Exogenous Calcium Enhances the Photosystem II Photochemistry Response in Salt Stressed Tall Fescue

Abstract

Calcium enhances turfgrass response to salt stress. However, little is known about PSII photochemical changes when exogenous calcium was applied in salinity-stressed turfgrass. Here, we probe into the rearrangements of PSII electron transport and endogenous ion accumulation in tall fescue (Festuca arundinacea Schreber) treated with exogenous calcium under salt stress. Three-month-old seedlings of genotype "TF133" were subjected to the control (CK), salinity (S), salinity + calcium nitrate (SC), and salinity + ethylene glycol tetraacetic acid (SE). Calcium nitrate and ethylene glycol tetraacetic acid was used as exogenous calcium donor and calcium chelating agent respectively. At the end of a 5-day duration treatment, samples in SC regime had better photochemistry performance on several parameters than salinity only. Such as the Area (equal to the plastoquinone pool size), N (number of [Formula: see text] redox turnovers until F_m is reached), ψE₀, or δRo (Efficiencdy/probability with which a PSII trapped electron is transferred from Q_A to Q_B or PSI acceptors), ABS/RC (Absorbed photon flux per RC). All the above suggested that calcium enhanced the electron transfer of PSII (especially beyond [Formula: see text]) and prevented reaction centers from inactivation in salt-stressed tall fescue. Furthermore, both grass shoot and root tissues generally accumulated more C, N, Ca²⁺, and K⁺ in the SC regime than S regime. Interrelated analysis indicated that ψE₀, δRo, ABS/RC, C, and N content in shoots was highly correlated to each other and significantly positively related to Ca²⁺ and K⁺ content in roots. Besides, high salt increased ATP6E and CAMK2 transcription level in shoot at 1 and 5 day, respectively while exogenous calcium relieved it. In root, CAMK2 level was reduced by Salinity at 5 day and exogenous calcium recovered it. These observations involved in electron transport capacity and ion accumulation assist in understanding better the protective role of exogenous calcium in tall fescue under salt stress.

Keywords: PSII photochemistry; carbon and nitrogen assimilation; exogenous calcium; salt stress; tall fescue.

Figure 1

Polyphasic rise of chlorophyll fluorescence in tall fescue leaves before (A) and after (B) subjected to salt stress with different calcium regimes. S (Salt stress) and SC (Salt stess combined with exogenous calcium application) shows no difference when compared to CK 5 days later, while SE (Salt stess combined with calcium chelator, EGTA) revealed a weakened P-step (B).

Figure 2

A “radar plot” of picked parameters characterizing different behavior of Photosystem II of tall fescue leaves exposed to diverse saline environment. All values are shown as percent of control (control plants = 1).

Figure 3

Calcium addition gently alleviated the reduction of chlorophyll content (A) and electrolyte leakage upsurge (B). Three independent experiments were performed in chlorophyll content determination specially. Different letters above the same columns indicate statistic significant difference at P < 0.05 by Tukey's multiple range tests. Comparisons were carried out among the same tissue at same time, respectively.

Figure 4

Exogenous calcium facilitated the carbon (A) and nitrogen (B) assimilation of tall fescue in salt environment. However, it showed no significant effect on Carbon Nitrogen ratio (C). Columns marked with different letters indicate statistic significant difference at P < 0.05 by Tukey's multiple range tests. “0-L” and “5-L” on the horizontal axis means experiments are carried out in shoots at 0 and 5 days after treatments (DAT), similarly, “5-R” represents roots at 5 DAT. Comparisons were carried out among the same tissue at same time, respectively.

Figure 5

The quantification of calcium, potassium content in shoots and roots Calcium concentration gradient in shoots was actually confirmed (A). In addition, the potassium absorption was promoted by calcium application (B), which was obviously reflected in roots. Columns marked with different letters indicate statistic significant difference at P < 0.05 (Tukey's multiple range test). Comparisons were carried out among the same tissue at same time, respectively.

Figure 6

ATP6E (A) and CAMK2 (B) transcriptional level in tall fescue exposed to salinity. Different letters above the same columns indicate statistic significant difference at P < 0.05 (Tukey's multiple range test). Calcium led to a 95.8% reduction in the ATP6E expression than the control under salinity stress conditions. Comparisons were carried out among the same tissue at same time, respectively.