Biochemical and Ultrastructural Results during Starch-Sugar-Conversion in Ray Parenchyma Cells of Populus during Cold Adaptation

Summary In wood ray parenchyma cells of Populus x canadensis Moench «robusta» the content of starch, individual sugars, protein and fat were followed biochemically during cold adaptation in fall, parallel to changes seen at their ultrastructural level. After leaf abscission, starch is hydrolysed by amylases. Initially, a large maltose pool is formed that is then converted into sucrose and its galactosides. The lower the temperature the more complete is the starch hydrolysis and its conversion into sucrose, raffinose and stachyose. There was no significant starch-fat conversion during this stage. Instead, fat decreased and glycerol increased. The total sugar content in the wood reached 27 and 35 μg mg -1 DW (in hexose units) at 0 °C and -5 °C, respectively. Sucrose, raffinose and stachyose increased to 19 and 27 μg mg -1 at the same time. This increase corresponded fairly exactly to the loss found in starch content. A sugar accumulation in the ray cell protoplast of 350 to 500 μg mg -1 was computed for the winter stage investigated. Parallel to the accumulation of sucrose and its galactosides, the protoplast becomes enriched with a population of tubular and vesicular smooth ER cisternae. This vesicle formation is still insignificant earlier in fall when the large maltose pool is present. Localization of sucrose and its galactosides in these compartments is therefore indicated. Structural changes observed in protein bodies suggest that they are possibly involved in the delivery of the protein moiety for the vast amount of biomembranes formed during this stage. Both the variations in ultrastructure and in individual sugars illustrate the continuing progress in the ray cell protoplast acclimation during fall.

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