Turgor-dependent efflux of assimilates from coats of developing seed of Phaseolus vulgaris L

Water relations of the cells involved in efflux

Wenhao Zhang, Brian J. Atwell, John W. Patrick*, N. Alan Walker

*Corresponding author for this work

Research output: Contribution to journalArticle

28 Citations (Scopus)

Abstract

The turgor-homeostat model of assimilate efflux from coats of developing seed of Phaseolus vulgaris L. was further characterised. The turgor pressure (P), the volumetric elastic modulus (ε) and hydraulic conductivity (Lp) of the seed coat cells responsible for assimilate efflux and cotyledon storage parenchyma cells were determined with a pressure probe. In addition, turgor of the seed coat and cotyledons was estimated by measuring the osmolalities of symplastic and apoplastic fluids extracted by centrifugation. Osmolality of symplastic and apoplastic saps collected from the seed coat declined significantly over the period of seed development from a cotyledon water content of 80% to 50%. However, the difference in osmolalities of the apoplastic and symplastic saps remained relatively constant. For cotyledons, osmolality of the apoplastic sap exhibited a significant decline during seed development, while the osmolality of symplastic sap did not change significantly. Hence cotyledon P increased as the water content dropped from 80% to 50%. For both detached and attached 'empty' seed coats, a small decrease (ca. 40mOSmol·kg-1) in the osmolality of the bathing solution, led to a rapid increase in P of cells involved in assimilate efflux (efflux cells) by about 0.07 MPa. Thereafter, cell P exhibited a rapid decline to the original value within some 20-30 min. When P of the 'efflux cells' was reduced by increasing the osmolality of the bathing solution, P exhibited a comparable rate of recovery for attached 'empty' seed coats but there was no P recovery to its original value in the case of detached seed coats. In contrast, the cotyledon storage parenchyma cells did not exhibit P regulation when the osmolality of the bathing solution was changed. The observations that the 'efflux cells' of P. vulgaris seed coats can rapidly adjust their P homeostatically in response to small changes in apoplastic osmolality are consistent with the operation of a turgor-homeostat mechanism. The volumetric elastic modulus (ε) of the seed coat 'efflux cells' exhibited a mean value of 7.3 ±0.8 MPa at P = 0.15 MPa and was found to be linearly dependent on cell P. The ε of the cotyledon storage parenchyma cells was estimated to be 6.1 ± 1.0 MPa at P = 0.41 MPa. Hydraulic conductivity (Lp) of the seed coat cells and the cotyledon cells was (8.2 ±1.5) × 10-8 m·s-1· MPa-1 and (12.8 ± 1.0) × 10-8 m·s-1· MPa-1, respectively. The relatively high ε, i.e., low elasticity, for the seed coat cell walls would ensure that small changes in water potential of the seed apoplast will be reflected in large changes in cell P. The high Lp values for both the seed coat and the cotyledon cells is consistent with the rapid changes in P in response to changes in water potential of the seed apoplast.

Original languageEnglish
Pages (from-to)25-33
Number of pages9
JournalPlanta
Volume199
Issue number1
Publication statusPublished - 1996

Keywords

  • Cell water relations
  • Cotyledon
  • Phaseolus (water relations)
  • Pressure probe
  • Seed coat
  • Turgor-homeostat

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