(1) The principle of "three combinations" breeding method is gene mutation and gene recombination. Artificial mutation such as "chemical mutation" and "physical mutation" can improve the mutation rate and create the mutation types needed by human beings. Because the genes of different parents are quite different, it is also possible to produce mutations through "distant hybridization".

(2) Seed coat develops from integument, endosperm develops from fertilized polar nucleus, and embryo develops from fertilized egg, so the chromosome numbers of seed coat, endosperm and embryo are 2N, 3N and 2n in turn.

(3) Trace elements are absorbed by the roots of colored wheat in the form of ions through active transportation.

(4) Due to the combined effects of space radiation and microgravity, genetic material in seeds is more prone to mutation, so crop space breeding technology has greater advantages than artificial mutation breeding.

So the answer is:

(1) Gene mutation Gene recombination mutation rate Mutation type Genetic material gene recombination produces variation.

(2)2N、3N、2N

(3) Roots are absorbed from soil by active transportation.

(4) Under the combined action of space radiation, microgravity and other space environmental factors, the genetic material in seeds is more prone to mutation.