Photosynthesis:What is the exact point in time it is initiated?

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Photosynthesis:What is the exact point in time it is initiated?

I want to be enlightened as to what is the point in time at any place when green plants initiate the process of photosynthesis? If it starts at dawn, then at what location of the sun below horizen?

Another relevant question is whether plants can distinguish between darkness and visible light? If they cant, how the process could be initiated for catching the light?

I shall be grateful if guided. Thanks.
 
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Your question is:what is the minimum light level required for photosynthesis?
The simple answer is that it depends on the species. Place a plant kept in darkness in a bell jar with a means of mesuring oxygen levels in the jar. Illuminate the plant with a controlled light source for about 10 minutes - the induction period. There
will be little change in the O2 reading during the first 5-10 minutes of illumination during which photosynthetic metabolites are synthesized until they reach the critical pool
sizes required for photosynthesis to occur. Once this has been achieved, the oxygen level will rise.

The light levels at which photosynthesis kicks in vary. And it doesn't start instaneously either.

Plants perceive day night length by changing phytochrome molecules between two different stereo isomers. Lengthening days means a buildup of one isomer; lengthening nights means a gradual buildup of the second isomer.
 
Thanks a lot jim mcnamara .

My question is not about specific plant. I want to know about the majority of plants, in general terms, the time at dawn before sunrise when the photosynthesis begins, in other words what exactly is the time or the position/location of sun, when intensity of light becomes sufficient to triger the process of photosynthesis. Thanks.