Nutrient storage is an important aspect of resprouting potential and production of Salix viminalis L., a pioneer species used for biomass production in Sweden. Seasonal dynamics of nitrogen (N), protein, soluble carbohydrates, starch, and lipids were studied in roots, cuttings, stems, and leaves during a full growth cycle induced by varying photoperiod and temperature in a growth chamber. Nitrogen was supplied at two rates. Both season and N availability significantly affected storage of N and carbohydrates. Reserves peaked in dormancy, and plants grown at a higher N availability were able to build up larger N reserves, whereas carbohydrate reserves were similar in the two N treatments. All perennial plant organs functioned as storage sites for N. Roots were prominent in carbohydrate storage, in good agreement with the notion of a pioneer species adapted to recurring disturbance by a pronounced resprouting capacity. Roots differed from aboveground plant parts in their exceptionally high starch levels, and in that N storage in roots to a greater extent involved nonprotein compounds. Triglycerides contributed to carbon storage in aerial plant parts but not in roots. Our results suggest that an increased N supply enhance both the accumulation of nutrient reserves and early season growth.Résumé : Le stockage de nutriments est un facteur important dans la production de rejets chez le Salix viminalis L., une espèce pionnière utilisée pour la production de biomasse en Suède. La dynamique saisonnière de l'azote (N), des protéines, des hydrates de carbone solubles, de l'amidon et des lipides a été étudiée dans les racines, les boutures, la tige et les feuilles durant un cycle complet de croissance induit en faisant varier la photopériode et la température en chambre de croissance. Les plants ont reçu deux niveaux d'azote. La saison et la disponibilité de N ont affecté de façon significative le stockage de N et des hydrates de carbone. Les réserves ont atteint le maximum pendant la période de dormance et les plants qui avaient plus d'azote disponible ont accumulé plus de N tandis que les réserves d'hydrates de carbone étaient semblables quel que soit le niveau de N. Tous les organes pérennes des plants ont agi comme sites d'emmagasinage de N. Les racines se démarquaient par le stockage d'hydrates de carbone conformément à la notion d'espèce pionnière, adaptée à des perturbations récurrentes grâce à une forte capacité de produire des rejets. Les racines se distinguaient des parties aériennes des plants par leur niveau exceptionnellement élevé d'amidon et par le fait que N était emmagasiné dans les racines sous forme de composés non protéiques. Les triglycérides contribuaient au stockage du carbone dans les parties aériennes des plants, mais non dans les racines. Les résultats suggèrent qu'un apport élevé de N favorise à la fois l'accumulation de réserves de nutriments et la croissance tôt en saison.[Traduit par la rédaction] Bollmark et al. 94

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Seasonal changes in starch were studied at the tissue and cellular levels in roots and stems of Salix viminalis L. cuttings. Cuttings were planted in pots containing sand and grown in a controlled environment chamber in which seasons were artificially induced by changes in temperature and photoperiod. Nitrogen was supplied at optimum and low rates, and during dormancy, one-half of the plants were decapitated. Starch concentrations in root and stem tissues were determined regularly during shoot extension growth, dormancy and resprouting after dormancy. We used light microscopy (LM) combined with image analysis (IA) to determine the cellular localization and amount of starch in different cell types of stem and root tissues. Chemical analysis confirmed that starch concentrations were lower in plants receiving a high-N supply rate than in plants receiving a low-N supply rate. In all plants, the highest concentration of starch was in the roots. Light microscopy and IA showed that starch accumulated mainly in the phloem and cortical cells of both root and stem tissues. Starch grains were also regularly found in ray parenchyma cells. The amount of starch as well as the size of the grains showed strong seasonal fluctuations. In both roots and stems, starch concentrations were highest during predormancy and lowest during periods of shoot extension growth. At the time of resprouting, root cells of decapitated plants were more depleted of starch than root cells of intact plants, supporting the hypothesis that starch reserves in roots are important during the early phase of resprouting in coppice systems.