Light Conditions Affect NaCl-Induced Physiological Responses in a Clonal Plant Species Hydrocotyle vulgaris

In order to understand if differences in light conditions can affect responses of a clonal plant species to increased soil salinity, the long-term effect of two substrate concentrations of NaCl on leaf growth, clonal plasticity and oxidative enzyme (peroxidase and polyphenol oxidase) activity in Hydrocotyle vulgaris L. plants grown at low, moderate and high light conditions was studied. H. vulgaris appeared to be a shade-tolerant species, since both leaf blade and petiole growth was inhibited at low light intensity. At low light intensity, H. vulgaris plants represented characteristics of halophytes with significant stimulation of leaf blade and petiole growth by NaCl, especially at low concentration. Secondary clonal growth was stimulated by increased salinity in low light and, for 25 mM NaCl treatment, even in moderate light. There was more than a five-fold increase of peroxidase activity by 100 mM NaCl treatment in high light conditions, in comparison to two-fold and four-fold increase in low and moderate light, respectively. The existence of different endogenous control mechanisms during H. vulgaris response to light and NaCl at the level of leaf growth, clonal growth and oxidative metabolism could be proposed. The possible ecological meaning of the presented results is discussed in detail.