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Effects of increasing inorganic carbon supply to roots on net nitrate uptake and assimilation in tomato seedlings
Author(s) -
Gao Z. F.,
Lips S. H.
Publication year - 1997
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.1997.tb01838.x
Subject(s) - lycopersicon , nitrogen , nitrate , aeration , chemistry , horticulture , botany , nutrient , biology , organic chemistry
We investigated the influence of an increased inorganic carbon supply in the root medium on NO − 3 uptake and assimilation in seedlings of Lycopersicon esculentum (L.) Mill. cv. F144. The seedlings were pre‐grown for 4 to 7 days with 0 or 100 m M NaCl in hydroponic culture using 0.2 m M NO − 3 (group A) or 0.2 m M NH + 4 (group B) as nitrogen source. The nutrient solution for group A plants was aerated with air or with air containing 4 800 μumol mol −1 CO 2 . Nitrate uptake rate and root and leaf malate contents in these plants were determined. The plants of group B were subdivided into two sets. Plants of one set were transferred either to N‐free solution containing 0 or 5 m M NaHCO 3 , or to a medium containing 2 m M NO − 3 and 5 m M NaHCO 3 . Both sets of group B plants were grown for 12 h in darkness prior to 2 h of illumination, and were assayed for malate content and NO − 3 uptake rate (only for plants grown in N‐free solution). The second set of group B plants was labeled with 14 C by a 1‐h pulse of H 14 CO − 3 which was added to a 5 m M NaHCO 3 solution containing 0 or 100 m M NaCl and 0 or 2 m M NO − 3 , and 14 C‐assimilates were extracted and fractionated. The roots of group B plants growing in carbonated medium accumulated twice as much malate as did control plants. This malate was accumulated only when NO − 3 was absent from the root medium. Both a high level of root malate and aeration with CO 2 ‐enriched air stimulated NO − 3 uptake. Analysis of 14 C‐assimilates indicated that with no NO − 3 in the medium, the 14 C was present mainly in organic acids, whereas with NO − 3 , a large proportion of 14 C was incorporated into amino acids. Transport of root‐incorporated 14 C to the shoot was enhanced by NO − 3 , while the amino acid fraction was the major 14 C‐assimilates in the shoot. It is concluded that inorganic carbon fixed through phosphoenolpyruvate carboxylase (EC 4.1.1.31) in roots of tomato plants may have two fates: (a) as a carbon skeleton for amino acid synthesis; and (b) to accumulate, mainly as malate, in the roots, in the absence of a demand for the carbon skeleton. Inorganic carbon fixation in the root provides carbon skeletons for the assimilation of the NH + 4 resulting from NO 3 reduction, and the subsequent removal of amino acids through the xylem. This ‘removal’ of NO − 3 from the cytoplasm of the root cells may in turn increase NO − 3 uptake.