Effects of Spraying Allantoin at Different Stages on Inorganic Nitrogen Assimilation, Endogenous Hormones, Yield, and Quality of Sugar Beet in Saline-Alkali Land

Soil salinization profoundly impacts the growth and development of sugar beet. As a urea cytokinin, allantoin effectively mitigates the adverse effects of salinity stress on plants. However, there has been limited research on the optimal application period of allantoin in practical production settings. This study aimed to ascertain the optimal timing for allantoin application in sugar beet cultivation under saline and alkaline conditions in Heilongjiang Province, China. The experiment involved spraying allantoin at a mass concentration of 0.1 mmol/L onto the foliage at the seedling stage (M), the rapid leaf cluster growth stage (Y), and the tuber expansion stage (K), as well as at the combined stages of MY and MYK, with clear water used as the control (CK). The results indicated that the combined MY and MYK treatments resulted in significantly higher aboveground and belowground dry matter accumulation compared to other treatments. Specifically, nitrogen content in the aboveground and belowground parts reached peak values of 15.77 g/plant and 7.045 g/plant under MY and MYK treatments, respectively. Furthermore, nitrogen metabolizing enzyme activities exhibited an initial increase followed by a decrease during beet growth and development, with enzyme activities in the MY treatments consistently remaining at elevated levels. Additionally, hormone levels in sugar beets were enhanced by allantoin application. Hormone levels were enhanced, with abscisic acid content significantly higher under MY and MYK treatments compared to other treatments, while gibberellin and cytokinin levels were notably higher compared to CK treatments. In comparison with CK, MY treatments exhibited an increase in yield by 0.26 percentage points, with sugar yield rising by 21.3%. However, there were no significant differences between MY and MYK treatments in each index. Therefore, spraying allantoin solely during the seedling and leaf cluster growth period can enhance sugar beet yield by augmenting nitrogen metabolism capacity and hormone levels, whereas application during the tuber expansion period can notably enhance sugar beet processing quality.