The authors have studied the spectroscopic characteristics and the fluorescence lifetime for the chloroplasts from spinach (Spinacia oleracea L.) and water hyacinth (Eichhornia crassipes (Mart) Solms.) plant leaves by absorption spectra, low temperature steady_state fluorescence spectroscopy and single photon counting measurement under the same conditions. The absorption spectra at room temperature for the spinach and water hyacinth chloroplasts are similar, which show that different plants can efficiently absorb light of same wavelength. The low temperature steady_state fluorescence spectroscopy for the water hyacinth chloroplast reveals a poor balance of photon quantum between two photosystems. The fluorescence decays in PSⅡ measured at the natural Q A state for the chloroplasts have been fitted by a three_exponential kinetic model. The slow lifetime fluorescence component is assigned to a collection of associated light harvesting Chl a/b proteins, the fast lifetime component to the reaction center of PSⅡ and the middle lifetime component to the delay fluorescence of recombination of P + 680 and Pheo -. The excited energy conversion efficiency (η) in PSⅡ RC is 87% and 91% respectively for the water hyacinth and spinach chloroplasts calculated on the 20 ps model. This interesting result is not consistent with what is assumed that the efficiency is 100% in PSⅡ RC. The results in this paper also present a support for the 20 ps electron transfer time constant in PSⅡ RC. On the viewpoint of excitation energy conversion efficiency, the growing rate for the water hyacinth plan is smaller than that for the spinach plant. But, authors' results show those plants can perform highly efficient transfer of photo_excitation energy from the light_harvesting pigment system to the reaction center (approximately 100%).
