Relative Availability of Nitrogen and Calcium Regulates the Growth of Poplar Seedlings Due to Transcriptome Changes

The concentrations of exogenous nitrogen and calcium can significantly regulate plant growth and photosynthesis and mutually affect their absorption and utilization. However, whether there is an optimal nitrogen-calcium ratio (N:Ca ratio) in poplar seedling growth and physiological adaptation and what the mechanism of changes in the transcriptome is remain unclear. In this study, three different N:Ca ratios were used to grow poplar seedlings, and physiological and transcriptomic methods were used to study the molecular mechanisms of poplar growth under nitrogen-calcium synergy and to determine the optimal N:Ca ratio for poplar seedling growth. The results of this study showed that maximum poplar seedling growth occurred in the treatment with an N:Ca ratio of 2, which resulted in significantly greater induction of growth than the two other treatments (p < 0.05). Under the lowest N:Ca ratio, poplar seedlings can ensure normal development by regulating photosynthesis, while under the highest N:Ca ratio, regulating nitrogen metabolism can achieve the same result. These results contribute to a better understanding of the molecular mechanisms by which poplar seedlings respond to different ratios of N:Ca. This study provides a valuable basis for exploring the synergistic effects of nitrogen and calcium on the growth of poplar shelterbelts.