Thermal imaging aids in early detection of glyphosate effects on trapoeraba

The occurrence of stress in plants caused by factors such as lack of water, diseases or pests can be monitored by thermal sensors, through the relationship with transpiration and stomatal conductance. Variations in leaf surface temperature are associated with the opening and closing of stomata present in leaf structures, which are responsible for gas exchange, regulating the entry of carbon dioxide and the release of water and oxygen in the form of vapor.

Cameras that capture thermal images, known as thermographic cameras, have a microbolometer sensor capable of identifying thermal energy in the region of the electromagnetic spectrum in which waves of length ranging from 8 μm to 14 μm are emitted. The sensor is composed of a matrix of heat-sensitive pixels, which transforms thermal energy into electrical signals and, from this, the thermogram or thermal image in which the conversion of electrical signals into temperature color tones is observed.

Thermal infrared remote sensing can then be performed using thermal cameras attached to ground or aerial vehicles (drones) to detect changes in crops related to changes in canopy temperature, as a result of changes in physiological variables resulting from stress. With the advancement of intensive computing and the development of new image processing algorithms, systems for detecting plant injuries using imaging sensors are increasingly accurate and can be incorporated into phytosanitary management.

Effects of herbicides

It is known that herbicides can cause a series of physiological effects on plants. Glyphosate, for example, one of the most widely used herbicides for weed control, acts by inhibiting the enzyme 5-enolpyrulvilshikimate-3-phosphate synthase (EPSPs), interrupting the production of essential amino acids in the production of the cell wall. It affects several fundamental processes in plants, such as inhibiting the synthesis of aromatic amino acids, causing damage to metabolism, changes in growth and morphology, disruption of the root system and inhibition of photosynthesis, causing damage to plant tissue cells, yellowing, wilting and necrosis, leading to plant death. It is a non-selective, systemic product, widely used in the field as a pre- and post-emergent agent in several crops.

Resistance mechanisms

With the excessive use of glyphosate, some weeds have developed resistance and/or tolerance mechanisms to its use, either by modifying the product's absorption route or even by altering the plant's own morphological characteristics. However, the use of adjuvants in the spray mixture is a strategy to enhance the herbicide's effect, and can improve characteristics such as translocation and absorption.

Study and its results

Using a thermal camera, a study was conducted in a protected environment on the premises of the Instituto Federal Goiano, in southeastern Goiás, in which the biochemical and leaf temperature associations of a weed species, trapoeraba (Commelina benghalensis), under the effect of glyphosate applied with and without adjuvant.

After two days of applying the herbicide, it was already possible to observe an increase in the temperature of the leaves of the plants that received it, thus demonstrating an early detection of phytotoxicity, that is, before the common visual symptoms caused by glyphosate appeared.

Regarding the physiological changes related to temperature changes, it was observed that management with glyphosate combined with the adjuvant promoted accumulation of shikimic acid, due to the inhibition of EPSPs.

The results also revealed a positive correlation between leaf surface temperature and superoxide dismutase (SOD), an essential enzyme in plants that plays a key role in protecting against oxidative stress. It specifically acts on the superoxide radical (O₂⁻), a highly reactive molecule produced in metabolic processes, especially during photosynthesis and cellular respiration.

The study also allowed us to verify a negative correlation between leaf temperature and malondialdehyde (MAD), the main degradation product of lipid peroxidation that occurs when cell membrane lipids are oxidized by free radicals, such as the superoxide radical (O₂⁻) and hydrogen peroxide (H₂O₂).

The thermal images were captured 80 cm from the targets, with a Flir C2 thermal imaging camera, between 8 am and 10 am, a period of high heat exchange.

* By Ana Caroline de Araújo, Anderson Rodrigo da Silva e Emerson Trogello (Goiano Federal Institute)