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Relative Thermal Stability of Malic Dehydrogenase From Heat-Hardened and Unhardened Phaseolus Sp.¹

Malic dehydrogenase was extracted from heat-hardened and unhardened plants of two varieties of Phaseolus vulgaris L., ‘Harvester’ and ‘G.N. Nebraska #1’ and P. acutifolius, Gray var. latifolius Freeman, ‘Tepary Buff.’ The leaf disc heat tests showed that Tepary was significantly the most heat-tolerant variety and that Harvester was significantly more heat-tolerant than Nebraska #1. Purified fractions of malic dehydrogenase were exposed to temperatures of 52 to 62 C for 15 min. This thermostability test showed that the enzyme from Harvester was significantly the most thermal stable, and the enzyme extracted from Tepary was significantly the least thermostable for both the heat-hardened and unhardened plants. Heat hardening increased the thermal stability of malic dehydrogenase extracted from all three varieties. Other data indicate that the leaf disc heat test correctly ranked the varieties for heat tolerance. Since the varieties were not properly ranked for heat tolerance by the thermal stability of extracted malic dehydrogenase, malic dehydrogenase does not appear to be a key enzyme in heat tolerance. The enzyme stability test ranked the two P. vulgaris varieties properly, but Tepary, a variety of P. acutifolius, did not appear to be correctly ranked. Enzyme variation based on species difference may tend to account for the discrepancy in ranking.

Key Words: Heat tolerance • Enzyme • Malic dehydrogenase • Heat hardening • Phaseolus

² Associate Professor, Department of Horticulture and Forestry, University of Nebraska, Lincoln, Nebraska 68503.

Received for publication September 20, 1969.