Effects of a localized supply of nitrate on NO3 - uptake rate and growth of roots in Lolium multiflorum Lam.

Roots of higher plants are usually exposed to varying spatial and temporal changes in concentrations of soil mineral nitrogen. A split root system was used to see how Lolium multiflorum Lam. roots adapt to such variations to cope with their N requirements. Plants were grown in hydroponic culture with their root system split in two spatially separated compartments allowing them to be fed with or without KNO₃. Net NO₃^- uptake, ¹⁵NO₃^- influx and root growth were studied in relation to time. Within less than 24 h following deprivation of KNO₃ to half the roots, the influx in NO₃^- fed roots was observed to increase (about 200% of the influx measured in plant uniformly NO₃^- supplied control plant) thereby compensating the whole plant for the lack of uptake by the N deprived roots. Due to the large NO₃^- concentrations in the roots, the NO₃^- efflux was also increased so that the net uptake rate increased only slightly (35% maximum) compared with the values obtained for control plants uniformly supplied with NO₃^-. This increase in net NO₃^- uptake rate was not sufficient to compensate the deficit in N uptake rate of the NO₃^- deprived split root in the short term. Over a longer period (>1 wk), root growth of the part of the root system locally supplied with NO₃^- was stimulated. An increase in root growth was mainly responsable for the greater uptake of nitrate in Lolium multiflorum so that it was able to fully compensate the deficit in N uptake rate of the NO₃^- deprived split root.