What is the phenotype of a plant with the genotype RR

To give a direct answer, I’d have to know what gene we were looking at. However, in a general sense, when a genotype has two capital letters, it means that it’s homozygous dominant. Take for example:

R = tall stalk

r = short stalk

The uppercase R is a dominant allele, which means if the plant has the gene with this in it (RR or Rr) then it will have that trait. If it has two lowercase letters (rr) then it will be the recessive trait.

Using this example, RR would be the tall stalk. For whatever your question is, the dominant phenotype would be the answer.