Insight into Membrane Stability and Physiological Responses of Selected Salt-Tolerant and Salt-Sensitive Cell Lines of Troyer Citrange (Citrus sinensis [L.] x Citrus trifoliata [L.] Raf.) under Salt Stress

The aim of this study was to evaluate the membrane integrity and some physiological responses of rootstock citrus calli under exposure to different concentrations of NaCl. Selected salt-tolerant cell lines were compared with salt-sensitive calli of Troyer's citrange (Citrus sinensis [L.] x Citrus trifoliata [L.] Raf.) (TC) with respect to growth, water content, Na+, K+ and Cl- ion content as well as cell membrane stability under exposure to different NaCl concentrations. The results show that the stressed sensitive lines have a consistently high ion efflux. The values recorded for these sensitive calli are 3 to 6 times higher than those of the tolerant calli. Thus, only selected halotolerant calli were able to maintain the integrity of their membranes under salt stress conditions. In the sensitive calli, NaCl always induces a slowing down of growth even from 4 g L--1,L- and the reduction in the relative growth rate is higher than 50% and reaches more than 90% for the three culture durations at 8 g L-1 NaCl. For the salt-tolerant selected lines, the relative growth rate seems to be slightly slowed down until the second month of culture but becomes equal to that of the control at the third month, whether at 4 or 8 g L-1 NaCl. At the end of the third month, the relative growth rate of the selected calli is 100% at 8 g L-1 NaCl. The water content is twice as high in the selected tolerant calli as in the sensitive ones after three months of salt treatment at 8 g L-1 NaCl. After long-term culture, the halotolerant calli absorbed similar or even higher amounts of Na+ and Cl- than the salt-sensitive lines. However, by the 3rd month, the recorded accumulation rate dropped in the unselected but continued to increase in the tolerant calli (4-fold higher at 12 g L-1 NaCl than the control). Furthermore, exposure of both types of calli (salt-sensitive and salt-tolerant) to equal concentrations of NaCl resulted in greater loss of K+ by the NaCl-sensitive lines. However, for tolerant lines, K+ uptake is not affected at 4 g L-1 NaCl and the decrease in tissue content is less than 25% at 8 g L-1 NaCl. From this observation, it can be concluded that growth and the ability to retain high levels of internal K+ are correlated.