Ascorbic Acid-Induced Photosynthetic Adaptability of Processing Tomatoes to Salt Stress Probed by Fast OJIP Fluorescence Rise

Abstract

In this study, the protective role of exogenous ascorbic acid (AsA) on salt-induced inhibition of photosynthesis in the seedlings of processing tomatoes under salt stress has been investigated. Plants under salt stress (NaCl, 100 mmol/L) were foliar-sprayed with AsA (0.5 mmol/L), lycorine (LYC, 0.25 mmol/L, an inhibitor of key AsA synthesis enzyme l-galactono-γ-lactone dehydrogenase activity), or AsA plus LYC. The effects of AsA on fast OJIP fluorescence rise curve and JIP parameters were then examined. Our results demonstrated that applying exogenous AsA significantly changed the composition of O-J-I-P fluorescence transients in plants subjected to salt stress both with and without LYC. An increase in basal fluorescence (F _o) and a decrease in maximum fluorescence (F _m) were observed. Lower K- and L-bands and higher I-band were detected on the OJIP transient curves compared, respectively, with salt-stressed plants with and without LYC. AsA application also significantly increased the values of normalized total complementary area (S_m), relative variable fluorescence intensity at the I-step (V_I), absorbed light energy (ABS/CS_m), excitation energy (TR_o/CS_m), and reduction energy entering the electron transfer chain beyond Q_A (ET_o/CS_m) per reaction centre (RC) and electron transport flux per active RC (ET_o/RC), while decreasing some others like the approximated initial slope of the fluorescence transient (M_o), relative variable fluorescence intensity at the K-step (V_K), average absorption (ABS/RC), trapping (TR_o/RC), heat dissipation (DI_o/RC) per active RC, and heat dissipation per active RC (DI_o/CS_m) in the presence or absence of LYC. These results suggested that exogenous AsA counteracted salt-induced photoinhibition mainly by modulating the endogenous AsA level and redox state in the chloroplast to promote chlorophyll synthesis and alleviate the damage of oxidative stress to photosynthetic apparatus. AsA can also raise the efficiency of light utilization as well as excitation energy dissipation within the photosystem II (PSII) antennae, thus increasing the stability of PSII and promoting the movement of electrons among PS1 and PSII in tomato seedling leaves subjected to salt stress.

Keywords: NaCl stress; ascorbic acid; fast OJIP fluorescence rise curve; photosynthesis; processing tomatoes.

Figure 1

Growth of salt-stressed tomato seedlings with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 2

Values of ascorbic acid (AsA) content (A), dehydroascorbate (DHA) content (B), total AsA content (C), and AsA/DHA (D) in the leaves of salt-stressed tomato seedlings with or without exogenous AsA and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, d, and e) indicate significant difference at P < 0.05 among treatments. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 3

Values of net photosynthetic rate (P_n) (A), stomatal conductance (G_s) (B), transpiration rate (_Tr) (C), and intercellular CO₂ concentration (C_i) (D) in the leaves of salt-stressed tomato seedlings with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, and d) indicate significant difference at P < 0.05 among treatments. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 4

Prompt fluorescence (PF) curves (A) and its normalized curves between O and P steps expressed as W_O−P = [(F_t−F_o)/(F_m−F_o)] (B) in the leaves of salt-stressed tomato seedlings with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 5

Normalized prompt fluorescence (PF) curves between O and J steps expressed as W_O−J = [(F_t−F_o)/(F_J−F_o)] (A) and ratio of variable fluorescence F_K to the amplitude F_J−F_O expressed as W_K = (F_K−F_o)/(F_J−F_o) (B) in the leaves of salt-stressed tomato seedlings with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, and d) indicate significant difference at P < 0.05 among treatments. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 6

Shape of the prompt fluorescence (PF) transient curves normalized between O and K steps expressed as W_O−K = [(F_t−F_o)/(F_K−F_o)], ΔW_O−K = W_O−K(stress)–W_O−K (control) (A), W_L = [(F_L−F_o)/(F_K−F_o)] (B) in leaves of salt-stressed tomato seedling with or without exogenous reduced ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, d, and e) indicate significant difference at P < 0.05 among treatments. Error bar is smaller than the line at the top of the graph. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 7

Spider plots of the JIP parameters deduced from chlorophyll a fluorescence OJIP transient curves in leaves of salt-stressed tomato seedlings with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 8

Parameters of absorption flux per reaction center (RC) (ABS/RC) (A), trapped energy flux per RC (at t = F_o) (TR_o/RC) (B), electron transport flux per RC (at t = F_o) (ET_o/RC) (C), and dissipated energy flux per RC (at t = F_o) (DI_o/RC) (D) in leaves of salt-stressed tomato seedling with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, d, and e) indicate significant difference at P < 0.05 among treatments. Error bar is smaller than the line at the top of the graph. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 9

Parameters of absorption flux per CS (at t = F_m) (ABS/CS_m) (A), dissipated energy flux per CS (at t = F_m) (DI_o/CS_m) (B), trapped energy flux per CS (at t = F_m) (TR_o/CS_m) (C), and electron transport flux per CS (at t = F_m) (ET_o/CS_m) (D) in leaves of salt-stressed tomato seedlings with or without exogenous reduced ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, d, and e) indicate significant difference at P < 0.05 among treatments. Error bar is smaller than the line at the top of the graph. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 10

Energy pipeline models of specific fluxes per reaction center (RC) and phenomenological fluxes per excited cross-section (CS_m) in the member (left) and the leaf (right) of salt-stressed tomato seedling with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. In the member and leaf models, ABS, TRo, ETo, and DIo indicate absorption, maximum trapping flux beyond Q_A, electron transport, and dissipation flux, respectively. In the leaf models (right), the leaf color represents the pigment concentration, and the active and inactive RCs are indicated by open circles and closed circles, respectively. The detailed calculation of each parameter is given in Table 1. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 11

Values of the maximal photochemical efficiency of PSII (F_v/F_m) (A) and performance index on absorption basis (PIABS) (B) in the leaves of salt-stressed tomato seedlings with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, d, and e) indicate significant difference at P < 0.05 among treatments. Error bar is smaller than the line at the top of the graph. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/L AsA.

Figure 12

Curves of light induced modulated 820 nm reflection (I/I_o) kinetics (A) and maximal redox capacity of PSI (ΔI/I_o) (B), efficiency with which an electron from the intersystem electron carriers is transferred to reduce end electron acceptors at the PSI acceptor side (δR_o) (C), and quantum yield for reduction of end electron acceptors at the PSI acceptor side (ϕR_o) (D) in the leaves of salt-stressed tomato seedling with or without exogenous ascorbic acid (AsA) and lycorine (LYC) spraying. Values are means ± SD (n = 3). Different letters (a, b, c, d, and e) indicate significant difference at P < 0.05 among treatments. Error bar is smaller than the line at the top of the graph. Control, no added NaCl, no sprayed AsA and LYC; NaCl, added 100 mmol/L NaCl; NA, added 100 mmol/L NaCl and sprayed 0.5 mmol/L AsA; NL, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC; NLA, added 100 mmol/L NaCl and sprayed 0.25 mmol/L LYC and 0.5 mmol/LAsA.