Plants get stressed when eaten

Do plants get stressed out? If so, what causes it and how do plants react to stress? 

Read the Malay version here.

When we think about stress, we usually paint it as a bad thing. But in reality it’s how we get along in the world. Our stress response helps us navigate an ever-changing environment—it’s the reason we’re able to react to new situations to figure out if they’re safe or dangerous. Without it, we wouldn’t survive. The same goes for plants, which are stressed by environmental changes that threaten their health, such as flood or drought, or by critters that would harm or kill them, like pathogens and plant-eating insects.

Plant stress

Plant stress is classified into exogenous stress and endogenous stress. Exogenous stress is stress caused by environmental factors and is classified into biotic and abiotic stress factors. Biotic stress factors include infections, herbivory stress and competition.

Herbivory stress

Plants get stressed when animals eat them and mount a defensive response, a biochemical or physical counterattack. Fungi, bacteria and protists that feed on living plants are termed plant pathogens. Microbes that feed on dead plants are saprotrophs. Flowering plants that obtain nutrition from other living plants are parasitic plants. 

Plants protect themselves by limiting the ability of herbivores (animals that eat plants) to eat them. Although herbivores are genuine threats to immobile plants, greater threats come in the form of viruses, bacteria and fungi that infect the wound left by the animals. 

Change dynamics

Plants respond to herbivore damage over spatial scales (from leaves to whole trees) and over temporal scales (from minutes to evolutionary time). Many plants are filled with toxins that kill herbivores or make them ill. Over 3000 plant species produce cyanide-containing compounds that break down cyanide when cells are damaged. The cyanide blocks cellular respiration, effectively damaging the herbivore.

Tobacco have elevated levels of nicotine that are lethal to hornworms. If plants cannot kill its attackers, it can overstimulate them with caffeine or sedate them with morphine. 

Protection by animals

Acacia trees and ants. Several species of ants provide small armies to protect Acacia trees from other herbivores. The ants attack other insects and small mammals. The Acacia tree in turn provides the ants with sugar or food at the tips of the leaves.

Parasitoid wasps, caterpillars and leaves. Caterpillars eat leaves before entering metamorphosis to turn into moths or butterflies. In some cases, leaves produces chemicals that are enough to deter the caterpillars. However, some plants have evolved to incorporate a more complex strategy. As caterpillars chew on leaves, plants release a volatile gas that attracts a female parasitoid wasp. The wasp lays her fertilized eggs in the body of the caterpillar. The eggs hatch and the larvae kills and eats the caterpillar.