THE INFLUENCE OF NITROGEN AVAILABILITY ON CARBON AND NITROGEN STORAGEIN THE BIENNIAL CIRSIUM-VULGARE (SAVI) 10 .1. STORAGE CAPACITY IN RELATION TO RESOURCE ACQUISITION, ALLOCATION AND RECYCLING

Plants of Cirsium vulgare (Savi) Ten. were cultivated under five diffe rent nitrogen regimes in order to investigate the effects of nitrogen supply on the storage processes in a biennial species during its first year of growth. External N supply increased total biomass production without changing the relationship between 'productive plant compartmen ts' (i.e. shoot plus fine roots) and 'storage plant compartments' (i.e . structural root dry weight, which is defined as the difference betwe en tap root biomass and the amount of stored carbohydrates and N compo unds). The amount of carbohydrates and N compounds stored per unit of structural tap root dry weight was not affected by external N availabi lity during the season, because high rates of N supply increased the c oncentration of N compounds whilst decreasing the carbohydrate concent ration, and low rates of N supply had the opposite effect. Mobilizatio n of N from senescing leaves was not related to the N status of the pl ants. The relationship between nitrogen compounds stored in the tap ro ot and the maximum amount of nitrogen in leaves was an increasing func tion with increasing nitrogen supply. We conclude that the allocation between vegetative plant growth and the growth of storage structures o ver a wide range of N availability seems to follow predictions from op timum allocation theory, whereas N storage responds in a rather plasti c way to N availability.