Salt-induced respiration in Bruguiera cylindrica - role in salt transport and protection against oxidative damage.

Leaves of Bruguiera cylindrica plants grown in the greenhouse and irrigated with fresh water (FW plants) and those from salt-adapted plants from the seacoast (SW plants) showed about 5-fold and 3-fold increase in respiration, respectively, when immersed in 4M NaCl solution. The increase in respiration was not due to dehydration effect of high salt concentration, since PEG-imposed dehydration stress to the leaves led to an inhibition of respiration rates in both FW and SW plants. The salt-induced increase in respiration rate was specific to monovalent cations, especially Na(+) and K(+), but not divalent or trivalent cations, and to Cl(-), but not other anions. Pretreatment of leaves of FW plants with 1mM amiloride, an inhibitor of the Na(+) / H(+) antiporter, reduced the NaCl-induced respiration surge. At least some part of the observed respiratory increase could therefore be for providing energy for ion transport, since the Na(+) / H(+) antiport activity is driven by activities of the tonoplast and plasma membrane H(+)-ATPases and H(+)-PPases. Respiration of the leaves from both FW and SW plants was accounted for by the COX pathway and was inhibited by KCN. But 4M NaCl-induced increase in FW, but not SW plants, was inhibited by the AOX inhibitor, SHAM. Also, generation of ROS was reduced by treatment with KCN, but increased with SHAM. This pointed to a protective role of AOX in reducing ROS generation during salt-induced respiration. Our results indicated that NaCl-induced increase in leaf respiration of B. cylindrica plants irrigated with fresh water was required for (a) salt transport and (b) reducing the harmful effects of ROS that are known to accompany increased respiratory activity.