Iron regulation of chloroplast photosynthetic function: Mediation of PS I development

Abstract

Photochemical and developmental aberrations of chloroplast function associated with severe iron deficiency were investigated by resupplying iron with foliar application to tobacco (Nicotiana tabacum). Foliar iron application began when leaf chlorophyll levels dropped to a value of 26 nmol cm^‐2. Photosynthetic CO₂ fixation rates were enhanced by 96% within the first 10 days of treatment per unit leaf area. Examination of photosynthetic electron transport function revealed that photosystem I (PS I) activity (DCPIP ? methylviologen) increased by 97% during the first ten days of the treatment on a chlorophyll cm^‐2 basis. Only modest increases were observed in photosystem II (PS II) activity (H₂O ? DBMIB) during this increase of PS I electron transport rates (approximately 2% of the initial rate). These observations prompted further investigations of the molecular association within the chloroplast membranes. Variation in the 77K fluorescence yield ratios (F₆₈₅/F₇₆₆) showed a steady decline with time of treatment, comparable with control levels after 20 days. Purified PS I particles were spectrophotometr‐ically examined for changes in P700, chlorophyll, cytochrome and protein content associated with foliar resupply of iron on a per unit area basis. These examinations revealed that while absolute concentrations per cm^‐2 increased, the ratios of the components remained constant. Electrophoretic examination of these PS I particles on an equal protein basis revealed 6 polypeptide bands. All band loci stained with equal intensity with coomassie blue, except for the 15kD band in the 0 day treatment. These data suggest that iron stress is involved in the regulation of PS I development, possibly by the direct regulation of a low‐molecular‐weight protein required for system assembly.