A COMPARISON OF THE RESPONSES OF LOLIUM PERENNE L. , HOLCUS LANATUS L. AND DESCHAMPSIA FLEXUOSA (L.) TRIN. TO A LOCALIZED SUPPLY OF NITROGEN

SUMMARY Three grasses were grown in a split-root solution culture experiment in which only a fraction of the root system of each plant was supplied with 4.0 mol m−3 N as NH4NO3. The solutions were labelled with 15NO3 or 15rNH4 during the final 24 h of the experiment. Deschampsia flexuosa, unlike Holcus lanatus and Lolium perenne, showed no reduction in total dry weight production relative to the controls. Proportionately greater increases in specific N-absorption rate were measured in D. flexuosa compared with H. lanatus and L. perenne. All three species exhibited large changes in root morphology (length : dry weight). In D. flexuosa such changes were confined only to those roots having direct access to N (+ N roots); in H. lanatus and L. perenne this response occurred also in the rest of the root system (-N roots). The proportion of 15N transported to the - N roots was greater in H. lanatus and L. perenne. Increased transport of ammonium-N to the shoot was found in D. flexuosa and, it is suggested, this stabilized the root: shoot ratio and maintained maximum relative growth rate. The smaller absolute N demand of D. flexuosa was such that responses to maximize N uptake, to confine to a greater extent the utilization of N to +N roots, and to maintain N supply to the shoot, ensured full compensation for the restricted N supply. The inherently higher N demands of H. lanatus and L. perenne could not be met by increases in specific absorption rates. In addition, the transport of relatively greater amounts of 15N to – N roots rather than to the shoot, and the consequent increase in root: shoot ratios prevented compensation from being achieved to the same extent. Thus, the efficient utilization of dry matter and nutrient resources appear to be important factors enabling D. flexuosa to grow effectively under restricted conditions of nutrient supply.

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