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

Abstract

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.