Gojon A, Wakrim R, Passama L, Robin P
Biochimie et Physiologie Végétales, INRA-ENSA URA CNRS 573, 34060 Montpellier Cedex 01, France.
Plant Physiol. 1991 Jun;96(2):398-405. doi: 10.1104/pp.96.2.398.
The regulation of NO(3) (-) assimilation by xylem flux of NO(3) (-) was studied in illuminated excised leaves of soybean (Glycine max L. Merr. cv Kingsoy). The supply of exogenous NO(3) (-) at various concentrations via the transpiration stream indicated that the xylem flux of NO(3) (-) was generally rate-limiting for NO(3) (-) reduction. However, NO(3) (-) assimilation rate was maintained within narrow limits as compared with the variations of the xylem flux of NO(3) (-). This was due to considerable remobilization and assimilation of previously stored endogenous NO(3) (-) at low exogenous NO(3) (-) delivery, and limitation of NO(3) (-) reduction at high xylem flux of NO(3) (-), leading to a significant accumulation of exogenous NO(3) (-). The supply of (15)NO(3) (-) to the leaves via the xylem confirmed the labile nature of the NO(3) (-) storage pool, since its half-time for exchange was close to 10 hours under steady state conditions. When the xylem flux of (15)NO(3) (-) increased, the proportion of the available NO(3) (-) which was reduced decreased similarly from nearly 100% to less than 50% for both endogenous (14)NO(3) (-) and exogenous (15)NO(3) (-). This supports the hypothesis that the assimilatory system does not distinguish between endogenous and exogenous NO(3) (-) and that the limitation of NO(3) (-) reduction affected equally the utilization of NO(3) (-) from both sources. It is proposed that, in the soybean leaf, the NO(3) (-) storage pool is particularly involved in the short-term control of NO(3) (-) reduction. The dynamics of this pool results in a buffering of NO(3) (-) reduction against the variations of the exogenous NO(3) (-) delivery.
在光照下的大豆（Glycine max L. Merr. cv Kingsoy）离体叶片中，研究了硝酸根（NO₃⁻）木质部通量对NO₃⁻同化作用的调节。通过蒸腾流供应不同浓度的外源NO₃⁻表明，NO₃⁻的木质部通量通常是NO₃⁻还原的限速因素。然而，与NO₃⁻木质部通量的变化相比，NO₃⁻同化率保持在较窄的范围内。这是由于在低外源NO₃⁻供应时，先前储存的内源性NO₃⁻有大量的再转运和同化，而在高NO₃⁻木质部通量时，NO₃⁻还原受到限制，导致外源NO₃⁻大量积累。通过木质部向叶片供应¹⁵NO₃⁻证实了NO₃⁻储存库的不稳定性质，因为在稳态条件下其交换半衰期接近10小时。当¹⁵NO₃⁻的木质部通量增加时，无论是内源性¹⁴NO₃⁻还是外源¹⁵NO₃⁻，被还原的可用NO₃⁻比例都类似地从近100%下降到不到50%。这支持了同化系统不区分内源性和外源性NO₃⁻的假设，并且NO₃⁻还原的限制同样影响了来自这两种来源的NO₃⁻的利用。有人提出，在大豆叶片中，NO₃⁻储存库特别参与了NO₃⁻还原的短期控制。这个库的动态变化导致了NO₃⁻还原对外源NO₃⁻供应变化的缓冲作用。
