Seaweed extract combined with boron promotes the growth of sugar beet by improving the photosynthetic performance under boron deficiency

Abstract Biostimulants can improve mineral nutrient effectiveness, but the physiological mechanisms by which seaweed extract, a natural biostimulant, and boron (B) fertilizer promote the growth of sugar beet under B deficiency are not clear. In this study, B and seaweed extract were applied under B‐deficiency (0.32 mg B kg −1 soil) and potentially B‐deficient conditions (0.69 mg B kg −1 soil), and the growth and photochemical properties of sugar beet seedlings were investigated. The results indicated that B application alone or with seaweed extract promoted sugar beet growth, with the combined application having a significantly enhanced effect. When comparing the two soils with different B content (deficient and moderately deficient), the spray of seaweed extract under the B‐deficient condition was more effective. After the addition of seaweed extract, the B content of the shoots and roots increased by 28.56% and 12.64%, respectively, under B deficiency(0.32 mg B kg −1 soil). Furthermore, the content of chlorophyll a (Chla), chlorophyll b (Chlb), and the net photosynthesis rate (P n ) increased by 8.96%, 30.57%, and 13.74%, respectively. The addition of seaweed extract significantly improved the light saturation point (P m ), with a 35.00% increase compared to the control. Additionally, it increased the quantum yield ( ET O /CS m ) of leaf electron transfer per unit area and reduced the absorption of light energy loss of the PS reaction center ( DI O /RC ) and per unit area ( DI O /CS m ), thereby improving photosynthetic performance and significantly increasing aboveground dry matter accumulation by 19.05%. In conclusion, seaweed extract can enhance B absorption, light energy capture and utilization ability. It provides the theoretical evidence for the regulation of B nutrition in sugar beet and the rational application of biostimulants.