EVALUATION OF PROLINE ACCUMULATION IN THE ADAPTATION OF DIVERSE SPECIES OF MARSH HALOPHYTES TO THE SALINE ENVIRONMENT

Proline accumulation by eight major species of salt marsh halophytes was examined under growth chamber and field conditions. When the plants were exposed to increasing salinities in the growth chamber, they accumulated proline after a threshold salinity had been reached. Three general patterns were apparent. Limonium carolinianum (Walt.) Britt. and Junius roemerianus Scheele began to accumulate proline at 0.25 m NaCl with accumulations up to 63.6 μmoles per gram fresh weight at higher salinities. The C₄ grasses, Spartina alterniflora Loisel., Spartina patens (Aiton) Muhl., and Distichlis spicata (L.) Greene, had threshold salinity levels around 0.5 m NaCl and accumulated proline to 27.4 μmoles per gram fresh weight. The succulents, Salicornia bigelovii Torr., Salicornia virginica L., and Borrichia frutescens (L.) DC, did not accumulate proline until very high salinities (0.7 m) were reached. Water stress imposed by polyethylene glycol instead of NaCl caused similar proline accumulation in the species studied, but to different extents. Field measurements of proline content and soil salinities correlated well with the findings from growth chamber experiments. Rates of proline accumulation and breakdown in L. carolinianum were sufficient for osmotic adjustment by the plant to the changes in interstitial salinity in the marsh. The significance of proline accumulation as an adaptation to the salt marsh environment was species specific. We suggest that proline accumulation is of considerable importance in L. carolinianum and J. roemerianus, important to the C₄ grasses at certain times and in certain locations in the marsh, and of little importance in the succulents.