Effects of Polyaspartic Acid Added to the Substrate on Tomato Seedling Growth

Polyaspartic acid (PASP) promotes plant growth by enhancing the absorption of mineral elements. To investigate the growth-promoting effects of PASP on greenhouse tomato seedlings, the effects of adding different proportions of PASP to the substrate on tomato seedling growth were compared using Zhongza 9 as the test material. The results showed that compared with the control, PASP added to the substrate promoted tomato seedling growth and dry matter accumulation, and increased.

Polyaspartic acid (PASP) has strong chelating, dispersing, and adsorbing properties and is completely biodegradable. It improves soil aggregate structure by increasing the content of physical clay in the soil. It also activates mineral elements adsorbed and fixed by the soil, allowing nutrients in fertilizers to be dispersed, stable, and easily absorbed by crops, thereby increasing soil nutrient content and fertilizer absorption efficiency. Research has shown that the application of polyaspartic acid can improve soil moisture conditions, promote root growth, increase nitrogen and potassium absorption by maize seedlings, and enhance drought resistance during the seedling stage. Polyaspartic acid can also stimulate plant enzyme activity and enhance the absorption of nitrogen, phosphorus, potassium, and trace elements, particularly zinc, manganese, and iron. Application of fertilizers containing polyaspartic acid can promote rice growth and development, significantly increasing plant height, ear length, number of effective ears, and yield. Root application of polyaspartic acid can increase leaf photosynthetic pigment content and light energy capture efficiency in Populus euphratica seedlings, promoting branch and leaf growth and dry matter accumulation. my country's greenhouse vegetable cultivation area is approximately 2.8 million hectares, and the annual demand for vegetable seedlings exceeds 680 billion plants (Liu Mingchi et al., 2018). Cultivating strong seedlings is a key issue in high-yield vegetable cultivation. Currently, research on polyaspartic acid in agriculture has primarily focused on field crops such as corn and rice, with few reports on the cultivation of strong greenhouse vegetable seedlings. This study, using Zhongza 9 as the test material, investigated the effects of polyaspartic acid added to the substrate on the growth of tomato seedlings, aiming to provide theoretical basis and technical support for the cultivation of strong greenhouse vegetable seedlings.

2.1 Effects of Polyaspartic Acid Added to the Substrate on Tomato Seedling Growth Indices

Compared with the control, the addition of PASP to the substrate significantly increased plant height, stem diameter, aboveground and underground dry and fresh mass, and seedling vigor index of tomato seedlings, promoting seedling growth. Among them, the treatment with 36.4 g kg⁻¹ PASP added to the substrate (T2) achieved the highest seedling index and the best results. Plant height, stem diameter, fresh weight of the whole plant, and dry weight of the whole plant were significantly higher than the control, increasing by 33.55%, 24.93%, 37.23%, and 35.29%, respectively.

2.2 Effects of Polyaspartic Acid Added to the Substrate on Chlorophyll Content and Photosynthesis in Tomato Seedling Leaves

With increasing amounts of PASP added to the substrate, leaf area, chlorophyll a content, total chlorophyll content, intercellular CO₂ concentration, stomatal conductance, net photosynthetic rate, and transpiration rate of tomato seedlings showed a trend of first increasing and then decreasing. The treatment with 36.4 g kg⁻¹ PASP added to the substrate (T2) showed the best results. Leaf area, chlorophyll a content, total chlorophyll content, intercellular CO₂ concentration, stomatal conductance, net photosynthetic rate, and transpiration rate all increased significantly compared to the control, by 31.36%, 35.58%, 33.33%, 8.31%, 39.10%, 38.67%, and 36.56%, respectively. This enhanced photosynthesis and promoted seedling leaf growth.

2.3 Effects of Polyaspartic Acid Added to the Substrate on Root Vitality and Root Architecture of Tomato Seedlings

With increasing levels of PASP in the substrate, root activity, total root length, root surface area, root volume, and number of root tips of tomato seedlings showed an initial increase followed by a decrease. The treatment with 36.4 g kg⁻¹ PASP (T2) exhibited the best overall effect, with root activity, total root length, root surface area, root volume, and number of root tips increasing by 30.62%, 28.96%, 35.20%, 44.58%, and 33.03%, respectively, compared to the control. All parameters, except total root length, were significantly higher than those in the control.

2.4 Effects of Polyaspartic Acid Added to the Substrate on Mineral Element Content in Tomato Seedlings

The addition of PASP to the substrate had no significant effect on the nitrogen, phosphorus, potassium, iron, and copper contents in the shoots and roots of tomato seedlings. However, with increasing PASP addition, the potassium and magnesium contents in the shoots and calcium, iron, copper, manganese, and zinc in the roots all showed an initial upward and then downward trend. The treatment with 36.4 g kg⁻¹ PASP (T2) showed significantly higher calcium, magnesium, manganese, and zinc contents in the shoots and phosphorus, calcium, magnesium, manganese, and zinc in the roots than the control, increasing by 10.47%, 27.61%, 38.00%, 13.51%, 38.55%, 36.42%, 30.15%, 36.53%, and 34.21%, respectively. This indicates that the addition of polyaspartic acid to the substrate significantly affects the mineral element contents in the shoots and roots of the plants.

3.1 Adding polyaspartic acid to the substrate can increase the mineral element content of tomato seedlings.

Polyaspartic acid has a much higher exchange and adsorption capacity for soil nutrient ions than soil's adsorption capacity for ions, forming a highly concentrated ion diffusion double layer, thereby dissociating nutrient ions from the soil. The unique peptide chain structure within its molecules forms a porous, complex, cyclic polymer group, which has a strong nutrient absorption capacity. These two effects synergistically facilitate plant absorption and utilization of fertilizer nutrients, promoting crop growth. Studies on crops such as corn, rapeseed, cucumber, and rice have shown that the application of polyaspartic acid can significantly increase plant absorption of nutrients such as N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, and B. Adding 36.4 g kg⁻¹ of polyaspartic acid to the substrate significantly increased the aboveground calcium, magnesium, manganese, and zinc contents of tomato seedlings, and the root phosphorus, calcium, magnesium, manganese, and zinc. This is likely due to the fact that polyaspartic acid increases the availability of mineral elements in the substrate, making them readily available for plant uptake, thereby enhancing the tomato plants' absorption of phosphorus, calcium, magnesium, manganese, and zinc. However, the addition of polyaspartic acid to the substrate had no significant effect on the nitrogen and potassium contents of the plants, likely because the nitrogen and potassium content in the substrate already met or exceeded the growth requirements of the tomato seedlings. Furthermore, with increasing levels of polyaspartic acid in the substrate, the aboveground potassium and magnesium contents of tomato seedlings, as well as the root calcium, iron, copper, manganese, and zinc contents, all showed an initial upward and then downward trend. This is likely due to the excessive concentration of polyaspartic acid in the substrate, which causes the polyaspartic acid particles to absorb water and swell, reducing the substrate's permeability. This reduces the amount of water and nutrients available to the plants, thereby inhibiting the growth of the roots and aboveground parts of the plants.

3.2 Polyaspartic Acid Added to the Substrate Promotes Growth of Tomato Seedlings

The experimental results showed that, compared with the control, the addition of 36.4 g kg⁻¹ of polyaspartic acid to the substrate significantly increased the net photosynthetic rate and chlorophyll a and total chlorophyll content in tomato seedling leaves, promoting the accumulation of photosynthetic products. It also significantly increased root surface area, root volume, root tip number, and root activity, enhancing the root system's ability to absorb soil nutrients. It also increased the accumulation of magnesium, manganese, and zinc, which are beneficial for photosynthesis, and calcium, which promotes root growth. The addition of polyaspartic acid to the substrate may promote the growth of tomato seedlings, possibly by increasing root nutrient accumulation, increasing the plant's absorption and accumulation of mineral elements, promoting root growth, increasing root activity, and enhancing the root system's absorption capacity, thereby promoting robust seedling growth. Another possible explanation is that the plants absorbed increased levels of calcium, magnesium, manganese, and zinc. Calcium, a key component of cell walls, participates in the formation of new cells in plants, promotes root growth and root hair formation, and increases water and nutrient absorption. Magnesium, a component of chlorophyll, enhances photosynthesis. Manganese is directly involved in photosynthesis and promotes seed germination and seedling growth. Zinc also participates in photosynthesis and improves plant stress resistance. The treatment with 36.4 g·kg⁻¹ polyaspartic acid added to the substrate showed the best overall effect, but the universal applicability of this dosage requires further verification. The effect of polyaspartic acid added to the substrate during seedling cultivation on plant growth after tomato seedling establishment also requires further study.

4  Conclusion

During tomato seedling cultivation, the addition of polyaspartic acid to the substrate can promote seedling growth, increase the absorption and accumulation of mineral elements, and improve seedling quality. The treatment with 36.4 g·kg⁻¹ polyaspartic acid added to the substrate showed the best results, contributing to the production of strong seedlings.