It is verified that inbreeding in cross-pollinated plants is leading to a decrease of vigor of the progeny. Hybrid organisms show inverse effects of an unusual vigor--hybrid vigor or heterosis. Several authors described ways to explain hybrid vigor by genetic mechanisms. But physiological explanations of hybrid vigor remain unclear. The nucleo-cytoplasmic interactions might be involved --above all the mitochondrial-nuclear interactions.

Mitochondria were isolated from the same corn hybrid genotype (F7 x F2) on various cytoplasms (N, T, C) and from the parental lines (F7N, F7T, F7C, F2). Mitochondria exhibited a large variability as judged by the following parameters: Respiratory Control Ratio (RCR), ADP/O Ratio and oxygen uptake. We have studied the mitochondrial activities after 2, 3, 4, 5, 6, 7 or 8 days of germination with NADH or malate as substrate.

The hybrids exhibit the highest activity for 3-day-old seedling with NADH and for 4- or 5-day-old seedling with malate. The lines exhibit the highest activity for 5- or 6-day-old seedling with NADH and for 4- or 5-day-old seedling with malate. In all the genotypes the highest protein quantity is shown from 2 to 4 days. At the 5-day stage the protein quantity decreases suddenly and from 6 or 8 days the quantity increases but never as high as it is at the first 4-day stage. We think that during the first 4 days the shoot growth is owed to the lipid storage of kernels. After 5 days the plants are built from the catabolism of glucose.

For explaining the variation of the protein quantities in the preps we think that once the lipids are used up the protein composition is changed; the lipolysis enzymes disappear. The glycolytic enzyme pathway takes place at the 6-day stage; this corresponds to an increasing of the protein quantity.

The same hybrids on N or T cytoplasm show similar mitochondrial activity variations, while the variations for F7C x F2 are different. The F7 lines on C and T cytoplasm show similar mitochondrial activity variations while the variations for F7N are different.

The mitochondrial activities depend both on the kind of cytoplasm and on the nuclear genotype, so that we cannot measure the mitochondrial activity from the N, T or C cytoplasm but always an interaction of N, T or C cytoplasm and a nuclear genotype. If we compare the activities of a hybrid and its parental lines we state that the hybrid follows always the same variation as the female line but it may be different. The male line follows a different scheme of variation.

Maryse Charbonnier and A. Bervillé

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