Figure from Das, K. and Roychoudhury, A. (2014). Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants. Frontiers in Environmental Science.

Reactive oxygen species act as important signaling molecules in a wide range of physiological processes and must be tightly controlled in order to benefit the plant without causing oxidative stress. While poor environmental conditions and unhealthy crops may allow ROS to accumulate to stressful levels, a healthy plant is able to regulate internal concentrations of ROS with complex antioxidant systems that plants have developed. Plants produce antioxidant compounds, including enzymes such as superoxide dismutase, which scavenge ROS within the cell and balance oxidizing and reducing conditions.

Reactive oxygen species play a significant role in many aspects of plant growth and development. ROS-producing enzymes are involved in the expansion and differentiation of plant cells and organs. ROS are required for leaf expansion, the creation of root hairs and root cell expansion, the strengthening of the cell wall, as well as the formation of pollen tubes.

Changes in environmental conditions cause temporary fluctuations in concentrations of ROS, which plants use to monitor the environment around them and react accordingly. For instance, in response to higher atmospheric CO2 concentrations after the sun sets, plants use ROS to send a signal that causes their stomata to close. There is evidence that ROS are involved in signaling for plant defense and immunity. Attack by a pathogen triggers the production of certain ROS, which ultimately stimulate an antimicrobial or insect-defense response from the plant. ROS also play a role in what is known as programmed cell death (PCD). PCD is a system which terminates specific plant cells when certain biotic or abiotic conditions are met. For instance, in response to infection or damage, a plant can trigger PCD and terminate those cells, thereby reducing the likelihood of the damage spreading to the rest of the plant. PCD is also used during regular developmental processes, such as cell differentiation, leaf senescence and embryo formation.

To promote plant health, growers should do their best to prevent other abiotic stresses which can cause oxidative stress in their crops. These stresses include drought, heat stress, salty soils, and heavy metal toxicity. Free from these pressures, a healthy plant is able to tolerate and regulate temporary increases in ROS. In fact, in moderation these transient oxidative stresses actually promote plant growth and are a natural part of plant development.