The relationship between malate dehydrogenase (MDH) isozyme genotype and plant vigor in peach [Prunus persica (L.) Batsch] was examined in two F2 populations (selfed `Belle of Georgia' and `Cresthaven') segregating at the Mdh1 locus. Total progeny examined were 1610 and 998 in the `Belle of Georgia' and `Cresthaven' populations, respectively. In both populations, plant vigor (as defined by total height and trunk caliper after 1 year of growth) was significantly less in Mdh1-1/Mdh1-1 homozygotes. Homozygous Mdh1-2/Mdh1-2 individuals showed the greatest vigor, and were significantly different in vigor from Mdh1-1/Mdh1-1 homozygotes in both populations and from Mdh1-1/Mdh1-2 heterozygotes in the `Belle of Georgia' population. A significant deviation from the expected 1 Mdh1-1/Mdh1-1: 2 Mdh1-1/Mdh1-1: 1 Mdh1-2/Mdh1-2 ratio was observed in the `Belle of Georgia' population, suggesting moderate lethality of homozygous Mdh1-1/Mdh1-1 genotypes.
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