Question

the shoot apex of tomato are cut away to improve the yield. why?

Answer 1

The shoot system of the tomato plant is made up of branching stems. A stem has a terminal bud at the tip or apex that is responsible for the increase in length of the stem. Branches grow out from axilary/lateral buds. In tomato, as in all typical dicot plants, lateral buds are found in the axil of the leaf--where the leaf connects to the stem.While the terminal bud (or shoot apex) of the stem is growing vegetatively (producing leaves), the growth of lateral branches is somewhat inhibited ("apical dominance"). Eventually, however, the terminal bud stops growing because it has aborted, been cut off by pruning, or it forms an inflorescence (branches producing flowers rather than leaves). When this happens, the lateral branches grow out.
2. Sympodial and Monopodial Stems 
a. In many plant species, the first stem grows vegetatively for many years, forming the main axis of the plant by itself. This kind of axis is called monopodial.b. In other plants, such as the tomato, the first stem terminates rather early. A lateral branch will take over and grow out for several nodes. Soon this branch terminates (often after a strict number of nodes bearing leaves) and a lateral branch takes over, and so on. In this kind of plant, the main axis is composed of many lateral branches, and is called sympodial.