The main technique used to distinguish C3, C4 and CAM plants is based on the quantification
of stable isotopes of 13C in plant leaves. However, this technique has a certain degree of
variability among individuals of the same species or between plant material derived from the
same plant, as in shaded leaves and leaves exposed to light of the same plant. In fact, for
several orchid species, values of 13C‰ typical of C3 plants were found, despite the fact that
they clearly exhibited the ability to fix some of the carbon at night, as reflected in the increased
nocturnal acidity in the tissues