Physiological changes in plants due to low light stress depends on duration of stress. Plants response differently due to variation of stress time. For instance, with the extension of low light stress from 5 to 15 days, the relative chlorophyll content, anthocyanin, net photosynthesis rate, transpiration rate, stomatal conductance, and antioxidant activity decreased significantly in Pak-choi. Similarly, the malondialdehyde content also increased under low light stress, which damage plant cells.
I would like to answer the first part of your question: The Physiological reason is that this luminic condition stimulates Calmodulie (Cm) production in the apical region and. consecuently, increases the number of cells, which leads to the lenthening of the stems. Certainly, in turn, cell elongation depends on the changes in the composition of the cell wall.
Cm-Ca regulate the process of cell elongation. Perhaps, thinking about this, the answer to the second part of your question might arise.
It depends on what crop you are investigating. You should first determine if the target plant you want to investigate is short day or long daylength. But generally, plants respond more in terms of yield if they receive more light (in higher wavelength) 600 to 900 nm than in low light.