Respiration in Developing Leaves in Relation to Vertical Profiles in Concentration of Potassium in Shoots of Nicotiana tabacum

Abstract

Summary In expanding leaves of tobacco ( Nicotiana tabacum ), O 2 -uptake in the dark per lamina as well as per 10 3 mesophyll cells increased from a low in the newly emerged leaf to a maximum in the 50 % expanded leaf. Thereafter, O 2 -uptake remained steady until expansion was completed. Expressed per unit fresh weight or per unit lamina area, the rate of O 2 -consumption in the dark, on the contrary, was highest in the newly emerged leaf and declined with advancing expansion to a minimum in the fully expanded leaf. The number of mesophyll cells per unit weight and per unit area was maximum in the newly emerged leaf and decreased with advancing expansion; this was influenced by K supply. The decrease in rate of respiration with advancing expansion of leaves reflected the decline in the density of mesophyll cells. In leaves expanded up to 25 %, O 2 -uptake per lamina and per 10 3 mesophyll cells was about 40 % greater in plants grown with a supply of 1 meq K + dm -3 in the substrate solution than in plants supplied with 6 meqdm -3 . However, after a leaf had expanded 50%, O 2 -uptake was 30% greater in plants supplied with the higher concentration of K, and remained higher until after expansion had ceased. The results have been discussed in relation to the level of substrate in the sink and source leaves.