Abstract: To study the mechanism of cell sap pH on flower color formation in Primula vulgaris，petals of pink，red，white，yellow and blue were used as materials. The flower color indices were measured using CIE L*a*b* system，and the cell sap pH values in different colored petals were measured by planar pH meter. Furthermore，the composition and content of anthocyanin，flavonoids and carotenoid were assessed qualitatively and quantitatively by UPLC. And then，the color change and absorbance change of cell sap in different external buffer solutions were observed using UV spectrophotometer. The results showed that cell sap pH value was the lowest in red（5.07），followed by pink，white and yellow，and the highest in blue petal（6.33）. The anthocyanidins were accumulated abundantly in pink，red and blue petals while the contents of flavonoids were the highest in white and yellow petals. Meanwhile，the cell sap pH values showed significantly positive correlations with the content of delphinidin-3-glucoside，hirsutin， petunidin-7- methoxy-3，5-diglucoside，malvidin-3-glucoside and hirsutin-4’-malonate，which all were specific in blue petals. Moreover，there was a bathochromic shift in the visible light region of the maximum absorption peak of pink，red and blue cultivars’ petal pigment extracting solution，and the color of pigment extracting solution of pink，red and blue petals was fading gradually with the increasing of buffer solution pH，and then turned into yellow. In addition，based on the results from expression profiles， the correlation between cell sap pH and gene expression，and the bioinformatics analysis，the relative expression of PvPH4 in blue petals was significantly higher than those in the other four colored petals. Furthermore，the relative expression of PvPH4 showed significantly positive correlation with cell sap pH. The conserved domain of PvPH4 was caca2，which belonged to ChaA superfamily. And the tertiary structure modeling of the protein showed that the similarity to the vacuolar calcium ion transporter was 39%. Thus，PvPH4 may play a role on flower color by encoding vacuolar calcium ion transporter to influence the petal cell sap pH.

Key words: Primula vulgaris, flower color, anthocyanin, cell sap pH, gene expression, PvPH4