Pests are unwanted organisms (plants or insects) that may cause damage to other organisms or property. Pesticides aim to eliminate these pests and protect target areas from further damage. For plants, pests include microorganisms, fungi, insects, and larger herbivores. In response to pests, plants have a slew of natural defense mechanisms against pests. According to the University of California Agriculture and Natural Resources, one should only use a pesticide after one is sure one needs it. Though pesticides have benefits and drawbacks compared to natural plant defense mechanisms, there are many caveats in the debate about when farmers should utilize them.
Natural Plant Defense Mechanisms
Plants are inevitably prey to herbaceous and pathogenic duress and undeniably need safeguards against these forces. They lack the array of defense mechanisms that complex heterotrophs contain. Therefore, varieties of vegetation have evolved various structures with a designated function in defense against herbivores and pathogens. In the subsequent sections, this post will list and describe some plant defense mechanisms. Following, this blog includes information concerning pesticides and evaluates their use.
Flowering plants, or angiosperms, have a variety of structures that act as defense mechanisms. Angiosperms, among other plant classifications, have a cuticle.
This cuticle gives a wax texture to the leaves. Consequently, the cuticle makes the plant impermeable and creates a seal between the plant epidermis and the surrounding environment. However, the cuticle does not make the plant completely impermeable. The cuticle works in conjunction with the stomata, allowing for the permeation of needed molecules into the cell, including oxygen and water. Stomata are tiny pores on leaves that allow for gas exchange.
The cuticle may differ from species to species. For example, aquatic plants have a thin cuticle, while cacti have a thick cuticle. This difference has much to do with water retention. The cuticle also prevents water from collecting on the surface of leaves. The hydrophobic character of the cuticle prevents the accumulation of standing water. As a result, fungal pathogens that thrive around water cannot breed and germinate spores harmful to the plant.
Beneath the wavy cuticle, plants have an epidermis layer. As one of the peripheral layers, the plant epidermis plays an essential role in the defense against pathogens. Moreover, the epidermis contains specialized and unspecialized cells that help in plant defense.
External Plant Structures
When examining a plant without a microscope, one can see the variations in texture aside from the cuticle. Specialized epidermal cells create plant textures, which provide some defense for the plant against outside forces. Trichomes create the velvety texture found on some plants and serve the purpose of preventing direct contact between insect eggs and the epidermis. They may be branched, unbranded, spiral, hooked, or pointed. Some plants have glandular trichomes that secrete volatile organic compounds (VOCs).
Efficacy of Trichomes
The symbiotic relationship between insects and the density of trichomes displays a direct correlation. A study concerning the defense of plants against herbivores found that adult leaf beetles, Phratora vulgatissima, may induce trichome density in Salix cinerea, which is a large grey willow. The willow has trichomes that inhibit the movement of insects on the plant surface, preventing interactions with the beetle. In leaves that already blocked P. vulgatissima from interactions with epidermal cells, the number of trichomes was stagnant. However, P. vulgatissima induces an increase in the density of trichomes in new willow leaves following interaction. Over time, trichome density increases from 25% to 100%.
Other Plant External Textures
Other plant species experience a similar correlation, like S. cinerea and P. vulgatissima. Phellem, or tree bark, is another structure that forms a barrier between the external environment and the external environment. Phellem does not contain the same cuticle found in the typical angiosperm, but it does have suberin, a hydrophobic compound. Aside from phellem and trichomes, plants may also have thorns, spines, or prickles. Thorns protect plants from grazing animals. Spines are the pointed appendages on cacti and serve a similar purpose as thorns, and prickles are small outgrowths on epidermal cells. Notably, roses have prickles, despite being known as thorns.
Sclerophylls, another classification group of plants, have sclerenchyma within their leaves. Sclerenchyma gives plants a hardened structure and decreases the digestibility of the plant leaves. Therefore, sclerophylls have protection from herbivores.
Plant Cell Defense Mechanisms
Plant defense mechanisms against herbivores and pathogens extend beyond the morphological makeup of the whole plant. The individual plant cells also plays a role in defense against predators. In other words, plant cells have structures that act as molecular barriers and receptor molecules that can detect an array of stimuli and generate a response.
The Plant Cell Wall
The cell wall is the first line of defense against pathogens. It consists of both a primary and secondary cell wall. Also, the primary cell wall has compounds that allow for intercellular communication. This characteristic is beneficial for cells to have a collective response of defense against invading pathogens. For example, glycans and hemicellulose form linkages between one cell and surrounding cells.
Pectin is also present between plant cells that acts as an extracellular matrix between cells that puts them in place. The primary cell wall is also responsible for turgor and flexibility, evident by its components, including cellulose and microfibrils. Different cells part of the epidermis has other molecules that contribute to rigidity, including Cutin, Suberin, and other waxes (like the cuticle). The secondary cell wall location is between the primary cell wall and the cytoplasm of the cell. It is present only after a cell stops growing and expanding.
Secondary Metabolites in the Cell Wall
On the cell wall, there are also callose deposits that may form. Callose has a high molecular weight and accumulates at the site where pathogens are invading plant cells. Considerably, callose is a secondary metabolite. But, it also forms the structural barrier to inhibit the infiltration of the cell. Callose may deposit in the papillae of the cell wall, which also helps prevent the infiltration of pathogens at the site of the infection.
Papillae are protrusions from the cell wall, serving the purpose of increasing the surface area of the cell wall. To explain, maximize contact with the surrounding environment through the cell wall. Callose deposition is a defense mechanism used in angiosperms.
Idioblasts are specialized plant cells that signal defenses against small insect herbivores. They may contain toxic chemicals or crystals as defense mechanisms. There are different types of idioblast cells.
One class of idioblasts is pigmented cells. Pigmented cells contain tannins that make the plant parts unpalatable to the insect herbivores due to their bitter taste. The bitter taste can deter an organism from attempting to consume a plant.
Another class of idioblast cells is sclereids, which cause the plant to be unpalatable for an insect herbivore. Sclereids are the individual cells found in sclerophylls. They are irregularly shaped cells with a thick secondary cell wall.
The third class of idioblasts is crystalliferous cells that contain the chemical compound calcium oxalate. Calcium oxalate causes irritation and swelling in the mouth and throat of the herbivore that consumes it and even holds the potential to be toxic if it is fully ingested. This defense mechanism, like pigmented cells, may deter an insect from consuming the plant subsequently.
The final class of idioblasts is silica cells. These cells are similar to sclereids, as both provide some extra rigidity to plant parts. Silica cells, however, occupy growing leaf blades. Like sclereids, silica cells make leaves unpalatable and deter insects from consuming growing leaf blades.
Secondary Metabolites and Volatile Organic Compounds
At a cellular and morphological level, there are a variety of defense mechanisms that plants display. The anatomy of plants holds the capacity to be a physical deterrent to predators. Anatomical structures may also secrete secondary metabolites as a defense mechanism. Glandular trichomes can secrete oils and metabolites, such as flavonoids, terpenoids, and poisonous alkaloids. These metabolites can repel, kill or trap organisms that jeopardize the plant; secondary metabolites are most effective against small insects.
Some plant tissues do not secrete these secondary metabolites. Instead, plant structures laced with secondary metabolites are almost always in action against herbivores. Secondary metabolites with insect-repellent properties include lemon, basil, sage, and peppermint. Pyrethroids are insect repellents produced by chrysanthemums and are common in commercial insecticides.
There are also secondary metabolites that directly come in contact with herbivores. Tannins, for example, make the herbivore sick by binding to digestive enzymes and disrupting the digestion process. Substances such as opium, cocaine, nicotine, and caffeine are considered drugs for humans. They can have similar effects on the insects and herbivores that ingest them off of a plant; they can disrupt normal nervous system function.
Requirements for VOCs
Though these secondary metabolites can protect the plant, they hold the disadvantage of requiring significant ATP input, as well as a multitude of invested materials to be created. Though the structure of plant cells and appendages such as trichomes allows for the production and secretion of secondary metabolites, it is not the most ATP-efficient option for defense against herbivores and pathogens.
Why are Pesticides Used on Plants?
Given the extensive details of natural plant defense mechanisms, it seems that plants have many measures to repel pests. So, why are pesticides used? Some target organisms resist natural defense mechanisms. Resistance is due to adaptations or just natural resistance. Natural resistance often occurs when the pest and plant are from different environments.
Benefits of Pesticide Use on Plants
The use of pesticides does have numerous benefits beyond saving crops.
Increased Shelf Life
Pesticides can prolong the shelf-life of crops. According to the United Nations, around 690 million people are food-insecure. In addition, 381 million people are undernourished, and a majority of this population lies within Asia and Africa. The use of pesticides prevents infection and damage to plants after harvest. Therefore, shipping companies can transport crops farther from the point of origin and have more time for transport. The situation is similar to a milk shed.
Milkshed refers to the functional region surrounding a milk farm. Milkshed is limited to the farm and the surrounding areas where one can transport milk without it spoiling. Correspondingly, the creation of a refrigeration system for transportation has expanded the milk shed. Like refrigeration systems, pesticides increase the distance that companies can transport crops.
Pesticides allow the production of crops throughout the year and for an affordable price. Take tangerines, for example. They are a reliable source of Vitamin C and are available at grocery stores year-round. But, the season for tangerines is between November and May. Pesticides allow tangerine harvests throughout the year, and therefore, help improve the health of the consumers. Pesticides increase the yield and quality of produce too.
Better Quality Produce
Given the natural plant defense mechanisms, pesticides also add an extra layer of protection to crops. As a result, pesticides increase crop yield while instances of plant infection decline. Additionally, pesticides decrease the risk of food-related illness in the consumer. Though pesticides prevent crop loss, they have some notable disadvantages.
Disadvantages of Pesticide Use on Plants
Pesticides are often synthetic chemicals that can damage the ecosystem and pollute the air, water, and soil. Pollution calls for methods of cleaning the environment because pollution adversely affects organisms. Also, frequent use of pesticides promotes pesticide resistance among target organisms.
Pesticides cause adverse health effects on those that work while exposed to them; symptoms of pesticide poisoning range from mild to severe. Mild exposure to pesticides may cause headaches, blurred vision, slowed heartbeat, nausea, and contracted pupils. More moderate exposure to pesticides can cause inability to walk, chest tightness, muscle twitching, and involuntary bowel movements. Severe poisoning can cause unconsciousness or seizures. Frequent exposure to pesticides may lead to conditions such as cancer.
When Should Farmers Use Pesticides on Plants?
Plants have a wide array of natural defense mechanisms. Despite these defense mechanisms, some microorganisms and fungi still find methods of infecting and damaging plants. The use of pesticides eliminates crop loss. Conversely, pesticides pollute the environment and threaten organisms. Given the current climate emergency, natural plant defense mechanisms are the most environmentally conscious methods for evading damage. But, we live in a world where a large portion of the population is food-insecure. Therefore, any means of increasing crop yield is most beneficial to food-insecure regions.
There are so many caveats to the use of pesticides. From the benefits and drawbacks of pesticides and the efficacy of plant defense mechanisms, pesticide use post-harvest seems the most viable option. But, this option is only sustainable if the population is not food-insecure and undernourished. For now, pesticide use seems like the only option for ensuring plants are healthy without genetic modification.