The Use of Internal Nitrogen Stores in the Rhizomatous Grass Calamagrostis epigejos During Regrowth After Defoliation

Roots route nitrogen for new shoot nutrition

Mowing the lawn is a frustrating business: the grass just keeps growing again. In natural habitats, this feature is an adaptation to defoliation by grazing animals. There are thus interesting questions about how the plant supplies nutrition and energy to fuel the regrowth of its aerial parts after severe reduction of photosynthetic capacity. This has been the subject of research by Monika Kavanov� and V�t Gloser (Brno, Czech Republic, pp. 457�463 ), working with the rhizomatous grass Calamagrostis epigejo s. For this species, there is the added interest that it is invasive and is proving difficult to control, although the authors state that repeated mowing reduces its competitiveness. In the experiments reported here, the grass was defoliated very extensively and regrowth was studied in relation to respiration and to aspects of N-metabolism. Leaf area and shoot length started to recover almost immediately, but total plant dry weight did not start to rise for about 6 days (following a slight drop), presumably indicating that in this initial phase net photosynthesis was not great enough to exceed the loss of carbon by respiration and other processes. In roots, respiration rate and specific nitrate uptake rate declined markedly in the first 4 days, after which they both partially recovered. The relationship between the two suggests that uptake of nitrate by roots is dependent on root respiration. Supply of N to the growing shoot therefore did not come from external nitrate but from amino acids and soluble proteins originating in the roots, which lost 50 % or more of their content of these compounds in the first week after defoliation. The loss of proteins appeared to be partially selective, with certain proteins declining much more than others. However, there was no evidence for a specific storage protein. Interestingly, rhizomes did not play a part in the N nutrition of the new shoots, although they clearly have an important role in plant establishment.