Leaf carbon isotopic composition (δ13C) and cold hardiness of wheat in relation to growth temperature and moisture stress

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Several cultivars of spring and winter wheats were grown simultaneously several times in environmental control rooms at (i) 25:20 °C (16 h light: 8 h dark), or (ii) 4:2 °C with continuous watering, or (iii) 25:20 °C with nighttime watering and daytime wilting. The mature leaf blades of late vegetative plants were collected, and their carbon isotope ratios (13C:12C) were measured by mass spectrometry and expressed as δ13C, with an accuracy of 0.2‰, to determine if physiological effects of environmental factors other than atmospheric isotope composition could be responsible for previously observed annual cycles in plant δ13C. In (i) and (ii), the values of leaf δ13C (−31 ± 1‰) were in the low end of the C3 range, but winter wheats were about 1.2‰ less negative than spring wheat. The values obtained in (i) were 2‰ lower (more negative) than in (ii) for both wheat types. These results might be indicative of lower carboxylation resistance in winter wheat than in spring wheat and at lower temperature than at higher temperature. The effect in the growth room of temperature alone quantitatively duplicated the seasonal effect in the field. Leaf δ13C values of plants in (iii) were at or beyond the high end of the observed C3 range, presumably as a result of daytime gas exchange limitations imposed by stomatal diffusion resistance and the consequent importance of nighttime phosphoenolpyruvate carboxylase activity. This drought effect was particularly pronounced in winter wheat.