Pea seedlings (Pisum sativum L. cv. Melbourne Market) were grown under light-limited conditions (12 h photoperiod of 1 klx; 19°C) so that both the development and senescence of the first true leaf could be studied within a period of 4 weeks from the beginning of germination. Rapid cell expansion and synthesis of protein and chlorophyll occurred between days 8 to 12. Whereas photosynthetic O2 evolution and RuBP carboxylase both showed maximum values per leaf at 16 days, declining thereafter, that of NADH:glyoxylate reductase, a peroxisomal marker, occurred between 11 and 14 days and was decreasing by 16 days. Total leaf ribose-5-P isomerase activity increased two-fold to a maximum at 14 days then fell sharply before 16 days. Dark O2 uptake by leaf slices and NADH oxidase activity showed extended maxima from the earliest stages to 20 and 16 days respectively. Soluble acid phosphatase also showed an extended period of maximum activity per leaf, from 12 to 21 days, then declined slowly.

As a proportion of leaf soluble protein, Fraction I protein varied from 17% (day 8) to 41% (day 10), 57%(day 14), 76%(day 16) then to about 60% from day 21 onwards. Between 14 and 16 days the amount of Fraction I protein in the leaf continued to increase, although the amount of total soluble protein had begun to decline. Aminopeptidase (leucyl β-naphthylamidase) activities per leaf also began to decline after 14 days. Since the maximum photosynthetic capacity of the leaf is not observed until 16 days, the data indicate the onset of leaf senescence prior to the completion of chloroplast development.