Plants' need for water "shown by leaf thickness and capacitance"

Leaf-mounted sensors can give growers timely indications of when to irrigate, according to recent US research.

Image: Penn State
Image: Penn State

The work at Pennsylvania State University identified water stress in a tomato plant by simultaneously measuring the thickness and electrical capacitance of its leaves at five-minute intervals over an 11-day period, during which the peat growing medium was allowed to dry out.

Electrical capacitance, or 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 capacitance variations decreased when soil moisture was below the wilting point and completely ceased below the soil volumetric water content of 11%, 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," lead researcher and doctoral degree candidate Amin Afzal said - unlike the relationship between leaf electrical capacitance and water status, which 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."

Afzal is now developing an algorithm to translate the leaf thickness and capacitance variations to meaningful information about plant water status.

The novel technique could lead to the development of a network of leaf clip sensors sending precise plant moisture information to a central unit controlling an irrigation system, he added.

The research is published in Transactions of the American Society of Agricultural and Biological Engineers.


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