Picosecond studies at 77 K of energy transfer in chloroplasts at low and high excitation intensities

Spinach chloroplast chlorophyll fluorescence at 685 and 735 nm (F₆₈₅ and F₇₃₅) has been time-resolved with a low-jitter streak camera system. Measurements are reported largely for 77 K, using single 30-ps 532-nm excitation pulses with pulse fluences of 2 · 10¹² to 2 · 10¹⁶ photons · cm^-2. A slightly fluence-dependent delay (16 ± 3 ps) found for the rise of F₇₃₅ relative to F₆₈₅ is too small to correspond to transfer into the species emitting F₇₃₅ from the species emitting F₆₈₅. The rise of F₇₃₅ is biphasic at lower intensities and monophasic at higher intensity. The delay in the rise of F₆₈₅ is smaller than approx. 2 ps. The time-resolved F₆₈₅ is fit with a three-component model in which energy is transferred from a large pool of antenna chlorophylls to a small pool closely connected to the reaction center, all exciton annihilation occurring in the small pool. The F₇₃₅ biphasic rise and its sensitivity to excitation fluence is explained by a similar but independent model. Data at our lowest intensities are consistent with those obtained by a photon-counting method using very low intensity excitation.