Definition of "structural underdominance"?
I understand heterozygote inferiority (also underdominance or heterozygote disadvantage) as the opposite of heterozygote advantage, that is, lower fitness of the heterozygous genotype than either homozygote (as reference, see Hedrick, 2009, p. 119). I haven't seen the term structural underdominance before. However, heterozygote disadvantage can sometimes be seen in species/subspecies hybrids and also chromosomal heterozygotes (see Hedrick, 2009, p 140), and this might be what the term structural underdominance is describing (structural as in chromosomal). This can then lead to unbalanced gametes with low viability, which means low fitness compared to homozygotes. The quote you have included also talks about plant hybrids, which makes this explanation likely, and it also makes sense since they mention artifical doubling as a solution. If I have understood the process correctly, chromosomal doubling should lead to better pairing of chromosomes and lower risk of unbalanced chromosome translocations.

Also note that Hedrick labels heterozygote disadvantage as an unstable equilibrium (p. 140ff), but I haven't looked more closely at the dynamics behind this.