Inhibition of photosystems I and II activities in salt stress-exposed Fenugreek (Trigonella foenum graecum)

Fenugreek (Trigonella foenum graecum) seedlings were exposed to increasing NaCl concentrations in the growth medium to examine the effect of salt stress on the electron transport reactions of photosynthesis. Activities of both photosystem II (PSII), measured by chlorophyll fluorescence, and photosystem I (PSI), measured by P700 photooxidation, were decreased by salt stress. The inhibition proceeded in a two step manner. At the lower salt concentrations used and shorter exposition periods, electron transfer between the quinone acceptors of PSII, QA and QB, was strongly retarded as shown by an increased amplitude of the OJ phase of the OJIP chlorophyll fluorescence induction traces and slowed chlorophyll fluorescence relaxation kinetics following a single turn-over flash. The above indicated a disturbance of the QB binding site likely associated with the first step of photoinhibition. In the second step, strong photoinhibition was observed as manifested by increased F0 values, declined Fv/F0 and loss of photoactive P700.