Alginate oligosaccharides (AOS) are functional substances in seaweed extracts that regulate crop quality and stress tolerance. In this paper, the effects of AOS spray application on the antioxidant system, photosynthesis and fruit sugar accumulation in citrus was investigated through a two-year field experiment. The results showed that 8-10 spray cycles of 300-500 mg L-1 AOS (once per 15 days) increased soluble sugar and soluble solid contents by 7.74-15.79% and 9.98-15.35%, respectively, from citrus fruit expansion to harvesting. Compared with the control, the antioxidant enzyme activity and the expression of some related genes in citrus leaves started to increase significantly after the 1st AOS spray application, while the net photosynthetic rate of leaves increased obviously only after the 3rd AOS spray cycle, and the soluble sugar content of AOS-treated leaves increased by 8.43-12.96% at harvest. This suggests that AOS may enhance photosynthesis and sugar accumulation in leaves by antioxidant system regulation. Moreover, analysis of fruit sugar metabolism showed that during the 3rd to 8th AOS spray cycles, AOS treatment increased the activity of enzymes related to sucrose synthesis (SPS, SSs), upregulated the expression of sucrose metabolism (CitSPS1, CitSPS2, SUS) and transport (SUC3, SUC4) genes, and promoted the accumulation of sucrose, glucose and fructose in fruits. Notably, the concentration of soluble sugars in citrus fruits was significantly reduced at all treatments with 40% reduction in leaves of the same branch, but the loss of soluble sugars in AOS-treated fruits (18.18%) was higher than that in the control treatment (14.10%). It showed that there was a positive effect of AOS application on leaf assimilation product transport and fruit sugar accumulation. In summary, AOS application may improve fruit sugar accumulation and quality by regulating the leaf antioxidant system, increasing the photosynthetic rate and assimilate product accumulation, and promoting sugar transfer from leaves to fruits. This study shows the potential application of AOS in the production of citrus fruits for sugar enhancement.

IntroductionSelenium (Se) is an essential trace element required for proper human and animal health.MethodsIn this paper, we investigated the uptake and distribution characteristics of a new Se fertilizer, which comprises algal polysaccharides–selenium nanoparticles (APS-SeNPs), in rice plants in both hydroponic and pot experiments.ResultsThe results from the hydroponic experiments revealed that the rice root uptake of APS-SeNPs fitted the Michaelis–Menten equation, with a Vmax of 13.54 μg g−1 root dry weight (DW) per hour, which was 7.69 and 2.23 times those of selenite and selenate treatments, respectively. The root uptake of APS-SeNPs was inhibited by AgNO3 (64.81%–79.09%) and carbonyl cyanide 3-chlorophenylhydrazone (CCCP; 19.83%–29.03%), indicating that the uptake of APS-SeNPs by rice roots is mainly via aquaporins and is also affected by metabolic activity. Moreover, sulfur deficiency caused rice roots to absorb more APS-SeNPs, but treatment with APS-SeNPs increased the expression of the sulfate transporter OsSULTR1;2 in the roots, suggesting that OsSULTR1;2 is probably involved in the uptake of APS-SeNPs. The application of APS-SeNPs significantly increased the Se content in rice plants and the apparent Se uptake efficiency compared with selenate and selenite treatments. Most of the Se in the roots of rice plants was distributed in the cell wall, while it was primarily located in the cytosol in the shoots when treated with APS-SeNPs. The results from the pot experiments indicated that the application of Se enhanced the Se content of each rice tissue. It is worth noting that the Se content in brown rice under APS-SeNP treatment was higher than that under selenite or selenate treatment and was mainly concentrated in the embryo end, with the Se in organic form.DiscussionOur findings provide important insights into the uptake mechanism and the distribution of APS-SeNPs in rice plants.

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