Exogenous GA₃ Enhances Nitrogen Uptake and Metabolism under Low Nitrate Conditions in 'Duli' (Pyrus betulifolia Bunge) Seedlings

'Duli' (Pyrus betulifolia Bunge) is one of the main rootstocks of pear trees in China. Gibberellin (GA) is a key plant hormone and the roles of GA in nitrate (NO₃^-) uptake and metabolism in plants remain unclear. In this study, we investigated the effects of exogenous GA₃ on the N metabolism of 'Duli' seedlings under NO₃^- deficiency. The results showed that exogenous GA₃ significantly improves 'Duli' growth under NO₃^- deficiency. On the one hand, GA₃ altered the root architecture, increased the content of endogenous hormones (GA₃, IAA, and ZR), and enhanced photosynthesis; on the other hand, it enhanced the activities of N-metabolizing enzymes and the accumulation of N, and increased the expression levels of N absorption (PbNRT2) and the metabolism genes (PbNR, PbGILE, PbGS, and PbGOGAT). However, GA₃ did not delay the degradation of chlorophyll. Paclobutrazol had the opposite effect on growth. Overall, GA₃ can increase NO₃^- uptake and metabolism and relieve the growth inhibition of 'Duli' seedlings under NO₃^- deficiency.