Influence of external salinity on the osmolality of xylem sap, leaf tissue and leaf gland secretion of the mangrove Laguncularia racemosa (L.) Gaertn

This study assessed if mature leaves of Laguncularia racemosa were able to demonstrate salt secretion, and if the magnitude of secretion was a function of soil salinity. Thus, salinity influence on the osmolality of leaf tissue, xylem sap and leaf secretion was assessed in field and glasshouse experiments. As salinity increased, solutes were accumulated in sufficient quantity to decrease osmotic potential over the whole range of water potential. In the field, xylem osmolality (mol m(-3)) increased with salinity from 32.4 +/- 2.9 at 17parts per thousand to 38.2 +/- 0.6 at 28parts per thousand. Similarly, in the glasshouse, xylem sap osmolality (mol m(-3)) increased from 33.4 +/- 1.8 (15parts per thousand) to 40.6 +/- 1.5 (30parts per thousand). Changes in Ne concentration explained about 51-58% of increase in xylem osmolality. Rates of secretion (mmol m(-2) day(-1)) in the field increased from 0.80 +/- 0.12 (17parts per thousand) to 1.16 +/- 0.14 (28parts per thousand), and in the glasshouse the secretion increased from 0.73 +/- 0.07 (15parts per thousand) to 1.25 +/- 0.07 (30parts per thousand). The Na' accounted for 40-53% of total secretion. This study presented evidence of the capability of mature leaves of L. racemosa to secrete salt for the first time, and that the rates of secretion were enhanced as soil salinity increased.