Solid Grain Waste Digestate as a Peat Substrate Amendment for Tomato Seedlings: Effects of Direct Sowing and Transplanting on Growth and Photosynthesis

The quality and productivity of tomato (Solanum lycopersicum L.) crops largely depend on the quality of the seedlings used for cultivation. Several factors, including cultivation strategy, fertilization practices, and abiotic and biotic stressors during early plant development, influence seedling quality. Recently, anaerobic digestate has attracted attention as a potential organic fertilizer and substrate component; however, information about its effects on tomato seedling quality remains limited, particularly when comparing different seedling establishment methods such as direct sowing and transplanting. Therefore, this study aimed to evaluate the effects of different concentrations of solid grain waste digestate (further digestate) in the peat substrate on the growth and physiological characteristics of tomato seedlings grown by means of direct sowing and transplanting. The experiment was conducted at the Institute of Horticulture of the Lithuanian Research Centre for Agriculture and Forestry in unheated greenhouses covered with double polymer film. Two cultivation strategies were applied (factor A): transplanting and direct sowing into pots. To evaluate the influence of digestate (factor B), different substrate compositions were used: peat (control) and peat mixed with 10%, 20%, 30%, 40%, and 50% digestate. The strong decline in growth parameters with increasing digestate concentration indicates that higher proportions of digestate created unfavorable conditions for seedling development in both cultivation stategies. A 10% digestate addition improved certain plant characteristics, while 20% improved some physiological indices but was associated with reduced growth. However, higher digestate concentrations (≥30%) negatively affected plant growth and physiological activity. Seedlings grown in substrates with higher digestate levels exhibited reduced transpiration rates and lower gas exchange indices, suggesting impaired water relations and stomatal regulation. These effects were more pronounced in transplanted plants compared with direct-sown seedlings, indicating greater sensitivity to changes in substrate composition after transplanting. Overall, the results demonstrate that digestate can be used as a substrate component for tomato seedling production. Still, its concentration must be carefully optimized to avoid negative effects on plant growth and physiological performance.