Photosynthetic electron transport and electrochromic effects at sub-zero temperatures

Area of Science:

• Plant biochemistry
• Photosynthesis research
• Chloroplast biophysics

Background:

• Photosystem I (PSI) is crucial for light-dependent reactions in photosynthesis.
• Understanding electron transport and membrane potential in chloroplasts is key to photosynthesis research.

Purpose of the Study:

• To investigate the light-induced absorbance changes in spinach chloroplasts at low temperatures.
• To elucidate the role of electron donors and acceptors in Photosystem I activity.
• To quantify the membrane potential generated during sustained electron transport.

Main Methods:

• Spectrophotometric analysis of spinach chloroplasts at -35 to -50°C.
• Suppression of Photosystem II activity.
• Use of electron acceptors (methyl viologen, silicomolybdate) and donors (N-methylphenazonium methosulphate).
• Measurement of light-induced absorbance changes at 700 nm and 518 nm.

Main Results:

• Reversible absorbance changes near 700 nm (P-700) and 518 nm (P-518) were observed.
• Decay kinetics were influenced by electron acceptors and donors, particularly N-methylphenazonium methosulphate.
• Sustained electron transport generated high membrane potentials (up to 500 mV) with slow dark decay, accelerated by gramicidin D.
• Absorbance changes correlated linearly with membrane potential.

Conclusions:

• Light-induced absorbance changes in spinach chloroplasts reflect Photosystem I activity and electrochromic shifts.
• Sustained electron transport generates significant, slowly dissipating membrane potentials across the thylakoid membrane.
• Membrane conductance is low, but can be modulated by ionophores like gramicidin D.