Received August 9, 2004
Returned for revision October 13, 2004
Accepted October 26, 2004
The Sources of Carbon and Nitrogen Supplying Leaf Growth. Assessment of the Role of Stores with Compartmental Models
Fernando Alfredo Lattanzi , Hans Schnyder *, and Barry Thornton
Lehrstuhl für Grünlandlehre, Technische Universität München, D-85350 Freising-Weihenstephan, Germany
Soil Plant Microbial Interactions Group, The Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, United Kingdom
* Corresponding author; email: schnyder{at}wzw.tum.de.
Patterns of synthesis and breakdown of carbon (C) and nitrogen (N) stores are relatively well known. But the role of mobilized stores as substrates for growth remains less clear. In this article, a novel approach to estimate C and N import into leaf growth zones was coupled with steady-state labeling of photosynthesis (13CO2/12CO2) and N uptake (15NO3-/14NO3-) and compartmental modeling of tracer fluxes. The contributions of current C assimilation/N uptake and mobilization from stores to the substrate pool supplying leaf growth were then quantified in plants of a C3 (Lolium perenne) and C4 grass (Paspalum dilatatum Poir.) manipulated thus to have contrasting C assimilation and N uptake rates. In all cases, leaf growth relied largely on photoassimilates delivered either directly after fixation or short-term storage (turnover rate = 1.6-3.3 d-1). Long-term C stores (turnover rate < 0.09 d-1) were generally of limited relevance. Hence, no link was found between the role of stores and C acquisition rate. Short-term (turnover rate = 0.29-0.90 d-1) and long-term (turnover rate < 0.04 d-1) stores supplied most N used in leaf growth. Compared to dominant (well-lit) plants, subordinate (shaded) plants relied more on mobilization from long-term N stores to support leaf growth. These differences correlated well with the C-to-N ratio of growth substrates and were associated with responses in N uptake. Based on this, we argue that internal regulation of N uptake acts as a main determinant of the importance of mobilized long-term stores as a source of N for leaf growth.
