Abstract
The development and growth of rye (Secale cereale L. cv. Rheidol) was studied in seedlings grown hydroponically in complete nutrient solutions containing between 10 and 600 μM K+. The phyllochron (defined as the interval between the appearance of successive leaves) was used as a developmental timescale to compare plants. The pattern of both shoot and root development was strictly ordered on a phyllochron basis and was unaffected by solution K+ concentration, with the exception that tillers in plants grown at the lowest K+ concentrations were occasionally either not initiated or aborted at an early stage of development. However, both the rate of leaf appearance on the main stem and successive tillers and the rate of tiller appearance were slower in plants grown at lower K+ concentrations. The rate of leaf appearance on the main stem was reduced to below 90% of its maximal value at solution concentrations below about 50 μM K+. Plant relative growth rate (RGR) was also reduced by lowering the K+ concentration of the nutrient solution and fell to below 90% of its maximal value at solution concentrations below about 200 μM K+. There was a complex relationship between tissue K+ concentration and the K+ concentration of the nutrient solution, which differed between leaves and root. Leaf K+ concentration increased steadily from about 50 μmol g-1 f. wt to about 200 μmol g-1 f. wt as solution K+ concentration was increased from 10 to 400 μM. In contrast, root K+ concentration exhibited a sigmoidal dependence on solution K+ concentration, maintaining a minimal value of approximately 20 μmol g-1 f. wt at concentration below 100 μM K+, then increasing progressively to about 120 μmol g-1 f. wt at a solution concentration of 600 μM K+. The 'critical' leaf K+ concentration, i.e. the concentration at which either plant RGR or plant development was reduced 90% of its maximal value, was 86 μmol g-1 f. wt for plant RGR and 150 μmol g-1 f. wt for plant development. The 'critical' root K+ concentration was 24 μmol g-1 f. wt K+ for both RGR and development. A decline in tissue K+ concentration below these thresholds reduced plant growth considerably. Root K+ concentration was a sensitive indicator of the K+ status of the plant with respect to potential growth since plant growth declined abruptly as root K+ concentration approached its 'critical' value, whereas plant growth showed a less defined relationship with shoot K+ concentration.