Do oxidative stress conditions impairing photosynthesis in the light manifest as photoinhibition?

We compared the effect of photoinhibition by excess photosynthetically acti ve radiation (PAR), UV-B irradiation combined with PAR, low temperature str ess and paraquat treatment on photosystem (PS) II. Although the experimenta l conditions ensured that the four studied stress conditions resulted in ap proximately the same extent of PS II inactivation, they clearly followed di fferent molecular mechanisms. Our results show that singlet oxygen producti on in inactivated PS II reaction centres is a unique characteristic of phot oinhibition by excess PAR. Neither the accumulation of inactive PS II react ion centres (as in UV-B or chilling stress), nor photo-oxidative damage of PS II (as in paraquat stress) is able to produce the special oxidizing cond itions characteristic of acceptor-side-induced photoinhibition.