Paraquat and photooxidative stress?

Question

You are a citrus grower in Izmir. In the internet, you read a long-term weather forecast report, which states that the temperature in Izmir will fall to extremely low values in two weeks. You know that citrus is very sensitive to low-temperature (chilling and/or freezing) stress, but you don't what you can do in order to protect your orchard. A friend of you is a plant physiologist and recommends you to spray your citrus trees with an extremely low dose of paraquat which will not cause any damage, before the cold weather front arrives in Izmir. What is the logic behind this recommendation?

Hint1: Plants under low-temperature stress are very sensitive to photooxidative stress.

Hint2: Adaptation

Answer 1

I doubt that there is much logic to this application of low doses of paraquat.  I'm skeptical.

The rationale in favor of this idea is based on the fact that at cold temperatures the enzymatically catalyzed reactions of CO2 fixation and carbon metabolism slow down considerably.  If the leaf is still subjected to strong illumination from the sun under these circumstances then the stage is indeed set for photooxidative damage.  The pigment system will continue absorb light and stimulate reaction centers involved in photosynthetic electron transport.  These processes are much less susceptible to low temperature so an extreme imbalance between the potential for generation of photochemical imtermediates and the capacity to use energy and reducing power is created in teh leaf.  This can lead to formation of reactive oxygen species that could damage or kill the leaf.

Paraquat will act in a very limited way as an alternate terminal acceptor in the electron transport system thereby mitigating effects of the loss of the Calvin cycle activity.  but, paraquat will also reduce O2 to hydrogen peroxide.  The latter is damaging to living tissue.  So, what has one gained?

On the other hand if in carefully constructed and executed experiments paraquat HAS been shown to confer resistance to cold then one has to take such results seriously.