*2.7. Carbon Isotope Discrimination (*∆*) Measurements*

Plant sampling was conducted with 6 h intervals during 24 h cycle: at the start of 18 h photoperiod and at 6, 12, and 18 h. Plant leaves were sampled in 4 replicates and dried in an oven at 65 ◦C.

Dry plant material was milled using Vibromill vibrator. After milling in powder, samples were weighed in tin containers and introduced by means of an autosampler into the elemental analyzer (FlashEA, Thermo Electron, Waltham, MA, USA), where in presence of O<sup>2</sup> and catalysts, they were quantitatively burnt to CO2. The formed gas was separated from other combustion gases on a chromatographic column and transferred via ConFlo interface to the isotope ratio mass-spectrometer (Delta V, Thermo) for analysis. Each sample was analyzed three times.

For calibration of IRMS instrument and control of results' accuracy we used IAEA standards of L-glutamic acid (USGS40, USGS41).

The values of the isotope ratio are expressed in δ‰ according to the formula

$$\boldsymbol{\delta}\% = \left(\mathbf{R}\_{\text{sample}} - \mathbf{R}\_{\text{standard}} / \mathbf{R}\_{\text{standard}}\right) \times 1000$$

where Rsample and Rstandard are the <sup>13</sup>C/12C ratios of the sample and the standard, respectively. By international convention, the standard used for the analysis was the carbon belemnite from the PeeDee formation (VPDB).
