Various external factors (pollutants available in the soil, a lack or insufficient amount of water and nutrients, etc.) lead to stressful conditions of plants and impossibility of their normal development. At the early stages it is difficult to identify visually the stressful situations of plants. Therefore development of methods and devices to detect stressful states is important.

A method of the laser-induced fluorescence is one of perspective methods for detection of stressful conditions of plants.

In spite of quite a great number of work presenting results of the pilot studies of fluorescence spectra of vegetation, there are some important issues, which are unclear.

The paper gives results of pilot studies of stability of a spectrum form of the laser-induced fluorescence of plants for different types of soil at the wavelength of excitation fluorescence of 532 nm.

Results of processing fluorescence spectra of plants show:

- fluorescence spectra of plants grown up under similar conditions have good repeatability of a spectra form for different samples of plants and different measurement time for each type of studied soil. The ratio value R of the fluorescence intensity at the wavelength of 685 nm to the fluorescence intensity at the wavelength of 740 nm has high stability. The standard deviation in sampling of the ratio R of different samples of a plant for one type of soil (for width of spectral ranges of recording fluorescent radiation of 10 nm) lies in the range ~ 0.055 - ~ 0.12;

- a difference in plant fluorescence spectra between themselves for different types of soil has the same order as a difference in fluorescence spectra of different samples of a plant for one type of soil. Difference in average value of the ratio R for different types of soil lies in the range ~ 0.01 - ~ 0.15.

Thus, the value of the ratio R is steady against a type of soil and can be used to control a condition of plants.