Estimation of the steady-state cyclic electron flux around PSI in spinach leaf discs in white light, CO₂-enriched air and other varied conditions

Cyclic electron flux (CEF) around PSI is essential for efficient photosynthesis and aids photoprotection, especially in stressful conditions, but the difficulty in quantifying CEF is non-trivial. The total electron flux through PSI (ETR1) and the linear electron flux (LEF_O2) through both photosystems in spinach leaf discs were estimated from the photochemical yield of PSI and the gross oxygen evolution rate, respectively, in CO₂-enriched air. ΔFlux = ETR1 - LEF_O2 is an upper estimate of CEF. Infiltration of leaf discs with 150 μM antimycin A did not affect LEF_O2, but decreased ΔFlux 10-fold. ΔFlux was practically negligible below 350 μolphotonsm ^-2s^-1, but increased linearly above it. The following results were obtained at 980 μolphotonsm^-2s^-1. ΔFlux increased 3-fold as the temperature increased from 5°C to 40°C. It did not decline at high temperature, even when LEF_{O 2} decreased. ΔFlux increased by 80% as the relative water content of leaf discs decreased from 100 to 40%, when LEF_{O 2} decreased 2-fold. The method of using ΔFlux as a non-intrusive upper estimate of steady-state CEF in leaf tissue appears reasonable when photorespiration is suppressed. Journal compilation