Ultrastructural and biochemical changes in chloroplasts during Brassica napus senescence

The ultrastructural and biochemical changes occurring in chloroplasts of canola (Brassica napus cv. Topas) cotyledons during senescence were characterized utilizing 1-, 2-, 3-, and 4-week-old plants. Chlorophyll a and b and carotenoid contents per unit cotyledon area were greatest 2 weeks after planting prior to the decline in total protein content. Photosystem II (PSII) activity per unit area of cotyledon was highest during the first week and declined through week 4. In contrast, photosystem I activity and CO2 assimilation remained relatively stable through week 2 and then declined. High levels of PSII activity were associated with large numbers of stacked grana; whereas loss of activity during week 2 was accompanied by reduced compactness of grana and the aggregation and enlargement of plastoglobuli.