Effects of phosphorus and nitrogen on nodulation are seen already at the stage of early cortical cell divisions in Alnus incana

Background and Aims The present work aimed to study early stages of nodulation in a chronological sequence and to study phosphorus and nitrogen effects on early stages of nodulation in Alnus incana infected by Frankia. A method was developed to quantify early nodulation stages in intact root systems in the root hair-infected actinorhizal plant A. incana. Plant tissue responses were followed every 2 d until 14 d after inoculation. Cortical cell divisions were already seen 2 d after inoculation with Frankia. Cortical cell division areas, prenodules, nodule primordia and emerging nodules were quantified as host responses to infection. Methods Seedlings were grown in pouches and received different levels of phosphorus and nitrogen. Four levels of phosphorus (from 0.03 to 1 mm P) and two levels of nitrogen (0.71 and 6.45 mm N) were used to study P and N effects on these early stages of nodule development. Key Results P at a medium concentration (0.1 mm) stimulated cell divisions in the cortex and a number of prenodules, nodule primordia and emerging nodules as compared with higher or lower P levels. A high N level inhibited early cell divisions in the cortex, and this was particularly evident when the length of cell division areas and presence of the nodulation stages were related to root length. Conclusions Extended cortical cell division areas were found that have not been previously shown in A. incana. The results show that effects of P and N are already expressed at the stage when the first cortical cell divisions are induced by Frankia.