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Plant-Based Sensors Tell Farmers When to Activate their Irrigation Systems

Plant-based sensors capable of measuring the electrical capacitance and thickness of leaves prove to be promising as they can tell  Farmers when to activate their irrigation systems, in order to prevent both parched plants and water waste, according to Researchers in Penn State's College of Agricultural Sciences.

Researchers integrated the capability to simultaneously measure leaf thickness and leaf electrical capacitance into a leaf sensor to monitor water stress in plants. Image: Penn State

It is important to continuously monitor plant "water stress" in arid regions and this has been traditionally performed by measuring soil moisture content or developing evapotranspiration models that have the potential to calculate the sum of ground surface evaporation and plant transpiration. However, the aim is to increase water-use efficiency with new technology that can detect when plants need to be watered in a more accurate manner.

This study was recently published in Transactions of the American Society of Agricultural and Biological Engineers. For this study, Lead Researcher Amin Afzal, a doctoral degree candidate in Plant Science, incorporated into a leaf sensor the potential to simultaneously measure leaf electrical capacitance and leaf thickness, which has never been done before.

The work was performed on a tomato plant in a growth chamber with a constant temperature and 12-hour on/off photoperiod for 11 days. The growth medium used was a peat potting mixture in which water content was measured by a soil-moisture sensor. For the first three days, the soil water content was maintained at a comparatively high level and then allowed to dehydrate over a period of eight days.

The Researchers randomly selected six leaves that were directly exposed to light sources and then mounted leaf sensors on them, avoiding the edges and the main veins. Measurements at five-minute intervals were recorded.

Variations in the daily leaf-thickness were minor, with no major day-to-day changes when soil moisture contents ranged from high to wilting point. However, leaf-thickness variations were more noticeable at soil-moisture levels below the wilting point, until leaf thickness steadied during the final two days of the experiment, when moisture content touched 5%.

The electrical capacitance, which exhibited the potential a leaf to store a charge, remained roughly constant at a minimum value during dark periods and rapidly increased during light periods, highlighting that electrical capacitance was a reflection of photosynthetic activity. The daily electrical-capacitance variations reduced when soil moisture was below the wilting point and totally stopped below the soil volumetric water content of 11%, signifying that the effect of water stress on electrical capacitance was observed via its impact on photosynthesis.

Leaf thickness is like a balloon — it swells by hydration and shrinks by water stress, or dehydration. The mechanism behind the relationship between leaf electrical capacitance and water status is complex. Simply put, the leaf electrical capacitance changes in response to variation in plant water status and ambient light. So, the analysis of leaf thickness and capacitance variations indicate plant water status — well-watered versus stressed.

Amin Afzal, Lead Researcher and a Doctoral Degree Candidate in Plant Science

The study is the most recent in a line of research Afzal believes will end in the development of a system in which leaf clip sensors will be able to send exact information about plant moisture to a central unit in a field, which at that point communicates in real time with an irrigation system to water the crop. He imagines an arrangement in which the central unit, sensors and the irrigation system all will communicate without wires, and also envisions that the sensors can be powered wirelessly with solar cells or batteries.

"Ultimately, all of the details can be managed by a smart phone app," said Afzal, who studied Electronics and Computer Programming at Isfahan University of Technology in Iran, where he received a bachelor's degree in Agricultural Machinery Engineering. Afzal is testing his working concept in the field at Penn State.

Two years ago, Afzal headed a team that won first place in the College of Agricultural Sciences' Ag Springboard contest and was awarded $7,500 to help develop the concept. This contest is an entrepreneurial business-plan competition.

Growing up in Iran, Afzal ia aware of the fact that water availability determines the fate of agriculture. In the last decade, the Zayandeh River in his home city of Isfahani has dried up, and a number of Farmers can no longer plant their usual crops.

Water is a big issue in our country. That is a big motivation for my research.

Amin Afzal, Lead Researcher and a Doctoral Degree Candidate in Plant Science

Afzal's technology proves to be very promising, noted Sjoerd Duiker, Associate Professor of Soil Management, a member of the research team and Afzal's adviser. The existing methods for determining irrigation are considered to be crude, whereas Afzal's sensors work directly with the plant tissue.

"I believe these sensors could improve water-use efficiency considerably," Duiker added. "Water scarcity is already a huge geopolitical issue, with agriculture responsible for about 70 percent of world freshwater use. Improvements in water use efficiency will be essential."

In a follow-up study, Afzal recently completed assessing leaf sensors on tomato plants in a greenhouse. The results proved the outcomes of the just-published study. In his new research, Afzal is creating an algorithm capable of translating the leaf thickness and capacitance variations to significant information about plant water status.

Jack Watson, Professor of Crop and Soil Science, and Dawn Luthe, Professor of Plant Stress Biology, also participated in the research.

This work was supported by the U.S. Department of Agriculture's National Institute of Food and Agriculture.

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