Expression levels of candidate reference genes in different plant samples. Expression data displayed as Cq values for each reference gene in all T. grandis samples. The horizontal lines of the box indicate the 25th and 75th quartiles. The central horizontal line across the box is depicted as the median. The whisker caps represent the maximum and minimum values. Dots represent outliers. Genes are in order from the most (lower Cq, on the left) to the least abundantly expressed (higher Cq, on the right).