Exploration of Chlorophyll a Fluorescence and Plant Gas Exchange Parameters as Indicators of Drought Tolerance in Perennial Ryegrass

Abstract

Perennial ryegrass (Lolium perenne L.) belongs to the common cultivated grass species in Central and Western Europe. Despite being considered to be susceptible to drought, it is frequently used for forming the turf in urban green areas. In such areas, the water deficit in soil is recognized as one of the most important environmental factors, which can limit plant growth. The basic aim of this work was to explore the mechanisms standing behind the changes in the photosynthetic apparatus performance of two perennial ryegrass turf varieties grown under drought stress using comprehensive in vivo chlorophyll fluorescence signal analyses and plant gas exchange measurements. Drought was applied after eight weeks of sowing by controlling the humidity of the roots ground medium at the levels of 30, 50, and 70% of the field water capacity. Measurements were carried out at four times: 0, 120, and 240 h after drought application and after recovery (refilling water to 70%). We found that the difference between the two tested varieties' response resulted from a particular re-reduction of P700⁺ (reaction certer of PSI) that was caused by slower electron donation from P680. The difference in the rate of electron flow from Photosystem II (PSII) to PSI was also detected. The application of the combined tools (plants' photosynthetic efficiency analysis and plant gas exchange measurements) allowed exploring and explaining the specific variety response to drought stress.

Keywords: MR 820; chlorophyll; delayed fluorescence; gas exchange; plant stress; turf grass varieties.

Figure A1

Differential curves of _ΔW_K (obtained by subtracting the control curve from the first sample in the time from 0.02 to 3 ms) of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacity of soil (50 and 30% FWC) and time (0, 120, 240 and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6. Relative units.

Figure A2

Differential curves of _ΔW_L (obtained by subtracting the control curve from the first sample in time from 0.02 to 0.3 ms) of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacity of soil (50 and 30% FWC) and time (0, 120, 240 and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6. Relative units.

Figure A3

Differential curves of _ΔW_H (obtained by subtracting the control curve from the first sample time from 1.9 to 30 ms) of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacity of soil (50 and 30% FWC) and time (0, 120, 240 and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6. Relative units.

Figure A4

Differential curves of _ΔW_G (obtained by subtracting the control curve from the first sample in time from 30 to 1000 ms) of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacity of soil (50 and 30% FWC) and time (0, 120, 240 and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6. Relative units.

Figure 1

Induction curves of chlorophyll a fluorescence of perennial ryegrass varieties (Roadrunner and Nira) grown under different field water capacities of soil (50 and 30% field water content (FWC)) at 0, 120, 240, and 400 h after stress application. (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6 relative units. rel. u., relative units.

Figure 2

Differential curves of ΔV_t (obtained by subtracting the control curve from the first sample) of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacities of soil (50 and 30% FWC) and times (0, 120, 240, and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6 relative units.

Figure 3

JIP-test parameters normalized to the values before stress application (0 h) as radar plots under different field water capacities of soil (50 and 30% FWC). (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. Relative units. Means of one parameter marked by an asterisk differ significantly (p < 0.05, n = 6).

Figure 4

Delayed fluorescence induction curves of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacities of soil (50 and 30% FWC) and times (0, 120, 240, and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6. Relative units.

Figure 5

(I₁-D₂)/D₂ and I₁/I₂ ratios ± S.D. of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacities of soil (50 and 30% FWC) and times (0, 120, 240, and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6. Relative units. Mean values within a term marked by the same letter did not show significant differences (p < 0.05, n = 6).

Figure 6

Kinetics of modulated light reflection at 820 nm (all data are normalized to the initial measured value of the signal) of perennial ryegrass varieties (Roadrunner and Nira) under different field water capacities of soil (50 and 30% FWC) and times (0, 120, 240, and 400 h after stress application): (A) Roadrunner under 50% FWC, (B) Roadrunner under 30% FWC, (C) Nira under 50% FWC, (D) Nira under 30% FWC. n = 6 relative units.

Figure 7

Relative rates of reaction centers of PS I (P700) oxidation (A,C) and P700⁺ re-reduction (B,D) ± S.D. calculated from the MR820 signal of Roadrunner (A,B) and Nira (C,D) under different field water capacities of soil (50 and 30% FWC) and times (0, 120, 240, and 400 h after stress application). Relative units. Mean values within a term marked by the same letter did not show significant differences (p < 0.05, n = 6).

Figure 8

Gas exchange parameters ± S.D. of Roadrunner and Nira under different field water capacities (70, 50, and 30% FWC) and times after the application of stress (0, 120, 240, and 400 h). (A) CO₂ assimilation (A) in Roadrunner; (B) CO₂ assimilation (A) in Nira; (C) H₂O transpiration (E) in Roadrunner; (D) H₂O transpiration (E) in Nira; (E) stomatal conductance (g_s) in Roadrunner; (F) stomatal conductance (G_s) in Nira; (G) internal CO₂ concentration (C_i) in Roadrunner; (H) internal CO₂ concentration (C_i) in Nira. Mean values within a term marked by the same letter did not show significant differences (p < 0.05, n = 6).

Figure 9

Pearson correlation coefficients (r) between prompt fluorescence parameters and CO₂ assimilation (A) (correlations significant at p < 0.05, n = 6). Relations between parameters marked by a solid line were calculated for plants in control conditions, and those marked by a dotted line were calculated for plants under 30% FWC.

Figure 10

Pearson correlation coefficients (r) between delayed fluorescence parameters and CO₂ assimilation (A) and between modulated reflection at 820 nm and CO₂ assimilation (A) (correlations significant at p < 0.05, n = 6). Relations between parameters marked by a solid line were calculated for plants in control conditions, and those marked by a dotted line were calculated for plants under 30% FWC.