Tolerance and avoidance responses to salinity and water stresses in Calotropis procera and Suaeda aegyptiaca

This study was designed to evaluate the response of 2 wild shrubs, Calotropis procera and Suaeda aegyptiaca, to salinity (100% seawater), drought, and waterlogging stresses. The 90-day-old plants were subjected to the stress treatments for 3 weeks, and growth and some physiological parameters were evaluated. The total plant dry mass of C. procera plants was reduced by 40%, whereas S. aegyptiaca was not significantly affected by salinity stress. Water deficit and waterlogging stresses significantly reduced the total dry mass of both species. Under all conditions, the root/shoot ratio in C. procera was 3-fold higher than in S. aegyptiaca. All applied stresses markedly increased leaf shedding in C. procera plants only. These plants appeared to have a higher salinity and waterlogging stress intensity index as manifested by chlorophyll levels lower than those in S. aegyptiaca. Under all conditions, Na+ levels of S. aegyptiaca were twice those of C. procera plants. All stresses reduced the K+/Na+ ratio in C. procera leaves. On the other hand, S. aegyptiaca plants were able to maintain this ratio near control levels under salinity and drought stresses. Consequently, S. aegyptiaca leaves had higher partial osmotic pressure than C. procera. The proline and total free amino acids levels in C. procera were between 1.3- and 2-fold higher than in S. aegyptiaca. Among all amino acids, the common change in both species under all stresses was an accumulation of free proline and a decrease in methionine levels. The results revealed that the response of both species to salinity and water stresses included avoidance and tolerance mechanisms with some differences between them.