Studies on potassium nutrition of plants IV.—Effect of potassium nutrition on the composition of leaves of cabbage seedlings (Brassica oleracea capitata)

The effect of potassium concentration in the nutrient medium on the composition of leaves of cabbage seedlings has been investigated. The plants were grown in sand culture with seven treatments ranging from extreme deficiency to relatively high potassium concentration. The total amounts of the mineral components were not influenced by the level of potassium nutrition. Similarly, proteins, amino acids and other organic nitrogen components, taken as a whole, did not vary appreciably as shown by the constancy of the Kjeldahl nitrogen values. Sugar (reducing sugars + sucrose) content rose with increasing potassium concentration in the medium to a maximum (11.0%) at 2.5 mequiv./l and fell slightly at the highest K+ concentration. Starch content varied in a similar manner but values were, in general, less than half those of the corresponding sugar contents. Total organic acids content was at a maximum (18.6%) when K+ concentration in the medium was 0.1 mequiv./l and fell to a minimum (7.7%) at a K+ concentration of 2.5 mequiv./l. ‘Other carbohydrates’ (the difference between content of 70% [by vol.] alcohol‐insoluble residue and Kjeldahl nitrogen × 6.25) also contributed to the balance of percentage composition; the content rose to a maximum (35.2%) at a K+ concentration of 2.5 mequiv./l and fell off both at higher and lower K+ concentrations. Within the limits of experimental error, the totals of the leaf components determined were constant over the range of potassium concentrations of 0.1–5.0 mequiv./l in the medium and, on average, 90.7% of the dried leaf was accounted for. Similar differences in mineral composition and ‘protein’, sugars and starch contents were found in plant fractions of different leaf ages in potassium deficiency. The differences were related to differences in potassium content which decreased with increasing leaf age.