Effects of Nitrate Supply and Deprivation and/or Defoliation on Potassium Absorption and Distribution in Ryegrass

Absorption and distribution of K⁺ by two ryegrass genotypesfrom steady-state supplies at low external concentrations werefollowed through periods of NO₃^– deprivation and afterdefoliation. Electron microprobe analysis was also used to examineK⁺ concentrations across the root cortex and the effects onvacuolar/cytoplasmic K⁺ concentrations in the cortex. In manyrespects, the two genotypes behaved in a similar way. Totalquantities of K+ absorbed were less than 50% of those previouslyrecorded for NO₃^–, and NO₃^– deprivation slightlyreduced cumulative K+ uptake. Unit absorption rates (_K) declinedwith time in the control plants, and removal of NO₃^– resultedin a progressive decline in _K, with an initial rapid responsewhich was independent of any effects of growth. In one genotypeabsorption rates did not recover but gradually did so in theother. Defoliation reduced _K only slightly for 6 d, thereafter_K, increased almost linearly so that values for plants withsustained NO₃^– supply were 2 ? those of entire plants.There was no evidence of an oscillatory cycle of K⁺ unit absorptionrates of the kind previously shown for NO₃^– during therecovery periods. Concentrations of K⁺ on both a dry matterand tissue water basis remained within a narrow range irrespectiveof treatment or time. The immediate decline in K⁺ unit absorption rate after NO₃^–withdrawal indicated that at least a proportion of their uptakeswere coupled. Calculated values for the length of the diffusivepathway indicated that the primary site of absorption into thesymplasm was either the epidermis or the outermost corticalcell. This was supported by evidence from the electron microprobeanalysis which indicated a strong gradient of K⁺ concentrationswhich declined from outer to inner cortex and was much steeperin plants with a sustained NO₃^– supply. Concentrationsin the cytoplasm of most cortical cells were generally greaterthan in the vacuoles: this difference was greater in low N plants.The changes in K+ in the cortex were related to the role ofK+ in the transport of NO₃^– in the xylem and effects onrecycling to the roots in the phloem. Key words: Cytoplasm, defoliation, diffusion, electron probe X-ray microanalysis, Lolium, nitrate, potassium, S.E.M.,, unit absorption, vacuole