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Increased Proline Loading to Phloem and Its Effects on Nitrogen Uptake and Assimilation in Water-Stressed White Clover (Trifolium repens)

Bok-Rye Lee, Yu Lan Jin, Jean-Christophe Avice, Jean-Bernard Cliquet, Alain Ourry and Tae-Hwan Kim
The New Phytologist
Vol. 182, No. 3 (May, 2009), pp. 654-663
Published by: Wiley on behalf of the New Phytologist Trust
Stable URL: http://www.jstor.org/stable/30224788
Page Count: 10
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Increased Proline Loading to Phloem and Its Effects on Nitrogen Uptake and Assimilation in Water-Stressed White Clover (Trifolium repens)
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Abstract

The aim of this study was to investigate the physiological significance of increased proline loading to phloem caused by water-deficit stress in relation to nitrogen (N) uptake and assimilation. N uptake and N assimilation were quantified by $^{15}N$ tracing in well-watered (control) and water deficit-stressed white clover (Trifolium repens). De novo proline synthesis and proline loading to the phloem were also compared between treatments. The relationships among proline concentrations in phloem exudates, N uptake, and assimilation of newly absorbed N were assessed. The newly synthesized proline in the phloem exudates increased rapidly after 3 d of water deficit. The water-deficit treatment significantly reduced the maximum nitrate reductase activity (NRA), and also attenuated de novo synthesis of amino acids and proteins in the roots. The increase in proline concentrations in phloem exudates was closely related to reductions in NRA in the roots, N uptake, and the assimilation of newly absorbed N. The accumulation of proline induced in roots by exogenous proline and NH₄Cl treatments was closely associated with the decrease in NRA. These results indicate that increased proline transport to roots via phloem caused by water deficit has a significant influence on the down-regulation of N uptake and the assimilation of newly absorbed N.

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