Local damage to plants can induce fast systemic physiological changes through generation and propagation of electrical signals. It is known that electrical signals influence numerous physiological processes including photosynthesis; an increased plant tolerance to actions of stressors is a result of these changes. It is probable that parameters of electrical signals and fast physiological changes induced by these signals can be modified by the long-term actions of stressors; however, this question has been little investigated. Our work was devoted to the investigation of the parameters of burning-induced electrical signals and their influence on photosynthesis under soil water shortage in pea seedlings. We showed that soil water shortage decreased the amplitudes of the burning-induced depolarization signals (variation potential) and the magnitudes of photosynthetic inactivation (decreasing photosynthetic CO2 assimilation and linear electron flow and increasing non-photochemical quenching of the chlorophyll fluorescence and cyclic electron flow around photosystem I) caused by these signals. Moreover, burning-induced hyperpolarization signals (maybe, system potentials) and increased photosynthetic CO2 assimilation could be observed under strong water shortage. It was shown that the electrical signal-induced increase of the leaf stomatal conductance was a potential mechanism for the burning-induced activation of photosynthetic CO2 assimilation under strong water shortage; this mechanism was not crucial for photosynthetic response under control conditions or weak water shortage. Thus, our results show that soil water shortage can strongly modify damage-induced electrical signals and fast physiological responses induced by these signals.

Electrical signals (ESs), which are generated in irritated zones of plants and propagate into their non-irritated parts, are hypothesized to be an important mechanism of a plant systemic response on the local action of adverse factors. This hypothesis is supported by influence of ESs on numerous physiological processes including expression of defense genes, production of stress phytohormones, changes in photosynthetic processes and transpiration, stimulation of respiration and others. However, there are several questions, which require solution to support the hypothesis. Particularly, the non-physiological stimuli (e.g., strong heating or burning) are often used for induction of ESs; in contrast, the ES induction under action of physiological stressors with moderate intensities requires additional investigations. Influence of long-term environmental factors on generation and propagation of ESs is also weakly investigated. In the current work, we investigated ESs induced by local action of the moderate heating and illumination in wheat plants under irrigated and drought conditions. It was shown that combination of the moderate heating (40°C) and illumination (blue light, 540 μmol m−2s−1) induced electrical signals which were mainly depolarization electrical signals near the irritation zone and hyperpolarization electrical signals (HESs) on the distance from this zone. The moderate soil drought did not influence HESs; in contrast, the strong soil drought significantly decreased amplitude of HESs. Finally, it was shown that the moderate heating could induce HESs without additional action of illumination. It was hypothesized that both hyperpolarization and depolarization ESs could be caused by the hydraulic wave.

Parameters of illumination including the spectra, intensity, and photoperiod play an important role in the cultivation of plants under greenhouse conditions, especially for vegetables such as lettuce. We previously showed that illumination by a combination of red, blue, and white LEDs with a high red light intensity, was optimal for lettuce cultivation; however, the effect of the photoperiod on lettuce cultivation was not investigated. In the current work, we investigated the influence of photoperiod on production (total biomass and dry weight) and parameters of photosynthesis, respiration rate, and relative chlorophyll content in lettuce plants. A 16 h (light):8 h (dark) illumination regime was used as the control. In this work, we investigated the effect of photoperiod on total biomass and dry weight production in lettuce plants as well as on photosynthesis, respiration rate and chlorophyll content. A lighting regime 16:8 h (light:dark) was used as control. A shorter photoperiod (8 h) decreased total biomass and dry weight in lettuce, and this effect was related to the suppression of the linear electron flow caused by the decreasing content of chlorophylls and, therefore, light absorption. A longer photoperiod (24 h) increased the total biomass and dry weight, nevertheless an increase in photosynthetic processes, light absorption by leaves and chlorophyll content was not recorded, nor were differences in respiration rate, thus indicating that changes in photosynthesis and respiration are not necessary conditions for stimulating plant production. A simple model to predict plant production was also developed to address the question of whether increasing the duration of illumination stimulates plant production without inducing changes in photosynthesis and respiration. Our results indicate that increasing the duration of illumination can stimulate dry weight accumulation and that this effect can also be induced using the equal total light integrals for day (i.e., this stimulation can be also caused by increasing the light period while decreasing light intensity). Increasing the duration of illumination is therefore an effective approach to stimulating lettuce production under artificial lighting.

In environmental conditions, plants can be affected by the action of numerous abiotic stressors. These stressors can induce both damage of physiological processes and adaptive changes including signaling-based changes. Development of optical methods of revealing influence of stressors on plants is an important task for plant investigations. The photochemical reflectance index (PRI) based on plant reflectance at 531 nm (measuring wavelength) and 570 nm (reference wavelength) can be effective tool of revealing plant stress changes (mainly, photosynthetic changes); however, its efficiency is strongly varied at different conditions. Earlier, we proposed series of modified PRIs with moderate shifts of the measuring wavelength and showed that these indices can be effective for revealing photosynthetic changes under fluctuations in light intensity. The current work was devoted to the analysis of sensitivity of these modified PRIs to action of drought and short-term heat stress. Investigation of spatially-fixed leaves of pea plants showed that the modified PRI with the shorter measuring wavelength (515 nm) was increased under response of drought and heat; by contrast, the modified PRI with the longer wavelength (555 nm) was decreased under response to these stressors. Changes of investigated indices could be related to parameters of photosynthetic light reactions; however, these relations were stronger for the modified PRI with the 555 nm measuring wavelength. Investigation of canopy of pea (vegetation room) and wheat (vegetation room and open-ground) supported these results. Thus, moderate changes in the measuring wavelengths of PRI can strongly modify the efficiency of their use for the estimation of plant physiological changes (mainly photosynthetic changes) under action of stressors. It is probable that the modified PRI with the 555 nm measuring wavelength (or similar indices) can be an effective tool for revealing photosynthetic changes induced by stressors.

Long-distance electrical signals caused by the local action of stressors influence numerous physiological processes in plants including photosynthesis and increase their tolerance to the action of adverse factors. Depolarization electrical signals were mainly investigated; however, we earlier showed that hyperpolarization electrical signals (HESs) can be caused by moderate stressors (e.g., local moderate heating) and induce photosynthetic inactivation. We hypothesized that HESs are related to stressor-induced increases in the hydrostatic pressure in the zone of action of the stressor and following the propagation of a hydraulic wave. In the current work, we tested this hypothesis through the direct investigation of electrical signals induced by the local action of artificially increased pressure and an analysis of the subsequent photosynthetic changes in the nonirritated parts of plants. The electrical signals and parameters of photosynthetic light reactions were investigated in wheat plants. The local action of the increased pressure was induced by the action of weights on the wheat leaf. Extracellular electrodes were used for electrical signal measurements. Pulse–amplitude–modulation fluorescent imaging was used for measurements of the quantum yield of photosystem II and nonphotochemical quenching of chlorophyll fluorescence in wheat leaves. It was shown that the local action of pressure on wheat leaf induced electrical signals near the irritated zone: HESs were caused by low pressure (10 kPa) and depolarization signals were induced by high pressure (100 kPa). The local action of moderate pressure (50 kPa) induced weak electrical signals near the irritated zone; however, HESs were observed with increasing distance from this zone. It was also shown that the local action of this moderate pressure induced the photosynthetic inactivation (decreasing the quantum yield of photosystem II and increasing the nonphotochemical quenching of chlorophyll fluorescence) in the nonirritated parts of the wheat leaves. Thus, our results show that the local action of the increased pressure and, probably, subsequent propagation of the hydraulic wave induce electrical signals (including HESs) and photosynthetic inactivation in nonirritated parts of plants that are similar to ones caused by the local action of moderate stressors (e.g., moderate heating). This means that both HESs and depolarization electrical signals can have a hydraulic mechanism of propagation.