Polyaspartic acid (PASP) is a polymer of aspartic acid (ASP) with molecular weight ranging from 1000 to hundreds of thousands. Owing to its function in promoting crop growth and nutrient uptake, PASP has been widely used in agriculture, while its structure and application effect varied when its molecular weight changed. Therefore, in this study, the structural characteristics of PASP with different molecular weights were analyzed by using infrared spectroscopy, and the effects of it on wheat root growth and nutrient uptake in hydroponic experiments were investigated to clarify the characteristics and efficiency-enhanced mechanism of PASP with different molecular weights, so as to provide a theoretical basis for the scientific application for PASP in the development of new efficiency-enhanced fertilizer and agricultural production.

A hydroponic experiment was carried out by using Hoagland nutrition solution and wheat (Jimai 22). Four kinds of test materials, including aspartic acid (ASP), PASP with low molecular weight (<1 kDa, PAL), PASP with medium molecular weight (3-5 kDa, PAM), and PASP with high molecular weight (>10 kDa, PAH), were separately added into the cultural solution in ratio of 10 mg·L^-1, 25 mg·L^-1 and 50 mg·L^-1, and the treatment only with Hoagland nutrient solution was set as the control group (CK). Thus, there were a total of 13 treatments, and each treatment was repeated four times. After 20-day growth, the wheat seedlings were harvested to determine shoot dry matter weight, root dry matter weight, nitrogen, phosphorus and potassium content as well as root morphology, root absorption area, root activity.

(1) The structure of polyaspartic acid with different molecular weights was different. With the increase of molecular weight of polyaspartic acid, the content of peptide bond increased gradually, and the content of carboxyl group increased first and then decreased, among which, PAH had the highest peptide bond content, and PAM had the highest carboxyl group content. (2) The addition of PASP with different molecular weights could significantly improve the dry matter weight of wheat. The total dry matter weight of wheat was shown in the descending order: PAM>PAH>PAL≈ASP. Compared with CK, PASP increased the dry weight of wheat root by 11.90%-19.06%. Compared with CK, at the additive amount of 10 mg·L^-1, 25 mg·L^-1 and 50 mg·L^-1, the total dry weight of wheat treated by PAM increased by 9.13%, 23.36% and 20.54%, respectively. (3) Polyaspartic acid with different molecular weights could optimize wheat root morphology and increase total absorption area, active absorption area and root activity. PAM had a better performance than other PASP treatment. (4) The addition of PASP with different molecular weights could promote the uptake of nitrogen, phosphorus and potassium, and the greatest promotion was obtained with PAM addition, followed by PAH addition. As for the treatments with the same material, the total uptake of nutrients was the highest at 50 mg·L^-1 of PASP addition. When 50 mg·L^-1 of PAM addition, the total uptake of nitrogen, phosphorus and potassium significantly increased by 16.88%, 25.97% and 21.61% than that of CK, respectively, and that 50 mg·L^-1 of PAH addition significantly increased by 16.28%, 23.36% and 18.16% than CK, respectively. (5) Correlation analysis showed that the contents of peptide bond and carboxyl group of polyaspartic acid with different molecular weights were significantly positively correlated with total dry matter weight, total uptake of nitrogen, phosphorus and potassium as well as total root length and total root absorption area. The dry matter weight and nutrient uptake of wheat were significantly or extremely significantly positively correlated with root surface area, total absorption area, active absorption area, and root activity.

Polyaspartic acid with different molecular weights could promote wheat growth, optimize root morphology, increase root absorption area and root activity, further promote nutrient uptake. The structure of polyaspartic acid with different molecular weights was different. With the increase of molecular weight of polyaspartic acid, the content of peptide bond increased gradually, and the content of carboxyl group increased first and then decreased. The content of peptide bond and carboxyl group of polyaspartic acid was positively correlated with wheat growth and nitrogen, phosphorus and potassium uptake. Under the conditions of this experiment, PASP with molecular weight of 3-5 kDa had the best performance on wheat root growth and nutrient uptake, followed by polyaspartic acid with molecular weight greater than 10 kDa. As for different additive amounts, PASP with high dosage (50 mg·L^-1) showed a more significant promoting effect on wheat growth and total nutrient uptake, while that with medium dosage (25 mg·L^-1) had the best performance on wheat root growth and root nutrient uptake.