Photoluminescence image captured during spectral analysis shows reaction of slice of sweet potato to nonpathogenic Fusarium oxysporum. (SAES)
Consecutive spectral analyses of ultraweak photon emission can allow researchers to chart the process of physiological transition in plants, leading to the eventual development of chemicals that work with plant physiology to fight infection, says Kimihiko Kato of the Shizuoka Agricultural Experiment Station (Shizuoka, Japan). "Many living organisms generate ultraweak photon emissions, which we call biophotons, generally as a response to external changes such as anaerobic treatment, growth hormone treatment, saline stress, temperature change, herbicide treatment, and attack by pathogens," Kato says.
The existence of biophotons was postulated by Gurvitsch in 1923. "A determination of the sources of ultraweak luminescence is of fundamental importance for a further development of the field," says Barbara Chwirot of the Institute of Biology and Protection of the Environment at Nicholas Copernicus University (Torun, Poland). "Such studies have already resulted in a qualitatively new approach to some features of cellular metabolic processes." Indeed, Kato's research team has used continuous spectral analyses to collect data confirming very strong intensity of biophotons associated with the defense response of sweet potatoes.
In the experiments, a multisample photon counting system (Hamamatsu Photonics K.K.; Hamamatsu, Japan) monitored the time-dependent intensity variations for the sample over several spectral regions. The device was equipped with an R329P photomultiplier tube (PMT) that provided a spectral response of 240 to 630 nm and a special dark box system with two rotating disks: one to hold 16 samples and another with band-pass filters.
The researchers prepared 16 cylindrical sweet potato samples 6 to 8 mm thick and 46 mm in diameter. Tubers live for months after they are harvested. Sweet potato was chosen for the experiments because the root contains no chloroplasts, which give off biophotons when engaged in photosynthesis. This helped reduce possible sources of biophoton emission noise.
After placing the sweet potato disks in 60-mm petri dishes, the researchers prepared a conidial suspension (1 x 107 ml-1) of nonpathogenic Fusarium oxysporum isolate. They applied either a 0.2 ml portion of the suspension or a similar amount of distilled water to the upper surface of the sweet potato disks, then measured ultraweak biophoton emissions for 36 hours at 20°C. They also exposed samples to 2,4-dichlorophenoxyacetic acid, which is known to induce sweet potato to form embryogenic calli. They monitored emissions from these samples for 65 hours at 20°C.
The spectrum of ultraweak luminescence from sweet-potato samples was represented as the ratio of intensity in each spectral region to the integrated intensity of all the spectral regions. "We plotted the spectral transition using the moving average of 10 measurements," Kato says. "Only four spectral regions showed emissions430 to 480 nm, 480 to 530 nm, 530 to 580 nm, and 580 to 630 nm. The other three regions showed almost no emissions."
As may be expected, the samples treated with water showed no appreciable variation in biophotonic activity, but those treated with Fusarium oxysporum showed a transient increase in ultraweak luminescence that appeared to be linked to a defense response. There was some fluctuation in the spectrum, but overall, it was quite constant.
"In addition, comparisons of samples inoculated with 2,4D and others that underwent changes in temperatures showed us that the emission intensity of samples treated with Fusarium oxysporum was 10 times greater than either of the other samples," Kato says. "It is reasonable, we think, to assume that the ultraweak biophotonic activity of the sweet-potato samples treated with nonpathogenic Fusarium oxysporum matches the strong physiological changes that occur when defense-related substances are first introduced in plants."
According to Kato, this noninvasive, real-time method can be used as a new parameter for identifying the physiological state of an organism. Kato's organization is also working with agricultural chemical producers to develop a new kind of chemical. In the past, chemicals were developed to directly attack the origins of diseases in plants. With the new information Kato's experiments provide, he and the corporate developers working with him believe that chemicals that stimulate defensive responses in plants can be effective against infection as well.