Adaptation and acclimation of higher plants at the enzyme level: thermal properties of NAD malate dehydrogenase of two species of Aster (Asteraceae) and their hybrid adapted to contrasting habitats

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Abstract
Thermal and kinetic properties of NAD malate dehydrogenase (MDH) were investigated in clonal populations of two species of Aster and their hybrid: A. acuminatus, a forest understory species; A. nemoralis, a sphagnum-bog species; and A. blakei, their hybrid occurring at the bog–forest ecotone. The populations were collected within a 300 m radius in southwestern Quebec. Compared with A. acuminatus, the MDH of crude extracts from A. nemoralis had lower thermostability in both 5- and 10-min assays at 55 °C, and reduced apparent energy of activation (Ea) in the temperature range of 15–25 °C. However, these differences were not maintained in purified extracts of the species and may be attributed to higher phenolase and peptidase activity in crude extracts of A. nemoralis. The ratio of MDH activity over total protein concentration, or fresh weight leaf tissue, was higher in A. acuminatus than in A. nemoralis. Most values for these MDH properties of A. blakei were intermediate between those of the two parents. No differences, however, were observed for the substrate binding ability (Km) of MDH in the three taxa. Electrophoretic analyses show no qualitative differentiation in the enzyme profiles of MDH of the three taxa, which consist of two mitochondrial and six cytosolic isozymes. Mitochondrial isozymes were more thermostable but no differences in thermostability were observed among the forms of the species. The thermal and kinetic properties of malate dehydrogenase, as measured in situ, have not been substantially modified by the contrasting microclimatic regimes associated with the habitats of A. acuminatus and A. nemoralis.