Influence of additional far-red light on the photosynthetic and growth parameters of lettuce plants and the resistance of the photosynthetic apparatus to high irradiance

Abstract

The effects of additional far-red light (FRL) on the photosynthetic and growth parameters of Lactuca sativa plants grown for 30 d and on the photosynthetic activity of the plants under high irradiance [4 h; 1,500 μmol(photon) m^-2 s^-1] were studied. The plants were grown under coloured light-emitting diodes at a ratio of red light (RL): blue light (BL): green light (GL): far-red light (FRL) = 0.7:1:0.3:0.4 or RL:BL:GL:FRL = 0.7:1:0.3:0.8 (test, T). Additional FRL led to an increase in plant biomass, height, and leaf area but to a decrease in photosynthesis and respiration rates. However, PSII activity was greater in plants with additional FRL. It is suggested that the increase in biomass occurred mainly due to an increase in leaf area but not in photosynthesis. In addition, PSII in the experiment was less resistant to high irradiance. The possible direct and indirect influences of the FRL on growth and photosynthesis were considered.

Keywords: Lactuca sativa; chlorophyll a fluorescence; far-red light; growth; photosynthesis; red light.

Fig. 1. Emission spectra of the control (C) (RL:BL:GL:FRL ratio = 0.7:1:0.3:0.4) and test (T) (RL:BL:GL:FRL ratio = 0.7:1:0.3: 0.8) LED (light-emitting diode) matrices. RL – red light, BL – blue light, GL – green light; FRL – far-red light.

Fig. 2. Dynamics of the growth parameters of lettuce plants grown in the control matrix with an RL/FRL ratio = 1.75 (control, C) and in the experimental matrix with an RL/FRL ratio = 0.875 (test, T): fresh mass of the plants (A), leaf area (B), and plant height (C). The data are presented as the means ± SE. The values between the ‘control’ and ‘test’ curves marked with an asterisk differ at the p<0.05 level, n = 10. RL – red light, FRL – far-red light.

Fig. 3. Dynamics of photosynthetic parameters and respiration in lettuce plants grown in the control matrix with an RL/FRL ratio = 1.75 (control, C) and in the experimental matrix with an RL/FRL ratio = 0.875 (test, T). Data are presented as the means ± SE. The values between the ‘control’ and ‘test’ curves marked with an asterisk differ at the p<0.05 level. Here, F_v/F_m is the PSII maximal quantum yield, PI_ABS is the PSII performance index, and P_N and R_D are the photosynthesis and respiration rates, respectively, n = 8. RL – red light, FRL – far-red light; P_N – net photosynthetic rate; R_D – respiration rate.

Fig. 4. Influence of additional FRL on the maximum quantum yield of PSII (F_v/F_m) and the PSII performance index (PI_ABS) of 30-d-old lettuce plants exposed to 4-h high light intensity [1,500 μmol(photon) m^–2 s^–1]. Control (C), RL/FRL ratio = 0.875. Test (T), RL/FRL ratio = 1.75. The data are presented as the means ± SE. The values between the ‘control’ and ‘test’ curves marked with an asterisk differ at the p<0.05 level, n = 6. RL – red light, FRL – far-red light.