Use of Gene Expression to Quantify Potato Nitrogen Sufficiency Under Field Conditions.

In-season measurement of crop N status is an important tool in fine-tuning fertilizer N management in potato production. Petiole nitrate and leaf chlorophyll content measured using a SPAD meter are currently the mostly commonly used in-season measures of potato N status. Previously we documented under greenhouse conditions that imposition of a significant reduction of nitrate supply could be detected using gene expression within 7 days. Here we evaluate the potential to use gene expression to monitor in-season potato N status over the growing season under field conditions. Potato cultivar Shepody was grown at six fertilizer N rates (0-250 kg N/ha). Leaf disks were collected from the last fully expanded leaf weekly for seven weekly sampling dates beginning at 42 days after planting. Expression of 22 genes involved in N assimilation, N transport and plant stress response and 5 housekeeping were quantified using the Nanostring nCounter digital gene expression system. Gene expression of the 22 genes was normalized using the geometric mean of the housekeeping genes. There was a significant response of 6 genes to fertilizer N rate where increasing N rate increased expression of glutamate synthase and decreased expression of nitrate reductase, ammonium transporter, nitrate transporter, nitrite transporter and plant stress response gene superoxide dismutase. For an additional 5 genes, there was a significant fertilizer N rate by sampling date interaction on gene expression where increasing fertilizer N rate either decreased or increased gene expression depending on the sampling date: nitite reductase, glutamine synthetase GS1, glutamine synthetase GS2, NADH-glutamate dehydrogenase and plant stress response gene ABA Insensitive 2 (ABI2). Correlation of ammonium transporter gene expression with total tuber yield was as good or better than the correlation between SPAD readings or petiole nitrate concentrations and total tuber yield on all sampling dates. Results of this study suggest that it may be possible to use gene expression to quantitatively assess potato N status under field conditions.