In this paper we present voltage-current-power characteristics of a plasma needle operating in the flow of helium at atmospheric pressure. In addition, we show some examples of how such a plasma affects plant tissues. In the characterization of the plasma needle, current and voltage waveforms were recorded by two derivative probes. These two probes are similar to the probes previously used by Puač et al for measuring transmitted power in low pressure CCP rf discharge. The instantaneous power was calculated from current and voltage waveforms and U -I characteristics of the discharge were determined. Regimes of operation with and without the grounding ring at the tip of the needle were considered. We have chosen two model systems to study the effect of the plasma needle on plant cells and tissues: sweet fern gametophyte (prothallus) and calli produced in vitro. Since the prothallus consists of a single layer of cells, the cytological effects could be easily examined. In addition, calli and prothallus are easy to manipulate and in vitro culture provides a possibility to work under constant and controlled conditions.
Nitrates have long been known to promote seed germination in many species, and various proposals have been made regarding the role of nitrates in this process. One hypothesis suggests a possible ecological role for nitrate, providing the seeds with a report of soil nitrogen status and the proximity of already established plants. However, nitrogen oxides (NO, NO2), which are present in the soil, also may be information carriers that indicate not only soil nitrate content, but also microbial activity and, therefore, soil quality. Because of annual variation of soil trace gas fluxes, seeds could be provided with information on seasonal and climate changes in their surroundings. Thus, nitrogen oxides would be the outer information carriers providing the seeds with integral data about many important factors required for successful germination and seedling establishment.
The germination of Empress tree (Paulownia tomentosa Steud.) seeds is phytochrome-controlled. Liquid smoke could not induce germination in darkness but red light irradiation of liquid smoke imbibed seeds induced a high percentage of germination. Maximum germination was achieved at liquid smoke concentration of 0.1% (v/v) when present during the imbibition phase or during the phase of phytochrome activity. The light requirement of these seeds could be completely substituted by exogenously applied gibberellins. In the presence of liquid smoke, optimal concentrations of GA 3 , GA 4 , and GA 9 necessary for inducing germination were several times lower than in the controls, while that of GA 7 was equally active when applied at a concentration one order of magnitude lower. The inhibitory effect of the applied growth retardants was strongly reduced and liquid smoke, in the presence of retardants, allowed light-induced germination, if applied simultaneously or after retardants treatment.
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