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, HO, and malondialdehyde than did night-stressed plants. We recommend that details regarding the time of stress be taken into consideration in physiological studies.