Photosynthetic Properties and Potentials for Improvement of Photosynthesis in Pale Green Leaf Rice under High Light Conditions

Abstract

Light is the driving force of plant growth, providing the energy required for photosynthesis. However, photosynthesis is also vulnerable to light-induced damage caused by the production of reactive oxygen species (ROS). Plants have therefore evolved various protective mechanisms such as non-photochemical quenching (NPQ) to dissipate excessively absorbed solar energy as heat; however, photoinhibition and NPQ represent a significant loss in solar energy and photosynthetic efficiency, which lowers the yield potential in crops. To estimate light capture and light energy conversion in rice, a genotype with pale green leaves (pgl) and a normally pigmented control (Z802) were subjected to high (HL) and low light (LL). Chlorophyll content, light absorption, chloroplast micrographs, abundance of light-harvesting complex (LHC) binding proteins, electron transport rates (ETR), photochemical and non-photochemical quenching, and generation of ROS were subsequently examined. Pgl had a smaller size of light-harvesting chlorophyll antenna and absorbed less photons than Z802. NPQ and the generation of ROS were also low, while photosystem II efficiency and ETR were high, resulting in improved photosynthesis and less photoinhibition in pgl than Z802. Chlorophyll synthesis and solar conversion efficiency were higher in pgl under HL compared to LL treatment, while Z802 showed an opposite trend due to the high level of photoinhibition under HL. In Z802, excessive absorption of solar energy not only increased the generation of ROS and NPQ, but also exacerbated the effects of increases in temperature, causing midday depression in photosynthesis. These results suggest that photosynthesis and yield potential in rice could be enhanced by truncated light-harvesting chlorophyll antenna size.

Keywords: chlorophyll; electron transport rate; light-harvesting chlorophyll antenna; non-photochemical quenching; photosynthesis; reactive oxygen species.

Figure 1

Genotype Z802 (right) and pgl (left) at the seedling stage.

Figure 2

Chlorophyll content (A,B), biomass (C), light-saturated photosynthesis (D) for Z802 and pgl from high (HL) and low light (LL) adapted plants. Data represent the means of three replicates per treatment ± standard deviation. The photosynthesis was measured at light intensity of 1,500 μmol m⁻² s⁻¹, temperature of 25°C with Li-6400.

Figure 3

Light micrographs of chloroplasts under low (LL; A,B) and high light treatment (HL; C,D) in Z802 (A,C) and pgl (B,D). Bars = 50 μm.

Figure 4

Electron micrographs of chloroplasts under low (LL; A,B) and high light treatment (HL; C,D) in Z802 (A,C) and pgl (B,D). Bars = 1 μm. Th, thylakoid lamellae; OB, osmophilic body; S, starch.

Figure 5

Leaf spectral absorptance rates of leaves of Z802 and pgl under low (LL) and high light treatment (HL).

Figure 6

(A) Rapid light curves (RLC) of Z802 and pgl under high (HL) and low light treatment (LL). The relative electron transport rate (rETR) is plotted against PAR irradiance (μmol photons m⁻² s⁻¹). Fitted curves are plotted with a dotted line, and the rETR_max, E_k, and α are displayed in the RLCs of Z802 under LL. (B) PSII efficiency (Φ_PSII), (C) photochemical quenching (qP), and (D) non-photochemical quenching (NPQ) derived from the RLCs under high (HL) and low light (LL) in Z802 and pgl as a function of PAR.

Figure 7

The relative abundance of ratio of light harvesting complex (LHC) binding proteins of pgl/Z802 in low light (LL) and high light (HL) treatments. The proteins of LHCI (Lhca1-4) and LHCII (Lhcb1-6) ligate chlorophylls and carotenes, which absorb light and transmit the excitation energy to the core complex.

Figure 8

Imaging of reactive oxygen species (ROS) in leaves of Z802 and pgl. ${\text{O}}_2^{-}$-dependent DHE fluorescence in leaves of Z802 (A,C,E) and pgl (B,D,F). (A,B) As a negative control, leaf samples were incubated with 1 mM N₃Na (peroxidase inhibitor). (C,D) Leaves under low light (PPFD: ~200 μmol m⁻² s⁻¹) and (E,F) under high light (~1,000 μmol m⁻² s⁻¹). Bars = 100 μm.

Figure 9

Lipid peroxidation (MDA, A), and antioxidant enzymes activity (superoxide dismutase, SOD, B; peroxidase, POD, C; catalase, CAT, D) in leaves of Z802 and pgl under low (LL) and high light treatment (HL). Different letters indicate significant differences (P < 0.05) between genotypes at LL or HL treatment.

Figure 10

Thermal images of the canopy of Z802 and pgl in the morning (9:00), at noon (13:00) and in the afternoon (17:00) at the flowering stage under different rates of nitrogen (0, 120, 240 N).

Figure 11

Canopy temperatures of Z802 and pgl in the morning (9:00), at noon (13:00) and in the afternoon (17:00) under different rates of nitrogen (0, 120, 240 N). Vertical bars denote standard deviations (n = 10). Different letters indicate significant differences (P < 0.05) between genotypes at the same level of N and at the same time of day.

Figure 12

Diurnal variation in the net photosynthetic rates of Z802 (A) and pgl (B) under different rates of nitrogen (0, 120, 240 N) at the flowering stage. Data represent the mean of four replicates with the standard deviation shown by vertical bars.