Shoot δ 13 C values as an indicator of cumulative stress: the role of re-watering during drought and salinity stress.

RATIONALE: The plant carbon stable isotope composition (δ13 C value) can reflect prolonged drought and salinity, as different isotopic signal resulting from drought and salinity can be retained in plant tissue. Commonly, drought and salinity are interrupted by intermittent precipitation or irrigation. It remains unclear whether the δ13 C values reflect the cumulative duration of intermittent drought or salinity stress.

METHODS: Drought (5% and 10% polyethylene glycol) and salinity (35 mM and 85 mM NaCl) were imposed on annual ryegrass consistently or cyclically; throughout the treatments, the stress duration for cyclic drought/salinity was half that of the corresponding prolonged stress treatment. The shoot δ13 C values were measured using isotope ratio mass spectrometry.

RESULTS: Prolonged drought restrained growth and increased shoot δ13 C values relative to the control group. However, the shoot biomass was even lower under cyclic drought than under prolonged drought. Furthermore, the shoot δ13 C value under cyclic drought was close to that of the control group. The low NaCl concentration treatment actually enhanced shoot growth. The shoot δ13 C value varied with both duration and intensity of salinity across all groups.

CONCLUSIONS: The shoot δ13 C value in annual ryegrass did indicate cumulative stress from cyclic low salinity, but not that from cyclic drought, in a manner that was mediated by the effect of re-watering on the mass and allocation of the photosynthates produced during stress.