Effects of shading on photosynthetic characteristics and fruit quality in strawberry

Abstract:【Objective】Light as the energy source, can affect plant photosynthesis. Both excessive and insufficient light conditions damage photosynthetic structures, and reduce photosynthetic efficiency and eventually fruit quality. Shading treatment is a practical method widely used in facility cultivation for high-quality strawberry, but the suitable shading level is not clear. To uncover the suitable shading level and provide theoretical basis for high-quality strawberry production during spring and summer in Zhejiang and in regions with similar climate, the effects of shading on leaf photosynthetic rate and fruit quality were investigated in this study.【Methods】In February, 'Akihime'cultivar seedlings with the same growth status were selected as the experimental materials. Sheds were built and covered with shading net to set 25% and 50% shading treatments in March, and the shed without shading net was used as the control (0% shading treatment) . During the strawberry ripening period (approximately 45-60 days after treatment) , chlorophyll fluorescence was measured with a Yaxin-1161 chlorophyll fluorescence analyzer, and various fluorescence parameters were calculated. All the leaves in each plant were harvested to measure the sugar content using anthrone colorimetry method, and the sugar content in fruits were determined using the same method. The contents of soluble proteins, total phenols and anthocyanins in fruitwere determined with the coomassie brilliant blue colorimetry method, the Folin-phenol method, and the spectrophotometric method with pelargonidin-3-O-glucoside as the standard, respectively. Titrable acidity was measured with titration method, and soluble solid content with an Abbe refractometer. The aroma compounds were extracted by the solid phase microextraction (SPME) method, analyzed with a gas chromatography-mass spectrometer (GC-MS) , and identified according to NIST library.【Results】In the treatments with 25% and 50% shading, the maximum quantum yield of primary photochemistry (φPO) in strawberry leaves decreased by 3.45% and 17.52% (p < 0.05) , respectively, compared with the control. Similar to φPO, the probability that a trapped exciton moves an electron into the electron transport chain beyond QA- (ΨO) , quantum yield for electron transport at t=0 (φEO) , and density of reaction centers per excited cross-section (RC/CSM) were not significantly influenced by 25% shading treatment, but they were significantly reduced by 50% shading treatment, with a reduction of 44.23% (p < 0.05) , 53.21% (p < 0.05) and 31.77% (p < 0.05) , respectively. Compared to the control, the maximum quantum yield of non-photochemical deexcitation (φDO) increased by 11.02% and 38.59% (p < 0.05) in 25%and 50% shading treatments, respectively. Shading treatments reduced the contents of glucose, fructose, reducing sugars and total sugars in the leaves, but there was no significant difference between 25% shading treatment and the control. For sucrose, its content in leaves decreased by 38.88% (p < 0.05) and5.88%, but the content in fruit increased by 21.17% (p < 0.05) and 7.23% in 25% and 50% shading treatments, respectively, indicating that the transportation of the main photosynthetic product sucrose from“source” (leaves) to“sink” (fruits) can be promoted by shading, especially 25% shading. The contents of total phenols, fructose, glucose, reducing sugars and total sugars in 50% shading treatment were16.93% (p < 0.05) , 6.63%, 20.07% (p < 0.05) , 17.45% (p < 0.05) and 14.66% (p < 0.05) lower than in the control, respectively. There was no significant difference in these parameters between 25% shading treatment and the control, except that the titratable acidity declined by 12.84% (p < 0.05) . Soluble proteins increased with increasing the shading level, but had no significant difference between 25% shading treatment and the control. Strawberry fruit released an abundance of aroma compounds, including esters, terpenoids, hydrocarbons, alcohols, acids, aldehydes, and ketones. Fifty-eight compounds were detected in the fruit without shading treatment, while 4 and 5 new compounds appeared in 25% and50% shading treatments, respectively. Among the fruit aroma, esters and terpenoids were the main types, as the esters were the most abundant components and the terpenoids showed the maximum emission amount. In the treatments with 25% and 50% shading, the emission amount of terpenoids increased by 22.53% (p < 0.05) and 9.84%, respectively. Trans-nerolidol was the typical component among strawberry aromas. It emission amount increased by 22.26% (p < 0.05) and 4.05% in 25% and 50% shading treatments respectively, which might result from the increased expression of nerolidol synthase 1 (Fa NES1) in the shading treatments, but the specific mechanism needs further study.【Conclusion】25%shading treatment promoted photosynthetic product transport from leaves to fruit, increased sucrose content and aroma emission, as well as reduced titratable acidity. Therefore, it is suitable to set 25% shading treatment to improve strawberry fruit quality in the facility cultivation during spring and summer in Zhejiang and its similar climatic province.