Photoinhibition of photosynthesis in Lemna gibba as induced by the interaction between light and temperature. II. Photosynthetic electron transport

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Abstract
Photoinhibition of photosynthesis in Lemna gibba L. was induced by exposing intact plants to a high photosynthetic photon flux density of 1 750 μmol m−2 s−1 at a low temperature of 3°C. Subsequently isolated chloroplasts showed pronounced reductions in the capacity of whole chain electron transport, measured as Hill activity, and in the efficiency of electron transport to the primary electron acceptor Q of photosystem II, measured as variable chlorophyll fluorescence at 20°C. These changes proceeded with similar kinetics (probably of the first‐order reaction), suggesting that the site of photoinhibition is in the electron transfer to Q. A partial uncoupling of the whole chain electron transport also occured. The capacity of electron transport mediated by photosystem I was unaffected. The extent of photoinhibition of photosynthetic electron transport, as produced by a 2 h exposure of L. gibba to three different combinations of photon flux density and temperature was studied. It was shown that intrinsically similar states of photoinhibition, on the evidence of their time courses of recovery, were induced by either a high photon flux density and 25°C or by a moderate photon flux density and 3°C.