The primary focus of this research is the retention of photosynthetic activity during extended photosynthesis and during dark intervals. The biological test materials are isolated spinach and Chlamydomonas chloroplasts and the parent tissues. The carbon dioxide fixing capability of the isolated intact spinach chloroplast falls off during photosynthesis after approximately 2-40 minutes, even though a functional electron transport chain and activity of stromal enzymes of the Calvin cycle are maintained. Chloroplasts kept in the dark for 10-20 minutes also lose their subsequent photosynthetic carbon dioxide fixing activity. Addition of a complex between magnesium ions and ATP or a chloroplastic respirable substrate to the external medium during photosynthesis or during the dark phase prior to photosynthesis allows retention of photosynthetic activity. Other nucleotides will not substitute for ATP nor do non-respirable chloroplastic substrates function in this capacity. Similarly, pre-treatment of the darkended spinach leaf results in an inactivation of the carbon dioxide fixing capacity. The level of inactivation seems to depend upon the nutritional status (perhaps carbohydrate content) of the leaf. Inactivation is extremely pH dependent as is senescence of the chloroplast. The basic events that result in the loss of photosynthetic carbon dioxide fixation will be elucidated as well as the underlying principles of the protective mechanisms. This research should provide substantial information supporting the concept that the retention of chloroplastic carbon dioxide fixing activity is dependent upon extrachloroplastic events such as cellular respiration and, to a limited extent, on chloroplastic respiration. Separation of the chloroplast from these events leads to its rapid irreversible destruction unless the organelle is protected by the products of respiration generated in the mitochondrion or within the organelle by starch break-down. The linkage of chloroplastic competency with other cellular events will add a basic component to our understanding of stress physiology, i.e. water status, temperature, ionic balance and other factors.
