Effect of salinity and PEG-induced water stress on water status, gas exchange, solute accumulation, and leaf growth in Ipomoea pes-caprae

This study was designed to evaluate the response of I. pes-caprae, a pantropical species growing down to the high tide mark on the beach, under low and high drought and saline stresses and under both stresses applied simultaneously. Leaf water relations, gas exchange parameters, and leaf expansion and production rates were evaluated using NaCl and Polyethylene glycol 6000 to adjust the iso-osmotic potential in the nutrient solution to -0.5 and -1.0 MPa. Additionally, both osmotica were combined in the quantities required to allow that both solutes contribute in the same proportion to the total reduction in the solution water potential. The leaf osmotic potential decreased more rapidly at low stress levels and was always lower in plants growing under salinity. At long term, the osmotic adjustment was reached in all treatments, but under high stress, the osmotic adjustment was delayed and plants experimented eventual turgor loss. The simultaneous addition of NaCl and PEG accelerated turgor recovery after the induction of osmotic stress, compared with both agents applied individually. Na+ accumulation was enhanced in plants by salt addition, while proline content per leaf water unit increased significantly only at high stress levels. In all treatments maximum assimilation rate and stomatal conductance decreased the first day after addition of salt and/or PEG, and thereafter progressively increased throughout the treatment. At short term and at high stress level, the leaf expansion rate (LER) decreased compared with control plants. When both stresses were applied simultaneously, the effects on LER and leaf area were less detrimental when compared with each stress separately. The results suggest that I. pes-caprae is more susceptible to high water limitations than to soil salinity, while the water and carbon balance were always enhanced when both stresses were applied simultaneously. Thus, although salinity and drought stress are physiologically related and the tolerance mechanisms overlap, some aspect of plant physiology and metabolism may differ when the plant experiments single saline and water or both stresses simultaneously. (C) 2010 Elsevier B.V. All rights reserved.