Photoinhibition Of Photosynthesis In Strawberry And Effect Of Light Quality On Its Growth And Fruiting

Strawberry is one of the most important baccate fruits in China. Its yield and cultivated area reached to 1338 thousands tons and 82 thousands hectares respectively in 2003, which ranked the first in the world. With the yield and cultivated area of strawberry exceeded 120 thousands tons and 6 thousands hectares respectively in 2003, Zhejiang province is one of the largest growing districts in china. After entry of China into WTO, our domestic strawberry market will surely face the competition from abroad. Therefore, the breeding and production of high quality strawberry fruit is urgent.The supplement of CO2 and light are two important cultural practices commonly used in strawberry culture under-glass.The photosynthetic response to CO2 can also depend on N supply, which is poorly known in strawberry. Growth, morphogenesis, photosynthesis, metabolism, and gene expression of plants are all regulated by light quality. Chromatic plastic film and light conversion film has been applied in protected cultivation, and plastic films with different light quality may serve as a non-chemical approach regulating growth and thermoperiodic responses of plants. A few studies showed that light quality affected growth and yield of strawberry, but little is known about its effects on photosynthesis and chlorophyll fluorescence in strawberry leaves, and fruit quality. So it is beneficial to scientific regulating growth and fruiting for strawberry protected cultivation to study on those effects and the interactions between CO2 and N supply on photosynthesis.Strawberry is thought to be a shade adapted plant. Leaves of strawberry plants grown in open field suffer from photoinhibition and photooxidative damage which may result in death in high light that usually occurs during the summer, which mechanism is still unknown. Therefore, it is needed to understand how high irradiation influences its photosynthesis.To clarify the mechanism of photoinhibition and responses of growth and fruiting to light quality, we studied the following physiological aspects (1) Response of photosynthesis to high light and its mechanism in strawberry leaves. (2) Effects of three levels of nitrogen nutrition on photochemistry and photoinhibition of photosynthesis in strawberry leaves growth in elevated CO2. (3) Effects of three levels of nitrogen nutrition on xanthophyll cycle and the antioxidant system in strawberry leaves growth in elevated CO2. (4) Response of Dl turnover to high light in strawberry leaves. (5) Effect of light quality on photosynthesis and chlorophyll fluorescence in strawberry leaves. (6) Effect of light quality on photosynthesis and chlorophyll fluorescence in strawberry leaves. The results were summarized below.1) PAM-2000 portable chlorophyll fluorometer and HCM-1000 photosynthesis measurement system were used to measure the apparent quantum efficiency(AQY), initial fluorescence (Fo), maximal photochemical efficiency of PSII (Fv/Fm), maximal fluorescence(Fm), photochemical quenching(qP), non-photochemical quenching(qN),actual quantum yield of PS II electron transport (Opsn), electron transport rate (ETR), amount of inactive PS II reaction centers (Fi-Fo) , rate of Qa reduction, proportion of Qe-non-reducing PS II reaction centers[(Fi-Fo)/ ( Fp-F ) ], and energy-dependent quenching (qE), photoinhibitory quenching (ql) and state-transition quenching (qT) of non-photochemical quenching in strawberry leaves. The results showed that Fv/Fm, Fm, Fi-Fo and the rate of Qa reduction decreased in strong light and increased during subsequent dark recovery, but (Fi-Fo) / (Fp-Fo) increased in strong light and decreased during subsequent dark recovery. They changed drastically within the first 10 minutes in strong light or in subsequent darkness. In the strong light, Opsh , ETR and qP increased first, and then decreased, but qN decreased drastically first, and then increased slightly. 'Toyonoka' showed less changes in strong light than 'Houkouwase' in Fo, Fv/Fm, Fm, ETR, Opsn and AQY. After treated with DTT, Fv/Fm and Fm were lower , but Fo was much higher than control.2) Biochemically based models of C3 photosynthesis was used to study effects of threelevels of nitrogen nutrition (12 mM, 4 mM, 0.4 mM) on acclimation of photosynthesis to elevated CO2 (700 ulT1) in strawberry leaves. The results showed that strawberry grown in 12 mM nitrogen, all of Pn, maximal carboxylation efficiency (Vc. raax), maximal linear electron flow through photosystemll (Jmax), electron flow to the photosynthetic carbon reduction cycle (Jc) and qP was significantly higher in plants grown and measured at elevated CO2 than for plants grown and measured at ambient CO2. This was due to a significant increase in Jc exceeding any suppression of electron flow to the photorespiratory carbon oxidation cycle (Jo). This increase in photochemistry with decreased non-photochemistry(qN or NPQ) at elevated CO2 alleviated photoinhibition at high light. For plants grown at 4 mM and 0.4 mM nitrogen, all of Pn , Vc, max, Jc and qP was significantly lower in plants grown and measured at elevated CO2 than for plants grown and measured at ambient CO2. Consistent with decreased photochemistry and increased non-photochemistry(qN or NPQ), for leaves grown at at 4 mM and 0.4 mM nitrogen, the photoinhibition was aggravated at elevated CO2. Elevated CO2 suppressed Jo leaves grown at 12 mM , 4 mM and 0.4 mM nitrogen. The results above suggested that deficient nitrogen( 4 mM and 0.4 mM )nitrogen elevated CO2 resulted in an acclimatory decrease of photosynthesis in leaves grown elevated CO2.3) Both the xanthophyll cycle pool size and the conversion of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z) at noon increased with decreasing N supply.As N supply decreased, activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), Guaiacol peroxidase (G-POD) and dehydroascorbate reductase (DHAR) on a Chi basis increased, and O2 generated rate and contents of H2O2 and Malondialdehyde (MDA) on a Chi basis also increased , but activitie of catalase (CAT) decreased with decreasing N supply. There were significant interactions between CO2 and N supply onxanthophyll cycle and the antioxidant system in strawberry leaves. For plants grown at 12 mM nitrogen, all of xanthophyll cycle pool size, activities of SOD, APX, G-POD and DHAR decreased at elevated CO2, and O2 generated rate and contents of H2O2 and MDA also decreased at elevated CO2, but plants grown at 4 mM and 0.4 mM nitrogen were on the contrary at elevated CO2.4) Responses of Dl turnover to high light in strawberry leaves were studied with Western-blotting. The results showed that Dl content in 'Toyonoka' leaves grown at high light for one week (4 hours/day) changed inapparently, but that in 'Houkouwase' increased significantly. Shortage in nitrogen supply facilitated degradation of Dl protein in photosystem II reaction center in high light , Dl content in 'Toyonoka' leaves grown at 4 mM and 0.4 mM nitrogen were lower than that in 12 mM at high light for one week (4 hours/day) significantly.5 )Studies on the photosynthetic characteristics of strawberry(Fragaria^ananassa Duch cv. 'Toyonoka') leaves under identical light intensity(55-57% natural light) and different light quality showed that chlorophyll contents, maximal photochemical efficiency of PSII (Fv/Fm), Fm/Fo, amount of inactive PS II reaction centers (Fi-Fo) and rate of QA reduction were correlated with red/blue ratios positively, but chlorophyll (a/b) ratios were correlated with red/blue ratios negatively. Carotenoid contents were in the order: blue film >green film> red film, white film and yellow film, which were correlated with red/far-red ratios negatively.The apparent quantum efficiency(AQY), photo-respiratory rate (Pr) and carboxylation efficiency(CE) also were strongly affected by light quality. The photosynthetic rate (Pn) of strawberry leaves under green film was significantly lower than that of all other film treatments.5) Studies on the photosynthetic characteristics of strawberry (Fragariaxananassa Duch cv.'Toyonoka') leaves under identical light intensity(55-57% natural light) and different light quality showed that chlorophyll contents, maximal photochemical efficiency of PSII (Fv/Fm), Fm/Fo, amount of inactive PS II reaction centers (Fi-Fo) and rate of QA reduction were correlated with red/blue ratios positively, but chlorophyll (a/b) ratios were correlated with red/blue ratios negatively. Carotenoid contents were in the order: blue film >green film> red film, white film and yellow film, which were correlated with red/far-red ratios negatively.The apparent quantum efficiency(AQY), photo -respiratory rate (Pr) and carboxylation efficiency(CE) also were strongly affected by light quality. The photosynthetic rate (Pn) of strawberry leaves under green film was significantly lower than that of all other film treatments.6) The influence of different light quality on plant growth and fruiting of 'Toyonoka' strawberry {Fragaria^ananassa Duch.) were studied. The optimum light quality for leaf and petiole growth was green film, red film was in the next place, and the last was bule film. The proportion of total plant dry biomass allocated to fruits, crowns and roots under blue film was the greatest, the next was red film, and the last was green...