Photochemical Efficiency of Photosystem II in Inverted Leaves of Soybean [ Glycine max (L.) Merr.] Affected by Elevated Temperature and High Light

Abstract

In summer, high light and elevated temperature are the most common abiotic stresses. The frequent occurrence of monsoon exposes the abaxial surface of soybean [Glycine max (L.) Merr.] leaves to direct solar radiation, resulting in irreversible damage to plant photosynthesis. In this study, chlorophyll a fluorescence was used to evaluate the functional status of photosystem II (PSII) in inverted leaves under elevated temperature and high light. In two consecutive growing seasons, we tested the fluorescence and gas exchange parameters of soybean leaves for 10 days and 15 days (5 days after recovery). Inverted leaves had lower tolerance compared to normal leaves and exhibited lower photosynthetic performance, quantum yield, and electron transport efficiency under combined elevated temperature and high light stress, along with a significant increase in absorption flux per reaction center (RC) and the energy dissipation of the RC, resulting in significantly lower performance indexes (PI_ABS and PI_total) and net photosynthetic rate (P _n ) in inverted leaves. High light and elevated temperature caused irreversible membrane damage in inverted leaves, as photosynthetic performance parameters (P _n , PI_ABS, and PI_total) did not return to control levels after inverted leaves recovered. In conclusion, inverted leaves exhibited lower photosynthetic performance and PSII activity under elevated temperature and high light stress compared to normal leaves.

Keywords: chlorophyll a fluorescence; elevated temperature; high light; leaf inversion; photosynthesis.

FIGURE 1

Meteorological data during treatment (A,C) and recovery (B,D) when measuring gas exchange and chlorophyll a fluorescence.

FIGURE 2

(A,B) The gas exchange parameters of both treatments during 2020 and 2021 at 9 a.m. and 4 p.m. Data are expressed as means ± SEs (n = 3). P_n, net photosynthetic rate; C_i, intercellular CO₂ concentration; gs, stomatal conductance; T_r, transpiration rate. Different letters represent significant differences (P < 0.05) between treatment and time of measurement. The vertical dotted line separates treatment from recovery.

FIGURE 3

Native fluorescence induction curves and double O–P normalized OJIP transients at 10 and 15 (recovery) days after soybean leaf inversion. Each curve presents the average kinetics of five repetitions. Chlorophyll a fluorescence transient curves normalized between F_O and F_P expressed as V_OP [V_OP = (F_t–F_O)/(F_P–F_O)], △V_OP = V_OP(treatment at 4 p.m.) – V_OP(measurement at 9 a.m.). The vertical dashed line separates treatment from recovery. The values in parentheses are the starting values for the graph on the right.

FIGURE 4

(A–D) Double normalization between F_O and F_K expressed as V_OK [V_OK = (F_t–F_O)/(F_K–F_O)], between F_O and F_J expressed as V_OJ [V_OJ = (F_t–F_O)/(F_J–F_O)], between F_O and F_I expressed as V_OI [V_OI = (F_t–F_O)/(F_I–F_O)], between F_J and F_I expressed as V_JI [V_JI = (F_t–F_J)/(F_I–F_J)], and between F_I and F_P expressed as V_IP [V_IP = (F_t–F_I)/(F_P–F_I)]. △V_OX = V_OX(measurement at 4 p.m.)–V_OX(measurement at 9 a.m.). Each curve presents the average kinetics of five repetitions. The vertical dotted line separates treatment from recovery. The values in parentheses are the starting values for the graph on the right.

FIGURE 5

The JIP-test parameters of soybean leaf inversion (10 days) and recovery (15 days) evaluated during 2020 (A) and 2021 (B) at 9 a.m. and 4 p.m.; L, the combined stress of elevated temperature and high light; T, leaf inversion; F_v/F_m, maximum quantum yield of photosystem II (PSII); ABS/RC, absorption flux per reaction center (RC) at t = 0; DI_O/RC, Dissipated energy flux per RC at t = 0; TR_O/RC, Trapped energy flux per RC at t = 0; ET_O/RC, Electron transport flux per RC at t = 0; Ψ_EO, Probability that a trapped exciton moves an electron into the trapped electron transport chain beyond Q_A^–; φ_EO, Quantum yield for electron transport at t = 0; δ_RO, Efficiency with which an electron can move from the reduced intersystem electron acceptors to the photosystem I (PSI) end electron acceptors; φ_RO, Quantum yield for the reduction of end acceptors of PSI per photon absorbed; V_K/V_J, Limitation/inactivation and possible damage of the oxygen-evolving complex; F_V/F_O, maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PSII; Area, Density area over the chlorophyll a fluorescence transient delimited by a horizontal line at F_m; F_O, Minimum fluorescence, when all PSII RCs were open; F_m, Maximum fluorescence, when all PSII RCs were closed; RC/CS, Density of active RCs (Q_A reducing RCs) per cross-section at t = 0; PI_ABS, performance index on absorption basis; PI_total, efficiency of energy conservation from absorbed photons to the reduction of PSI end acceptors. Data are expressed as means ± SEs (n = 5). Different letters represent significant differences (P < 0.05) between treatment and time of measurement. The vertical dotted line separates the treatment from the recovery.

FIGURE 6

Principal component analysis of variability of JIP-test and gas exchange parameters of soybean leaves for 10 and 15 days (recovery) after leaf inversion in 2020 and 2021.