Assertion :In some plants flowering depends only on a combination of light and dark exposure.
Reason :The site of perception of light or dark duration are the shoot apices of plants.

Some plants require a periodic exposure to light to induce flowering. It is now also known that not only the duration of a light period but the duration of the dark period is also of equal importance. Hence, it can be said that flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations. This response of plants to periods of day/night is termed photoperiodism. The shoot apices modify themselves into flowering apices prior to flowering, shoot apices cannot perceive photoperiods by themselves. The site of perception of light/dark duration are the leaves.

So the correct answer is 'Both Assertion and Reason are incorrect'.