Picosecond fluorescence decay studies on water-stressed pea leaves: energy transfer and quenching processes in photosystem 2

Detached leaves of pea (Pisum sativum) were submitted to water stress at different relative air humidities. The photosynthetic activity of photosystem 2 (PS2) was monitored by time-resolved picosecond chlorophyll (Chi) fluorescence spectroscopy. In the first days the well-known fast Chi fluorescence decay was observed which indicated high PS2 activity. After a few days the average fluorescence decay time tau(m) reached a maximum, depending on the wilting conditions, but always at a relative loss of leaf mass of 80 %. After this maximum, tau(m) decreased within a few hours, the fluorescence decay became similar to that one of an intact leaf, but an additional fluorescence decay component with a lifetime of 3.6 ns appeared. At first the primary quinone Q(A) was reduced due to inhibition of the electron transfer to the secondary quinone Q(B). Simultaneously, water deficiency caused an electron lack at the oxidizing site of PS2. This disabled the primary electron donor of PS2, tyrosine Z, from reducing the oxidized reaction centre of PS2 (P680(+)). Thus a recombination of P680(+)-pheophytin-Q(A)(-) took place, and the energy was lost as heat. With further water stress, Q(A) was decoupled from PS2. The new fluorescence decay component could therefore be assigned to energetically decoupled antenna complexes.