Decreased Photosynthetic Efficiency in Nicotiana tabacum L. under Transient Heat Stress

Abstract

Heat stress is an abiotic factor that affects the photosynthetic parameters of plants. In this study, we examined the photosynthetic mechanisms underlying the rapid response of tobacco plants to heat stress in a controlled environment. To evaluate transient heat stress conditions, changes in photochemical, carboxylative, and fluorescence efficiencies were measured using an infrared gas analyser (IRGA Licor 6800) coupled with chlorophyll a fluorescence measurements. Our findings indicated that significant disruptions in the photosynthetic machinery occurred at 45 °C for 6 h following transient heat treatment, as explained by 76.2% in the principal component analysis. The photosynthetic mechanism analysis revealed that the dark respiration rate (Rd and Rd^*_CO2) increased, indicating a reduced potential for carbon fixation during plant growth and development. When the light compensation point (LCP) increased as the light saturation point (LSP) decreased, this indicated potential damage to the photosystem membrane of the thylakoids. Other photosynthetic parameters, such as A_MAX, VC_MAX, J_MAX, and ΦCO₂, also decreased, compromising both photochemical and carboxylative efficiencies in the Calvin-Benson cycle. The energy dissipation mechanism, as indicated by the NPQ, qN, and thermal values, suggested that a photoprotective strategy may have been employed. However, the observed transitory damage was a result of disruption of the electron transport rate (ETR) between the PSII and PSI photosystems, which was initially caused by high temperatures. Our study highlights the impact of rapid temperature changes on plant physiology and the potential acclimatisation mechanisms under rapid heat stress. Future research should focus on exploring the adaptive mechanisms involved in distinguishing mutants to improve crop resilience against environmental stressors.

Keywords: chlorophyll fluorescence; dark respiration; electron transport chain; gas exchange; nonphotochemical quenching; plant stress; thermal conditions.

Figure 1

Response curves for light, CO₂, and fluorescence from tobacco plants under control and heat stress conditions. (A) Thermal images of plants under normal and heat stress conditions. The colour gradient, ranging from blue to red, indicates an increase in the temperature. (B) The temperature of the leaves was increased from 0 to 2500 μmol m⁻² s⁻¹ PPFD. (C) Transpiration rate (E) and stomatal conductance (g_s). The white circle represents transpiration (E) in the control plants, the white triangle represents stomatal conductance (g_s) in the control plants, the black circle represents transpiration (E) under heat stress plants, and the black triangle represents stomatal conductance (g_s) under heat stress plants. (D) Net photosynthetic light (A-PPFD) response. (E) Net photosynthetic CO₂ (A-C_i) response. (F) Intrinsic water use efficiency (iWUE) response curves. Statistically significant differences according to the t-test (p < 0.01). Mean ± SE. (n = 10).

Figure 2

Response curves for chlorophyll a fluorescence in tobacco plants under control and heat stress conditions. (A) Operational efficiency of photosystem II (ΦPSII). The inset in the bar graph indicates the maximum quantum yield of PSII (Fv/Fm) in the dark-adapted leaves. (B) Effective quantum yield of PSII (Fv/Fm). (C) Electron transport rate (ETR). (D) Nonphotochemical quenching (NPQ). (E) Photochemical dissipation quenching (qP). (F) Nonphotochemical dissipation quenching (qN). The white circle represents the control plants and black circle represents under heat stress plants. Asterisks over the bars indicate statistically significant differences according to t-test (p < 0.01). Mean ± SE. (n = 10).

Figure 3

Multivariate analysis of control and stress tobacco plants. The 2D PCA biplot of principal component analysis (PCA) displayed two principal components (PC1 and PC2) and the contribution of the most responsive vectors.

Figure 4

Putative mechanisms underlying the effects of thermal stress on photosynthetic and intra- and intercellular responses in leaves. Heat stress (indicated in red) triggers a series of cellular responses, including photochemical, carboxylative, and fluorescence changes. This leads to alterations in transpiration and stomatal opening/closing as well as intracellular signalling cascades that adjust photosynthetic rates. Intracellular communication within the leaf arranges chloroplast crosstalk, adapting the plant to thermal fluctuations and modulating cellular and photosynthetic activity in chloroplasts. If transient heat modulation does not occur, the electron transport chain may become compromised, leading to reduction and disintegration at the electron transport chain level in the thylakoids. Heat stress can also induce the expression of heat shock proteins (HSPs), heat shock factors (HSFs), hypersensitive responses (HR), reactive oxygen species (ROS), programmed cell death (PCD), heat-induced susceptibility (HIS), and heat-induced protective immunity (HIPI). ROS were generated under thermal stress. (1) Tobacco plants under transient heat stress trigger intercellular and intracellular thermal responses in their leaves. (2) Intercellular signalling is required for the recognition of photochemical, carboxylative, and fluorescence alterations due to heat stress, the initiation of transpiration, stomatal opening/closing, and intracellular signalling cascades to adjust photosynthetic rates. (3) Intracellular signalling within the leaf orchestrates chloroplast crosstalk, driving plant adaptation to thermal fluctuations and consequently modulating cell and photosynthetic activity in chloroplasts. (4) The electron transport chain is compromised if early modulation does not occur, leading to reduction and disintegration at the level of the electron transport chain in thylakoids. The sizes of the arrows indicate the efficiency of the electron transport chain in thylakoid membranes. The figure legends were created using https://www.biorender.com (accessed on 26 December 2023).

Figure 5

Flowchart of the methodology used for assessing photochemical, carboxylative, and chlorophyll a fluorescence analyses in Nicotiana tabacum L. under transient heat stress. 1° Stage: Plants were grown in a greenhouse. 2° Stage: The plants were subjected to transient heat stress at 45 °C overnight for 6 h and evaluated using thermal imaging. 3° Stage: Photosynthetic and chlorophyll a fluorescence analyses were conducted. 4° Stage: The directional flow of electrons in the electron transport chain and the flow of carbon in the Calvin–Benson cycle were evaluated. 5° Stage: Statistical analyses were performed, including principal component analysis. * shows the flow of electrons.