Haploid plants are preferred over diploids for mutation study because in haploids
Haploid plants are useful in mutation studies because a recessive mutation will be observed immediately in a haploid plant as the genotype can be identified from the phenotype. In case of diploid plants, a recessive mutation of a dominant gene will be covered by a corresponding dominant allele and the mutation will not be observed until segregation. So haploid plants are preferred over diploids for mutation study because of their clear/obvious expression.