Remobilization of leaf Na+ content and use of nonstructural carbohydrates vary depending on the time when salt stress begins in woody species.

Basic mechanisms are known to promote salt tolerance in plants: a delay in Na+ uptake or rapid Na+ remobilization from leaf tissue. We measured dynamics of the Na+/K+ ratio and components of carbon metabolism during the first 72 h after saline stress (200 mM NaCl) began in Cenostigma pyramidale, a woody species, under controlled conditions. Saline stress at two times: one plant group at the beginning of the morning and the other in the evening. Stressed plants had three times more Na+ in leaves than did control plants in the first 24 h. However, in the next few hours, despite new applications of saline solution, the Na+/K+ ratio continued to decline. Several samples, including night treatments, provided evidence that this species uses Na+ recirculation mechanisms to endure salt stress. Effects of salt on the traits evaluated differed depending on the time when stress began. Between the two saline treatments, in the first 24 h after saline stress, gas exchange decreased more strongly in morning-stressed plants, when large amounts of Na+ reached the leaf and K+ left this organ. Nevertheless, when stress was applied in the evening, leaf Na+ remobilization was faster, and the soluble sugar/starch ratio remained greater than did the control. Our data suggested that time of the beginning of salt stress could change the level of damage. Morning-stressed plants synthesized greater amounts of proline, H2O2, and malondialdehyde than did night-stressed plants. We recommend that details regarding the time of stress be taken into consideration in physiological studies.