Thermal Adaptation and Acclimation of Higher Plants at the Enzyme Level: Thermostability of Nad Malate Dehydrogenase in Three Species of Viola

Abstract

(1) Thermal properties of NAD malate dehydrogenase (MDH) were investigated in three violet species, Viola blanda, V. fimbriatula. and V. sororia. Plants from three clones of each species, collected over the latitudinal distribution of the species in eastern North America, were acclimated under controlled conditions at three different thermoperiods: 7-15 'C, 15-22 'C and 22-28 'C (night/day: 16 h photoperiod). (2) No clear acclimatory modifications of the thermostability of MDH were found although there was a tendency in some clones for thermostability to increase following cold acclimation. (3) The thermostability of MDH of Viola blanda and V.fimbriatula increased with the temperature at the original site of collection. For V. sororia this trend was not observed, but MDH from the most northern clone showed lower thermostability. (4) Adaptive trends were readily observed in between-species comparisons and the results correlate well with the habitat preferences of the species. The most thermostable MDH was that of Viola fimbriatula, an early successional species and the most thermolabile that of V. blanda, a species growing in cool, humid and shaded forest understoreys. For Viola sororia, a species occupying a broad range of micro-climates, MDH showed intermediate levels of thermal stability.