Short-term (up to 1 h) systemic responses of tobacco (Nicotiana tabacum cv. Samsun) plants to local burning of an upper leaf were studied by measuring the following variables in a distant leaf: extracellular electrical potentials (EEPs); gas exchange parameters; fast chlorophyll fluorescence induction; and endogenous concentrations of three putative chemical signaling compounds-abscisic (ABA), jasmonic (JA), and salicylic (SA) acids. The first detected response to local burning in the distant leaves was in EEP, which started to decline within 10-20 s of the beginning of the treatment, fell sharply for ca. 1-3 min, and then tended to recover within the following hour. The measured gasometric parameters (stomatal conductance and the rates of transpiration and CO(2) assimilation) started to decrease 5-7 min after local burning, suggesting that the electrical signals may induce stomatal closure. These changes were accompanied by systemic increases in the endogenous ABA concentration followed by huge systemic rises in endogenous JA levels started after ca. 15 min, providing the first evidence of short-term systemic accumulation of these plant hormones in responses to local burning. Furthermore, JA appears to have an inhibitory effect on CO(2) assimilation. The correlations between the kinetics of the systemic EEP, stomatal, photosynthetic, ABA, and JA responses suggest that (1) electrical signals (probably induced by a propagating hydraulic signal) may trigger chemical defense-related signaling pathways in tobacco plants; (2) both electrical and chemical signals are interactively involved in the induction of short-term systemic stomatal closure and subsequent reductions in the rate of transpiration and CO(2) assimilation after local burning events.
We studied the temperature dependence of chlorophyll fluorescence intensity in barley leaves under weak and actinic light excitation during linear heating from room temperature to 50 degrees C. The heat-induced fluorescence rise usually appearing at around 40-50 degrees C under weak light excitation was also found in leaves treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) or hydroxylamine (NH(2)OH). However, simultaneous treatment with both these compounds caused a disappearance of the fluorescence rise. We have suggested that the mechanism of the heat-induced fluorescence rise in DCMU-treated leaves is different than that in untreated or NH(2)OH-treated leaves. In DCMU-treated leaves, the heat-induced fluorescence rise reflects an accumulation of Q(A) (-) even under weak light excitation due to the thermal inhibition of the S(2)Q(A) (-) recombination as was further documented by a decrease in the intensity of the thermoluminescence Q band. Mathematical model simulating this experimental data also supports our interpretation. In the case of DCMU-untreated leaves, our model simulations suggest that the heat-induced fluorescence rise is caused by both the light-induced reduction of Q(A) and enhanced back electron transfer from Q(B) to Q(A). The simulations also revealed the importance of other processes occurring during the heat-induced fluorescence rise, which are discussed with respect to experimental data.
Heat tolerance of plants related to cell membrane thermostability is commonly estimated via the measurement of ion leakage from plant segments after defined heat treatment. To compare heat tolerance of various plants, it is crucial to select suitable heating conditions. This selection is time-consuming and optimizing the conditions for all investigated plants may even be impossible. Another problem of the method is its tendency to overestimate basal heat tolerance. Here we present an improved ion leakage method, which does not suffer from these drawbacks. It is based on gradual heating of plant segments in a water bath or algal suspensions from room temperature up to 70-75°C. The electrical conductivity of the bath/suspension, which is measured continuously during heating, abruptly increases at a certain temperature T (within 55-70°C). The T value can be taken as a measure of cell membrane thermostability, representing the heat tolerance of plants/organisms. Higher T corresponds to higher heat tolerance (basal or acquired) connected to higher thermostability of the cell membrane, as evidenced by the common ion leakage method. The new method also enables determination of the thermostability of photochemical reactions in photosynthetic samples via the simultaneous measurement of Chl fluorescence.
The chlorophyll fluorescence (F) temperature curves in a linear time-temperature heating/cooling regime were used to study heat-induced irreversible F changes in primary green leaves of spring barley (Hordeum vulgare L. cv. Akcent). The leaf segments were heated in a stirred water bath at heating rates of 0.0083, 0.0166, 0.0333, and 0.0500 °C s −1 from room temperature up to maximal temperature T m and then linearly cooled to 35 °C at the same rate. The F intensity was measured by a pulse-modulated technique. The results support the existence of the two critical temperatures of irreversible F changes postulated earlier, at 45−48 and 53−55 °C. The critical temperatures are slightly dependent on the heating rate. Two types of parameters were used to characterize the irreversibility of the F changes: the coefficient of irreversibility μ defined as the ratio of F intensity at 35 °C at the starting/ending parts of the cycle and the slopes of tangents of linear parts of the F temperature curve. The dependence of μ on T m revealed a maximum, which moved from 54 to 61 °C with the increasing heating/cooling rate ν from 0.0083 to 0.0500 ºC s −1 , showing two basic phases of the irreversible changes. The Arrhenius and Eyring approaches were applied to calculate the activation energies of the initial increase in μ. The values varied between 30 and 50 kJ mol −1 and decreased slightly with the increasing heating rate.
Gradual heating of green leaves up to non-physiological temperatures is often used to estimate thermal stability of photosynthetic apparatus. However, a complete sequence of heat-induced disassembly and denaturation of chlorophyll-containing protein complexes (CPCs) has not been reported yet. In this work, we heated (1 degrees C x min(-1)) barley leaves to temperatures selected according to the changes in the chlorophyll fluorescence temperature curve (FTC) and we analyzed CPC stability by two-dimensional native Deriphat/SDS-PAGE. The first distinct change in both structure and function of photosystem II (PSII) appeared at 40-50 degrees C. PSII core (CCII) dimers began to dissociate monomers, which was accompanied by a decrease in PSII photochemistry and reflected in FTC as the first fluorescence increase. Further changes in CPCs appeared at 57-60 degrees C, when FTC increases to its second maximum. Photosystem I (PSI) cores (CCI) partially dissociated from light-harvesting complexes of PSI (LHCI) and formed aggregates. The rest of CCI-LHCI complexes, as well as the CCI aggregates, degraded to the PSI-A/B heterodimer in leaves heated to 70 degrees C. Heating to these temperatures led to a complete degradation of CCII components and corresponding loss of PSII photochemistry. Trimeric light-harvesting complexes of PSII (LHCII) markedly dissociated to monomers and denatured, as evidenced by a release of large amount of free chlorophylls. Between 70 and 80 degrees C, a complete degradation of LHCII occurred, leaving the PSI-A/B heterodimer as the only detectable CPC in the membrane. This most thermostable CPC disappeared after heating to 90 degrees C, which corresponded to a loss of PSI photochemistry.
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