ObjectivesIn this study, we investigated the effect of humic acid addition on growth, nitrate reductase activity and photosynthetic parameters of tomato plug seedlings, which will provide a theoretical reference for the application of humic acid in plug seedling production.

MethodsThe substrate used for tomato seedlings was composed of peat, vermiculite and perlite in volume ratio of 4:3:2, in which humic acid content was 54.5 g/L. Five treatments were designed according to the amount of humic acid added before seed sowing and recorded as CK (no addition), T1 (10 g/L), T2 (20 g/L), T3 (30 g/L) and T4 (40 g/L). Each treatment was repeated three times and each replication had three plug trays. When the fifth leaf appeared in tomato seedlings (about 29-31 days after cotyledons fully developed), the growth parameters, root activity, nitrate reductase activity, photosynthetic pigment contents, photosynthetic parameters and chlorophyll fluorescence parameters were measured.

ResultsTreatment of T1, T2 and T3 significantly increased the plant heights and stem diameters, and the effects on the root dry weights were more significant than on the shoot dry weights. Compared with CK, T1 and T2 significantly increased the ratio of root to shoot and health seedling index, while T3 did not. T2 showed a distinct advantage in all the growth parameters, while T4 showed the worst performance. T1, T2 and T3 treatments significantly promoted the root activity and nitrate reductase activity, while T4 treatment decreased these two parameters. The contents of photosynthetic pigment were increased firstly and then decreased with the increase of humic acid content. All treatments except T4 significantly increased the Pn and Gs, and T2 had the highest values. All treatments increased the Ci compared to CK. The Tr was decreased with the increase of humic acid concentration, and the maximum value was in CK. The Fo and NPQ were decreased in T1 and T2, while increased in T3 and T4. The minimum and maximum values were observed in T2 and T4, respectively. The order of the Fv/Fm and ΦPSⅡ content in different treatments was T2 > T1 > CK > T3 > T4, in which T1 and T2 significantly increased these two parameters, while T3 and T4 significantly decreased these two parameters.

ConclusionsFor the substrate of containing 54.5 g/L humic acid, the addition of some more humic acid will be able to enhance physiological activities, promote growth and cultivate vigorous tomato seedlings. The suitable amount of humic acid addition is 20 g/L of the substrate for plug seedlings of tomato.