Solute Leakage from Solanum nigrum L. Seeds Exposed to High Temperatures during Imbibition

Exposure of Solanum nigrum L. seeds to high temperatures duringimbibition affected their leakage pattern: (1) The rate of leakageof total electrolytes was markedly increased with elevationof temperature. The increase was highest during the first 3h of imbibition but with a reduced rate thereafter. (2) Leakageof Na⁺ was almost complete after 6 h of imbibition at both temperatures,but much more Na⁺ leaked out at 50?C than at 25?C. (3) A markedincrease in leakage of K⁺ occurred after 24 h of exposure to50?C so that after 96 h three times more K⁺ leaked out at 50?Cthan at 25?C. (4) After 6 h of imbibition Ca11 and Mg⁺⁺ continuedto leak out at 25?C and at 50?C at a similar rate. (5) Imbibitionat an elevated temperature induced a marked increase in theleakage of both nucleic acids and proteins. (6) Malate dehydrogenasewas not detected in the leachate at 25?C, but was found after48 h at 50?C. It is assumed that this enzyme was of cytoplasmicorigin, indicating heat damage to membranes. The possible roleof the above phenomena in the loss of viability of the seedsdue to exposure to high temperature during imbibition is discussed. Key words: Leakage, Germination, S. nigrum     

Changes in Properties of Malate Dehydrogenase Isolated from Solanum nigrum L. Seeds Imbibed at High Temperatures

Exposure of Solanum nigrum L. seeds for 48 h to high temperature(50°C) during imbibition, lowered considerably the activityof malate dehydrogenase in the mitochondria and in the cytosol.The affinity of both isozymes for their substrates and co-enzymeswas strongly depressed by this treatment. Optimal pH and optimaltemperature for maximal specific activity were also changed.Enzyme from treated seeds, although much less active, was moreheat tolerant. Separation on a DEAE cellulose column showedtwo distinct peaks of activity for cytosolic malate dehydrogenasefrom both treated and control seeds. Gel filtration througha G-75 Sephadex column showed only one peak of activity in bothcases. Gel electrophoresis of s-MDH from control seeds showedtwo bands of enzyme activity for each of the peaks eluting fromthe DEAE column. Only one activity band could be detected forenzyme from heat treated seeds and this band corresponded toone of the activity bands found in the control. From analysisof the data it appears that while part of the enzyme was denatured,due to exposure of the seeds to 50°C, a conformational changein the other part of the enzyme might have occurred which resultedin a change of enzyme properties and activity. However, on thebasis of the reported results an induction of synthesis of anew ‘heat tolerant’ enzyme, due to the treatment,cannot be ruled out. Key words: Solanum nigrum L., malate dehydrogenase, heat treatment, seeds