Effects of nitrate deficiency on nitrate assimilation and chlorophyll synthesis of detached apple leaves

Nitrogen is one of the most important nutrients for plant growth and development. Nitrate nitrogen (NO3--N) is the main form of nitrogen taken up by plants. Understanding the effects of exogenous NO3--N on nitrogen metabolism at the gene expression and enzyme activity levels during nitrogen assimilation and chlorophyll synthesis is important for increasing nitrogen utilization efficiency. In this study, cell morphology, NO3--N uptake rates, the expression of key genes related to nitrogen assimilation and chlorophyll synthesis and enzyme activity in apple leaves under NO3--N deficiency were investigated. The results showed that the cell morphology of apple leaves was irreversibly deformed due to NO3--N deficiency. NO3--N was absorbed slightly one day after NO3--N deficiency treatment and effluxed after 3 days. The relative expression of genes encoding nitrogen assimilation enzymes and the activity of such enzymes decreased significantly after 1 day of NO3--N deficiency treatment. After treatment for 14 days, gene expression was upregulated, enzyme activity was increased, and NO3--N content was increased. NO3--N deficiency hindered the transformation of 5-aminobilinic acid (ALA) to porphobilinogen (PBG), suggesting a possible route by which NO3--N levels affect chlorophyll synthesis. Collectively, the results indicate that NO3--N deficiency affects enzyme activity by altering the expression of key genes in the nitrogen assimilation pathway, thereby suppressing NO3--N absorption and assimilation. NO3--N deficiency inhibits the synthesis of the chlorophyll precursor PBG, thereby hindering chlorophyll synthesis.