable plants. Non-organic farming methods use synthetic herbicides on a large scale to remove weeds, but cause a massive loss of pollinating insects, reduce soil quality, and increase herbicide resistance in weeds. Important global crops, such as varieties of rice, wheat, corn, rye, and sorghum, are allelopathic, and are being exploited to manage weeds in the field. Widespread use of these crops as mulch, crop rotation, and selective breeding of those crops with increased allelochemicals would eliminate the need for synthetic herbicides.
Space invaders
A plant is considered invasive when it is non-native and causes economic or environmental damage locally. It can easily grow out of control and out-compete native plants, partly due to its allelopathic nature to affect native plant performance and soil microbes. Water hyacinth, spotted knapweed, Japanese knotweed, and garlic mustard are some invasive plants that exhibit allelopathy. Surprisingly, scientists have discovered that allelochemicals from water hyacinth control aquatic weeds and algal growth, thereby cleaning waterways.
Invasive plants need to be managed cautiously as they could decrease (by harming other plants) or increase (by producing more flowers themselves) the availability of nectar for pollinators. Complete removal of invasive plants is very demanding and almost impossible, given their resilience. Studying allelochemicals of invasive plants could lead to revolutionary approaches to control their spread.
Plants control where and how they can grow through molecular conversations with their neighbours. It’s good to talk!
Dr Radhika Desikan.
