The interrelationship between HCO 3 − , NO 3 − , and Cl− as effective anions in the photosynthetic electron transport

Abstract

Thylakoids were isolated from the leaves of three different plants (Pisum sativium L., Lactucasativa L., and Raphanus sativus L.). The addition of HCO3− to a suspension of salt-and HCO3−-epleted thylakoids (suspended in salt-free medium) raised the rate of O2 evolution up to fourfold. This stimulation could be partially replaced by the addition of chloride or nitrate ions. However, the addition of HCO3− in the presence of Cl− or NO3− resulted in a higher stimulation of O2 evolution (sixfold in the presence of nitrate and sevenfold in the presence of chloride). On the other hand, the addition of HCO3− to the thylakoids depleted from salt only raised the rate of O2 evolution by 10–15%, whereas 40–70% was obtained by the addition of nitrate or chloride ions. The fluorescence induction studies indicated a significant decrease in the yield of the variable fluorescence of the salt- and HCO3−-depleted thylakoids. A partial increase in the fluorescence yield was obtained by the addition of HCO3− alone. A typical fluorescence induction curves were obtained by the addition of HCO3− in the presence of Cl− or NO3− ions. The data obtained suggest a similar role for chloride and nitrate ions in O2 evolution in the Hill-reaction, which is restricted at the donor side of photosystem II, whereas bicarbonate plays its role at both sides (acceptor and donor sides). The presented data are those obtained for the thylakoids of P. sativium, which were more or less similar to those obtained for L. sativa and R. sativus.