The release of water from storage compartments to the transpiration stream is an important functional mechanism that provides the buffering of sudden fluctuations in water potential. The ability of tissues to release water per change in water potential, referred to as hydraulic capacitance, is assumed to be associated with the anatomy of storage tissues. However, information about how specific anatomical parameters determine capacitance is limited. In this study, we measured sapwood capacitance (C) in terminal branches and roots of five temperate tree species (Fagus sylvatica L., Picea abies L., Quercus robur L., Robinia pseudoacacia L., Tilia cordata Mill.). Capacitance was calculated separately for water released mainly from capillary (CI; open vessels, tracheids, fibres, intercellular spaces and cracks) and elastic storage compartments (CII; living parenchyma cells), corresponding to two distinct phases of the moisture release curve. We found that C was generally higher in roots than branches, with CI being 3-11 times higher than CII Sapwood density and the ratio of dead to living xylem cells were most closely correlated with C In addition, the magnitude of CI was strongly correlated with fibre/tracheid lumen area, whereas CII was highly dependent on the thickness of axial parenchyma cell walls. Our results indicate that water released from capillary compartments predominates over water released from elastic storage in both branches and roots, suggesting the limited importance of parenchyma cells for water storage in juvenile xylem of temperate tree species. Contrary to intact organs, water released from open conduits in our small wood samples significantly increased CI at relatively high water potentials. Linking anatomical parameters with the hydraulic capacitance of a tissue contributes to a better understanding of water release mechanisms and their implications for plant hydraulics.

Changes in root hydraulic resistance in response to alterations in nitrate supply were explored in detail as a potential mechanism that allows plants to respond rapidly to changes in their environment. Sunflower (Helianthus annuus cv. Holiday) plants grown hydroponically with limited nitrate availability (200 micromol l(-1)) served as our model system. Experimental plants were 6-9-weeks-old with total dry mass of 2-4 g. Root pressurization of intact plants and detached root systems was used to elucidate the temporal dynamics of root hydraulic properties in sunflower plants following changes in external nitrate availability. The response was rapid, with a 20% decrease in hydraulic resistance occurring within the first hour after the addition of 5 mM nitrate and the magnitude of the effect was dependent on nitrate concentration. The change in root hydraulic resistance was largely reversible, although the temporal dynamics of the response to nitrate addition versus nitrate withdrawal was not symmetric (a gradual decrease in resistance versus its fast increase), raising the possibility that the underlying mechanisms may also differ. Evidence is presented that the observed changes in root hydraulic properties require the assimilation of nitrate by root cells. The hydraulic resistance of roots, previously stimulated by the addition of nitrate, increased more than in control plants in low nitrate under anoxia and that suggests a key role of aquaporin activity in this response. It is proposed that a rapid decrease in root hydraulic resistance in the presence of increased nitrate availability is an important trait that could enhance a plant's ability to compete for nitrate in the soil.

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Production of new leaf area was entirely reliant, during the first week after defoliation, on N stores present in the plant. Mobilized N originated mainly from free amino acids and soluble proteins located in roots, and less so from proteins in stubble. Presence of VSP in the roots was not confirmed. The data suggest that rhizomes played an important role in N transport but not in N storage.

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The building and use of internal N stores in the grass Calamagrostis epigejos was investigated in context of complex ecological study focused on mechanisms underlying competitive ability of this highly successful invasive species. Induced changes in nitrogen availability in the course of two subsequent vegetation seasons were used as a tool for finding (i) to what extent high N availability in substrate is important for building N reserves in autumn that support spring regrowth and, (ii) if contrasting contents of N storage compounds may result in differences in growth in the next season. Plants were grown in solely inorganic substrate and received a nutrient solution containing 5 mol m À3 of NH 4 NO 3 . The nitrogen supply was reduced in a low nitrogen (LN) treatment to 0.25 mol m À3 in August whereas in high nitrogen (HN) treatment remained high till December. During the following growing season were plants from both treatments grown at the low N supply (0.25 mol m À3 ). An increase in the content of N storage compounds was observed from September to December in both treatments. Plants in the LN treatment showed significantly lower total N content and also N allocated to mobilizable reserves (20-50% of HN plants), namely due to a smaller accumulation of amino acids and soluble protein in autumn. External nitrogen availability in autumn is hence highly important for building N reserves in this species. A major portion of the nitrogen stored in HN plants during winter was taken up from growth medium in late autumn, whereas translocation from senescing shoots dominated in LN treatment. During the winter about 50% of N in plants was permanently present in shoots bearing several frost resistant green leaves. Spring regrowth was accompanied by a fast decrease of both total N and the content of N storage compounds in both treatments. Amino acids were identified as the most prominent source of mobilizable N during spring regrowth. Development of leaf area in LN plants was significantly slower in March and April than in HN plants namely due to smaller number of tillers and green leaves per plant. Low N availability in autumn, therefore, may result in restrictions of plant growth and development in the following season.

We investigated how patchy nitrate availability influences growth and functioning of plant roots and generates, through vascular constraints on long-distance transport, aboveground heterogeneity in plant growth and chemistry. We examined two broadleaf tree species, Acer rubrum L. and Betula papyrifera Marsh. Plants were grown either in a split-root setup where a single root received full nutrient supply and the rest of the root system received all nutrients except nitrogen (patchy treatment), or in a single pot with full nutrient supply (homogeneous treatment). In both species, fine roots proliferated in the nitrogen patch, but B. papyrifera produced twice as much fine root biomass in response to patchy nitrate availability as did A. rubrum. There was no difference between treatments in nitrogen uptake rate in either species. In general, specific water uptake was higher in A. rubrum than in B. papyrifera, especially in the nitrogen-rich side pot. When nitrate availability was patchy, nitrate reductase activity in roots and leaves was unaffected in either species. In A. rubrum, but not in B. papyrifera, patchy nitrate supply resulted in aboveground heterogeneity, with leaves above the N-fertilized roots being larger and having a higher relative chlorophyll concentration than those inserted in the opposite quater of the stem.

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