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Relative rates of delivery of xylem solute to shoot tissues: Possible relationship to sequential leaf senescence

1984; Wiley; Volume: 62; Issue: 3 Linguagem: Inglês

10.1111/j.1399-3054.1984.tb04591.x

1399-3054

Peter Neumann, Zipporah Stein,

Plant nutrient uptake and metabolism

The rates of delivery of regulatory solutes such as cytokinins and mineral ions from the roots to competing shoot tissues can influence rates of metabolism and development. A 15 min pulse of a synthetic xylem mobile and phloem‐immobile solute, acid fuchsin, was used to quantify relative rates of solute delivery to competing organs on excised transpiring bean shoots ( Phaseolus vulgaris L. cv. Contender) at different stages of development. Stem, flower and fruit tissues received comparatively low rates of solute delivery. The relative rate of solute delivery to newly opened leaves was initially low, but increased during rapid leaf expansion and then declined progressively as the leaves exceeded 70% of their final area. The relative rate of solute delivery to tissues of the basal primary leaves declined progressively from 2 weeks onwards. This decline appeared to be caused by progressive internally regulated increases in both stomatal resistances and hydraulic resistances to xylem flow up to and into the leaf blade. Thus combined abaxial and adaxial stomatal resistance values in the primary leaves (Rs) increased from 3 to ≥ 7 s cm −1 between 2 and 5 weeks. Similarly, mean values for the connection resistances (Rc) to hydraulic flow into the primary leaves rose from 7 to 13 TPa · s · m −1 between 2 and 4 weeks. In the same period pathway resistance from stem to primary leaf petioles (Rp), as determined by direct pressure flow assay, increased from 7 to 15 TPa · s · m −1 . Senescence‐associated declines in protein and chlorophyll levels in the primary leaves were initiated in parallel with, or after, declines in relative rates of solute delivery. The provision of extra illumination at the basal leaf level between 2 and 5 weeks did not prevent declines in chlorophyll and soluble protein or increases in stomatal resistance. We suggest that internally programmed changes in the hydraulic architecture of the plant progressively divert xylem‐transported root supplies of nutrients and cytokinins from basal to more apical leaves and thus regulate the progressive senescence of leaves from the base upwards.