POTATO GENOTYPES DIFFER IN PETIOLE NITRATE-NITROGEN CONCENTRATIONS OVER TIME

Petiole NO3-N concentrations (PNCs) of seven potato (Solanum tuberosum L.) genotypes grown under four N treatments were studied. In 1986-88, the cultigens were planted in plots with a gradient of available N cr eated by adding 0, 140, 280, or 420 kg N/ha ammonium nitrate split bet ween preplant and periodic seasonal applications. PNCs were significan tly (P less-than-or-equal-to 0.05) affected by year, sampling time (fo ur times per season), N rate, and cultigen. All first- and second-orde r interactions were also significant (P less-than-or-equal-to 0.05). T he relative PNC ranking among cultigens remained nearly constant acros s years when averaged across sampling dates and N rates. Regression-eq uation distinctiveness for each cultigen relating PNC to sampling time demonstrated a genotypic influence on seasonal PNC and allowed separa tion into four response classes. Using a data subset consisting of the 1988 trial, an optimal N rate was determined and regression equations were computed relating PNC to sampling date for each cultigen at the applied N rate nearest to the optimum. Tests for distinction separated the equations of the seven cultigens into six unique classes; 'Fronti er Russet' and 'Ranger Russet' equations were coincident.