if a homozygous red plant is crossed with homozygous white plant what could be the phenotype of the F1 generation

Depends on the kind of interaction the alleles have with each other.

Dominant, co dominant or incomplete dominant

Let the homozygous red plant be of genotype RR and the homozygous white plant be of the genotype rr, denoting dominant homozygous and recessive homozygous respectively. Crossing the two plants would result in offsping genotype of Rr, heterozygous dominant, in the F1 generation. In this case, as I have denoted the dominant allele to result in red phenotype, Rr would result in red phenotype. However, if you were to assign R to the white phenotype, the phenotype for Rr would be white instead.

It is also worth noting that such cases are normally not that straightforward and it is important to check if there is incomplete dominance or codominance which can result in different phenotypes for intermediate heterozygous genotypes.