Decrease in sunflower (Helianthus annuus) seed viability caused by high temperature as related to energy metabolism, membrane damage and lipid composition

The aim of the present work was to investigate whether loss of germination ability and viability of sunflower ( Helianthus annuus L.) seeds during incubation at a high temperature (45degreesC) was related to changes in energy metabolism, loss of membrane integrity, and/or changes in lipid composition. Pre-treatment of seeds at 45degreesC progressively reduced subsequent germination at the optimal temperature (25degreesC). Seeds did not germinate at 45degreesC and almost all of them were dead after 72 h of soaking at this high temperature. This loss of seed viability was associated with a large increase in leakage of K+ and total electrolytes into the incubation medium, and with production of malondialdehyde in the embryonic axis and cotyledons, suggesting a loss of membrane integrity probably due to lipid peroxidation. ATP and ADP levels increased sharply during the first hours of imbibition at 45degreesC, remained high for about 24 h and then decreased. As a consequence, the energy charge followed a similar pattern. If the treatment at 45degreesC did not exceed 48 h, seeds recovered an apparently normal energy metabolism after transfer to 25degreesC, even though they lost their ability to germinate at this temperature. Therefore, energy metabolism at the whole embryo level cannot be considered as an indicator of germination ability. Incubation of seeds at 45degreesC resulted in an increase in triacylglycerols and diacylglycerols without a significant change in their fatty acid composition. It also induced a slight increase in phospholipid content with an increase in C-16:0, C-18:0 and C-18:1, but with no change in C-18:2. In phospholipids, the C18:2/C18:1 and (C-18:1 + C-18:2)/(C-16:0+C-18:0) ratios thus declined during treatment at 45degreesC. The results obtained suggest that deterioration of sunflower seeds during incubation at a high temperature is mainly related to membrane damage and alteration of energy metabolism, and that accumulation of malondialdehyde, which is an index of lipid peroxidation, does not correspond to a decrease in total lipids and phospholipids nor to a significant change in fatty acid composition, except in PL in which the C-18:2/C-18:1 and (C-18:1+C-18:2)/(C-16:0+C-18:0) ratios slightly declined.