Effect of Time of Day of Sampling on Potato Foliar Gene Expression Used to Assess Crop Nitrogen Status

Abstract

The potential to use gene expression as an indicator of plant N status has been identified, however how such expression indicators may vary with time of day (TOD) has not been examined. This study quantified variation in gene expression at four times during the day (8:00, 11:00, 14:00 and 17:00) on potato plots receiving 0 or 180 kg N ha−1. Expression of 24 experimental genes was quantified by nCounter and normalized by reference genes. Expression of all but one of the experimental genes varied significantly with time of day. Most (15) genes, across a range of gene function categories, had greater expression at 8:00 and 17:00 than during midday, and 10 of these genes had least expression at 14:00, the sampling time with greatest solar radiation. For 7 genes, expression was greater during midday, and for carbohydrate metabolism or chlorophyll-related genes maximum expression occurred at 14:00. Expression of ammonium transporter AT1, previously identified as a good predictor of potato N status, varied strongly with TOD and became insensitive to potato N status during midday. TOD effects on both experimental and reference genes must be considered in developing gene expression based diagnostic tests of crop N status, and ideally, genes which are sensitive to crop N status, but insensitive to TOD, can be identified for such diagnostic purposes.