J . theor . Biol . (1998) 194, 29–47 Article No . jt980740 0022–5193/98/170029 + 19 $30.00/0  1998 Academic Press Modelling of Allocation and Balance of Carbon in Walnut (Juglans regia L.) Seedlings during Heterotrophy–autotrophy Transition A. J. E-G´ *†§, F.-A. D*, J.-P. G` ‡, P. M*  J.-S. F* *U.A. Bioclimatologie -PIAF, Institut National de la Recherche Agronomique , Domaine de Crouelle , F-63039 Clermont -Ferrand Cedex 02, France and ‡Unite´ d ’Agronomie , Institut National de la Recherche Agronomique , Centre de Recherche de Bordeaux , BP 81, F-33140 Villenave -d ’Ornon Cedex , France (Received on 15 July 1997, Accepted in revised form on 5 May 1998) A deterministic and dynamic model of carbon allocation in walnut seedlings is described. Two experimental data sets were used to calibrate and validate the model. These data included: time course of the carbon content, chemical and isotope composition ( 12 C and 13 C) of the kernel and growing organs (roots, stem, leaves), and gas exchange rates during the first 55 days of the life of the plant with continuous 13 CO 2 feeding. The plant is modelled as a network with nodal organs acting as sources or sinks for carbohydrates. In a sink organ the demand for carbon is the sum of four elementary demands: maintenance respiration, structural growth, growth-associated respiration and carbon storage. The organs of the plant are assumed to be in exponential growth phase. The supply of carbon readily accessible for the organs is the store of soluble sugars present in a local reservoir. Carbon flow in the network is determined by the source/sink activities of the organs and local levels of demand and supply. Two carbon sources are considered: soluble sugars from the kernel, and gross photosynthesis. The rate of synthesis of soluble sugars in the kernel, and measured photosynthesis in the leaves are inputs for the plant model. The outputs are the predicted fluxes of carbon within the seedling; 13 C composition, carbohydrate allocation to the growing organs, starch and soluble sugars accumulation, and respiration. The mathematical equations were translated into PSPICE software instructions. After optimisation of the parameter values, the model provided an accurate description of experimental observations in the seed–plant system during the critical transition from heterotrophy to autotrophy, especially C allocation to organs and C partitioning between storage, structural growth and respiration in each organ. The growth of the young plant is supply-limited, except at the earliest stages. A sensitivity analysis suggests that intense competition for carbohydrates dominates the relations among and within organs.  1998 Academic Press †Author to whom correspondence should be addressed. §Present address: Northern Research Station, British Forestry Commission, Roslin, Edinburgh, Midlothian EH25 9SY, U.K.; Permanent address: Instituto de Recursos Naturales, Colegio de Postgraduados, 56230 Chapingo- Montecillo, Mexico.  Present address: Unite´ d’Ecophysiologie Forestie` re, Institut National de la Recherche Agronomique, Centre de Nancy, F-54280 Champenoux, France. Introduction Germination has been extensively studied at agronomic, physiological and biochemical levels. Agronomically, germination begins when the seed is placed in moist soil, and ends when the seedling emerges above the soil surface and