Effects of sulfur treatment on chloroplast ultrastructure and fruit quality in kiwifruit (Actinidia deliciosa)

Abstract: 【Objective】Sulfur (S), as a structural constituent of amino acids, iron-sulfursulfur clusters, proteins, membrane sulpholipids, glutathione, glucosinolates and coenzymes, is essential for plant growth anddevelopment. Low-S soils, continuous cropping and use of high-yielding varieties may have increased sulfur starvation in crops. Absence or low sulfur concentration in the soil severely retards growth. And the research on soil sulfur and sulfur effect is receiving more and more attention. However, combinatorial examination of sulfur application on kiwifruit (Actinidia deliciosa) production has rarely been reported. The development of chloroplasts is an important factor in crop yield and quality formation. And the changes ofthe ultrastructure of chloroplasts can be used to evaluate plant tolerance to fertilizer. Therefore, in this pa⁃per, chloroplast structure and fruit quality in kiwi under sulfur treatments at different concentrations (0.0,0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 kg·m^-3) was observed in order to provide scientific reference for reasonable application of sulfur in kiwifruit production.【Methods】The pot experiment was conducted to regularly ob⁃serve the new growth after transplanting the 1-year-old grafted‘Miliang No.1’seedlings in pots filledwith soil, organic fertilizer and sulfur powder. The field experiment was conducted in the winter of 2014 tostudy the effects of sulfur fertilizer on the ultrastructure of mesophyll cells, kiwifruit exterior (single fruitweight, longitudinal diameter, transverse diameter, pleurocaulis and fruit shape index) and interior quality(firmness, soluble solid content, vitamin C content, soluble sugar content, titratable acid content, sugarcontent/titratable acidity and soluble protein content), and the content of total sulfur in kiwifruit collectedin the following September after the annul furrow application of base fertilizers of sulfur at above men⁃tioned doses and 10 kg organic fertilizer per plant.【Results】The growth of new shoots was stimulated byapplying appropriate amount of sulfur. When the concentration was greater than 2.5 kg·m^-3, the new shootwas shorter with smaller, curly and yellow leaves compared with the control group, and with the growth ofthe kiwi vines, the leaves withered and abscised. The results showed that kiwifruit plant growth was inhib⁃ited by excessive sulfur. Soil properties changed obviously after applying sulfur fertilizer for 2 years. ThepH in the soil treated with sulfur at 1.0-2.0 kg·m^-3 was in the range that was suitable for kiwi growth.Moreover, the soil organic matter, total nitrogen contents and available phosphorus content in the soil treat⁃ed with sulfur at 1.5 kg·m-3 were significantly higher than those in the other treatment groups. The chloroplast ultrastructure was destroyed under sulfur deficiency and excessive sulfur. The main symptoms in⁃cluded the poor uniformity in cell size and arrangement, increased intercellular space, thinner cell walls,more osmium-phillic particles, and decreased number of chloroplast and starch grains compared with thenormal leaves. Under the treatment of excessive sulfur, the granum thylakoids in chloroplast expandedwith disordered and irregular arrangement, and the chloroplast structure was destroyed and even cracked.In all treatments with sulfur, the shape of chloroplast was in regular fusiform around the intracellular sur⁃face with clear grana and stroma; the grana had many layers of thylakoids densely and neatly arranged.There were a lot of starch grains in the chloroplasts and the leaf cells were uniform and neatly arranged inthe treatment with sulfur at 1.0 kg·m-3. In the treatment groups with sulfur higher than 2.0 kg·m^-3, fruitweight and fruit shape index increased and fruit length became longer. Therefore, the treatments improvedthe exterior quality of kiwi fruit. The treatments with low and medium concentrations of sulfur could significantly improve the intrinsic quality, and the effect of sulfur treatment at 2.0 kg·m^-3 was the best, withfruit firmness, soluble solids content, vitamin C content, soluble sugar content and soluble protein contentincreased by 32.65%, 5.45%, 9.16%, 11.76% and 26.92%, respectively, compared with those in the con⁃trol. The total sulfur content in fruit increased with the increase of sulfur concentration applied. However,in all treatments, the total sulfur content in fruit was below the safety limit in crops.【Conclusion】Using ap⁃propriate concentration of sulfur and organic fertilizer could improve soil nutrient, increase thylakoid layers in the grana and starch grains in the chloroplasts, improve the appearance and nutritional quality of kiwi fruit, and ensure fruit safety. It was recommended that annul furrow application of sulfur at doses of1.5-2.0 kg·m^-3 and organic fertilizer in winter could be used in kiwifruit production.