Impact of Sodium Nitroprusside on the Photosynthetic Performance of Maize and Sorghum

Abstract

Nitric oxide (NO) is an important molecule in regulating plant growth, development and photosynthetic performance. This study investigates the impact of varying concentrations (0-300 µM) of sodium nitroprusside (SNP, a donor of NO) on the functions of the photosynthetic apparatus in sorghum (Sorghum bicolor L. Albanus) and maize (Zea mays L. Kerala) under physiological conditions. Analysis of the chlorophyll fluorescence signals (using PAM and the JIP-test) revealed an increased amount of open PSII reaction centers (qP increased), but it did not affect the number of active reaction centers per PSII antenna chlorophyll (RC/ABS). In addition, the smaller SNP concentrations (up to 150 μM) alleviated the interaction of Q_A with plastoquine in maize, while at 300 μM it predominates the electron recombination on Q_AQ_B^-, with the oxidized S₂ (or S₃) states of oxygen evolving in complex ways in both studied plant species. At the same time, SNP application stimulated the electron flux-reducing end electron acceptors at the PSI acceptor side per reaction center (REo/RC increased up to 26%) and the probability of their reduction (φRo increased up to 20%). An increase in MDA (by about 30%) and H₂O₂ contents was registered only at the highest SNP concentration (300 µM). At this concentration, SNP differentially affected the amount of P700⁺ in studied plant species, i.e., it increased (by 10%) in maize but decreased (by 16%) in sorghum. The effects of SNP on the functions of the photosynthetic apparatus were accompanied by an increase in carotenoid content in both studied plants. Additionally, data revealed that SNP-induced changes in the photosynthetic apparatus differed between maize and sorghum, suggesting species specificity for SNP's impact on plants.

Keywords: JIP parameters; chlorophyll fluorescence; nitric oxide; photosynthetic performance; photosystem I; photosystem II.

Figure 1

Effects of different SNP concentrations on (a) MDA and (b) H₂O₂ contents in the leaves of maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Albanus). Mean values ± SE (n = 8) are presented. Different letters indicate significant differences between the values (p < 0.05).

Figure 2

Effects of different SNP concentrations on the membrane stability index (MSI) in the leaves of maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Albanus) expressed in percentages from untreated plants. Mean values ± SE (n = 8) were presented and different letters indicate significant differences among treatments at p < 0.05.

Figure 3

Effects of different SNP concentrations on PAM chlorophyll fluorescence parameters of maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Albanus) leaves. (a) The ratio of quantum yields of photochemical to non-photochemical processes (Fv/Fo); (b) photochemical quenching (qP); (c) effective quantum yield of photochemical energy conversion in PSII (Φ_PSII); (d) excitation efficiency of open PSII centers (Φ_exc). Mean values ± SE (n = 8) were presented and different letters indicate significant differences among treatments at p < 0.05.

Figure 4

Effects of different SNP concentrations on the chlorophyll fluorescence decay ratio) (R_Fd) on leaves from maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Albanus). Mean values ± SE (n = 8) are presented and different letters indicate significant differences among treatments at p < 0.05.

Figure 5

Selected JIP parameters of untreated plants from maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Albanus). ABS/RC (absorption flux per RC, apparent antenna size of an active RC); ETo/RC—electron transport flux (further Q_A) per RC; REo/RC—electron flux-reducing end electron acceptors at the PSI acceptor side per RC; φPo—maximum quantum yield of primary photochemistry (at t = 0); φRo—quantum yield of reduction in end electron acceptors at the PSI acceptor side; Vj—relative variable fluorescence at the J-step; ψEo—movement of an electron into the electron transport chain beyond Q_A; Wk—the ratio of the K-phase to the J-phase; RC/ABS—the number of active RC per PSII antenna chlorophyll. Mean values ± SE (n = 20) were presented and asterisks indicate the significant differences between the two plant species at p < 0.05.

Figure 6

Effects of different SNP concentrations on selected JIP parameters in the leaves of maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Albanus) plants treated with (a) 25 μM of SNP, (b) 50 μM of SNP, (c) 150 μM of SNP and (d) 300 μM of SNP. ABS/RC (absorption flux per RC, apparent antenna size of an active RC); ETo/RC—electron transport flux (further Q_A) per RC; REo/RC—electron flux-reducing end electron acceptors at the PSI acceptor side per RC; φPo—maximum quantum yield of primary photochemistry (at t = 0); φRo—quantum yield of reduction in end electron acceptors at the PSI acceptor side; Vj—relative variable fluorescence at the J-step; ψEo—movement of an electron into the electron transport chain beyond Q_A; Wk—the ratio of the K-phase to the J-phase; RC/ABS—the numbers of active RC per PSII antenna chlorophyll. The parameters are normalized to the respective control. Mean values ± SE (n = 20) are presented.

Figure 7

Effects of different SNP concentrations on (a) PItotal—the performance index for energy conservation from exciton to the reduction in PSI end acceptors and (b) PI_ABS—the performance index for the energy conservation from exciton to reduction in intersystem electron acceptors in the leaves of maize (Zea mays L. Kerala) and sorghum (Sorghum bicolor L. Albanus). Mean values ± SE (n = 20) are presented and different letters indicate significant differences among treatments at p < 0.05.