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Evidence supporting a non‐phloem source of water for export of solutes in the xylem of soybean root nodules
Author(s) -
STREETER J. G.,
SALMINEN S. O.
Publication year - 1992
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1111/j.1365-3040.1992.tb01016.x
Subject(s) - xylem , phloem , apoplast , stele , exudate , nodule (geology) , transpiration stream , shoot , root nodule , botany , vascular tissue , biology , chemistry , transpiration , symbiosis , photosynthesis , cell wall , paleontology , genetics , bacteria
The vascular anatomy of soybean nodules [ Glycine max (L.) Merr.] suggests that export of solutes in the xylem should be dependent on influx of water in the phloem. However, after severing of stem xylem and phloem by shoot decapitation, export of ureides from nodules continued at an approximately linear rate for 5h. This result was obtained with decapitated roots remaining in the sand medium, but when roots were disturbed by removal from the rooting medium prior to shoot decapitation, export of ureides from nodules was greatly reduced. Stem exudate could not be collected from disturbed roots, indicating that flow in the root xylem had ceased. Thus, ureide export from nodules appeared to be dependent on a continuation of flow in the root xylem. When seedlings were fed a mixture of 3 H 2 O and 14 C‐inulin for periods of 14–21 min, nodules had higher 3 H/ 14 C ratios than roots from which they were detached. The combined results are not consistent with the proposal that export of nitrogenous compounds from nodules is dependent on import of water via the phloem. The results do support the view that a portion of the water required for xylem export from soybean nodules is supplied via a symplastic route from root cortex to nodule cortex to the nodule vascular apoplast.