If two plants that were both true-breeding for the same trait had offspring, how would the traits of the P and F1 generations compare?

True-breeding plants have two of the same alleles for a particular trait. Therefore, the genotypes of true-breeding plants are homozygous recessive or homozygous dominant. Thus, true-breeding plants have only one type of allele to pass onto their offspring. If two individuals that were each true-breeding for the same characteristic had offspring, then their offspring would also be true-breeding for that trait.  

For example, say both parents were homozygous dominant (DD) for a trait. Then, the cross could be represented as such:

DD x DD

                    D                                      D

D               DD                                      DD

D               DD                                      DD

All offspring would also be homozygous dominant for that trait.

Likewise, if all two parents that were homozygous recessive (dd) for a trait had offspring, then the following cross would result:

                     d                                      d

d                 dd                                      dd

d                 dd                                      dd

All offspring would also be homozygous recessive for that trait.