Leaf chemistry of a willow clone (Salix aquatica Smith) differed significantly when grown at constant relative growth rates depending upon the relative availability of nutrients and light. Concentration of amino acids and nitrate were high in plants grown with a relative surplus of nutrients. Concentrations of starch, tannin, and lignin, on the other hand, were high in plants grown with a relative surplus of carbon. Photosynthetic rates, expressed per unit leaf area, were similar when plants were grown under high light conditions, regardless of nutrient availability. Dark respiration was much higher in plants supplied with abundant nutrients than in those with a more limited supply, reflecting differences in nitrogen concentration of the tissue. The experimental approach allows plants to be grown to a standard size with differing, but highly uniform chemistry. Plants grown in such a manner may provide good experimental material to evaluate interactions between herbivores or pathogens and their hosts.

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The objective of the present study was to assess whether, in barley, nitrogen supply limits the rate of leaf elongation through a reduction in (relative) cell elongation rate and whether this is attributable to a reduced turgor, a reduced availability of osmolytes or, by implication, changed wall properties. Plants were grown on full-strength Hoagland solution (``Hoagland''-plants), or on N-de®cient Hoagland solution while receiving N at a relative addition rate of 16 or 8% N á plant-N A1 á d A1 (``16%-'' and``8%-plants''). Hoagland-plants were demand-limited, whereas 16%-and 8%-plants were supply-limited in N. Third leaves were analysed for leaf elongation rate and ®nal epidermal cell length, and, within the basal growing region, for the spatial distribution of relative segmental elongation rates (RSER, pin-pricking method), epidermal cell turgor (cell-pressure probe), osmotic pressure (OP, picolitre osmometry) and water potential (Y). During the development of the third leaf, plants grew at relative growth rates (relative increase in fresh weight ) of 18.2, 15.6 and 8.1% á d A1 (Hoagland-, 16%-and 8%-plants, respectively). Final leaf length and leaf elongation rate were highest in Hoagland plants (ca. 34.1 cm and 2.33± 2.60 mm á h A1 , respectively), intermediate in 16%-plants (31.0 cm and 1.89±1.96 mm á h A1 ) and lowest in 8%-plants (29.4 cm and 1.41±1.58 mm á h A1 ). These dierences were accompanied by only small dierences in ®nal cell length, but large dierences in cell-¯ux rates (146, 187 and 201 cells á cell-®le A1 á d A1 in 8%-, 16%-and Hoagland-plants, respectively). The length of the growth zone (32±38 mm) was not much aected by Nlevels (and nutrient technique). A decrease in RSER in the growth zone distal to 10 mm produced the signi®-cant eect of N-levels on leaf elongation rate. In all treatments, cell turgor was almost constant throughout the growing region, as were cell OP and Y in 16%-and 8%-plants. In Hoagland-plants, however, cell OP increased by ca. 0.1 MPa within the zone of highest elongation rates and, as a consequence, cell Y decreased simultaneously by 0.1 MPa. Cell Y increased considerably where elongation ceased. Within the zone where dierences in RSERs were highest between treatments (10±34 mm from base) average turgor was lowest, OP highest and Y most negative in Hoagland-compared to 8%-and 16%-plants (P < 0.001), but not signi®cantly dierent between 8%-and 16%-plants.Abbreviations and symbols: LER = leaf elongation rate; OP = osmotic pressure; Y = water potential; RAR = relative addition rate; RIFW = relative increase in fresh weight; RSER = relative segmental elongation rate

Small birch plants were grown for up to 80 d in a climate chamber at varied relative addition rates of nitrogen in culture solution, and at ambient (350 μmol mol‐1) or elevated (700 μmol mol‐1) concentrations of CO2. The relative addition rate of nitrogen controlled relative growth rate accurately and independently of CO2 concentration at sub‐optimum levels. During free access to nutrients, relative growth rate was higher at elevated CO2. Higher values of relative growth rate and net assimilation rate were associated with higher values of plant N‐concentration. At all N‐supply rates, elevated CO2 resulted in higher values of net assimilation rate, whereas leaf weight ratio was independent of CO2. Specific leaf area (and leaf area ratio) was less at higher CO2 and at lower rates of N‐supply. Lower values of specific leaf area were partly because of starch accumulation. Nitrogen productivity (growth rate per unit plant nitrogen) was higher at elevated CO2. At sub‐optimal N‐supply, the higher net assimilation rate at elevated CO2 was offset by a lower leaf area ratio. Carbon dioxide did not affect root/shoot ratio, but a higher fraction of plant dry weight was found in roots at lower N‐supply. In the treatment with lowest N‐supply, five times as much root length was produced per amount of plant nitrogen in comparison with optimum plants. The specific fine root length at all N‐supplies was greater at elevated CO2. These responses of the root system to lower N‐supply and elevated CO2 may have a considerable bearing on the acquisition of nutrients in depleted soils at elevated CO2. The advantage of maintaining steady‐state nutrition in small plants while investigating the effects of elevated CO2 on growth is emphasized.

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