Role of Macronutrients

Magnesium is absorbed by plants in the form of the divalent ion $Mg^{2+}$.
It is a constituent of the ring structure of chlorophyll: the chlorophyll molecule has a porphyrin head made of four pyrrole rings, and all four rings are bound to a magnesium atom ($Mg^{2+}$) present at the centre.
So a plant requires magnesium for chlorophyll synthesis; its deficiency causes chlorosis.
Magnesium also activates the enzymes of respiration and photosynthesis, is involved in the synthesis of DNA and RNA, and helps to maintain the structure of ribosomes.
Cell wall development and holding the cells together are functions of calcium, which forms calcium pectate of the middle lamella.
So the answer is chlorophyll synthesis.

Plants obtain sulphur in the form of sulphate ($SO_4^{2-}$).
Sulphur is present in the amino acids cysteine and methionine, and is a constituent of several coenzymes, vitamins (thiamine, biotin, coenzyme A) and ferredoxin.
Oilseed crops have a particularly high requirement of sulphur, because it is needed for the synthesis of oil and of sulphur-containing compounds.
For example, the pungent oils of mustard and other crucifers (glucosinolates) contain sulphur.
An adequate supply of sulphur increases both the seed yield and the oil content of oilseed crops such as mustard, groundnut and sunflower.
Pulses also respond to sulphur, but its importance for productivity is greatest in oilseeds.
So sulphur is an important nutrient for optimum growth and productivity in oilseed crops.