From what I know, with a magnesium deficiency, a plant's overall growth will be reduced and the leaves will appear chlorotic (pale green or yellow).
The reason for this deficiency symptom is that magnesium is required for chlorophyll formation - or the production of the green photosynthetic pigment.
Thus, without magnesium, a plant will not be able to produce chlorophyll which is vital for photosynthesis. A plant not being able to photosynthesise means that it would be unable to get energy from light, which for a plant is life threatening.
The reason for this deficiency symptom is that magnesium is required for chlorophyll formation - or the production of the green photosynthetic pigment.
Thus, without magnesium, a plant will not be able to produce chlorophyll which is vital for photosynthesis. A plant not being able to photosynthesise means that it would be unable to get energy from light, which for a plant is life threatening.