Nitrogen effects on yield, quality and K/Na selectivity of sugar beets grown on clays under semi-arid, irrigated conditions

In a four-year experiment, five nitrogen rates (0, 60, 120, 180 and 240 kg N ha-1) were tested over irrigated sugar beets grown on clays, under Mediterranean conditions, in central Greece. There, sugar beets are commonly grown under water shortages, high temperatures and high soil Na concentrations. Contrary to previous reports, N rates did not affect significantly population density (as assessed by root number at harvest) and sucrose content in fresh and dry root weight (SC and SCD, respectively). Yield response to N was year dependent and only in one out of four seasons, was there a positive effect of N on sugar yield and white sugar yield. In that case, the estimated optimum N dose was high (220 kg N ha-1). Increasing N rates increased significantly N assimilation (as assessed by petiole NO3-N and root ?-amino N) and water content in root (WCR) but decreased biomass partitioning to root (lower harvest index). Selective absorption (SA, the preferential uptake of K over Na in roots) decreased with increasing N rates and it was negatively correlated with sugar beet N nutrition indices (petiole NO3-N and root ?-amino N). A negative correlation between SA and petiole NO3-N was also evident when data combined over years, indicating that strong Na exclusion was associated with poor N nutrition, a contradiction to previous reports. The higher the SA, the lower the WCR indicating less dilution of sucrose in root and thus, the higher the SC. Moreover, high SA evoked sucrose accumulation in roots as it was shown by its positive correlation with SCD.