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 (NO-N) is the main form of nitrogen taken up by plants. Understanding the effects of exogenous NO-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, NO-N uptake rates, the expression of key genes related to nitrogen assimilation and chlorophyll synthesis and enzyme activity in apple leaves under NO-N deficiency were investigated. The results showed that the cell morphology of apple leaves was irreversibly deformed due to NO-N deficiency. NO-N was absorbed slightly one day after NO-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 NO-N deficiency treatment. After treatment for 14 days, gene expression was upregulated, enzyme activity was increased, and NO-N content was increased. NO-N deficiency hindered the transformation of 5-aminobilinic acid (ALA) to porphobilinogen (PBG), suggesting a possible route by which NO-N levels affect chlorophyll synthesis. Collectively, the results indicate that NO-N deficiency affects enzyme activity by altering the expression of key genes in the nitrogen assimilation pathway, thereby suppressing NO-N absorption and assimilation. NO-N deficiency inhibits the synthesis of the chlorophyll precursor PBG, thereby hindering chlorophyll synthesis.