In the present study, the yields of goji fruits under different phosphorus fertilizer levels at different harvest times were analyzed, and we found that the total yield of goji fresh fruit (FF) and dry fruit (DF) were both highest in P0 and lowest in P2. Shi et al. reported that the correlation between yield and various fertilizer factors and the first-order coefficient showed a larger effect for potassium, followed by phosphorus and then nitrogen26. The goji is an economic tree species with continuous flowering and fruiting from summer to autumn, and can be harvested four to five times per year15,27,28,29. Thus, each harvest of goji is very important for the total yield, and there has so far been no research analyzing the yield differences according to different harvest times and different levels of fertilizer. Here, we found that the yield of fresh fruits (FF) and dry fruits (DF) had a negative and strong linear correlation (− 0.98, − 0.89) with phosphorus fertilizer levels for the first and third harvests, and a positive correlation (0.99) for the fourth harvest. Furthermore, the significant difference in the total yield of goji fruits for different phosphorus fertilizer levels was due to differences in the third harvest. However, the results of this study are different from those of previous studies which indicated that fertilization had a positive effect on the yield of goji11,12,13,14,30. We think the third harvest time is key for the yield of goji fruit, where low phosphorus at this time is beneficial for yield. Meanwhile, we studied the ratio of goji fruit fresh weight (FW) and dry weight (DW) in three treatments under different harvest times. FW/DW means the number of kilograms of fresh fruits needed for one kilogram of dry fruit. Yuan et al. set up a comprehensive evaluation system for fresh goji fruit drying process indicators, which indicated that the 100-grain dry weight and tissue moisture content are the key indicators that reflect the processing characteristics of dried fresh goji fruit31. Although there were significant differences in the FW/DW in the first, second, and fourth harvest under different phosphorus level, this still had no significant impact on the total FW/DW of goji. We infer that the FW/DW may be affected by climatic conditions of cultivated areas and the characteristics of the specific variety.
The amino acids, polysaccharides, betaines, flavonoids, anthocyanins, and other functional ingredients in goji fruits have been shown to enhance human immunity, inhibit the growth of tumor cells, delay aging, contribute to fatigue resistance, lower blood pressure, protect the liver, protect vision, and provide antioxidant activity1,32. By characterizing the yield of fruits in response to P0, P1, and P2, a significant difference was observed for the third harvest. Therefore, we detected the main nutritional contents of fresh goji fruits in P0, P1, and P2 from the first harvest and third harvest. Meanwhile, we analyzed the metabolic profiles of nutritional contents in goji fruit from the first harvest using an integrated UPLC–ESI–MS/MS detection system. In the present study, the first and third harvests of goji fruit corresponding P0, P1, and P2 treatments presented 17 amino acids, among which six essential amino acids and 11 non-essential amino acids were found. The results were consistent with previous studies, which detected 17 kinds of amino acids from summer and autumn fruits of ‘Jingqi No.1’, ‘Jingqi No.2’, ‘Ningqi No.1’, ‘Ningqi No.5’ and ‘Ningqi No.7’3,21,30,33,34,35. The essential and non-essential amino acids levels were both higher for all three treatments in the third harvest than compared to the first harvest. The results corresponded with those of a previous report, which indicated that the dry goji fruits harvested in July and October had the highest nutritional contents; the fresh goji fruits harvested in summer are easier to eat fresh, and in autumn, they are suitable for making dry fruits30,31. Meanwhile, the total contents of amino acids in goji fruits increased with the phosphorus level for a range of phosphorus level of 32.5–65 g/plant. A previous study reported that the increasing phosphorus level inhibits Cd accumulation and promotes the synthesis of amino acids in plants, but the correlation between each amino acid and phosphorus level has not been clarified36. We found that there were significant negative linear correlations between contents of amino acids (Ser, Gly, His, Arg, Ala, Pro, Tyr, Met, Val, Thr, Leu, Phe, and Lys) and phosphorus fertilizers. We infer that phosphorus may affect the conversion between amino acids, but how it affects the conversion between amino acids requires further study.
The flavonoid contents of goji fruits showed no significant difference between the first harvest and third harvest for P0, P1, and P2. However, a significant difference in fruits was observed between P0, P1, and P2 for the first harvest, where the flavonoid contents of fruits in the first harvest were positively correlated with phosphorus level (0.99). Wang et al. reported that the flavonoid content of the goji was highly correlated with altitude (r = 0.914, p < 0.01) and average diurnal temperature (r = 0.851, p < 0.05). Thus, the divergence of flavonoid contents in each harvest time may be caused by phosphorus level and diurnal temperature3,15,27. The flavonoid biosynthetic pathway is initiated by the catalytic action of phenylalanine ammonia lyase (PAL) on the precursor amino acid phenylalanine, and then cinnamate 4-hydroxylase (C4H) enzyme, leading to the production of the entry compound to flavonoid biosynthesis, chalcone. Chalcone was catalyzed by chalcone isomerase to naringenin, which is the main product of metabolism and then enters other different metabolic pathways37,38,39. We analyzed the metabolic profiling of flavonoids and found 117 metabolites (42 flavones, 27 flavonols, 2 flavonolignans, 24 flavone C-glycosides, 16 flavanones, and 6 isoflavones) and 13 anthocyanins in goji fruits. The flavanones of goji fruits included four metabolites (naringin, naringenin, eriodictyol, and butein) which were significantly downregulated and others that exhibited no change. There were also some metabolites of flavone, flavonol, flavonolignan, flavone C-glycoside, isoflavone, and anthocyanin in goji fruits which were upregulated or downregulated. We infer that the change in flavonoids may be initiated by the divergence of phenylalanine.
Lycium barbarum polysaccharide (LBP) is a glycoprotein complex in which sugar chains (glucose, arabinose, galactose, mannose, xylose, and rhamnose) account for 70% of its total content2,31,40. We found that LBP was significantly negatively correlated with phosphorus level (− 0.87). However, Zhang et al. indicated that the correlation between total sugar and available potassium (0.608) was positive, and polysaccharides were positively correlated with available potassium (0.626)41. Their descriptions are different from the results presented here, which may mean that the samples were different. Our samples were from the first harvest and third harvest, but their samples were mixed year-round. A previous study also provides an opinion, which show that potassium deficiency can lead the root respiration per fresh or dry matter increased, but decreased by deficiency of either phosphorus or all nutrients42. So, the LBP may be will be influenced by phosphorus fertilizer level. But, through the metabolic profiling of carbohydrates, we found only three monosaccharides (DL-arabinose, L-fucose, and glucosamine) belonging to LBP and no change with changing phosphorus level was observed. Betaine is a small molecule belonging to the active components of goji43,44. According to the metabolic profiling of alkaloids and terpenoids, we found six alkaloids and two terpenoids of goji fruits. Only betaine showed a significant change, which had a strong positive correlation with phosphorus level. Chung et al. indicated that the synthesis of betaine in goji was highly related to nitrogen levels, with a decrease in betaine concentrations with increasing N fertilizer45. Thus, we can infer that the betaine of goji had a positive correlation with phosphorus level and negative correlation with nitrogen level.
In conclusion, the yield of goji fruits under different harvest times had a highly negative correlation with phosphorus fertilizer levels, especially in the third harvest time. The amino acids, flavonoids, polysaccharides, and betaine contents of goji fruits in the first harvest time were significantly affected by phosphorus level. The data of the metabolic profiling of goji fruits showed that the phosphorus fertilizer levels mainly affected the conversion between amino acids and the biosynthesis of flavonoids.