An Exploration of the Carbon Economy of the Tobacco Plant. III. Gas Exchange of Leaves in Relation to Position on the Stem, Ontogeny and Nitrogen Content

Tobacco plants were grown in sunlit, controlled-environment cabinets, and their growth and gas exchange were followed from shortly after emergence to 90 days from sowing. There were three major findings: 1. Summed over all leaves, dark respiration remained at 6-7% of net photosynthesis for a lengthy period (50-90 days from sowing), but in younger plants the fraction reached as high as 18%. 2. In the 12 leaves monitored from their emergence to full expansion, net photosynthesis (Pn) followed a similar time course, even though the first and last leaves in this group emerged nearly 40 days apart and there was a 100-fold difference in final area (Amax). For a sequence of nine of these leaves, the agreement was so close that their photosynthetic histories could be represented by a single relationship with developmental time. Peak Pn was consistently attained at about 37% Amax, when peak dA/dt occurred, and it was held for only 3-5 days. The subsequent decline reduced Pn to less than one-third of peak Pn at Amax. 3. Data for the nitrogen content (w/w) of the leaves after 37% Amax could be combined in a manner similar to that described for Pn. Pn, nitrogen content and leaf ontogeny were therefore directly related after peak Pn had been attained. The bearing of these findings on the study of photosynthesis in dicotyledonous species is discussed.