Does microwaving or freezing reduce the losses of non-structural carbohydrates during plant sample processing?

Context: Non-structural carbohydrates (NSC) mediate tree survival, but precise determinations of NSC concentrations remain challenging. An unclear aspect is whether carbohydrate losses during sample processing can be reduced by different sample treatments. Aims: We postulated that due to higher metabolic rates, leaves should be more responsive to sample treatments than branch sapwood, and resource-acquisitive species should be more responsive to sample treatment than resource-conservative species. Methods: In leaves and branch sapwood of six tree species, we compared the effects of three sample treatments on the concentrations of NSC, starch, and low-molecular weight sugars: sample microwaving before drying (microwave), sample freezing at -20ºC before drying (freezing), and only drying (stove). Results: We found that across species and tissues, freezing led to significantly higher NSC and sugar concentrations than microwave and stove. This effect was, however, entirely driven by the leaves. Although the effect of sample treatments on NSC, starch, and sugar concentrations differed among species, resource-acquisitive species were not necessarily more affected by sample treatments than resource-conservative species. Conclusion: Results suggest that either freezing or microwaving does not reduce carbohydrate losses in branch sapwood during sample handling and processing; however, freezing reduces leaf sugar and NSC losses when compared to microwave or direct stove-drying.