farming irrigation

Leaf sensors allow farmers to know exactly how much water they need for crops

Plant-based sensors that continuously monitor “water stress” have been developed by a team at Penn State’s College of Agricultural Sciences.

The sensors are capable of measuring both the thickness and electrical capacitance of leaves which help farmers to identify when they should activate their irrigation systems and how much water is needed.

The technology is particularly critical in arid regions and traditionally has been done by measuring soil-moisture content or developing evapotranspiration models that calculate the sum of ground surface evaporation and plant transpiration.

The new leaf sensors provide a boost to water-use efficiency as they are able to more accurately predict when plants need to be watered.

One of the leaf sensors developed by the team

Lead researcher Amin Afzal began the project by working on a tomato plant in a growth chamber with a constant temperature and 12-hour on/off photoperiod for 11 days.

The growth medium was a peat potting mixture, with water content measured by a soil-moisture sensor.

The soil-water content was maintained at a relatively high level for the first three days and allowed to dehydrate thereafter, over a period of eight days.

The researchers randomly chose six leaves that were exposed directly to light sources and mounted leaf sensors on them, avoiding the main veins and the edges. They recorded measurements at five-minute intervals.

The daily leaf-thickness variations were minor, with no significant day-to-day changes when soil moisture contents ranged from high to wilting point.

Leaf-thickness changes were, however, more noticeable at soil-moisture levels below the wilting point, until leaf thickness stabilised during the final two days of the experiment, when moisture content reached 5 per cent.

The electrical capacitance, which shows the ability of a leaf to store a charge, stayed roughly constant at a minimum value during dark periods and increased rapidly during light periods, implying that electrical capacitance was a reflection of photosynthetic activity.

The daily electrical-capacitance variations decreased when soil moisture was below the wilting point and completely ceased below the soil volumetric water content of 11 per cent, suggesting that the effect of water stress on electrical capacitance was observed through its impact on photosynthesis.

“Leaf thickness is like a balloon – it swells by hydration and shrinks by water stress, or dehydration,” Afzal said. “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.”

He envisions an arrangement in which the sensors, central unit and irrigation system all will communicate without wires, and the sensors can be powered wirelessly with batteries or solar cells.

“Ultimately, all of the details can be managed by a smart phone app,” said Afzal.

The study is the latest in a line of research Afzal hopes will end in the development of a system in which leaf-clip sensors will send precise information about plant moisture to a central unit in a field, which then communicates in real-time with an irrigation system to water the crop.

“I believe these sensors could improve water-use efficiency considerably,” said Professor Sjoerd Duiker, who also worked on the project. “Water scarcity is already a huge geopolitical issue, with agriculture responsible for about 70 per cent of world freshwater use. Improvements in water use efficiency will be essential.”

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