the purple allele(P) - dominant
white allele (p) - recessive
for the recessive allele to be expressed, both alleles should be recessive. The white plant therefore should have both p alleles. Genotype of white plant is pp.
the other parent is a purple plant. For the dominant allele to be expressed, at least one of the alleles should be P. Therefore genotype of the purple plant should have one P allele. the other allele needs to be determined.
For all the offspring, they receive one p allele from the white plant, since they are purple the other allele should be P.Genotypes of offspring should be Pp. For all the offspring to receive P from purple plant then purple plant should be homozygous PP, to give only P alleles.
Therefore genotype of parent purple plant is PP.