Oakwood University

Oakwood University, founded in 1896, is a historically black, four-year private, liberal arts institution located in Huntsville, Alabama. Its mission embodies access to educational opportunity, academic excellence, and spiritual development for persons reflecting demographic, economic, cultural and educational diversity.

Low temperature (cold) plasma finds an increasing number of applications in biology, medicine and agriculture. The nature of cold plasma interactions with bio-matter is the subject of active research. It is known that sensitive plants such as the Venus flytrap and Mimosa pudica can change shape and dimensions under external perturbations. This morphing behavior involves processes associated with the activation of ion channels and aquaporins in these plants.

We have experimentally observed that argon plasma jet in atmospheric air can activate plant movements and morphing structures in Venus flytrap and Mimosa pudica similar to stimulation of their mechanosensors in vivo. Application of atmospheric pressure argon plasma jet to the inside or outside of a lobe, midrib, or cilia in Dionaea muscipula Ellis induces trap closing.

Schematic 1: Diagram of the cold plasma jet interacting with Venus flytrap (A). Picture of the device with the shielding cover off (B). Typical voltage and current waveform for one pulse (C).

The cold plasma jets used in our experiments have an annular geometry using a pair of nested quartz tubes (Schematic 1A). The working gas flows through a quartz tube with an outer diameter of 6 mm and inner diameter of 4 mm, which is open at both ends. The inner tube has an outer diameter of 3 mm and an inner diameter of 2 mm and is closed at one end. The tubes are held inside a plastic compression Tee fitting with the gas line at the perpendicular leg of the tee. This creates a sealed volume with the only outlet being the annular space between the inner and
outer quartz tube.

The entire system is placed in a metal enclosure to reduce electromagnetic interference (Schematic 1B). A 1 mm diameter tungsten pin is inserted into the inner tube while a stainless-steel ring is held around the outer tube. A high voltage coaxial power connector provided shielded power delivery. The central pin carries the high voltage signal from the pulsed dc power source to the tungsten pin. The grounded shield wire of the coaxial connector and line are
connected to the metal enclosure around the plasma source. The ring is grounded via contact with the metal enclosure. This design prevents directly contact between the plasma generated in the annulus and either electrode, thus preventing arcing which would damage the plants.

The pulsed dc power source was driven at 9 kV, 16 mA, 6 kHz frequency, and 1 µs pulsed width (Schematic 1C). Bottled helium and argon, and air from an air compressor were metered with MKS digital mass flow controllers. The helium flow rate was set at 2.9 L/min and air was added at 0.149 L/min, ~5% air by volume. The plasma jet with pure helium is
approximately 4 cm long. With air added, the jet length shrinks to 2 cm. Argon was flown at a rate 1.55 L/min between the two tubes. The gas was ionized by the strong electric fields between the electrodes, and formed a plasma jet approximately 2 cm long measured from the exit of the outer tube.

The plasma was driven with a high voltage pulsed DC circuit consisting of a Matsusada AU-10P60 10 kV DC power supply, a IXYS PVX-4110 pulsed generator, and a SRS DG-645 delay generator. The voltage and current were monitored by the pulse generator had a square pulse shape with a slight sloping during the rising edge. The current trace had two distinct current pulses at the rising and failing fronts of the voltage (Schematic 1C).

Our experiments and simulations narrowed possible causes of the observed effects. We have confirmed that gas flow and UV radiation associated with plasma jet are not the primary reasons for the observed effects. Reactive oxygen and nitrogen species (RONS) produced by argon plasma jet in atmospheric air appear to be the primary reason for plasma-induced activation of phytoactuators in plants. Some of these RONS are known to be signaling molecules, which
control plants’ developmental processes. Further studies of underlying mechanisms are underway. Understanding details of plasma interactions with plants could promote plasma-based technology for plant developmental control and future use for plant protection from pathogens. Plasma treatment could induce desirable changes of developmental and physiological processes in plants, improve seed resistance to stress and diseases, modify seed coat structures, increase the permeability of seed coats, and stimulate seed germination and seedling growth for several types of
commercially significant food plants for human and animal consumption.

Data acquisition for electrical signaling of plants

 The experimental setup is shown in Schematic 2. Three different data acquisition systems were used in the present work. High speed data acquisition was performed using microcomputers with simultaneous multifunction I/O plug-in data acquisition board NI-PXI-6115 (National Instruments, Austin, TX, USA) interfaced through a NI SCB-68 shielded connector block to Ag/AgCl electrodes. The system integrates standard low-pass anti-aliasing filters at one half of the sampling frequency. Data acquisition board NI-PXI-6115 (National Instruments, Austin, TX, USA) has a maximum sampling rate of 4,000,000 samples/s.  

The NI 9206 16-Bit Analog Input Module was also used with a NI cDAQ-9174 CompactDAQ chassis (National Instruments, Austin, TX, USA) for data acquisition. The NI 9206 features 16 differential analog inputs, 16-bit resolution, and a maximum sampling rate of 250 kS/s.  The power supply for the data acquisition system was Energizer XP-1800 external battery.

The NI USB-6210 data acquisition card (National Instruments, Austin, TX, USA) was used in some experiments. All experimental results obtained with NI PXI-6115, 9206 and USB-6210 coincides.

Schematic 2: Diagram of the experimental setup for electrophysiological experiments. Ag/AgCl electrodes were used for measurements of plant electrical responses. Results were reproduced on 3 different data acquisition cards NI PXI-6115, NI USB 6210 and NI cDAQ 9174.

Schematic 3: Schematic diagram of possible low-temperature plasma effects in seeds and plants.