What follows is an article I submitted to the Economist for their tech and science internship. I’m working on exploring testing of yields in my home garden currently and will have a separate post on that when the results are in.
Helping Plants Help Themselves
A well-tested biological polymer, chitosan, could be used to improve yield and reduce water consumption
For many readers right now, they think back to warm summer days and the smell of fresh hexanol longingly waiting for the next summer to come about. Hexanol is better known as one of the main components of fresh cut grass and signals to the plant being damaged to induce a chemical response to protect against mould and predators. The defence response in kind produces a more resilient plant, and researchers have been looking for ways to reproduce this response without actually damaging the plant.
One such successful stressor is chitosan, a sugar polymer derived from chitin, found in abundance in crustaceans and fungus. Chitosan elicits a defence response similar to a fungal attack due to it sharing what is called a microbe-associated molecular pattern, MAMP, with known pathogens. This response causes both a local cellular and systemic response eventually building a systemic acquired resistance against further attacks.
At a local level, cells respond to the MAMP by fortifying the walls and boarding the windows. Most plant cells under attack will begin fortifying the cell wall by depositing lignins, which strengthen the cell, help make it impermeable to pathogens, and catalyse a burst of oxidative chemicals capable of damaging incoming pathogens and signalling to other cells of the attack. Boarding of the windows is accomplished by guard cells which can close the stomata which regulate the flow of carbon dioxide and water vapor needed for photosynthesis. This local response has been evidenced produce plants that are more resistant to rot and require less water due to decreased water vapor loss through the stomata. Studies have shown that some plants can require as much as 43% less water with no loss in yield due in part to the MAMP-induced response.
While producers benefit greatly from the plant’s cellular response, consumers gain the most from the plant’s systemic response. The flavours and colours that consumers enjoy come from the secondary metabolites that the plant uses primarily for defence. The major classes are terpenoids, the essential oils that citrus and herbs produce; phenolics, flavonoids which provide most of the pigment for flowers’ colours, tannins, and other antioxidants; and alkaloids such as caffeine, capsaicin, and nicotine. Chitosan has shown to boost terpenoids up to two and half times and phenolics up to three and a half times as reported by Hyun-Jin Kim in one study done of chitosan-treated sweet basil, thereby improving the taste, appearance, and antioxidant properties of basil as well as the improving yield.
Studies so far have focused primarily on the objective qualities such as yield, crop loss, and other measurable changes in the plant itself, but less work has been done exploring the impact of chitosan-treated produce on the consumer. Chitosan has been regulated by both the Environmental Protection Agency (EPA) and European Commission (EC) for the use in crop treatment. Both have determined that chitosan is safe for the environment and the consumer however they differ as to the specifics of its application. The EPA in 2005 determined it to be an acceptable adjuvant when used with an active pesticide but explicitly denied its use as a plant defence booster or growth enhancer. The European Commission petitioned by ChiPro, a German company, determined it to be admissible as a “basic substance” under new guidelines. A basic substance is any substance found safe that are useful for plant protection but not typically used as such. The EC’s decision was made in 2014 and has given little time for producers to explore the addition of chitosan on the impact on crops and the market for their crops. As it stands chitosan has yet to see wide application by any producer in the USA or EU, despite having proven safe and efficacious for agriculture. The question remains: while chitosan has helped plants win the battle, will it help them win the war?