Objective To clarify the effects of nitrogen forms and concentrations on the growth, root morphology, photosynthetic pigment contents and nutrient uptake of pineapple seedlings, this study screened the optimal form and concentration of nitrogen for pineapple seedling stage, so as to provide technical support for the scientific nitrogen application for pineapple seedling stage.

Method Seedlings of pineapple cv. 'Bali' were used as test materials and cultivated with nutrient solution. Two nitrogen forms, ammonium nitrogen(AN) and nitrate nitrogen(NN), were set in three nitrogen concentration gradients of 0.5, 2.0, and 4.0 mmol/L, generating six treatments, namely AN0.5, AN2.0, AN4.0, NN0.5, NN2.0 and NN4.0. Indicators including dry matter accumulation, leaf and root morphology, chlorophyll content, nitrate nitrogen and ammonium nitrogen concentrations, as well as the accumulation amounts of nitrogen, phosphorus, and potassium were determined.

Result Ammonium nitrogen facilitated aboveground dry biomass of pineapple seedlings. The dry weight of leaves under the AN0.5 and AN2.0 treatments increased significantly by 106.02% and 55.83%, respectively compared to the NN treatment in equivalent nitrogen concentrations. The dry weights of leaves and aboveground stems decreased with increasing nitrogen concentration under AN treatments, indicating that high ammonium nitrogen concentration inhibited aboveground growth. Underground stem dry weight decreased first and then rose with rising nitrogen concentration. Both AN and NN treatments exhibited the lowest underground stem dry weight under the nitrogen concentration of 2.0 mmol/L.Root dry weight increased with elevated nitrogen concentration. Only the AN2.0 treatment had significantly higher root dry weight than the NN2.0 treatment, while no significant differences in root dry weight were detected between AN and NN treatments in all other corresponding nitrogen concentrations. The total dry weight of pineapple seedlings under AN0.5 and AN2.0 treatments was significantly increased by 54.51% and 37.45%, respectively, compared with the NN treatments in the corresponding nitrogen concentrations. Pineapple seedlings under AN treatments exhibited superior morphological indices relative to NN treatments, and the total leaf area of the AN0.5 treatment was markedly higher than that of all NN treatments. With the increase of nitrogen concentration, root length of AN treatments first increased and then decreased, root length of NN treatments increased continuously. The AN0.5 treatment had the highest leaf total chlorophyll content, reaching 0.56 mg/g. Ammonium nitrogen was more conducive to the absorption of nitrogen, phosphorus, and potassium by leaves. Compared with NN treatments in equivalent nitrogen concentrations, leaf nitrogen contents under AN treatments were significantly increased by 11.33%-14.78%, and leaf phosphorus contents under AN treatments were significantly elevated by 66.21%-162.55%. Compared with NN treatments in equivalent nitrogen concentration, leaf potassium contents of AN0.5 treatments were significantly increased by 18.75%, while leaf potassium content of AN4.0 treatment was significantly reduced by 10.92%. There was no significant difference in leaf potassium content between AN2.0 and NN2.0 treatments. Principal component analysis extracted three principal components with a cumulative variance explained ratio of 85.75%. Comprehensive evaluation via the membership function method showed that the AN0.5 treatment exhibited the optimal comprehensive performance in pineapple seedling growth and nutrient uptake.

Conclusion Ammonium nitrogen is the optimal nitrogen form for pineapple seedlings. A 0.5 mmol/L ammonium nitrogen concentration maximizes pineapple seedling growth, photosynthetic pigment synthesis and nutrient uptake, whereas high ammonium nitrogen concentrations suppress aboveground growth.