English: The OPLS model of the metabolite profile of Populus leaves at different sampling positions. (A) The leaf numbers (leaf) refers to the sampling position, where leaf one is the first leaf longer than 1 cm. We sampled leaves from position 1 (leaf 1) to position 20 (leaf 20). (B) Expansion of upper side including apex, leaf 1 to leaf 8 of an aspen plant. (C) The number of components in the OPLS model was determined as two (one orthogonal and one predictive) according to seven-fold full cross-validation (Wold, 1978). The model explains 98.1% of the variation in Y (R2Y = 0.981) and the estimated ability to predict 92.1% of the variation in Y (Q2Y = 0.921) according to cross-validation. The model was able to model 47.8% of the variation in X, 22.9% of the variation is correlated to leaf position (Y) and 24.9% is uncorrelated.