The effect of N supply on plant growth and leaf demography of a deciduous and an evergreen Ericaceae was studied in relation to their internal cycling of N. Mature ramets of Vaccinium myrtillus (deciduous) and Vaccinium vitis-idaea (evergreen) were established in sand culture for 1 year with an adequate supply of a balanced nutrient solution. During one growing season, the plants were given two levels of N supply enriched with 15N and eight sequential destructive harvests were taken. Recovery of unlabelled N in the new shoot was used to determine the remobilization of N from storage. Initially, growth was unaffected by N supply. After May, High N enhanced growth for both species but the nature of their growth response differed. For both species, new shoot biomass and leaf number increased but root biomass production was affected for V. myrtillus only. Whole plant biomass production was similar for both species under High N, but was greater for V. vitis-idaea under Low N. The amount of N remobilized to support new shoot growth was similar for the two species and was independent of N current supply. N was remobilized predominantly from previous year leaves for V. vitis-idaea and from previous year stems and roots for V. myrtillus. The contribution of remobilization to new shoot N was similar for the two species, but depended on N supply. Remobilization was faster in V. myrtillus, but lasted longer in V. vitis-idaea. The results are discussed in relation to species growth in N-poor environments, focusing on the extent to which species-differences in the dynamic of N remobilization and growth may explain their adaptation to constant and/or changeable N supply.

No abstract

No abstract

No abstract

A closed growth chamber was designed to study the acquisition of autotrophy by seedlings of walnut (Juglans regia L. cv. Lara) in controlled conditions (22°C, 12‐h photoperiod) during the first two months of growth. The chamber consisted of two airtight compartments, in which continuous gas exchange was measured on the aerial and subterranean parts of several batches of tree seedlings. Long‐term labelling with 13CO2 was used in the chamber to study the import, distribution, and respiratory losses of photoassimilates (autotrophic carbon) in relation to the partitioning and use of reserves of the maternal seed (heterotrophic carbon). The carbon economy of walnut seedlings was estimated by measurements of gas exchange, carbon content, and 13C/12C isotopic ratio in dry matter and respiratory CO2. The seedlings were entirely heterotrophic for energy and structural growth during the first 21 days after sowing. From day 22, photosynthesis appeared. At day 29, autotrophic carbon accounted for 25% and 30% of respiration in the root and shoot respectively; these proportions increased to 45% and 65% at day 54. The autotrophic carbon was incorporated into the dry matter of the shoot from day 32 but only after day 40 into the dry matter of the taproot. From day 32, the total contribution of heterotrophic carbon decreased regularly, and until day 43, it was essentially used for root growth. Thereafter, the contribution of heterotrophic carbon was negligible, and at day 54 the walnut seedlings were entirely autotrophic.