The major portion of the dry weight of plants comprises of : -

The dry matter of plants is mostly made of organic compounds such as cellulose, starch, lignin, proteins and fats.
These are built mainly of carbon, hydrogen and oxygen. Carbon and oxygen come from $CO_2$ and hydrogen from water, through photosynthesis.
Together C, H and O make up about 94–96 percent of the dry weight of a plant; all the mineral elements together (N, P, K, Ca, Mg, S and micronutrients) form only about 4–6 percent.
So the major portion of the dry weight of plants comprises carbon, hydrogen and oxygen.