Morphometric analysis of cucurbit mitochondria: The relationship between chondriome volume and DNA content

Abstract

Electron micrographs of cucurbit seedling cells were subjected to morphometric analysis in order to determine organellar volumes. Serial sections revealed that for watermelon, zucchini squash, cucumber and muskmelon there were hundreds of small, ovoid mitochondria in cotyledon, leaf and hypocotyl cells, but only about 7 or 8 large, branching mitochondrial reticula in apical meristem cells of watermelon and muskmelon. For a given tissue there was little species to species difference in the volume of the cells, nuclei, total plastids and total mitochondria (the chondriome), although for each species the volumes were widely variable for cells and plastids, and somewhat variable for nuclei and the chondriome. Despite our previously determined 7- to 8-fold range in mitochondrial DNA content among cucurbit species, there was no variation among species in the volume of the chondriome for any of the four tissues analyzed. We conclude that for both individual, ovoid mitochondria and the entire chondriome, there is no correlation between volume and DNA content, as has been sometimes reported for the nucleus. Calculations show that in cells of differentiated tissues, the number of ovoid mitochondria may be larger than the number of mitochondrial genomes.