Continuous monitoring of plant “water stress” can help let farmers know when to activate irrigation systems, preventing both water waste and parched plants. Traditionally this has been done by measuring soil moisture content or with the use of models that calculate ground surface evaporation and plant transpiration.
But, say the researchers, water-use efficiency could be improved with new technology that is able to more accurately detect when plants need to be watered. The leaf sensor they developed can do just that, by simultaneously measuring, for the first time, both leaf thickness and leaf electrical capacitance – the ability of a leaf to store a charge.
“Leaf thickness is like a balloon — it swells by hydration and shrinks by water stress, or dehydration,” says lead researcher Amin Afzal. “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.”
The researchers performed their tests by mounting leaf sensors on random leaves of a tomato plant in a growth chamber under controlled temperature and lighting conditions. Measurements were recorded at five-minute intervals.
The electrical capacitance of the leaves stayed roughly constant at a minimum value during dark periods and increased rapidly during light periods, implying that it was related to photosynthetic activity. The daily variations in electrical capacitance decreased when soil moisture was below the wilting point and stopped completely below a soil volumetric water content of 11 percent, suggesting that “the effect of water stress on electrical capacitance was observed through its impact on photosynthesis.”
The researchers foresee a wireless system where leaf sensors 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. “Ultimately, all of the details can be managed by a smart phone app,” says Afzal.
Afzal is testing his working concept in the field at Penn State. He is currently developing an algorithm to translate the leaf thickness and capacitance variations to meaningful information about plant water status.
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